Source of the July 2006 West Java tsunami estimated from tide gauge records

GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L24317, doi:10.1029/2006gl028049, 2006 Source of the July 2006 West Java tsunami estimated from tide gauge records Yushiro Fujii 1 and Kenji Satake 2 Received 13 September 2006; revised 26 October 2006; accepted 22 November 2006; published 29 December 2006. [1] The source of the West Java tsunami of July 17, 2006, which was generated during a large earthquake near the Sunda trench, is constrained by tsunami waveforms that were recorded on six tide gauges around the Indian Ocean. The tsunami travel times poorly constrain the source area, probably because shallow bathymetry near these gauges is not well known. Inversion of tsunami waveforms, however, reveals that the tsunami source was about 200 km long. The largest slip, about 2.5 m for instantaneous rupture model, was located about 150 km east of the epicenter. Most of the slip occurred on shallow parts of the fault, indicating that this earthquake shares the same characteristics with tsunami earthquakes which generate abnormally large tsunamis compared with ground shaking. The slip distribution yields a total seismic moment of 7.0 10 20 Nm (Mw = 7.8). Citation: Fujii, Y., and K. Satake (2006), Source of the July 2006 West Java tsunami estimated from tide gauge records, Geophys. Res. Lett., 33, L24317, doi:10.1029/ 2006GL028049. 1. Introduction [2] An earthquake off the south coast of Java (9.222 S, 107.320 E, Mw = 7.7 at 8:19:28 UTC according to USGS) on July 17, 2006 generated a tsunami that left more than 800 persons dead or missing in western Java (International Federation of Red Cross and Red Crescent Societies, Indonesia: Western Java earthquake and tsunami, 2006, available at http://www.ifrc.org/cgi/pdf_appeals.pl?rpts06/ IDtseq26070603.pdf). The source process of the 2006 Java earthquake has been inferred from teleseismic body waves. Y. Yagi (see http://www.geo.tsukuba.ac.jp/press_hp/yagi/ EQ/20060717Jawa/) and C. Ji (Preliminary result of the 2006 July 17 magnitude 7.7 south of Java, Indonesia earthquake, 2006, available at http://neic.usgs.gov/neis/ eq_depot/2006/eq_060717_qgaf/neic_qgaf_ff.html) estimated long source duration of 150 s and 200 s with rupture velocities of 1.5 km/s or less and 1.1 km/s, respectively. Ammon et al. [2006] estimated a low rupture velocity of 1.0 1.5 km/s and long source duration of about 185 s, using long-period body waves and Rayleigh waves. Hara [2006] also found that the duration of high energy radiation was 156 s, longer than typical earthquake of M7 class, through his early determination of magnitude. W. Kongko et al. (Rapid survey on tsunami Jawa 17 July 2006, available at 1 International Institute of Seismology and Earthquake Engineering, Building Research Institute, Tsukuba, Japan. 2 Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan. Copyright 2006 by the American Geophysical Union. 0094-8276/06/2006GL028049 http://ioc3.unesco.org/itic/files/tsunami-java170706_e.pdf) and Y. Tsuji et al. (Field survey of the tsunami inundated heights due to the Java tsunami (2006/07/17) along the coast on the Indian Ocean in Java Island, available at http:// www.eri.u-tokyo.ac.jp/tsunami/javasurvey/index_e.htm) interviewed local residents along 200 km of coast northeast of the epicenter, and reported that the residents felt slight or no shaking. The long duration, slow rupture velocity and slight ground shaking would indicate that this earthquake was a tsunami earthquake [Kanamori, 1972] that generates abnormally large tsunamis than expected from seismic waves. [3] Tsunami waveform analyses show that tsunami earthquakes share a common feature: narrow rupture width and shallow faulting near the trench axis. The 1992 Nicaragua earthquake was the first tsunami earthquake recorded on modern broad-band seismic instruments, and seismological studies showed that the duration was very long for its size, about 100 s [Kanamori and Kikuchi, 1993]. Comparison of numerically computed tsunami waveforms with tide gauge records [Satake, 1994] showed that a narrow rupture width (40 km) and shallow fault near trench axis is responsible for the abnormal tsunami. More typical tsunami earthquakes, the 1896 Sanriku earthquake and the 1946 Aleutian earthquake, also show a similar character to the Nicaragua earthquake [Satake and Tanioka, 1999; Tanioka and Satake, 1996a, 1996b; Tanioka and Seno, 2001a, 2001b]. [4] To the east of the 2006 source, an earthquake occurred on June 2, 1994 and its tsunami killed more than 250 people [Tsuji et al., 1995]. Polet and Kanamori [2000] classified the 1994 event as a slow tsunami earthquake which has an anomalously low energy release in frequency range of 1 20 s, with the centroid located close to the trench (shallow rupture). In contrast, Abercrombie et al. [2001] reported that there is no evidence for slow, shallow rupture for the 1994 event, which does not support tsunami earthquake. Tsunami analysis could not distinguish them, because this tsunami was not recorded instrumentally [Tanioka and Satake, 1996b]. The 2006 tsunami, on the contrary, was instrumentally recorded at several tide gauge stations. In this paper we analyze the tsunami waveforms to examine if the 2006 earthquake was a tsunami earthquake. 2. Tide Gauge and Bathymetry Data [5] The 2006 West Java tsunami was recorded at more than 12 tide gauge stations around the Indian Ocean (Figure 1). Some of them, namely Benoa and Rodrigues, have been collected by Global Sea Level Observing System (GLOSS) and were stored on the web site of University of L24317 1of5

Table 1. List of Tide Gauge Stations Station Latitude a Longitude a Agency b AT, c min Rodrigues 19:40S 63:24E U 400 Benoa 8:46S 115:12E U 86 Christmas 10:32S 105:40E B 16 Cocos 12:08S 96:52E B 96 Broome 18:00S 122:14E B 284 Hillarys 31:50S 115:42E B 254 a Latitude and longitude are given in degrees and minutes. b U is the University of Hawaii Sea Level Center (UHSLC) and B is the Bureau of Meteorology, Research Centre (BoM), Australian Government. c AT is arrival time of observed tsunami. Figure 1. Epicenters of the July 2006 West Java earthquake (solid star) and the June 1994 Java earthquake (open star). Triangles indicate the location of available tide gauge stations; we used only stations in black. Harvard CMT solution of the mainshock is also shown (lower hemisphere equal-area projection). Hawaii Sea Level Center (UHSLC). We downloaded digital data from the web site (http://ilikai.soest.hawaii.edu/rsl/). We also use the waveforms recorded on four tide gauges operated by Australian Bureau of Meteorology (BoM). These tide gauge records usually include ocean tides, which we removed by applying a high-pass filter. Amplitudes of the initial tsunami wave range from several cm to less than 0.4 m (see Figure 2). These six tsunami waveforms will be used for the analysis. We did not use data from six other stations available at UHSLC web site, because of no clear tsunami signals (at Gan, Male, Hanimaadhoo, Colombo and Sibolga) or obvious clock error (at Sabang). [6] Since the phase velocity of shallow-water waves depends only on the water depth, accurate bathymetric data is essential for the tsunami numerical computation. For the global ocean, the gridded bathymetry data digitized from contour maps is available from GEBCO [British Oceanographic Data Centre, 1997]. We use this bathymetry data for calculating tsunami travel times and waveforms. 3. Source Region From Tsunami Arrival Times [7] We first estimate the tsunami source area from the observed tsunami travel times, by calculating the initial wavefronts through back projection of tsunamis from tide gauge stations toward the source [e.g., Lay et al., 2005]. We calculate tsunami travel times on the original GEBCO data of 1 0 (arc-minute) interval. [8] The tsunami source is not well constrained by the travel times (Figure 3). The source is bounded by three arcs to the west (Rodrigues, Christmas and Cocos) but they differ by more than 100 km. The eastern arcs (Benoa and Broome) also differ by more than 100 km. The southern edge is bounded by only travel time to Hillarys. These computed travel times imply a tsunami source area much larger than the aftershock area, or a need for adjustments in travel times. The poor bathymetry data in shallow coastal area especially around Broome and Hillarys, where shallow and wide continental shelf is developed, is probably responsible for the poor constraints. 4. Inversion Method 4.1. Fault Parameters [9] In order to estimate the extent of the tsunami source and the slip distribution, we divide the tsunami source into Figure 2. Comparison of observed (gray lines) and synthetic (black lines) tsunami waveforms computed from the slip distribution estimated with instantaneous rupture propagation (Figure 4a). The gray-dashed line is the original record and the gray-solid line is the time shifted waveform segment. Time ranges shown by solid curves are used for the inversion; the dashed parts are not used for the inversion, but shown for comparison. Arrow indicates the onset time of tsunami for each trace as listed in Table 1. Figure 3. Constraint for tsunami source from arrival times of tsunami. Solid and dotted line indicate estimated initial wavefronts from the observed and adjusted tsunami arrival times, respectively (see text for details). Solid circles show the epicenters of aftershocks occurring one day after the mainshock located by the USGS. The 2006 and 1994 epicenters are also shown. 2of5

10 subfaults that cover the aftershock area during one day after the mainshock (Figure 4). The subfault size is 50 km 50 km (Figure 4 and Table 2). The top depths are 3 km and 11.7 km for shallow (odd numbers) and deep (even numbers) subfaults, respectively. The epicenter is located on the southwestern subfault. The focal mechanisms for all the subfaults are strike = 289, dip angle = 10 and slip angle = 95 from the Harvard CMT solution of the mainshock. We initially assume an instantaneous rupture on all the subfaults, because the tsunami propagation velocity is 0.24 km/s for the water depth of 6000 m, much smaller than the typical rupture velocity. We then vary the rupture velocity from 1.0 to 3.0 km/s at a 0.5 km/s interval (Table 2). The rupture is assumed to propagate unilaterally from the epicenter to the east. Subfaults 1 2 3 4 begin to slip simultaneously and the next slip begins in sequence at subfault pairs 5 6, 7 8, and 9 10. Figure 4. Slip distribution estimated by inversion of tide gauge data assuming (a) instantaneous rupture propagation and different rupture velocities (b) 2.0 3.0 km/s, (c) 1.5 km/s, and (d) 1.0 km/s). Star shows the mainshock epicenter. Circles indicate aftershocks within one day after the mainshock. Subfault numbers are also shown. 4.2. Finite Difference Computation [10] To calculate tsunami propagation from each subfault to stations, the linear shallow-water, or long-wave, equations were numerically solved by finite-difference method [Satake, 1995]. The details of governing equations are described by Fujii and Satake [2006]. The computation area extends from 55 E to 130 E and 40 S to 15 N (Figure 1). The bathymetric grid interval is basically 2 0 (2 arc-minutes, about 3.7 km), hence there are 2,250 1,650 grid points along the longitude and latitude directions, respectively. Near the coastal tide gauge stations, we use a finer grid interval of 24 00 (24 arc-seconds, about 0.75 km) to better model nearshore propagation. Since the GEBCO dataset is gridded at 1 0 interval, we resample it at 2 0 interval for the basic grid and 24 00 interval for finer grids around tide gauge stations, respectively. A time step of 2 s is used to satisfy the stability condition for the finitedifference method. [11] As the initial condition, static deformation of the seafloor is calculated for a rectangular fault model [Okada, 1985]. We also consider the effect of coseismic horizontal displacement in region of steep bathymetric slopes [Tanioka and Satake, 1996b]. Tsunami waveforms at tide gauge stations are calculated assuming a constant rise time (or slip duration) of 1 min on each subfault. Because the subfault size is 50 km 50 km, the assumed rise time includes the effect of rupture propagation within each subfault. 4.3. Inversion [12] We used non-negative least square method and delete-half jackknife method to estimate the slip and error, respectively; the details of inversion method are described in Fujii and Satake [2006]. The observed tsunami waveforms at tide gauges were sampled at 1 min interval, hence the synthetic waveforms are also computed at 1 min interval. We used the first cycle of tsunami waveforms, because the poor bathymetry data may prevent accurate modeling of later phases such as reflected waves. The total number of data points used for the inversion is 148. We weight Cocos data twice the other tide gauge data, because its waveform is fairly sensitive to the solution. The waveforms are aligned in 3of5

Table 2. Slip Distributions Estimated by Tsunami Waveform Inversion With Different Rupture Velocities a Slip and Error, m Lat., deg S Lon., deg E Vr = 1 2.0 3.0 km/s 1.5 km/s 1.0 km/s 1 9.515 107.274 0.53 ± 0.51 0.75 ± 0.27 0.38 ± 0.14 0.50 ± 0.17 2 9.096 107.420 0.15 ± 0.31 0.10 ± 0.17 0.42 ± 0.25 0.19 ± 0.32 3 9.661 107.705 0.51 ± 0.13 0.27 ± 0.10 0.57 ± 0.21 0.67 ± 0.26 4 9.242 107.851 0.00 ± 0.08 0.00 ± 0.06 0.27 ± 0.24 0.30 ± 0.37 5 9.807 108.136 1.38 ± 0.58 2.24 ± 0.72 1.05 ± 0.42 1.81 ± 0.70 6 9.388 108.282 0.79 ± 0.23 0.00 ± 0.54 1.52 ± 0.60 0.95 ± 0.68 7 9.953 108.568 1.43 ± 0.64 3.73 ± 1.34 1.01 ± 0.42 2.42 ± 1.11 8 9.534 108.714 0.00 ± 0.40 0.00 ± 0.72 0.98 ± 0.76 2.73 ± 0.97 9 10.100 109.000 2.47 ± 0.97 2.82 ± 1.11 1.97 ± 0.91 0.26 ± 0.71 10 9.681 109.146 2.12 ± 0.77 2.36 ± 1.00 2.61 ± 1.35 3.69 ± 1.67 a Vr is rupture velocity. Locations (latitude (Lat.) and longitude (Lon.)) indicate the right bottom corner of each subfault. such a way that first arrivals of the observed and synthetic waves match. 5. Results and Discussion [13] The inversion results are shown in Table 2 and Figure 4. Differences in synthetic waveforms for different rupture velocities are much smaller than the differences between the observed and synthetic waveforms. The result for an instantaneous rupture (Vr = 1; Figure 4a), which is our preferred model, shows a tsunami source length of about 200 km. Around the epicenter, the estimated slip is up to 0.5 m, but slip on the westernmost subfault is not well resolved as indicated by the large error. The largest slip of 1.4 to 2.5 m is estimated on the eastern part of the source region. Most of the larger slip is located at the shallower region of the fault, which is responsible for generating abnormally large tsunami. The total seismic moment is calculated from this slip distribution as 7.0 10 20 Nm (Mw = 7.8), assuming the rigidity of 3.0 10 10 N/m 2. [14] The synthetic waveforms generally agree with the observed ones at most stations (Figure 2). Although the amplitudes at distant stations are not well reproduced, the phases are well explained. To match the first arrivals of the observed and synthetic waveforms, the observed waveforms are delayed by 3 min at Christmas, and 5 min at Benoa, Broome and Hillarys. These adjustments imply that the longer travel times than the observed ones are needed because of the poor bathymetry data. We redrew the initial wavefronts by using the longer travel times (dotted lines in Figure 3), which would indicate smaller tsunami source similar to the aftershock area. [15] The obtained slip distribution is similar to those of Y. Yagi (http://www.geo.tsukuba.ac.jp/press_hp/yagi/ EQ/20060717Jawa/), Ammon et al. [2006], and C. Ji (http:// neic.usgs.gov/neis/eq_depot/2006/eq_060717_qgaf/ neic_qgaf_ff.html). Their results show an asperity with the largest slip around the epicenter and other asperities with large moment at eastern shallow region. Our eastern major asperity may correspond to their asperities on shallower region. Ammon et al. [2006] found that this event consists of five to six asperities: slip near the epicenter and larger slip to the east. The large slips around the epicenter and our subfaults 5, 7 and 9 may correspond to their asperities. [16] Locations of large slip are stably resolved on the eastern and shallower part of the source area, regardless of the rupture velocity chosen although the slip amounts on each subfault are slightly different (Table 2). Figure 4 shows the slip distribution estimated with different rupture velocities. The rupture velocities from 2.0 to 3.0 km/s give the same results, because the time delays due to the rupture propagation for 2.0 3.0 km/s are the same within 1 min sampling interval of waveforms. Although it is difficult to judge which rupture velocity best explains the observations, the largest slip is always located to the eastern part of the epicenter. We also confirmed that the slip distribution is stable for slight changes (less than 5 min in time) of waveforms used for inversion, such as onset time, time duration, or weight of each waveform. 6. Conclusion [17] We have estimated the source of the 2006 West Java Tsunami by using tsunami waveforms at tide gauge stations around the Indian Ocean. Tsunami travel times poorly constrain the source area, and we needed to adjust the travel times at some stations. Inversion of tsunami waveforms, including these adjustments, indicates that the 2006 Java tsunami source was about 200 km long, extending from the epicenter to east. For instantaneous rupture model, the largest slip is 1.4 to 2.5 m on the eastern part of the fault, up to 0.5 m around the epicenter. The large slip concentrated on shallower region of the fault, as inferred from tsunami waveforms, supports that this earthquake was a tsunami earthquake. [18] Acknowledgments. We thank University of Hawaii Sea Level Center (UHSLC) and Australian Bureau of Meteorology for providing us tide gauge data. We thank T. Lay, B. Atwater, H. Horikawa, T. Hara and H. Kanamori for their valuable comments to improve our manuscript. We also thank for comments by A. Rabinovich and an anonymous reviewer. Most of the figures were generated by using Generic Mapping Tools [Wessel and Smith, 1998]. References Abercrombie, R. E., M. Antolik, K. Felzer, and G. Ekström (2001), The 1994 Java tsunami earthquake: Slip over a subducting seamount, J. Geophys. Res., 106(B4), 6595 6608. Ammon, C. J., H. Kanamori, T. Lay, and A. A. Velasco (2006), The 17 July 2006 Java tsunami earthquake, Geophys. Res. Lett., 33, L24308, doi:10.1029/2006gl028005. British Oceanographic Data Centre (1997), The Centenary Edition of the GEBCO Digital Atlas [CD-ROM], Liverpool, U. K. Fujii, Y., and K. Satake (2006), Tsunami source of the 2004 Sumatra-Andaman earthquake inferred from tide gauge and satellite data, Bull. Seismol. Soc. Am., 97, S192 207. Hara, T. (2006), Determination of earthquake magnitudes using duration of high-frequency energy radiation and maximum displacement amplitudes: Application to the July 17, 2006 Java earthquake and other tsunami earthquakes, Eos Trans. AGU, 87(52), Fall Meet. Suppl., Abstract S21A-0132. 4of5

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