J. SE Asian Appl. Geol., Sep–Dec 2010, Vol. 2(3), pp. 261–266

OBSERVATION AND SIMULATION OFTSUNAMIS INDUCED BY THE 2003TOKACHI-OKI EARTHQUAKE (M 8.0)

Tatsuo Ohmachi∗1, Shusaku Inoue1, and Tetsuji Imai2

1Department of Built Environment, Tokyo Institute of Technology, Japan2Civil Engineer, Tokyo Gas Company, Ltd., Japan

Abstract

The 2003 Tokachi-oki earthquake (MJ 8.0) occurredoff the southeastern coast of Tokachi, Japan, and gen-erated a large tsunami which arrived at Tokachi Har-bor at 04:56 with a wave height of 4.3 m. Japan Ma-rine Science and Technology Center (JAMSTEC) re-covered records of water pressure and sea-bed accel-eration at the bottom of the tsunami source region.These records are first introduced with some find-ings from Fourier analysis and band-pass filter anal-ysis. Water pressure disturbance lasted for over 30minutes and the duration was longer than those ofaccelerations. Predominant periods of the pressurelooked like those excited by Rayleigh waves. Next,numerical simulation was conducted using the dy-namic tsunami simulation technique able to repre-sent generation and propagation of Rayleigh waveand tsunami, with a satisfactory result showing va-lidity and usefulness of this technique.Keywords: Earthquake, Rayleigh wave. tsunami,near-field.

1 Introduction

About a decade ago, the dynamic tsunami sim-ulation technique was developed by Ohmachiet al. (2001a) to simulate the generation oftsunamis followed by their propagation. Ittakes into acount not only the dynamic dis-placement of the seabed induced by seismic

∗Corresponding author: T. OHMACHI, Department ofBuilt Environment, Tokyo Institute of Technology, Japan.E-mail: ohmachi@enveng.titech.ac.jp

faulting but also the acoustic effects of the sea-water. As the dynamic technique assumes weakcoupling between the seabed and seawater, thesimulation consists of a two-step analysis. Thefirst is on the dynamic seabed displacement re-sulting from seismic faulting, and the second ison the seawater disturbance induced by the dy-namic seabed displacement. In the first step, theboundary element method (BEM) is used. Inthe second, the finite difference method (FDM)is used to solve the Navier-Stokes equation.A series of dynamic tsunami simulations havedemonstrated that in the near-field of tsunamisources there are some significant differencesfrom those resulting from conventional tech-nique because water waves associated with wa-ter compressibility and dynamic sea surfacedisturbance are not accounted in the latter tech-nique.

Although the features from the dynamicanalysis seems quite reasonable, they have tobe verified by observed data (Ohmachi et al.2001b). For this reason, the data obtained byJAMSTEC (Japan Marine Science and Technol-ogy Center) during the 2003 Tokachi-oki earth-quake and the dynamic tsunami simulation areused in the present study.

2 The 2003 Tokachi-oki earthquake andoffshore monitoring system

The 2003 Tokachi-oki earthquake of magni-tude 8.0 occurred off the southeastern coast ofTokachi at 04:50 on September 26, 2003. Report-

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edly, the tsunami arrived at Tokachi Harbor at04:56 with a wave height of 4.3 m, while theJapanese Nationwide Coastal Wave Observa-tion Network (NOWPHAS) located on the 23m-deep seabed off Ohtsu Harbor recorded thefirst tsunami arrival at 04:51 (Nagai and Ogawa2004). The locations of these harbors and epi-center of the earthquake are shown in Figure1, where the location of the offshore monitor-ing system deployed by JAMSTEC and a faultmodel dipping northwest (Koketsu et al. 2004)are also shown. The system is equipped with3 sets of 3-component broadband seismometers(OBS1-3) and 2 high-precision pressure gauges(PG1-2). The distances between OBS1 and PG1and between OBS3 and PG2 are 4.0 km and 3.4km, respectively. As these two pairs are nearlylocated, the locations of these pairs are referredto as St. A and St. B in Figure 1, and the distancebetween these two stations is about 72 km. Ta-ble 1 shows elements of the offshore monitor-ing system and seawater depth at each sensor(JAMSTEC 2009). Sampling rates of the pres-sure gauges and seismometers are 1 Hz and 100Hz, respectively.

Figure 1: Locations of the JAMSTEC monitor-ing system equipment and epicenter of the 2003Tokachi-oki earthquake.

Table 1: The JAMSTEC offshore monitoring sys-tem and water depth of equipment.

3 Tsunami generation process inferredfrom observed data

Time histories of water pressure and groundmotion acceleration (vertical component) at St.A and St. B are shown in Figure 2. Appar-ently, the water pressure change lasted longerthan the ground motion acceleration. Fourierspectra of the time histories are shown in Fig-ure 3. Peak periods of 7.0 s at St. A and 6.5 s atSt. B are observed in both water pressure andground motion acceleration, and each period Tcan be approximated by T=4H/c, where H(=2.2- 2.3 km) is the seawater depth and c (=1.5 km/s) is an acoustic wave velocity of water (Nosov etal.2005).

Figure 2: Time histories of water pressure andvertical ground motion acceleration.

Time histories of the water pressure changein Figure 2 are full of short period componentswith amplitudes several times larger than thetsunamiinduced water pressure. When the pe-riod components shorter than 50s are filteredout, the water pressure time histories result inthose indicated by the gray curves in Figure 4.It is interesting to note that after the main shock,the baseline of the water pressure was shiftedby about 40 cm at PG1 and about 10 cm at PG2in terms of the water depth. These baselineshifts were supposedly caused by the uplift of

262 c© 2010 Department of Geological Engineering, Gadjah Mada University

OBSERVATION AND SIMULATION OF TSUNAMIS INDUCED BY THE 2003 TOKACHI-OKIEARTHQUAKE

Figure 3: Fourier spectra of time historiesshown in Figure 2.

the seabed due to the seismic faulting (Watan-abe et al. 2004).

The gray curves in Figure 4 indicate the shapeof the recently-generated tsunami. At PG2, thetsunami looks like a solitary wave with a heightof about 20 cm and a period of 15 min. At PG1,the wave height and the period are not so clearas those at PG2, and a gentle slope only can beobserved. The difference in the shape of thetsunami is due to the difference in the relativelocation between the observation stations andthe tsunami source.

In addition, the gray curves indicate anothergroup of water waves preceding the tsunami atthe beginning of the short-period water pres-sure change which are referred to be precedingwaves.

4 Water waves induced by Rayleighwaves

Band-pass filtering between 3s and 15s was ap-plied to the original time histories of water pres-sure and vertical ground motion displacementat St. A and St. B, with the result shown inFigure 5. It can be seen that the water pressurechange in this period range had the largest am-plitude at the beginning of the earthquake mo-tion as the seabed displacement occured. Fig-ure 6 shows the particle orbit of the ground mo-tion displacement plotted on the vertical NS-UD plane during 4:50:20 and 4:50:40. During

Figure 4: Time histories of water pressure afterlong-pass filtering of 50 s.

the 20s, the particle orbit showed an elliptic ret-rograde motion but not so clear on the EW-UDplane, which suggests the directionality of thewave motion. From the particle motion andthe directionality, the motion is considered to bemainly due to Rayleigh waves.

Propagation of vertical ground displace-ments associated with the 2003 earthquake isillustrated in Figure 7, which was obtainedfrom double integration of acceleration timehistories observed at K-net stations in andaround Hokkaido. Large amplitude arrived atthe southeast coast of Hokkaido at about 20sand propagated in the northwestern direction.The waves are probably due to Rayleigh waveswith the wave length of about 100 km and wavevelocity of about 3.5 km/s.

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Figure 7: Observed ground displacement of UD-component at K-net

Figure 5: Time histories of water pressureand vertical ground motion displacement afterband-pass filtering of 3-15 s

5 Tsunami simulation

A fault model proposed by Koketsu et el. (2004)is used in this simulation (see Figure 1). Thearea of the simulation is 250 km long in theNS direction and 350 km wide in the EW di-rection. The grid size for the seabed and water

Figure 6: Particle orbit of ground motiondisplacement at OBS1 during 04:50:20 and04:50:40.

layer is 5km and 1km, respectively. Fault rup-ture starts at the southeastern edge and extendsto the northwestern direction with a velocity of3km/s. The total duration of the fault rupturingis about 30s.

Snapshots in Figures 8 and 9 show the simu-lation results. At 20s the faulting progresses upto the middle of the fault plane. From 60s to 80s,

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the water wave induced by the Rayleigh waveis propagated to the northwestern coast. At 80swhen the fault rupture finishes, static uplift andsubsidence remain in the near-fault area and thetsunami profile is almost the same as that of thestatic displacement of the seabed.

The dynamic simulation technique can dealwith not only the tsunami generation butalso the water wave induced by Rayleighwaves which was actually observed beforethe tsunami arrival.

6 Conclusions

Analysis of the JAMSTEC monitoring data haveshown that three kinds of water waves were in-volved in the dynamic tsunami generation pro-cess following the 2003 Tokachi-oki earthquake;that is, water pressure waves of short period, atsunami of long period, and preceding waves ofintermediate period.

As for the tsunami of long period, two typesof recently generated tsunamis were detectedby the water pressure gauges. One was a soli-tary wave with a height of about 20 cm anda period of 15 min, and the other had a gen-tle slope and its wave height and period werenot so clear as the first type. The first typewas detected at a station located a little distantfrom the seismic fault, while the second was de-tected at a station just above the fault plane. Thewater waves preceding the tsunami were de-tected in an early stage of the water pressurechange and they were seemingly induced by alarge seabed displacement associated with theRayleigh waves because of the directivity, an el-liptical particle orbit, and a predominant periodranging between 3 s and 15 s.

These three kinds of water waves can be an-alyzed by the dynamic simulation technique.

Further studies are needed for early detectionand warning of near-field tsunamis, especiallyon the dynamic process of the tsunami genera-tion, using both observation and simulation asshown in the present paper.Acknowledgements

The authors are grateful to JAMSTEC for thedata from the deep-sea monitoring used in thisstudy.

References

JAMSTEC (2009), ”Submarine Cable Data Center,”http://www. jamstec.go.jp/scdc/ K. Koketsu, K.Hikima, S. Miyazaki and S. Ide (2004), ”Joint in-version of strong motion and geodetic data for thesource process of the 2003 Tokachi-oki, Hokkaido,earthquake,” Earth Planets Space, 56(3), 329-334.

T. Nagai and H. Ogawa (2004), ”Characteristic ofthe 2003 Tokachi-off Earthquake Tsunami Pro-file,” Technical Note on the Port and Airport Re-search Institute, No. 1070.

M. Nosov, S. V. Kolesov, A. V. Ostroukhova, A. B.Alekseev and B. M. Levin (2005), ”Elastic oscil-lations of the water layer in a tsunami source,”Doklady Earth Sciences, 404(7), 1097-1100.

T. Ohmachi, H. Tsukiyama and H. Matsumoto(2001a), ” Simulation of Tsunami Induced by Dy-namic Displacement of Seabed due to SeismicFaulting,” Bull. Seis. Soc. Am., 91(6), 1898-1909.

T. Ohmachi, H. Tsukiyama and H. Matsumoto(2001b), ” Sea Water Pressure Induced by Seis-mic Ground Motions and Tsunamis,” ITS2001,595-609 (2001b).

T. Watanabe, H. Matsumoto, H. Sugioka, H. Mikadaand K. Suyehiro (2004), ” Offshore MonitoringSystem Records Recent Earthquake Off Japan’sNorthernmost Island,” Eos Trans. AGU, 85(2),doi:10.1029/2004EO020003.

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Figure 8: Contour map of simulated vertical displacement of the ground and sea-bed

Figure 9: Snapshots of simulated water level

266 c© 2010 Department of Geological Engineering, Gadjah Mada University