November 8-11, 2006 The 6th Joint Meeting of the UJNR Panel on Earthquake Research in Tokushima, Japan

This study is aimed at improving the quality of tsunami height data obtained by satellite altimetry by reducing the effects of oceanographic phenomena other than tsunamis. Datasets of the tsunami profiles in the eastern Indian Ocean with fewer backgrounds along five tracks from four satellites after the occurrence of the Sumatra-Andaman earthquake were successfully obtained.

1. IntroductionFour satellites equipped with altimeters (Table

1) passed over the eastern Indian Ocean approximately 115–545 min after the occurrence of the 2004 Sumatra-Andaman earthquake (Fig. 1). Altimeters are sensors that use microwave radar to determine the distance from the sea surface directly below. Tsunamis can be detected by the change in the height of the sea surface during oceanographic monitoring missions if they have proper tracks (Fig. 2). The 2004 Indian Ocean tsunami was the very first event for which distinct tsunami profiles were observed in the oceanic region by satellite altimetry [1].

Detection of the 2004 Indian Ocean tsunami by satellite altimetryYutaka HAYASHI ( Meteorological Research Institute )

AcknowledgementsSome of the figures were prepared by using General mapping tools [ 9]. The satellite altimetry data and processing codes [10] were

provided by AVISO, CNES (Archivage, Validation et Interprétation des données des Satellites Océanographiques, Centre National d’Etudes Spatiales).

References[1] Gower, J. (2005), EOS Trans. AGU, 86, 37-38. [2] Song, Y.T., et al. (2005), GRL, 32, L20601.[3] Hirata, K. et al. (2006), Earth Planets Space, 58,195-201. [4] Fujii, Y., and K. Satake (2006), submitted to BSSA.[5] NEIC, USGS (2005), ftp://hazards.cr.usgs.gov/weekly/manuscript/ [6] Bird, P. (2003), Geochem. Geophys. Geosyst., 4, 1027.[7] Hayashi et al. (2005),Technical Memorandum of PWRI, 3983, 63-69. [8] Ducet et al. (2000), J. Geophy. Res., 105, 19477-19498.[9] Wessel, P. and W.H.F. Smith (1999), The Generic Mapping Tools Technical Reference and Cookbook, Version 3.3.[10] AVISO (2006), SSALTO/DUACS User Handbook: (M)SLA and (M)ADT Near-Real Time and Delayed Time Products.

2. Data(1) Satellites (ref. Table 1)Jason-1, TOPEX/Poseidon, GFO, and ENVISAT

(2) DatasetsAlong track DT-SLA (delayed time sea level

anomaly) products compiled and distributed by AVISO. (Table 2).(3) Sampling points

a) (For searching tsunami) Sampling points on all satellite tracks in the Eastern Indian Ocean within 12 h after the occurrence of the main shock.

b) (For reference) All sampling points in eastern Indian Ocean (ref. Fig. 3).(4) Period

a) At sampling points (3) a).The cycle of the day of Indian Ocean tsunami (for searching tsunami), and 5 cycles each before and after the tsunami (for background estimation).

b) At sampling points (3) b).From 280 days (8-cycles period for ENVISAT)

before the main shock to 280 days after it.

Fig. 1. Satellite tracks which has passed near by the source area of the 2004 Sumatra-Andaman earthquake within 12 hours from its origin time (December 26, 2004, 00h58m53s;UT).Observation times of each tracks are shown in the Table 3. Solid circles plot the epicenters (M>=4.0) [5]

within 12 hours from the main shock. The curved pink line indicates plate boundaries [6].

4. ResultsThree tracks in the eastern Indian Ocean recorded

clear tsunami profiles ([A,B,C] in Table 3 and Fig. 5). Two other tracks ([D,E] in Table 3 and Fig. 5) also recorded the tsunami wave partly. In comparison with the tsunami profiles given by the difference in the two cycles [1], the products in this study have simpler wave forms, smaller peak heights, indistinct double peaks near S4° ([A]), and a defined tsunami height corresponding to more points ([A,B]). The residual errors have been notably reduced (Table 3) nearly to the accuracies of the observation by the catalogue (e.g. <2.5cm (goal) for Jason-1; 4.2cm for TOPEX/Poseidon).

Fig. 2. An ocean tsunami caused by a huge earthquake might be observed by satellite altimetry.Satellite altimetry observes the distance from the satellite to the sea surface just below. Satellites with altimeters in favorable tracks and schedules might observe ocean tsunamis in propagation. (Illustrated by F. Hayashi)

3. Methods3.1 Multisatellite time-spatial interpolation

Multisatellite time-spatial interpolation was performed in order to define the reference height, which was estimated as the SSH under the assumption of no tsunami occurrence. The reference height function is defined by the weighted mean as Eq. 1 and 2.3.2 Definition of tsunami height

The scale parameters are determined as R = 45 km, T =10 days, referring to the correlation function of the SSH [8]. The data collected within 24 h of the main shock was not used for interpolation because of the possibility of the data being affected by the tsunami. Thus, the tsunami height (htsunami) at any sampling point is derived as Eq. 3.3.3 Estimation of background level

If the date and time under no tsunami occurrence is substituted for t, instead of tsunami heights, residual errors of multisatellite time-spatial interpolation methods at the sampling point (Φ,θ) is provided.

5. Discussion and Summary5.1 Alternative method

As a method of acquiring the reference value (SSHref) in Eq. 3, interpolation of the grid point values on the mean sea level anomaly, which are periodically produced, easily comes up to mind. In Fig. 6, DT-MSLA (delayed time mean sea level anomaly) mapped every 3.5 days by AVISO was notably affected by observation data including tsunami. Therefore, it does not fulfill "SSH under the assumption of no tsunami", which is the necessary condition for the reference height.5.2 Newly discovered facts on the Indian Ocean tsunami observed by satellite altimetry

The tsunami datasets obtained in this study may have the potential to contribute towards a more accurate analysis of the Indian Ocean tsunami. The results of the previous study, which are obtained from the tsunami height calculated using the simple method described above, need to be verified by paying attention to newly discovered facts that the peak heights have been overestimated, the existence of double peaks are doubtful, the defined tsunami heights in shallow sea regions include large noises, etc.

SSHref (Φ,θ,t) = Σwi・SSHobs,i / Σwi (Eq. 1)wi = exp (-ri

2 / R2 - ti2 / T2 ) (Eq. 2)

ri: distance between the tsunami observation point (Φ,θ,t) and location of ith datumti: difference in observation time of the tsunami observation point (Φ,θ,t) and ith datumR, T: scale parameter

htsunami (Φ,θ,t) = SSHobs (Φ,θ,t) - SSHref (Φ,θ,t) (Eq. 3)SSHobs: observed SSH SSHref: reference sea surface height (defined by Eq. 1,2)

The simplest technique to estimate the height of a tsunami is to subtract the observed height at the same point in the previous cycle from the data in the cycle that includes the tsunami effect. Estimated tsunami heights by this technique has been utilized [e.g. 2,3,4]. However, the changes in the sea surface height (SSH) that were observed by satellite altimetry included various effects of oceanographic, meteorological, geodetic, and seismic phenomena. This study aims at enhancing tsunami height data of the Indian Ocean tsunami by eliminating the effects from phenomena other than tsunamis and by extracting tsunami components from the SSH anomaly products.

Fig. 4. Example of distribution of weight for calculating weighted mean by multisatellite time-spatial interpolation.Size of the circle is set to be proportional to the weight defined by Eq. 2. The figure is for the sampling point where peak tsunami height has recorded along track 109 of Jason-1.

SSH include following noises

• Geoid locality• Ocean tides• Air pressure• Atmospheric vapor• Offsets of each satellite

SSH anomaly still have various effects•Non-seismic effects

Sea currentsTemperatureWinds … etc.

•Tsunami•Coseismic geoid change

reduced by routine processing of AVISO

products "DT-SLA"

oceanographers' interest

seismological interestvanishingly small [7]

separated by this study

Table 2. Map of processing sea surface height (SSH) data obtained by satellites altimetry.

35.0000ESA, CNESENVISAT17.0506NOAAGFO

TOPEX/Poseidon9.9156CNES, NASA

Jason-1

cycle (day)agencies in operationsatelliteTable 1. List of satellites equipped with altimeters.

Fig. 6. Distribution of reference for Indian Ocean tsunami height in case of using DT-MSLA products by AVISO. The positive-negative sequence pattern of interpolated MSLA on the day of the mainshock (mapped) show good agreement to the patterns of tsunami height by this study on the satellite tracks (lines) especially along track 129 of Jason-1, 208 and 210 of GFO.

Table 3. Residual errors in defined tsunami heights by multisatellite time-spatial inter-polation (this study) and by the conventional study [1].

[E] GFO track210 cycle143

[D] GFOtrack208 cycle143

[C] ENVISATtrack352 cycle033

[B] TOPEX/Poseidontrack129 cycle452

[A] Jason-1track129 cycle109

satellite, track and cycle number conventionalthis study

10.94.6544 - 523.5S48 - N21.2°

9.14.6441 - 422.7S39 - N21.8°

10.83.8201 - 189.6S19 - N21.8°

8.04.9121 - 130.0S6 - N20.1°

7.03.5114 - 123.1S6 - N19.4°

RMS residual error (cm)detected tsunami time (min after main shock), and range (lat.)

Fig. 5. Profiles of tsunami heights detected from satellite altimetry along five tracks.Red dots indicate tsunami heights defined by this study (Eq. 3) or conventional one. Conventional study defined tsunami height as the difference of SSH (sea surface height) with reference of the SSH in the previous cycle. Light blue lines show oceanographical effects estimated by this study (Eq. 2). Blue and dark blue dots indicate SSH data immediately after the main shock and at the previous cycle, respectively. Gray dots indicate background level of tsunami heights by this study or conventional study.

5.3 Tsunami detectability of satellite altimetry missionThe RMS background level obtained in this study probably gives the limit of the detectability of

satellite altimetry detection of tsunamis in the eastern Indian Ocean in terms of delayed time analysis. Tsunamis with wavelengths longer than the sampling resolutions (e.g. 15 km for Jason-1) and with peak heights exceeding the background levels (3.5–4.9 cm; Table 3) by a few times along a certain satellite track in this area could probably be extracted by the multisatellite time-spatial interpolation method employed in this study.

On the other hand, the RMS background level obtained by the method involving the determination of the tsunami height from the differences between two continuous cycles gives the typical magnitude of a non-seismic oceanographical disturbance in one cycle (Table 1) of each satellite. If ideal data acquisition, processing, and monitoring techniques were available, the relationship between the RMS background levels and the cycles of satellites would reveal the theoretical limits of near real-time tsunami detection by altimetry.

Fig. 3. Distribution of sampling points of four satellites equipped altimeters in northeastern Indian Ocean. Bold red line indicates plate boundaries [6]. Light green, sky blue, red purple and red thin lines indicate sampling points of Jason-1, TOPEX/Poseidon, GFO and ENVISAT, respectively.

Digital data files of these results are prepared. Please contact me. e-mail: yhayashi@mri-jma.go.jp

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