Proceedings of the Japan-Indonesia bi-lateral workshop on

Subduction processes and related topics along the Sumatra-Java arc

March 11 (Wed), 2009 Venue: Seminar room-B, JAMSTEC Tokyo Office

Sponsors:

Japan Society for the Promotion of Science (JSPS) and Indonesian Institute of Sciences (LIPI)

Preface

It already passed more than four years from a tragedy of the great Sumatra-Andaman

earthquake of December 26, 2004. The great earthquake generated a giant tsunami that

caused tremendous damage to the countries surrounding the Indian Ocean and Andaman

Sea. The giant tsunami most affected the northwest tip of Sumatra, implying a very large

tsunamigenic (co-seismic) deformation on the sea floor off northwest Sumatra. However, its

tsunami generation mechanism remains unresolved.

This workshop has been proposed to discuss various topics relating to geological and

tectonic processes on-going in Sumatran and Java arcs including the 2004 tsunami

generation problem, and a kick-off meeting for the three year research project between Japan

and Indonesia.

In the workshop, we had fifteen oral presentations with three poster presentations.

Various scientific topics including characteristics of the large tsunami observed off northwest

coast of Sumatra and its waveform modeling, new interpretation of aftershock distribution

determined by OBS network, geological features on seafloor morphology off northwest

Sumatra, and GPS-based crustal deformation and modeling studies, etc, were presented and

extensively discussed to interchange recent findings and knowledgements between Japanese

and Indonesian researchers. We hope that the workshop will give us an opportunity to

advance our research activity.

The workshop is financially supported by the Japan Society for the Promotion of

Science (JSPS) and Indonesian Institute of Science (LIPI) under the JSPS-LIPI joint research

program of "Geological and Geophysical study in the southern part of the 2004 Indian Ocean

Tsunami source". We really appreciate JSPS and LIPI for their financial support.

Toshiya Fujiwara and Haryadi Permana

The Japan-Indonesia bi-lateral joint workshop on Subduction processes and related topics along the Sumatra-Java arc

Date: March 11 (Wed), 2009

Venue: Seminar room-B, JAMSTEC Tokyo Office Access: http://www.jamstec.go.jp/e/about/bases/tokyo.html

Sponsorship: This workshop is supported by Japan Society for the Promotion of Science

(JSPS) and Indonesian Institute of Sciences (LIPI) under the JSPS-LIPI joint research program of "Geological and Geophysical study in the southern part of the 2004 Indian Ocean Tsunami source"

PROGRAM Oral Session 08:30 Welcome: Toshiya Fujiwara (JAMSTEC) 08:35 Seismicity change preceding the 2004 (M9.0), 2005 (M8.6), and 2007 (M8.5)

great earthquake series : Shozo Matsumura (National Research Institute for Earth Science and Disaster Prevention)

09:20 Detection of crustal deformation following large earthquakes in Indonesia by using SAR: Agustan (Nagoya University)

09:45 . Cumulative process of strain in the northern part of the Great Sumatran Fault based on GPS observation: Endra Gunawan (Nagoya University)

10:10 Geotectonics in Java: Nuraini Rahma Hanifa (Nagoya University) 10:35-10:50 coffee break Chair: Mamoru Nakamura (Univ. of Ryukyus) 10:50 Characteristics of the large tsunami observed by fishermen far away from the

northern Sumatra coast during the 2004 Sumatra-Andaman earthquake : Yoshinori Hayashi (Shizuoka University)

11:30 Aftershock distribution off NW Sumatra determined with an emergent OBS network: Ei'ichiro Araki (JAMSTEC)

12:10-14:10 lunch (poster session)

Chairs: Haryadi Permana (LIPI), and Toshiya Fujiwara (JAMSTEC) 14:10 Preliminarily results from seismic surveys with SONNE in 2008: Yusuf S.

Djajadihardja (BPPT) 14:50 Heat Flow Study of Acretionary Prism off 2004 Great Sumatra-Andaman

Earthquake's Area: Udrekh (BPPT) 15:20 Geological approaches to the large earthquakes off northern Sumatra Island --

Surface and shallow subsurface structure analyses by deep-tow SBP and side-scan sonar and turbidite paleoseismology by piston coring -- :Kohsaku Arai and Ken Ikehara (GSJ-AIST)

15:50 Numerical simulation of the large tsunami observed far away from the northern Sumatra coast during the 2004 Sumatra-Andaman earthquake: Mamoru Nakamura (University of the Ryukyus)

16:30 – 16:45 coffee break Chairs: Yusuf S. Djajadihardja (BPPT), and Kohsaku Arai (AIST) 16:45 Geological structure off northwest Sumatra deduced from detailed seafloor

morphology and seismic reflection survey (Preliminary): Haryadi Permana (LIPI) 17:25 Magnetic susceptibility of coring samples from the trench of the subduction

zone west of Sumatera Island, Indonesia: Eddy Z. Gaffar (LIPI) 17:55 Baruna Jaya II, New Multi-channel – seismic vessel in Indonesia: Udrekh

(BPPT) 18:25 Source Model of the 2007 Bengkulu Earthquake Determined from Tsunami

Waveforms and InSar Data: Aditya R. Gusman (Hokkaido University) 18:55 Monitoring of seafloor deformation in Japan Trench: Yoshihiro Ito (Tohoku

University) 19:25 – 19:40 coffee break 19:40 discussions and future plan 20:40-20:45 Closing: Haryadi Permana (LIPI) Poster Session (core time: 12:10 -14:10) 1. Discovery of surface break of a thrust fault that initiated the Indian Ocean Tsunami in the Sumatra–Andaman Earthquake of 26 December 2004: Wonn Soh et al.(JAMSTEC)

2. Detailed seismicity around Nanaki Trough determined with ocean-bottom seismographs: Akira Yamazaki (MRI)

3. Prospect of tsunami early warning system based on earthquake rupture propagation : Kenji Hirata (MRI)

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 1 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Seismicity Change preceding the 2004(M9.0), 2005(M8.6), and 2007(M8.5) Sumatra Giant Earthquake Series Shozo Matsumura National Research Institute for Earth Science and Disaster Prevention, Tennodai 3-1, Tsukuba, Ibaraki, 305-0006, Japan (shozo@bosai.go.jp)

A giant earthquake of M9.0 attacked the Sumatra island on 26, Dec. 2004, and caused severe damages on the vast areas around the Indian Ocean with strong Tsunami disasters. Including this event, an earthquake series of M8 and greater succeeded along the Sunda trench after 2000, comprizing M8.3 in 2000, M9.0 in 2004, M8.6 in 2005 and M8.5 in 2007 (The first one may be a little overestimated). We examined the seismicity along the Sunda trench from the point of view whether or not some significant change preceded the major events. Sampling M5 and greater earthquakes from the USGS catalogue and treating it as a background seismicity, we depicted a contour map indicating seisimicity rate change. The seismicity during an examined period of 5 years is compared to the standard of 17 years from 1973 till 1989, and the examined period is moved every 2.5 years. A typical example of the seismicity change mapping is shown in Fig.1, the left of which indicates the rate distribution for the examined period just before the 2004 M9.0 earthquake (stage D), where red (blue) zones correspond to activation (quiescence) of the background seismicity. The right picture shows slips (asperities) of the actual earthquakes. Comparing both pictures, we recognize that those asperities of the forthcoming earthquakes are projected in advance on the seismogenic area as activated zones of the background seismicity prior to the focused

Figure 1 Seismicity change map just before the 2004 M9.0 earthquake (left picture), and real slips of the focused earthquakes analyzed by Yamanaka (right picture). The ellipses in the left mark the activated zones. The light green and the dark green zones in the right are the respective asperities of the 2004 and 2005 events. The red ellipses are activated zones yet unresolved at this time.

2 Matsumura ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ events, excepting only one positionned at the 2000 M8.3 event, which projects the aftershock performance asociated with the main shock.

We interpret this phenomenon based on the following hypothesis. The seismogenic area should have inhomogeneity in its locking strength. In this case, approaching the final breakage, a preceding quasi-static slip on weakly locked portions can be expected. Then, the slips leaving the stronger portions behind them, will induce stress concentrations on the asperities. The activations in the background seismicity may thus project the existence of such asperities.

We also depicted the similar figure for the succeeding period (stage E: from July 2002 till June 2007) following Fig.1, within which both the 2004 M9.0 and the 2005 M8.6 had already happened, but the 2007 M8.5 had not yet happen. In stage E, we recognize three activated zones, a newly one (red ellipse) as well as the previous two, all of which had been unresolved by M8 earthquakes. However, three months after stage E, one of these was resolved by the 2007 M8.5 (the bluish green in the right picture). As a result, two ellipses are left yet unresolved at present (red arrowed ellipses in the right). One is the Mentawai islands region off-coast of Padang city, and the other is the western area of the Sunda strait. The former is especially feared due to historical reoccurrence of an M8 earthquake series, which happened about two hundred years ago, and furthermore this region is recognized to form a remarkable seismic gap at present.

Figure 2 Seismicity change map after the 2004 M9.0 and the 2005 M8.6 events (left picture), and real slips analyzed by Yamanaka (right picture). The bluish green zone in the right is the asperity of the 2007 M8.5 event, which has resolved one of red ellipses in Fig.1. A newly activated zone appeared in the left. Including this, two activated zones (red arrowed ellipses in the right) are as yet unresolved at present.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 3 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Ground Deformation Assessment Following Large Earthquakes in Indonesia using DInSAR Technique Agustan1,2, Fumiaki Kimata1 1Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, JAPAN (agustan@seis.nagoya-u.ac.jp)

2PTISDA-BPPT, Jl. M.H. Thamrin No. 8 Jakarta, INDONESIA Ground deformation associated with an earthquake is the change in shape that can occur prior to, during or after an earthquake, and known as pre-, co- and post-seismic deformations. Previous studies show that there is a correlation of one earthquake to the next earthquake related to stress transfer caused by fault slip. A quantification of deformation rates, including relative movements over large regions and local displacements associated with large earthquake is increasingly used in earthquake hazard assessment. Interferometric Synthetic Aperture Radar (InSAR) is one technique in active remote sensing based on radar characteristics that can provide spatially dense coverage of ground deformation. The launching of Advanced Land Observing Satellite (ALOS) that is equipped with a L-band radar sensor (Phased Array type L-band Synthetic Aperture Radar or PALSAR) gives possibilities to enhance the ground deformation studies. The most of advantage of this system is the ability to penetrate high density vegetation and thick cloud-covered area. Therefore, this system is suitable for tropical region such as Indonesia. In addition, Indonesia is the second country in term of the number of earthquake events due to its location in a junction of plate boundary: Eurasia, Australia, Pacific and Philippine plates. This study discusses the ground deformation detected by InSAR in Indonesia, especially in Aceh region (including Bandaaceh area and Beutong area) related to the Mw 9.2 Sumatra-Andaman earthquake on December 26th 2004; and Manokwari region related to the Mw 7.6 Papua earthquake on January 3rd 2009. For Aceh region, the northern part of Sumatran Fault System is also covered in ALOS-PALSAR data which are selected to be processed. Time range of data observation is one year spanning from February 2007 to February 2008 consisted of 7 images. The perpendicular baselines are ranging from 88 m to the longest 450 m. The interferograms are stacked to get the average displacement rates. For Manokwari region, only two ALOS-PALSAR data are utilized that cover before and after the earthquake and its perpendicular baseline are only 1 m and therefore the topographic effects can be neglected. The data are processed using GAMMA SAR Software (Wegmuller and Werner, 1997). To improve interferogram coherence and noise reduction, adaptive smoothing window filter (Goldstein and Werner, 1998) is applied before unwrapping the interferogram using minimum cost flow algorithm. For longer perpendicular baseline, the SRTM-3 is used to model the topographic phase. It is found that there is a positive displacement along line-of-sight direction that can be interpreted as post-seismic deformation on Bandaaceh area. The quantification is approximately 3 cm in one year. Also there is no clear phase difference detected on approximate location of Sumatran fault. On the other hand, Beutong area indicates different pattern of ground deformation. There is an opposite direction of line-of-sight displacement for the north-east and south-west area which is bordered by the Sumatran fault. It represents the right-lateral creeping happening on this area. For Manokwari region, the co-seismic deformation is clearly detected by ALOS-PALSAR data and similar result by JAXA (2009). By converting the line-of-sight displacement to vertical component, it can be said that the maximum uplift is 25 cm and subsidence is 45 cm. The fault parameters are then modeled by constraining the line-of-sight displacement. It is found that the fault dimension is of this model is smaller compare to other model, i.e. Yamanaka (2009) and USGS model (Hayes, 2009) with maximum slip is 3 m.

4 Agustan et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Fig.1 The InSAR results for Aceh region in February 2007 – February 2008

Fig.2 The InSAR and Model results for Manokwari Earthquake

References Goldstein, R. M. and Werner, C. L. (1998) Radar interferogram filtering for geophysical

applications, Geophysical Research Letters, Vol. 25 (21), pp. 4035-4038. Hayes, G. (2009) Preliminary Result of the Jan 3, 2009 Mw 7.6 Papua Earthquake, USGS,

Available at http://earthquake.usgs.gov/eqcenter/eqinthenews/2009/us2009bjbn/finite_fault.php

JAXA (2009) Observation results of ALOS/PALSAR relating to the magnitude 7.6 and 7.4 earthquakes in Papua, Indonesia (1), Available at http://www.eorc.jaxa.jp/ALOS/en/img_up/dis_indoc_eq_090115.htm

Luis, J. F. (2007) Mirone: A multi-purpose tool for exploring grid data. Computers & Geosciences, 33, 31-41.

Wegmuller. U and Werner, C. (1997) Gamma SAR Processor and Interferometry Software, 3rd ERS Symposium, Florence.

Yamanaka, Y (2009) NGY 地震学ノート No.13, 名古屋大学地震火山・防災研究センター , Available at http://www.seis.nagoya-u.ac.jp/sanchu/Seismo_Note/2009/090103.jpg

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 5 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Cumulative process of strain in the northern part of the Sumatran fault based on GPS observation (2005 – 2008) Endra Gunawan1, Takeo ITO, AGUSTAN, Fumiaki KIMATA, Takao TABEI2, Hasanuddin Z. ABIDIN3, Irwan MEILANO3, Mipi A. KUSUMA3, Didik SUGIYANTO4, IRWANDI4, Muksin UMAR4 1Nagoya University, D2-2(510), Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan (endra@seis.nagoya-u.ac.jp)

2 Kochi University, 2-5-1 Akebono-cho, Kochi, 780-8520, Japan 3 Institute of Technology Bandung, Jl. Ganesha 10, Bandung, 40132, Indonesia 4 Syiah Kuala University, Jl. T. Nyak Arif Darussalam, Banda Aceh, 23111, Indonesia

After the great 2004 Sumatra-Andaman earthquake, several Global Positioning System (GPS) observations were conducted in Aceh, northern Sumatra. At the very beginning, most of the observations were campaign and only 1 continuous GPS station was constructed. Located in the city of Banda Aceh, we named this continuous site as USKL. Since then, we repeated the campaign observation every year across the Aceh region. In 2008, another 5 continuous GPS stations were constructed using a permanent pillar. We named these continuous stations as UGDN, MALO, TANG, MANE, and BTBW. Our GPS networks were divided into 2 categories. One is located in the southern part of the region with a perpendicular direction with Sumatran fault, named by Beutong line. Second is located in the northern part of the region with a north-south direction across the Sumatran fault, named by Geumpang line. For more analysis purpose, we also include one SUGAR continuous GPS station in the region, named by UMLH. Data of this site could be downloaded from the SOPAC website (http://garner.ucsd.edu). Our calculation result shows difference GPS velocities in the southern part and northern part of the Aceh region. In the southern area, velocities ~ 5 cm/yr were calculated. Meanwhile, in the northern area, our calculation shows velocity to ~10 cm/yr. With south-west direction, it is thought that most of these are the post-seismic velocities of the great 2004 Sumatra-Andaman earthquake (Fig. 1). As the distribution of GPS sites are irregularly distributed, we re-calculate the strain on the node of a regular planar grid. Our strain results also shows that the large stain accumulation were observed in the north-western part and south-eastern part of the network. Strain in this area was including the post-seismic effect and the fault slip of Sumatran fault. However, in the middle part, the strain results suggest that this area is weak of fault slip. Our result also shows that in the south-eastern part, strain is changing from extension to compression mechanism across the Batee fault (Fig. 2).

6 Gunawan et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Fig.1 Average horizontal site velocities in Aceh region with respect to Sundaland between

2005 and 2008 with 95% confidence ellipse. Sundaland motion parameter was based on the work by Simons et al., 2007. Different colors in station location (light blue, red and yellow) suggest the GPS observation site description. Different red color arrow shows the different in cumulative GPS observation span time. The lighter red color means shorter cumulative GPS observation time. Inland Sumatran fault and Batee fault are shown in dash blue color line [Sieh et al., 2000].

Fig.2 Cumulative strain in Aceh region based on horizontal site velocities. Blue color shows

an extension strain mechanism and red color shows compression strain mechanism. Circle indicates the location of GPS station.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 7 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Geotectonic of Java: The Geodynamics in Java, Indonesia, Deduced from GPS Data and The Java 2006 Tsunami Earthquake N.R. Hanifa1, Fumiaki Kimata1, Takeshi Sagiya1, Hasanuddin Z. Abidin2, Irwan Meilano2 1Research Center for Seismology, Volcanology and Disaster Mitigation, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601 Japan (hanifa@seis.nagoya-u.ac.jp)

2Geodesy Research Division, Institut Teknologi Bandung, Jl. Ganesha 10, Bandung, 40132, Indonesia

In contrast with Sumatera region, the plate interface near Java and the Lesser Sunda Islands was considered to slip a seismically with low seismic potential (Newcomb and McCann, 1987), while the Sumatera region were recognized by the scientist as a high seismic potential to produce great thrust earthquake due to historical data. GPS result by Bock et al (GRL 2003), Simon et al (GRL 2007) and Setyadji et al, (Annual of DPRI-Kyoto University, 1997) suggested a week coupling in the subduction zone between Australia plate and Java trench. GPS result also infers that as there is different oblique angle of the subduction of Australia plate, which is accommodated in several left lateral strike slip fault along the Java Island. After the Aceh 2004 event, a magnitude 7.7 earthquake occurred in Southwest Java at July 17, 2006, triggering a tsunami that caused at least 378 deaths. Kato et al (2006), Ammon et al (2006) and Fujii and Satake (2006) considered this event as tsunami earthquake based on the fact that the earthquake generated a much larger tsunami than expected from its seismic waves, its unusual long rupture, its source mechanism, and its location which is near the trench. The modeling of this event revealed discrepancy between the calculated model with the GPS displacement observation, tide gauge observation and run-height observation (Hanifa, 2008a,b). This discrepancy might be attributed to two effects; first, if the source process was accompanied by splay faulting, vertical displacement of the sea floor become larger and caused large tsunami. Another possibility is an effect of structural heterogeneity. Reviewing the historical data in Java from Utsu catalogue, three tsunami occurred previously in the offshore of South of Java; 1867 Yogyakarta with unrecorded magnitude, 1921 M.7.5 South East Java, and 1994 M.7.2 South East Java. The latest was also recognized as tsunami earthquake event. Since 1900, in inland of Java, there were only three occurrences of earthquakes with magnitude up to 7; in 1903 with M 8.1, in 1937 with M 7.2, and in 1943 with M 8.1. In May 26, 2006, a M 6.2 earthquake occurred in Yogyakarta, causing deaths of 5749 people. These events raised question about the geodynamic in Java. Thus, I study the geodynamics of Java, geodetic analysis, seismological analysis and numerical modeling. The purpose of this study is to understand the mechanism of tsunami earthquake beneath Java island and do evaluation of seismic potential where interseismic strain accumulation is not apparent, by making clear the stress, strain and coupling condition in Java.

8 Hanifa et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Fig 1. Sunda Trench Earthquake History and Seismicity. In left figure, red dot represent data from UTSU catalogue in period of 1770-1975,

with M>7, d<50 km, black moment tensor represent data from HCMT in period of 1976-2008, with M>6, d<50 km.

In right figure, moment tensor solution from HCMT catalogue, in period of 1976-2008. References Ammon, C.J., Kanamori, H., Lay, T., Velasco, A.A. The 17 July 2006 Java tsunami earthquake.

Geophys. Res. Lett., 33, L24308, doi:10.1029/2006GL028005, 2006. Bock et.al. Crustal motion in Indonesia from Global Positioning System Measurements.

Journal of Geophysical Research, Vol. 108, No. B8, 2367, 2003. Hanifa, N.R., Irwan Meilano, Takeshi Sagiya, Fumiaki Kimata, Hasanuddin Z. Abidin.

Numerical Modeling of The 2006 Java Tsunami Earthquake. Advances in Geosciences, accepted, 2008a.

Hanifa N.R., Takeshi Sagiya, Fumiaki Kimata, Hasanuddin Z. Abidin, Irwan Meilano. Numerical Modeling of The Java 2006 Tsunami Earthquake, Constrain with Coseismic Displacement from GPS Observation, Tidae Gauge Data, and Tsunami Run-up Height. Presentated in 7th General Assembly of Asian Seismological Commission and Seismological Society of Japan, 2008b Fall meeting.

Kato, T., Ito, T., H.Z. Abidin, Agustan. Preliminary report on crustal deformation surveys and tsunami measurements caused by the July 17, 2006 South off Java Island Earthquake and Tsunami, Indonesia. Earth Planets Space, 59, 1055-1059, 2007.

Newcomb, K.R., and McCann, W.R. Seismic History and Seismotectonics of the Sunda Arc. Journal of Geophysical Research, Vol. 92, No. B1, pages 421-439, January 10, 1987.

Setyadji et.al. Analysis of GPS measurement in West-Java, Indonesia. Annual of Disas. Prev. Res. Inst., Kyoto Univ., No. 40 B-1, 1997.

Simons et.al. A decade of GPS in Southeast Asia: Resolving Sundaland motion and boundaries. Journal of Geophysical Research, Vol. 112, B06420, 2007.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 9 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Characteristics of the large tsunami observed by fishermen far away from the northern Sumatra coast during the 2004 Sumatra-Andaman earthquake Yoshinari Hayashi1, Masataka Ando2, Mizuho Ishida3, Didik Sugiyanyo4, Mamoru Nakamura5 1 Center for Integrated Research and Education of Natural Hazards, Shizuoka University, Shizuoka, 422-8529, Japan (oyhayas@ipc.shizuoka.ac.jp)

2 Institute of Earth Sciences, Academia Sinica, Taipei, 11529, Taiwan 3 Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokohama, 236-0001, Japan

4 Tsunami and Disaster Mitigation Research Center, Syiah Kuala University, Banda Aceh, 23111, Indonesia

5 Faculty of Science, University of Ryukyus, Nishihara, Okinawa, 903-0213, Japan 1. Introduction

The purpose of this study is to clarify the physical behaviors of 2004 Indianocean tsunami from eyewitnesses. Many video and photographs were taken by people at some places in this tsunami disaster; nevertheless these were few restricted points. We didn’t know the tsunami behavior in another place. In this study, we tried to collect extensive information about tsunami behavior not only in many places but also wide time range from witness. 2.Method

To collect detail information about the tsunami, we contrived the interview method (Hayashi and Kimura, 2008). This method contain making pictures of tsunami experience from the scene of victims’ stories. These materials encourage the discussion through researchers and bring new idea for modeling of tsunami behavior and its disaster. Furthermore, we could use in purpose of verification of eyewitness on time of re-interview.

During the present study, about 50 local people were interviewed . In average, a person was interviewed for a period of more than 1 hour that was recorded on IC recorder. In some instances, some of the interviewees were interviewed again to be certain on the content of their accounts. The interview process was accomplished in three periods: 3 days in November 2006, 6 days February 2007 and 5 days in March 2008 that amounted to a total of two weeks. The translation from Acehenis (local language) or from Bahasa Indonesia (standard Indonesian language) was carried out by lecturer and his students of Syiah Kuala University.

3.The offshore tsunami observed by fishermen onboard

There are about 30 fishermen in Banda Aceh city and west of Banda Aceh County were interviewed during the survey periods. 10 of these fishermen encountered the tsunami on their boats or fishing vessel during December 24, 2004 earthquake.

These are some of accounts of the interviewed fishermen who experienced the offshore tsunami. All fishermen in the sea felt the earthquake shocks except one interviewee who was cooking inside the vessel. Several fishermen immediately noticed the shaking because of their previous earthquake experiences at sea. However, even an interviewee who had no prior experience of a tsunami, he and his colleagues recognized it as an earthquake.

During the shaking, three interviewees had difficulty controlling their vessels and decided to switch off the engine. The vessel drifted in an S-shape curve or its speed seemed to be slower than normal. The shaking continued 5 min or 10-20min.

Immediately after, or about 2-5 min, or 10-20 min after the shaking, unusual waves were seen in short distances, 0.2 to 1 km ahead of them or more than 4-5 km coming from the open sea and/or from different directions. The maximum height of the unusual waves were 10 to 20 m high, or even as high as 30m. Large waves repeatedly struck their vessels and the sea calmed down within 6 to 20 min.

10 Djajadihardja et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Among the 10 vessels, four of them were overturned and destroyed by the unusual waves. The other boats narrowly escaped the unusual waves by successfully steering their vessels in between the big isolated waves(Figure.1).

Since almost all fishing vessels were not equipped with GPS or any device establish their location, the location was deduced based on the kind of fish they were catching, which are garapu tabang that are similar to alfonsino. These kinds of fishes live near the rocky seafloor at depths shallower than 50m. 4.Summary

Based from these interviews, it is apparent that tsunami can pose danger even offshore when the tsunami amplitudes are high and water depths are shallower than about 50m. These observations provided evidence that the 2004 tsunami already grew high enough to damage or overturn ships 0.5 to 20 km from the shore.

Figure.1 Sketch of the tsunami encountered by the interviewee in west of Banda Aceh coast based on his accounts, painted by Tetsuya Fujita in 2007. References Hayashi, Y. and R. Kimura, How is it Possible to Let People Visualize Disasters that They have

Never Experienced, 14th WCEE Proceedings, CD-ROM (8pp.).

Hayashi et al.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 11 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Aftershock distribution off NW Sumatra of the 26 December 2004 Sumatra-Andaman earthquake determined with an emergent OBS network Ei'ichiro Araki1, Masanao Shinohara2, Koichiro Obana1, Tomoaki Yamada2, Yoshiyuki Kaneda1, Toshihiko Kanazawa2, and Kiyoshi Suyehiro1 1 Department of Oceanfloor Network System Development for Earthquake and

Tsunami (DONET). Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan (araki@jamstec.go.jp)

2 2Earthquake Research Institute, the University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan

We deployed an OBS network in February–March 2005 in the rupture area of the Sumatra Andaman earthquake on 26 December 2004. We placed 17 short-term OBSs and two long-term OBSs, and recovered OBSs after observation for 19–22 days. The hypocenter distribution from 10-day data of 17 OBS revealed the detailed structure of aftershock seismicity offshore of Sumatra Island. Aftershock seismicity associated with the subducting slab starts 40 km inward from the Sunda trench axis; it ceases at 50 km depth beneath the Aceh Basin, approximately 240 km inward from the trench axis. Aftershocks in 120–170 km from the trench axis consist of a surface with a dip of 10–12◦ dominated by a dip-extension type mechanism. Beyond the southwestern edge of the Aceh Basin, the aftershock activity becomes higher, and dominated by dip-slip type earthquakes, with a slightly increased dipping angle of 15–20◦. Three along-arc bands of shallow seismicity were identified at 70 km inward from the Sumatra trench, 110 km inward from the trench, and in the south of the Aceh Basin. These locations correspond to steep topographic slopes in the accretionary prism, suggesting the present evolutional activity of the accretionary prism offshore Sumatra Island.

12 Djajadihardja et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Araki et al.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 13 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Preliminarily results from seismic surveysof Seacause II and Sumatra Cruise using R/V SONNE 2006 Yusuf S. Djajadihardja1, Christoph Gaedicke2, Stefan Ladage2, Won Soh3

1 Agency for the Assessment and Application of Technology. BPPT Building II,

Jl M.H. Thamrin No.8, Jakarta 10340, Indonesia. Phone; +62-21-3169700 (iyung@ceo.bppt.go.id).

2 Bundessanstalt fur Geowissenschaften und Rohstoffe, Stilleweg 2, 30655 Hannover, Germany, Phone; +49 511 6430,

3 Japan Marine Earth-Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan.

Seacause II and Sumatra Cruises were carried out in the western part of Sumatra Island on 21st January – 24 February 2006 and 6th September – 8th October 2006 using R/V Sonne. The main objectives of the two cruises are to investigate the southern part of the rupture zone of the 9.3 magnitude earthquake of 26th December 2004 and the rupture zone of the 8.7 magnitude earthquake of 28th March 2005 and also would identify the internal structures of the Sunda Trench, Accretionary wedge, Outer Arc High, and the Fore Arc Basin of the western Sumatra offshore. On the other hand also would like to identify the segment boundary between the two earthquakes as recognized by the distribution of their aftershock. During the cruises a total length of Multichannel Seismic Reflection lines were carried out of about 9733 km, including swath bathymetry, gravity, magnetic and high resolution parasound data. Multichannel Seismic images showed that the sediment in the trench are decrease in the thickness toward the south of the Sunda Arc, this is due to the sediment source of the trench fill are came from the Bengal Fan in the eastern part of India. Both the ocean basin and the trench sediments are dissected by numerous normal faults. Magnetic map anomaly show that the plate boundary of the Indo-Australian plate was subduct beneath the Simeulue Island, this is also identified by the profile of Multichannel Seismic line BGR06-119, 121 and 122, and 124. On line SO86-121-122 clearly that the oceanic basement reflector as continuation of the line SO06-119 in the left side of line SO06-121-122 is clearly identified, also in the right side of the line SO06-121-122 as a continuation of the oceanic basement in line SO06-124 is clearly identified in the right side of the line SO06-121-122. To the middle side of this line, the reflector of oceanic basement was not clear. It means that this area interpreted as a plate boundary as also identified in the magnetic anomaly map. This plate boundary was a boundary for the co-seismic of 26th December 2004 and the Nias earthquake that already happened 3 month letter after the 26th December 2004, that is on 28 March 2005 with the magnitude 8.7. Tectonic element of the Sunda Strait region having similar structure with the structure founded in offshore Aceh region. In Aceh region, Bati fault segment is cross cutting the accretionary prism, while the geometric of the Sunda Trench having curve form in this region. Those similarities of the structure is also founded in the Sunda Strait area. Here, the Sunda Trench is also curved in the southern part of the Sunda Strait. On the other hand, Yokosuka cruise in 2001 and 2002 have discover the prolongation of the Sumatra Fault Zone to the southern part of the west Java waters. This prolongation of the Sumatra fault to the southern of west Java water is most likely act as Bati fault in the Aceh region. GPS data show that the Sumatra is being rotated clockwise with a pivot of this rotation is located in the Sunda Strain region. One GPS station located close to Bengkulu area is show that the little

14 Djajadihardja et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ movement of the crust (locked area), this mean that in this region is a compression area came from the accretionary prism block that pushed by the collision between Indo-Australian plate and a continental crust of Sumatra and west Jawa. The prolongation of the Sumatra fault to the southern part of the west Java water act as a slipper for the block movement above. Seimicity data also show that big earthquake with magnitude ? already happened in the southern end of Sumatra Island 1908. Not only the similar geometric of the tectonic element between Sunda Strait and Aceh, but those data are remain us that the Sunda Strait region is also a high risk region for incoming a big earthquake in the future.

Djajadihardja et al.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 15 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Heat Flow Study of Acretionary Prism off 2004 Great Sumatra-Andaman Earthquake's Area Udrekh1, Masa Kinoshita2, Yuka Masaki 3 1The Agency for the Assessment and Application of Technology, BPPT 1st building, 20 floor, Jl. M.H. Thamrin 8, Jakarta 10340, Indonesia 2 IFREE/JAMSTEC, Natushima-cho 2-15, Yokosuka 237-0061, Japan. 3 Graduate school of Kochi University / IFREE/JAMSTEC Bilateral Japanese – Indonesia cooperation on marine research had been conducted more than 20 years. This collaboration covered various aspects such as Oceanography, Climate, Geology and Geophysics. After 26th December 2004, when the Great Sumatra-Andaman Earthquakes occurred in Indonesian Ocean off Aceh – Sumatra, which generated tsunami and sacrificed more than 16000 peoples, the government of Japan gave a quick response to give scientific support by sending R/V Natsushima. Geology / Geophysical investigation called “Natsushima NT_05_02 cruise” was conducted in February 2005. This survey objective is to examine geomorphology and aftershock in and around the overriding plate off Aceh, Sumatra, Indonesia. During this cruise, rich collections of deep-sea living organism, rock, fluid and sediment samples have been obtained; as well as heat flow measurement using ROV Hyper-Dolphin. We focused on heat flow data processing in order to better understand heat flow values around accretionary prism of 2004 Sumatra – Andaman great earth- quakes area. Various data such as seismic profile, bathymetry, and CTD are used to support analysis and interpretation. References Ashi, J, Machiyama, H, Soh, W, Tokuyama H, 2003, Natural gamma ray survey at the active

submarine faults southwest off the Sumatra Island, Indonesia, The Earth Monthly, Special No.56,73-78, 2006 (in Japanese)．

Goto, S, Yamano, M, Kinoshita,M, 2005, Thermal response of sediment wit vertical fluid flow to periodic temperature variation at the surface.

Soh, W, Hirata, K, Fujiwara, T, et al, Cruise Report of Natsushima NT05-02, 2005.

16 Udrekh et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 17 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Geological approaches to the large earthquakes off northern Sumatra Island –Surface and shallow subsurface structure analyses by deep-tow SBP and side-scan sonar and turbidite paleo- seismology by piston coring– Kohsaku Arai1 and Ken Ikehara1 1Geological Survey of Japan-AIST, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki, 305-8567, Japan (ko-arai@aist.go.jp)

Geological Survey of Japan-AIST (GSJ-AIST) has conducted researches on the large earthquakes. We will introduce about our activity on the Sumatra Andaman Earthquake in this presentation. 26th December 2004, the Sumatra Andaman Earthquake and its subsequent devastating Tsunami attacked not only the Sumatra Island but also many countries which face to the Indian Ocean. The epicenter was located at off Simuelur Island, off Aceh, northern Sumatra, Indonesia. The seismic rupture was propagated northward along the Nicobal and Andaman Islands more than 1,000 km long. Urgent offshore survey program (NT05-2 Natsushima cruise) was carried out by the JAMSTEC and the BPPT in order to examine geomorphological changes and to monitor aftershocks in and around the overriding plate off Ache, Sumatra, Indonesia. The swath bathymetry survey by the multi-beam echo sounder and the single channel seismic (SCS) profiles indicated the thrust ridge and landmass wasting patterns. Near-bottom observations were carried out successfully by the ROV, Hyper Dolphin, to find abundant deformed features of the seafloor, such as ruptures and fissures (Sho et al., 2005). The speculation onboard is that some of the observed deformation on the seafloor surface was co-seismic, possibly caused by exceeding gravity acceleration during the grand motion of the catastrophic earthquake. Surface and shallow subsurface structure analyses (SBP and SSS) A study using DAI-PACK (Deep-sea Acoustic Imaging Package) was carried out off Sumatra Island during urgent offshore survey on R/V Natsushima NT05-2 cruise. The DAI-PACK was attached to the Hyper Dolphin to clarify the deformation and faulting below/on the seafloor. Bottom penetration of SBP was from 10 to 20 meters throughout the NT05-2 cruise. We found stratified sediment broken into large clumps at the foot of steep slope of the thrust ridge during the ROV Dive #386. Strong reflectors occur in the steep slope area and some strata tilted to landward covered with thin soft sediment. On the top of the thrust ridge, we saw many ruptures and fissures in video images of ROV from the Dive #391. The SSS imagery of the fissure/rupture shows strong reflectors and structural patterns. The SBP image shows that the sediments above the strong reflector at about 7.5 m is highly deformed in general. Strong reflector at about 9 m depth can be traced continuously below the deformed strata. We could not identify a thrust rupture on the seafloor, however, very intense earthquake shaking was inferred from the deformed features on the young sediments at the top of thrust ridge. Turbidite paleoseismology by piston coring Cooperation study between GSJ-AIST and BPPT desires to concerning examination of the recurrence pattern of large earthquake in Aceh basin which along the West Andaman fault using turbidite paleoseismology by piston coring. Understanding the recurrence pattern of large earthquake such as the Sumatra-Andaman Earthquake should be useful for fortunately decreasing the human damage under the geo-hazard. Three cores corrected from Ache basin on last year Mirai cruise (MR07-07 cruise: See figure). We could not find obvious evidences of the collapse of steep cliff along the West Andaman fault in the top of the pilot core. The results indicate that the large collapse of 2004 Sumatra-Andaman earthquake was not occur around the piston core site. In the other hand, many sand layers can be observed the lower part of piston core. In particular, the lower remarkable thick sand layer that correlated each core between 5-7 m below core top has been

18 Arai and Ikehara ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ observed. In the several ten cm below the sand layer, many fining upward cycles were well developed along the West Andaman fault (PC-03). It is concluded therefore, the large rupture or collapse was recorded in the lower part of piston cores. The events, which shallower sand sediments have been transported into the Aceh basin, were completely decreased at upper part of the piston cores. decreased at upper part of the piston cores.

Figure The study areas off northern Sumatra Island. Data of deep-tow SBP and SSS was corrected on front of outer arc high. Position of the piston core MR07-07 located in the Ache Basin. References Soh et al., Cruise Report on NT05-02 “Survey off northwest Sumatra Island”, 2005. http://www.jamstec.go.jp/jamstec-e/sumatra/natsushima/bm/contents.html

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo, 2009 19 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Geological structure off northwest Sumatra deduced from detailed seafloor morphology and seismic reflection survey (Preliminary)

H. Permana1. A. Laesanpura2, R. Rahardiawan3 1Research Center for Geotechnology-LIPI, Jl. Sangkuriang, Bandung, W. Java, 40135, Indonesia. permhp@yahoo.com

2 Institute Technology of Bandung. Jl. Ganesha no. 10, Bandung, W. Java, 40135. Indonesia

3 Marine Geological Institutes. Jl. Terusan Pasteur, Bandung, W. Java, Indonesia

The geological structure offshore of West of Sumatra is more complex and less understands then we expected. Our knowledge until now could not answer seismic activities and rupture propagation occur in that area. A segmented of subducted slab hypothesis is one possibility to explain such kind a geological process. An oceanic crust slab or present of a micro-continent block as well changes of plate motion rate or plate age are possibly as main role in such structural pattern variety. The aim of this study is determining seafloor morphology pattern which expressed a geological structure using all available data and high resolution bathymetry combine with available seismic data. Covered by thick of water column where the erosion and weathering are weak, the seafloor morphology could be recorded certain structural pattern during geological processes. The main geological structure in this area is elongated (NNW to NW) subduction front or front of deformation, accretional prism complex (NNW, NW, N pattern) and fore-arc basin (NNW pattern). Structure lineament represent thrust fault within thrust fold system, strike slip system, normal fault and basin opening system. A complex lateral and longitudinal variation of morphological pattern demonstrate variety of geological structures. Landward or seaward vergence structures commonly occur in close area. Whilst, a step deformation front and rough accretional prism complex in the north compare to this in the southern part indicate a sediment subduction involve as well a plate motion rate.

20 Permana et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 21 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Magnetic susceptibility of coring samples from the trench of the subduction zone west of Sumatera Island, Indonesia Eddy Gaffar1, Udrekh 2, Dayuf 2 1The Agency for the Assessment and Application of Technology, Center for Technology for Natural Resourcess Inventory, BPPT 1st building, 20 floor, Jl. M.H. Thamrin 8, Jakarta 10340, Indonesia

2The Agency for the Assessment and Application of Technology, Center of Technology for Marine Survey, BPPT 1nd building, 18 floor, Jl. M.H. Thamrin 8, Jakarta 10340, Indonesia

Indonesian and American scientist have conducted research on the west trench Sumatera after earthquake and tsunami Aceh at December 2004 with more than 200.000 victims. 105 cores sampling from deepsea have taken with several kinds of coring as Gravity core, Kasten core, Multi core, Piston core and Trigger core. Cores have description as the mud to sand cyclus. This cyclus is called the turbid cyclus. In this study, we measure magnetic susceptibility of the core samples. It seems, it is good correlation between sediment grain, RGB and magnetic susceptibility results.

22 Gaffar et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 23 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Baruna Jaya II, New Multi-channel – seismic vessel in Indonesia Udrekh1, Yudi Anantasena2, Rahadian 2 1The Agency for the Assessment and Application of Technology, Center for Technology for Natural Resourcess Inventory, BPPT 1st building, 20 floor, Jl. M.H. Thamrin 8, Jakarta 10340, Indonesia

2The Agency for the Assessment and Application of Technology, Center of Technology for Marine Survey, BPPT 1nd building, 18 floor, Jl. M.H. Thamrin 8, Jakarta 10340, Indonesia

Indonesia is archipelago country, which consist of more than sixteen thousand islands. Beside it, Indonesia is also located in active margin, where tectonic and volcanic activities are very high. After 26th December 2004, when the Great Sumatra-Andaman Earthquakes occurred in Indonesian Ocean off Aceh – Sumatra, Indonesian Government gives more attention for marine Geophysics and Geology activities. In order to do some scientific activity as well as oil exploration, R/V Baruna Jaya was finally modified and was equipped with new multi-channel seismic system. Relatively high-end hardware such as: streamer, gun, recording, navigation system, and well known software system for acquisition, QC, navigation and trigger have been installed at the end of 2007. This system is designed to fulfill standard requirement of oil industry as well as for scientific activity. This system has been finally running well and ready for seismic acquisition. In 2008, some acquisition activity, mostly for oil industry has been carried out successfully. We would like to share any information about facilities and capability of Baruna Jaya II, recent activity and future planning.

24 Udrekh et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 25 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Source Model of the 2007 Bengkulu Earthquake Determined from Tsunami Waveforms and InSAR data Aditya Riadi Gusman1, Yuichiro Tanioka1, T. Kobayashi1, Hamzah Latief2, Wahyu W. Pandoe3 1Institute of Seismology and Volcanology, Hokkaido University, Kita 10 Nishi 8 Kita-ku, Sapporo City, Hokkaido 060-0810, Japan (adit@mail.sci.hokudai.ac.jp, tanioka@mail.sci.hokudai.ac.jp, t.kobayashi@mail.sci.hokudai.ac.jp)

2 Center for Marine and Coastal Development, Bandung Institute of Technology, Ganesha 10, Bandung, West Java 40132, Indonesia (hamzah@ppk.itb.ac.id)

3 BPPT, Jakarta, DKI Jakarta, Indonesia On September 12, 2007 at 11:10:26 UTC, an earthquake with moment magnitude of 8.4 occurred off the west coast of Sumatra. The epicenter of the earthquake located at 4.52°S-101.374°E about 130 km southwest of Bengkulu. In this study, we estimate the slip distribution of the 2007 earthquake using tsunami waveforms and Synthetic Aperture Radar Interferometry (InSAR) data. The tsunami waves generated by the earthquake were recorded by tide gauge stations around Indian Ocean and two tsunami buoys deployed in the deep ocean northwest Sumatra and off the Sunda Strait. We select tsunami waveforms recorded in Padang, Cocos Islands, and on the tsunami buoys. Surface deformation on Pagai Islands was detected by “Daichi” (ALOS) satellite and the maximum crustal deformation located on the southern part of the island is approximately 1.8 m in the line of sight direction. The synthetic tsunami waveforms at the four stations are calculated by solving the non linear shallow water equations. The synthetic dislocation in the line of sight direction is scaled from displacements calculated by the Okada (1985) formula. We estimate source model of the earthquake by performing joint inversion using recorded tsunami waveforms and crustal deformation on Pagai Islands detected InSAR. In estimating the slip distribution we include smoothing factor (� 2) in the calculation. On the ruptured area, we create a fault segment area of 150 km width by 300 km length and divide it into 72 subfaults. We use single focal mechanism (strike=327°, slip=12°, rake=114°) determined by Global CMT solution for each subfault. The slip distribution inverted from tsunami waveforms and InSAR data shows that the major slip regions are located about 100 km north from the epicenter and on the southern part of the Pagai Islands. Assuming the rigidity of 4 x 1010 N/m2, the total seismic moment obtain from the slip amount is 3.4 x 1021 Nm (Mw=8.3) which is consistent with the Global CMT solution on the seismic moment determination of 5.05 x 1021 Nm.

26 Gusman et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 27 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Monitoring of seafloor deformation in Japan Trench –A challenge to observation of slow earthquakes– Yoshihiro Ito1, Motoyuki Kido1, Yukihito Osada1, Takeshi Tsuji2, Ryota Hino1, Juichiro Ashi3, Hiromi Fujimoto1 1Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, 6-6, Aramaki-Aza-Aoba, Aoba-ku, Sendai, 980-8578, Japan (yito@aob.geophys.tohoku.ac.jp)

2Department of Civil and Earth Resource Engineering, Kyoto University, Kyoto Japan, Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto, 615-8540, Japan

3 Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, 164-8639, Japan

It is important to understand the process of stress accumulation to an asperity of a

large, or megathrust earthquakes. Recently, several different types of slow earthquakes—non-volcanic tremors, very-low-frequency (VLF) earthquakes, or short-term slow slip—have been reported in the Nankai, Cascadia, and Costa-Rica subduction zone [Dragert et al., 2001; Obara, 2002; Rogers and Dragert, 2003; Obara et al., 2004; Brown et al., 2005; Ito et al., 2007]. VLF earthquakes are detected only in Nankai subduction zone. The periods of the seismic waves corresponding to VLF earthquakes along the Nankai subduction zone are predominantly in the range of 10–20 s [Obara and Ito, 2005]. The earthquake hypocenters are distributed at a depth of ~10 km above the upper surface of the subducting Philippine Sea Plate. The focal mechanism indicates reverse faulting [Ito and Obara, 2006].

Hydrotectonic events coincident with some seismic signals observed by ocean-bottom-seismometers were first reported by Brown et al. (2005); they used the CAT-meters in the shallow subduction system in certain regions in the Costa Rica subduction zone. Their results suggested that anomalously rapid flow events coincide simultaneously with bursts of seismic noise.

The Pacific plate subducts beneath Tohoku, northeastern (NE) Japan, along the Japan Trench. The seismicity along the plate boundary is the highest in the world. The regional seismicity varies from north to south along the Japan Trench. From off Sanriku to off Miyagi, the northern part of the subduction zone, there exist asperities of large earthquakes, that is, earthquakes whose magnitude exceed 7, and some clusters of small or intermediate earthquakes around the asperities. An aseismic slip has been observed as a post-seismic slip after large events. Tsunami earthquakes, a type of slow earthquakes, have also occurred near the Japan Trench. However, non-volcanic tremors, VLF earthquakes, and short-term slow slips have not been observed in NE Japan; this may be attributed to poor detectability for long-period events near the trench. In order to detect various types of slow earthquakes, we have installed temporal ocean-bottom seismic, geodetic, and hydraulics observation instruments in NE Japan by using Research/Vessel “YOKOSUKA” and submersible “SHINKAI 6500” of Japan Agency for Marine-Earth Science and Technology.

We have carried out an investigation of cold seeps by using SHINKAI 6500; we deployed two simplified ocean-bottom benchmarks (SOBBs), two ocean-bottom pressure gauges (OBPs), and deployed six long-term ocean-bottom seismometers (OBSs) (Fig. 1). We also used osmotically-driven fluid flow meters (CAT-meters) to observe the transience of fluid flow related to some slow earthquakes (Fig. 1). Since data showing the existence of cold seeps in the research area were unavailable, we first investigated about the distribution of chemosynthetic benthic colonies by diving surveys involving the SHINKAI 6500. Then, we installed CAT-meters on cold seeps by using the SHINKAI 6500. We carried out six dives with the SHINKAI 6500 in order to search for chemosynthetic benthic colonies, which indicate the existence of a cold seep, and to deploy two CAT meters. We found more than 10 calyptogena colonies (Fig.1) in six regions at a depths ranging from 5702 m to 5861 m.

We deployed two SOBBs with three types of sensors: short-period seismometers,

28 Ito et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ broadband seismometers, and a pressure gauge. These SOBBs were thrown down from the surface of the sea. One of the SOBBs was deployed on the footwall near the possible faults estimated by traces of calyptogena colonies. The other was deployed on the hanging wall of a possible fault along the calyptogena colonies, and is also located seaward of the possible fault. All the observations are now ongoing. In this region, we have already established GPS/Acoustic seafloor positioning system in order to observation of seafloor deformation related to the subducting plate [Osada et al., 2006]. In the future, we will investigate slow earthquakes in NE Japan based on seismological, geodetic, and cold seepage observations.

Fig.1 (Left) Cruise track, dive points, and deployed instruments in the research area. (Right) Calyptogena colonies near the Japan Trench. References Brown, K. M., M. D. Tryon, H. R. DeShon, L. M. Dorman, and S. Y. Schwartz, Correlated transient

fluid pulsing and seismic tremor in the Costa Rica subduction zone, Earth Planet. Sci. Lett., 238, 189-203, 2005.

Dragert, H., K. Wang, and T. S. James, A silent slip event on the deeper Cascadia subduction interface, Science, 292, 1525-1528, 2001.

Ito, Y., and K. Obara, Dynamic deformation of the accretionary prism excites very low frequency earthquakes, Geophys. Res. Lett., 33, L02311, doi:10.1029/2005GL025270.

Ito, Y., K. Obara, K. Shiomi, S. Sekine, and H. Hirose, Slow earthquakes coincident wih episodic tremor and slow slip event, Science, 315, 503-506, 2007.

Obara, K., Nonvolcanic deep tremor associated with subduction in southwest Japan, Science, 296, 1679-1681.

Obara, K., and Y. Ito, Very low frequency earthquakes excited by the 2004 off the Kii peninsula earthquakes: A dynamic deformation process in the large accretionary prism, Earth Planet Space, 56, 347-351, 2004.

Obara, K., H. Hirose, F. Yamamizu, and K. Kasahara, Episodic slow slip events accompanied by non-volcanic tremors in southwest Japan subduction zone, Geophys. Res. Lett., 31, L23602, doi:10.1029/2004GL020848.

Osada, Y., H. Fujimoto, T. Kanazawa, S. Nakao, S. Sakai, S. Miura, J. A. Hildebrand, and C. D. Chadwell, Development of a GPS/Acoustic seafloor positioning system for 6,000 m water depth and its trial experiments at sea, J. Geodetic Soc. Japan, 52, 172-182, 2006.

Rogers, G. and H. Dragert, Episodic tremor and slip on the Cascadia subduction zone: The chatter of silent slip, Science, 300, 1942-1943, 2003.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 29 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Surface break of a thrust that initiated the Indian Ocean Tsunami

in the Sumatra-Andaman Earthquake of 26 December 2004 W.Soh1, H. Machiyama1, K. Hirata3, E. Araki2, K. Obana2, K. Arai4, T. Fujiwara2, Y. Djajadihardja5, S. Burhanuddim6, C. Muller7, L. Seeber8 and K. Suyehiro9 1Kochi Institute for Core Sample Research, JAMSTEC, B200, Monobe, Nankoku, Kochi 783-8502, Japan (soh@jamstec.go.jp)

2Institute for Research on Earth Evolution, JAMSTEC, 2-15, Natsushima, Yokosuka 237-0061, Japan

3Seismology and Volcanology Research Department, Meteological Research Institute, 1-1, Nagamine, Tsukuba 305-0052, Japan

4Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1, Central 7 Higashi, Tsukuba 305-8567, Japan 5Center for Assessment and Application of Technology for National Resources Inventory, Agency for Assessment and Application of Technology (BPPT), Thamrin 8, Jakaruta 10340, Indonesia 6Laboratory of Marine Geology, Department of Geology, Univerisity of Hasanuddin, Jl. Perintis Kemerdekaan, Makassar 90245, Indonesia 7Marine Seismic Survey Technique Development, Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, Hannover 30655, Germany 8Lamont Doherty Earth Observatory, Columbia University, Palisader, N.Y. 10964, U.S.A. 9Headquater, JAMSTEC, 2-15, Natsushima, Yokosuka 237-0061, Japan

Just after the Sumatra–Andaman Earthquake of 26th December 2004 (Ammon et al., 2005) a giant tsunami spread though the Indian Ocean and struck the coasts of Sumatra, Malaysia, India, and Sri Lanka. Maximum tsunami run-up heights of 13–35 m were observed on the west coast of Banda Aceh at the northern tip of Sumatra (cf. Borrero, 2005; Jeffe et al., 2006). Even though the earthquake was the second largest ever recorded, the tsunami was larger and more devastating than predicted (McClosky et al., 2005; Seno and Hirata, 2007). Generation of the giant tsunami has been ascribed to co-seismic rupture, occurring across the outer arc along the main megathrust (Sibuet et al., 2007). However, the giant tsunami is not successfully accounted for only by the displacement in a deep part of the co-seismic thrust fault (Geist et al., 2007), and requires an understanding of the shallow-to-surface rupture propagation of the co-seismic fault (Plafker et al., 2007). Until now, the rupture propagation model to explain the tsunami generation has been discussed based on tsunami model (Seno and Hirata, 2007; Hanson et al., 2007) and tectonomorphologic analysis (Henstock et al., 2006; Sibuet et al., 2007), however, there is no consensus yet.

Approximately two months after the 2004 Andaman-Sumatra Earthquake we conducted a direct observation by using the ROV Hyper-Dolphin during R/V Natsushima cruise to investigate the seafloor disturbance in the outer arc, off Sumatra, considered to have experienced the largest co-seismic slip (Fig. 1). By using an ROV, we discovered shattering of seafloor on the most trenchward splay fault along western margin of the outer arc. Here, the seafloor was intensively and freshly deformed by fracturing, folding as well as gravity-driven collapse. The water column was unusually cloudy by re-suspension of sediment, suggesting that it can be elucidated by the main shock of the Sumatra-Andaman Earthquake. But it is very likely that the tsunami wave source (upheaval area) amounted to more archside and it cannot explain all only in the displacement model of the front of the

30 Soh et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ outer arc if the tsunami travel time of being surged against the Aceh west coast is considered.

To test the tsunami generation, examination of two fault hypotheses was conducted: the splay-fault hypothesis, which involves both a megathrust and a splay fault, and the rupture-to-accretionary-prism-toe hypothesis that considers a rupture of the megathrust alone. The two models clearly show differences in the amount of fault slip, and the location and magnitude of uplift. For the splay-fault hypothesis, the amount of slip was relatively small (~ 33 m). Uplift greater than 10 m was concentrated along the margin of the outer arc high above the splay fault. However, no uplift was observed in the trench floor or close to the deformation front. On the other hand, 55 m of slip was needed in the model based on the rupture-to-accretionary prism-toe hypothesis. The OBS observation shows that the aftershocks were entirely distributed beneath the outer arc but it did not extend to the toe of the accretionary prism particularly in the study area where the maximum disturbance took place. On the other hands, Moran et al. (2005) discovered enigmatic structures along the deformation front. However, it did not show any significant fault structure, and few, if any, of significant geomorphic features to be predicted by the inversion experiment were observed here. We conclude, therefore, that the splay-fault model is more appropriate to explain generation of the hazardous Sumatran tsunami.

Figure 1 Bathymetric features and ROV dive locations at the western margin of the outer arc high.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 31 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Detailed seismicity around the Nankai trough determined with ocean-bottom seismographs Akira Yamazaki (ayamazak@mri-jma.go.jp) MRI/JMA, 1-1 Nagamine, Tsukuba 305-0052, Japan

Large interplate earthquakes, called Tonankai and Nankai earthquake, have occurred periodically with an interval of about 100-200 years along the Nankai trough, southwestern Japan. The next these earthquakes are estimated to occur in the first half of this century. In order to investigate the precise seismicity around the Nankai trough, we have conducted pop-up type ocean bottom seismometer (OBS) observations. The OBS stations, which were deployed in latest five years by MRI, are shown in Figure 1.

Fig. 1 OBS stations deployed by MRI form 2004 to 2008 around the Kii Peninsula TN041,TN042,TN043 : Observed in 2004FY TN051,TN052 : Observed in 2005FY TN061,TN062 : Oserved in 2006FY TN071,TN072 : Observed in 2007FY TN081 : Observed in 2008FY

Seismicity around the Nankai trough axis south off the Kii Peninsula: Around the Nankai trough axis south off the Kii Peninsula, the OBS observations were conducted four times intermittently during 2005 to 2008 with shift of the observed area to westward. A one-dimensional seismic velocity structure for hypocenter determination was derived from the survey result of the previous seismic refraction study. A station correction method using a PS conversion wave was applied to improve the hypocenter determination. We detected the relatively high seismic activity exists around the Nankai trough axis (Fig. 2). The detected seismic activity was invisible by land seismic observation network. It is found that most of the earthquakes occur at the depth of 10km to 25km in the oceanic crust or in the uppermost mantle of the Philippine Sea plate.

32 Yamazaki ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Fig. 2 OBS-located hypocenters compiled by four OBS observations. The small + marks denote hypocenters derived from JMA's earthquake catalog of which the period from October 1, 2005 to the end of 2008. The broken line denotes the Nankai trough axis. The periods of each observation are as follows; red reverse triangles, 2005.10.18-2005.11.25 ; thick black plus, 2006.05.21-2006.07.24 ; red stars, 2007.10.16-2007.12.01 ; blue reverse triangles, 2008.05.12-2008.09.12.

Aftershock observation of the 2004 off the Kii Peninsula earthquake: The 2004 off the Kii Peninsula earthquake (Mj7.4) occurred near the Nankai trough axis, southeast off the Kii Peninsula, Japan, on September 5, 2004. In order to investigate the precise distribution and time change of the aftershock, we conducted pop-up ocean bottom seismometer (OBS) observations around the aftershock region. It was found that the aftershock distribution can be divided roughly into two groups, a relatively shallower group with a depth range of 5 to 10km, and a deeper group with a depth range of 15 to 30km. The shallower group, which is located inside the PHS or the accretionary prism just over the PHS, was distributed from the center to the north of the aftershock region. The deeper group is located in the uppermost mantle of the PHS near the Nankai trough axis, which is inferred to be the main ruptured zone of the main shock. We also detected several seismic clusters in the shallower earthquake group. They form vertical planes going down from the accretionary prism to the PHS. We are interested in the relation between the detected seismic clusters and the splay fault system in the accretionary prism. References Yamazaki, A., S. Aoki, Y. Yoshida, A. Kobayashi, A. Katsumata, M. Abe, K. Moriwaki, N.

Okawara, Y. Osada, H. Matsuoka, T. Yoshida, H. Sekitani, K. Niinou, and H. Hiramatsu, Aftershock observation of the 2004 off the Kii Peninsula earthquake using ocean bottom seismometers, Papers in Meteorology and Geophysics, Vol. 59, 65-82, 2008.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 33 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ The 5th hypothetical model to explain the 2004 huge tsunami generation off northwest Sumatra Kenji Hirata 1 (khirata@mri-jma.go.jp), Jeffrey A Hanson 2, Eric L Geist 3, Tetsuzo Seno 4, Wonn Soh 5, Toshiya Fujiwara 5,Christian Muller 6, Hideaki Machiyama 5, Eiichiro Araki 5, Kohsaku Arai 7, Kazuki Watanabe 8, Leonard Seeber 9, Yusuf S Djajadihardia 10, Safri Burhanuddin 11, Bdrul M Kemal 12, Nugroho D Hananto 13, Hananto Kurnio 14, Yudi Anantasena 10, Kiyoshi Suyehiro 5 1 MRI/JMA, 1-1 Nagamine, Tsukuba 305-0052, Japan 2 Science Applications International Corporation, 10260 Campus Point Dr, San Diego, CA 92121, United States

3 USGS, 345 Middlefield Rd, Menlo Park, CA 94025, United States 4 ERI, Univ of Tokyo, Tokyo 113-0032, Japan 5 JAMSTEC, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan 6 BGR, Stilleweg 2, Hannover 30655, Germany 7 AIST, 1-1-1 Central 7 Higashi, Tsukuba 305-8567, Japan 8 Hydrographic and Oceanographic Department, Japan Coast Guard, 5-3-1 Tsukuji, Tokyo 104-0045, Japan

9 Lamont Doherty Earth Observatory, Palisades, New York 10964, United States 10 BPPT, J1.M.H Thamrin 8, Jakarta 10340, Indonesia 11 Agency for Marine and Fisheries Research, J1.Batu Mandi Blok J No.15, Pondok Gede Bekasi 17411, Indonesia

12 Andalas University, Kampus Limau Manih, Padang 25163, Indonesia 13 LIPI, J1.Sangkuriang Komplek, Bandung 40135, Indonesia 14 Marine Geological Institute of Indonesia, J1.Dr.Junjunan 236, Bandung 40174, Indonesia

The 2004 Mw 9.2 Sumatra-Andaman earthquake caused a huge tsunami of more than 20 m on average along the west coast of Aceh. Four hypothetical models have been proposed for tsunami generation.

The first model is that coseismic slip along the Sumatran megathrust is responsible for generation of the huge tsunami (Fig.1a) [e.g.,Henstock et al., 2006]. In this case, however, an additional tsunami generation mechanism such as inelastic deformation of soft accretionary sediment may be needed [Seno and Hirata, 2007]. The second model is that the most trenchward splay fault branching updip from the megathrust displaced coseismically (Fig.1b) [Soh et al., 2005; Seeber et al., 2007]. Splay faults can generate larger tsunamis, primarily because of their steeper dip. The third model is that the most landward splay fault, located at the eastern margin of the Sumatran outer-arc high, displaced coseismically (Fig.1c) [Sibuet et al., 2007]. The fourth model is that the West Andaman Fault, just west of the Aceh (forearc) basin, displaced coseismically (Fig.1d) [Plafker et al., 2006, 2007], though ROV diving surveys did not find any signature of coseismic fault motion along this fault [NT0502 scientific party, 2005; SEATOS scientific party, 2005].

Recent studies help constrain possible 2004 tsunami sources off northwest Sumatra. Coseismic slip limited to the deeper (landward) part of the megathrust cannot explain the observed Sea Surface Heights [Geist et al., 2007], indicating that the rupture reached the more trenchward portion of the fault. Calculated tsunami backward wave-fronts suggest that the trenchward boundary of the tsunami source off northwest Sumatra was located near the accretionary prism toe and that long-wave-approximated, maximum uplift area was located in the middle of the outer-arc high [e.g., Fine et al., 2005].

34 Yamazaki et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Array analysis of short-period tsunami dispersed waves, observed with hydrophone

arrays in the Indian Ocean, suggests that the source is located at 4.3 degN and 93.8 degE [Hanson et al., 2007]. This location coincides with along Middle Thrust, depicted by Sibuet et al. [2007], which is located in the middle outer arc-high. The estimate is considered precise but it is possible that the source location may be a few tens of kilometers at most trenchward of this position. The source size was much smaller than 30 km in length [Hanson et al.,2007]. All the studies above provide critical constraints on the location and mechanism of anomalous tsunami generation offshore northwest Sumatra.

Taking these constraints into account, we can construct a new model to explain the unusual tsunami generation off northwest Sumatra. We thus propose the fifth model that the 2004 earthquake ruptured updip along the megathrust (plate interface) near the deformation front, but branched onto one of the outer-arc high splay faults: either the Middle Thrust or possibly the Lower Thrust of Sibuet et al.[2007] (Fig.1e). References Fine,I.V., B. et al., 2005, GRL, 32, L16602, doi:10.1029/ 2005GL023521. Geist, E. L., et al., 2007, Bull. Seism. Soc. Am. 97,S249–S270. Hanson, J.A.,et al., 2007,BSSA, 97, No. 1A, pp. S232－S248, January 2007, doi: 10.1785/0120050607 Henstock,T.J., et al., 2006, Geology, v34, pp485-488. NT05-02 Scientific Party, 2005. http://www.jamstec.go.jp/jamstec-e/sumatra/natsushima/

bm/contents.html Plafker, G., et al., 2006, Seism. Res. Lett. 77, 231. Plafker, G., 2007. New evidence for a secondary tectonic source for the cataclysmic tsunami of

12/26/2004 on NW Sumatra, BIO. SEATOS onboard scientific party, 2005. SEATOS 2005 Cruise Report, 40.pp. Seeber, L., et al., 2007, Earth Planet. Sci. Lett., 263, 16-31, 2007. Seno,T., and K.Hirata, Bull.Seismiol. Soc.Am., 97, S296-S306, doi: 10.1785/0120050615, 2007. Sibuet, J.-C., et al., 2007, Earth Planet. Sci. Lett., 263, 88-103. Soh, W., Y. S. Djajadihardja, et al., 2005, EOS Trans. AGU 86, no. 52, Fall Meet. Suppl., U11B-0842.

Fig.1 The 2004 tsunami generation models off

NW Sumatra. (a) Coseismic rupture reaches the

deformation front (DF) along the plate

interface. (b) Coseismic rupture branches

onto the main thrust (M'T).(c) Coseismic

rupture branches onto the upper thrust (UT)

(d)West Andaman Fault displace coseismically.

(e) Coesismic rupture propagates updip along

the plate interface near the deformation

front, but branched onto one of the outer-arc

high splay faults: either the Middle Thrust or

possibly the Lower Thrust (the 5th model)．

Hirata et al.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 35 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Appendix A: List of Participants (A-Z) Agustan Graduate student (D2) Research Center for Seismology Volcanology and Disaster Mitigation, Graduate School of Environmental Studies, Nagoya University D2-2 (510) Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan e-mail: agustan@seis.nagoya-u.ac.jp Arai, Kohsaku Senior Scientist Geological Survey of Japan-AIST Central 7, Higashi 1-1-1, Tsukuba, Ibaraki, 305-8567, Japan e-mail: ko-arai@aist.go.jp Araki, Ei'ichiro Technical Scientist Department of Oceanfloor Network System Development for Earthquake and Tsunami (DONET) Japan Agency for Marine-Earth Science and Technology (JAMSTEC) 2-15 Natsushima-cho, Yokosuka 237-0061, Japan e-mail: araki@jamstec.go.jp Djajadihardja, Yusuf S. Director Center for Technology for Natural Resources' Inventory Agency for the Assessment and Application of Technology(BPPT) BPPT Building II, Jl. M.H. Thamrin No.8, Jakarta 10340, Indonesia. e-mail: iyung@ceo.bppt.go.id Fujiwara, Toshiya Technical Scientist Institute for Research on Earth Evolution (IFREE) Japan Agency for Marine-Earth Science and Technology (JAMSTEC) 2-15 Natsushima-cho, Yokosuka 237-0061, Japan e-mail: toshi@jamstec.go.jp Gaffar, Eddy Z. Researcher Research Center for Geotechnology Indonesian Institute of Science (LIPI) Jl. Sangkuriang, Bandung, W. Java, 40135, Indonesia. e-mail: gaffar@geotek.lipi.go.id Gunawan, Endra Graduate student (D1) Research Center for Seismology Volcanology and Disaster Mitigation, Graduate School of Environmental Studies, Nagoya University D2-2 (510) Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan e-mail: endra@seis.nagoya-u.ac.jp

36 Yamazaki et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Gusman, Aditya Riadi Graduate student (D2) Institute of Seismology and Volcanology, Hokkaido University, Kita 10 Nishi 8 Kita-ku, Sapporo City, Hokkaido 060-0810, Japan e-mail: adit@mail.sci.hokudai.ac.jp Hanifa, Nuraini Rahma Graduate student (D1) Research Center for Seismology Volcanology and Disaster Mitigation, Graduate School of Environmental Studies, Nagoya University D2-2 (510) Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan e-mail: hanifa@seis.nagoya-u.ac.jp Hayashi, Yoshinori Associate Professor Center for Integrated Research and Education of Natural Hazards, Shizuoka University Shizuoka, 422-8529, Japan e-mail: oyhayas@ipc.shizuoka.ac.jp Hirata, Kenji Senior Researcher Department of Seismology and Volcanology Meteoroligical Research Institute (MRI)/Japan Meteorological Agency (JMA) 1-1 Nagamine, Tsukuba 305-0052, Japan e-mail: khirata@mri-jma.go.jp Ikehara, Ken Group Leader Geological Survey of Japan-AIST Central 7, Higashi 1-1-1, Tsukuba, Ibaraki, 305-8567, Japan e-mail: k-ikehara@aist.go.jp Ishida, Mizuho Principal Scientist Institute for Research on Earth Evolution (IFREE) Japan Agency for Marine-Earth Science and Technology (JAMSTEC) 3173-25 Showa-cho, Kanazawa-ku, Yokohama 236-0001, Japan e-mail: ishidam@jamstec.go.jp Ito, Yoshihiro Assistant Professor Research Center for Prediction of Earthquakes and Volcanic Eruptions Tohoku University 6-6, Aramaki-Aza-Aoba, Aoba-ku, Sendai, 980-8578, Japan e-mail: yito@aob.geophys.tohoku.ac.jp

Appendix A.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 37 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Kinoshita, Masataka Group Leader Institute for Research on Earth Evolution (IFREE) Japan Agency for Marine-Earth Science and Technology (JAMSTEC) 2-15 Natsushima-cho, Yokosuka 237-0061, Japan e-mail: masa@jamstec.go.jp Matsumura, Shozo Guest Scientist National Research Institute for Earth Science and Disaster Prevention, 3-1 Tennodai, Tsukuba, Ibaraki, 305-0006, Japan e-mail: shozo@bosai.go.jp Nakamura, Mamoru Associate Professor Faculty of Science, University of the Ryukyus, Nishihara, Okinawa, 9030213, Japan e-mail: mnaka@sci.u-ryukyu.ac.jp) Nakamura, Takeshi Researcher (Post-doc) Department of Oceanfloor Network System Development for Earthquake and Tsunami (DONET) Japan Agency for Marine-Earth Science and Technology (JAMSTEC) 3173-25 Showa-cho, Kanazawa-ku, Yokohama 236-0001, Japan e-mail: ishidam@jamstec.go.jp Permana, Haryadi Researcher Research Center for Geotechnology Indonesian Institute of Science (LIPI) Jl. Sangkuriang, Bandung, W. Java, 40135, Indonesia. e-mail: permhp@yahoo.com Soh, Wonn Director Kochi Institute for Core Sample Research Japan Agency for Marine-Earth Science and Technology (JAMSTEC) B200, Monobe, Nankoku, Kochi 783-8502, Japan e-mail: soh@jamstec.go.jp) Udrekh Researcher Center for Technology for Natural Resources' Inventory Agency for the Assessment and Application of Technology(BPPT) BPPT Building II, Jl. M.H. Thamrin No.8, Jakarta 10340, Indonesia. e-mail: udrekh@gmail.com

38 Yamazaki et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Yamazaki, Akira Senior Researcher Department of Seismology and Volcanology Meteoroligical Research Institute (MRI)/Japan Meteorological Agency (JMA) 1-1 Nagamine, Tsukuba 305-0052, Japan e-mail: ayamazak@mri-jma.go.jp

Appendix A.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 39 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ Appendix B: Photographs in the Workshop

Dr. Toshiya Fujiwara

Dr. Shozo Matsumura

Mr. Agustan

40 Yamazaki et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Mr. Endra Gunawan

Discussion

Ms.Nuraini Rahma Hanifa

Appendix B.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 41 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Dr. Yoshinori Hayashi

Dr. Ei'ichiro Araki

poster session #1

42 Yamazaki et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

poster session #2

Dr. Yusuf S. Djajadihardja

Dr. Udrekh

Appendix B.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 43 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Dr. Kohsaku Arai

Dr. Ken Ikehara

Dr. Mamoru Nakamura I

44 Yamazaki et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Dr. Haryadi Permana

Discussion

Mr. Eddy Z.Gaffar

Appendix B.

Workshop on Subduction Process along the Sumatra-Java arc, Tokyo 2009 45 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Dr.Udrekh

Mr. Aditya R. Gusman

Dr. Yoshihiro Ito

46 Yamazaki et al. ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

Appendix B.