IUGG2003 SS04/10P/D-034

Characterized Source Models for Shallow Intraslab Earthquakes in JapanKimiyuki ASANO, Tomotaka IWATA, and Kojiro IRIKURA(Disaster Prevention Research Institute, Kyoto University, JAPAN)k-asano@egmdpri01.dpri.kyoto-u.ac.jp

Abstract Large shallow intraslab earthquakes, occurring within subducting slabs at 30-100km depths, generate earthquake damages by strong ground motions (e.g. the 1993 Kushiro-oki earthquake, the 2001 Geiyo earthquake). Ground motion characteristics of intraslab earthquakes have been pointed out to have some different features compared to those of inland crustal earthquakes or interplate earthquakes by several papers. We have examined seven shallow intraslab earthquakes that recently occurred around Japan (MJMA5.1~7.0) using strong motion network data. We carried out broadband ground motion simulations based on the empirical Green's function method (Irikura, 1986) to investigate the source characteristics of shallow intraslab earthquakes. Using the empirical Green's function method, we can construct the source model to explain observed waveforms in broadband frequency range (Kamae and Irikura, 1998; Miyake et al., 1999). We used the observed waveform of a small event occurring in each source region as the empirical Green's function so that we could take account of propagation path and site effects implicitly. First, we determined the rupture plane by comparing synthetic waveforms from two possible fault planes obtained from focal mechanism solutions, because of the insuffcient aftershock distribution information for these events. Next, we estimated the size of strong motion generation area (SMGA), rise time, and rupture propagation velocity of the target event by forward modeling. The misfit function is selected as the sum of residuals of displacement waveforms and those of acceleration envelopes for explaining wide frequency band ground motions. For inland crustal earthquakes, the self-similar relation between the asperity area derived from the kinematic waveform inversion and the seismic moment was shown by Somerville et al. (1999). Some studies (Kamae and Irikura, 1998; Miyakoshi et al., 2000; Miyake et al., 2001) demonstrated that the SMGA obtained by the broadband ground motion simulation coincides with the asperity. We compared the SMGA size obtained in our study with the empirical relation for inland crustal earthquakes proposed by Somerville et al. (1999). The size of SMGA obtained for each earthquake is about 14-90% of prediction from that empirical relation. Consequently, the stress drops on SMGA of shallow intraslab earthquakes are higher than those of inland crustal earthquakes. The ratios between the combined area of SMGA and the value predicted from the empirical relation decrease with focal depth, that is, stress drops on SMGA of shallow intraslab earthquakes increase with focal depth.

Recent large earthquakes in Japan1993/01/15 Kushiro-oki (M7.8) Intraslab event1993/07/12 Hokkaido Nansei-oki (M7.8) Interplate event1994/10/04 Hokkaido Toho-oki (M8.2) Intraslab event1994/12/28 Sanriku Haruka-oki (M7.6) Interplate event1995/01/17 Hyogo-ken Nanbu (M7.3) Inland event2000/10/06 Tottori-ken Seibu (M7.3) Inland event2001/03/24 Geiyo (M6.7) Intraslab event2003/05/26 Miyagi-ken Oki (M7.0) Intraslab event

Japan is located along two major subduction zones, and its tectonic setting is very complicated so that we have various types of earthquakes in Japan. Historically many shallow intraslab earthquakes have occured below populated areas with strong ground motion causing earthquake disasters. However, source characteristics of shallow intraslab earthquakes have rarely been examined with the exeption of several recent large earthquakes [Kikuchi and Kanamori, 1995; Kakehi and Yamauchi, 2001; Morikawa et al., 2002], and there are many things remained to make clear compared

ReferencesAsano, K., T. Iwata, and K. Irikura, Earth Planet Space, 55, e5-e8, 2003.Birgoren, G. H. Miyake, and K. Irikura, Eos Trans. Am. Geophys. Union, 82, Fall Meet. Suppl., Abstract S52E-0689, 2001.Irikura, K., Proc. 7th Japan Earthq. Eng. Symp., 151-156, 1986.Kamae, K. and K. Irikura, Bull. Seism. Soc. Am., 88, 400-412, 1998a.Kamae K. and K. Irikura, Proc. 10th Japan Earthq. Eng. Symp., 643-648, 1998b. (in Japanese)Kakehi, Y. and M. Yamauchi, Eos Trans. Am. Geophys. Union, 82, Fall Meet. Suppl., Abstract S42C-0679, 2001.Kikuchi, M. and H. Kanamori, Geophys. Res. Lett., 22, 1025-1028, 1995.Miyake, H., T. Iwata, and K. Irikura, Zisin 2 (J. Seism. Soc. Jpn.), 51, 431-442, 1999. (in Japanese with English Abstract)Miyake, H., T. Iwata, and K. Irikura, Geophys. Res. Lett., 28, 2727-2730, 2001.Miyakoshi, K., T. Kagawa, H. Sekiguchi, T. Iwata, and K. Irikura, Proc. 12th World Conf. Earthq. Eng., 1850, 2000.Morikawa, N., T. Sasatani, and H. Fujiwara, Proc. 11th Japan Earthq. Eng. Symp., 2002. (in Japanese)Somerville, P. G., K. Irikura, R. Graves, S. Sawada, D. Wald, N. Abrahamson, Y. Iwasaki, T. Kagawa, N. Smith, A. Kowada, Seism. Res. Lett., 70, 59-80, 1999.Wessel, P. and W. H. F. Smith, Eos Trans. Am. Geophys. Union, 76, 329, 1995.

Studied Area

Methodology

2001/4/3 23:57(JST) Shizuoka-ken Chubu Earthquake (MW5.2)

Scaling Relationship between SMGA and Seismic Moment

Epicenter of the mainshock and Station distributions. Red triangles indicate the strong motion observation stations, which were used to estimate the source parameters, and blue ones are other stations.

AcknowledgmentsWe sincerly thank Kyosin Network (K-NET), Kiban Kyosin Network (KiK-net), and Full Range Seismograph Network of Japan (F-net) operated by National Institute for Earth Science and Disaster Prevention (NIED) for providing strong motion data and moment tensor solutions, Harvard University for centroid moment tensor solutions, and Japan Meteorological Agency (JMA) for hypocentral information. We used Generic Mapping Tools [Wessel and Smith, 1995] to draw some figures in this presentation.

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Comparison of observed and synthesized waveforms. Numbers indicates the maximum values of observed waveforms. We used bandpassfiltered (0.3-10Hz) records. Lower limit of filter was determined from SN ratio of small event records. Fitting of waveforms are good in broadband range.

Horizontal strong ground motions were simulated using the empirical Green's function method [Irikura, 1986; Miyake et al., 1999]. Observed record from a small event having similar focal mechanism, that occurred near the hypocenter of the target event, was used as the empirical Green's function.

Seismic Moment:8.17×1016 Nm (F-net)

SMGA: 4.0 km2

Stress drop: 23 MPa

Latest Result 2003/5/26 18:24(JST) Miyagi-ken Oki Earthquake (MW7.0)

Epicenters of the mainshock and the aftershock used as the empirical Green's function and Station distributions. Green and orange triangles indicate the strong motion observation stations of K-NET and KiK-net, respectively. The focal mechanisms of the mainshock and aftershock determined by F-net with the moment tensor inversion are also indicated.

Overview of the empirical Green's function method (After Miyake et al., 1999). (a) The strong motion generation areas of the mainshock and a small event. L/l=W/w=N. (b) Correction function to ajust a difference in slip velocity function between the mainshock and the small event. (c) Displacement amplitude spectra following the ω-2 source model assuming stress drop ratio C between the mainshock and the mall event.

(b)

(a)

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Source model of this event. Green rectangular area is SMGA. Blue area is background area roughly estimated from aftershock distributions. Open circles indicate aftershock occurring within a day after the mainshock.

Conclusions■We have succesively simulate the observed strong motion accelerations, velocities, and displacements of 8 shallow intraslab earthquakes in

the broadband frequency range by the forward modeling based on the empirical Green's function method.■Strong motion generation area, that is equivalent to asperity in characterized source model, of a shallow intraslab earthquake is smaller than

that of a inland crustal earthquake with comparable seismic moment.■Consequently, the stress drop on asperities of a shallow intraslab earthquake is higher than that of a inland crustal earthquake, and it

increases with the focal depth. This is one of the important features for shallow intraslab earthquakes.

Locations of epicenters and focal Mechanisms (lower hemisphere projection). These focal mechanisms were determined from the moment tensor inversion by F-net or Harvard University.

No. Origin time Lat. Long. Depth MW1 1997/03/16 14:51 34.9N 137.5E 39.1km 5.62 1999/08/21 05:33 34.0N 135.5E 65.8km 5.63 2000/01/28 23:21 43.1N 146.7E 58.5km 6.74 2001/03/24 15:27 34.1N 132.7E 46.5km 6.85 2001/04/03 23:57 35.0N 138.1E 30.3km 5.26 2001/04/25 23:40 32.8N 132.3E 39.3km 5.77 2002/11/04 13:36 32.4N 131.9E 35.2km 5.78 2003/05/26 18:24 38.8N 141.7E 70.7km 7.0*Origin time, location of hypocenter were determined by JMA*Seismic moment was determined by F-net

8 shallow intraslab events that occurred in Japan were studied. The focal depth ranges are between 30 km and 70 km. The list of earthquakes in this study is shown below.

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Observed (black) and synthetic (red) velocity waveforms (0.3 - 10 Hz) at each station. Waveforms at each station are normalized by the observation. These stations are not used in constructing the source model. These calculation is the validation of obtained source model.

Our source model is composed several rectangular strong motion generation areas (SMGAs) on fault plane. Strong motion is assumed to radiate only from SMGA. Appropriate fault plane is assumed based on the moment tensor solution by F-net (NIED) or Harvard Univeristy. Length and width of SMGA, rise-time, and rupture propagation velocity were determined by forward modeling to minimize the summation of the residuals of acceleration envelopes and those of displacement waveforms [Miyake et al., 1999] The SMGA estimated from the broadband strong motion simulation are considered to be the asperity derived from the kinematic waveform inversions using strong motion records [e.g., Kamae and Irikura, 1998; Miyakoshi et al., 2000; Miyake et al., 2001].

Source model of the 2003 Miyagi-ken Hokubu earthquake. Green rectangular area is SMGA. Open circles indicate aftershock occurring within a day after the mainshock. The estimated stress drop on SMGA is very high. Rupture propagation velocity is 2.75 km/s.

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MY G H01

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MY G H11

MY G H12

IWT H04

IWT H05

IWT H17

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IWT 018

MY G H01

MY G H03

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IWT H04

IWT H05

IWT H17

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IWT H19

IWT H20

IWT H21

IWT H22

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IWT H19

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Acceleration (0.3-10 Hz)

Acceleration (0.3-10 Hz)

Velocity (0.3-10 Hz)

Velocity (0.3-10 Hz)

137° 30'E

137° 30'E

138° 00'E

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34° 30'N 34° 30'N

35° 00'N 35° 00'N

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0 50

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HK W

S ZO014

S ZO019

S ZO026

A IC 007A IC 008

A IC 010A IC 013

NG N025

S ZO001

S ZO003

S ZO005

S ZO006

S ZO008

S ZO009S ZO011

S ZO012

S ZO013

S ZO015

S ZO016

S ZO017

S ZO018

S ZO022

S ZO023

S ZO024S ZO025

Y MN006

Y MN007

The size of SMGAs of shallow intraslab earthquakes was estimated to be 14 - 90 % of values predicted from the empirical relationship for inland events. Namely, the size of SMGA or asperities of a shallow intraslab event is smaller than that of an inland event with comparable seismic moment. This result led us to a conclusion that the stress drop on asperities of a shallow intraslab earthquake is much higher than that of a inland crustal earthquakes. It is one of the important features to understand the source characteristics or physics of shallow intraslab earthquakes.

Relationship between the combined area of SMGAs and seismic moment. Solid circles, solid triangles, and solid squares indicate intraslab earthquakes investigated in this study, large intraslab earthquake arround Hokkaido, Japan, and inland crustal earthquakes, respectively. The solid line indicates the empirical relationship between the combined area of asperities and seismic moment for inland crustal earthquakes proposed by Somerville et al. (1999). The broken portion is the extension of the relationship for smaller and larger events. The dotted line is the relationship when stress drop on asperities is five times higher than that assumed from the empirical relationship of Somerville et al. (1999) . This figure is the update version of Asano et al. (2003)

Sa : Total size of SMGA for each event obtained in this studySa' : Predicted combined area of asperities from Somerville et al. (1999)

The ratio Sa / Sa' decreases with focal depth. This fact means that thedeeper event has relatively larger stress drop than shallower eventsamong shallow intraslab ebents. Stress drop on SMGAs or asperitiesof a shallow intraslab earthquake depends on focal depth.

Relationship between Sa / Sa' and focal depth for intraslab events. Solid circles (red) and solid triangles (orange) indicate the results obtained in this study and Morikawa et al. (2002), respectively. Solid lines indicate the spatial extent of SMGA in the direction of depth. The Sa / Sa' value is decreasing with focal depth.

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Hyogo-ken Nanbu by Kamae and Irikura (1998a)Northridge by Kamae and Irikura (1998b)Inrand Crustal Earthq. by Miyake et al. (2001)Duzce(Turkey) by Birgoren et al. (2001)Intra-slab Earthq. by Morikawa et al. (2002)This study

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2003/ 05/ 26 18: 24- 2003/ 05/ 27 18: 23( Det er mi ned by J MA)

Aftershock distribution within a day after the mainshock.

Observed (black) and synthetic (red) acceleration and velocity waveforms (0.3 - 10 Hz) at several stations. Waveforms at each station are normalized by the observation. Observed waveforms fairly well-reproduced by synthetics. For KiK-net stations, borehole seismograph records are used to get good signal-to-noise ratio of small event's data.