Yuichiro Tanioka

Tsunami generation by horizontal displacement of ocean bottom

Tsunami generation by an earthquake is generally modeled by water surface displacement identical to the vertical deformation of ocean bottom due to faulting. The effect of horizontal deformation is usually neglected. However, when the tsunami source is on a steep slope and the horizontal displacement is large relative to the vertical displacement, the effect becomes significant. We show this for two recent earthquakes which generated much larger tsunamis than expected from seismic waves. In the case of the 1994 June 2 Java, Indonesia, earthquake, the focal mechanism was a very shallow dipping thrust and the source was near a very steep trench slope. In the case of the 1994 Nov. 14 Mindoro, Philippines, earthquake, strike-slip faulting extended from ocean to land perpendicular to the coast line. In both cases, we found that the horizontal motion of slope had an important contribution to the tsunami generation.

Fault parameters of the 1896 Sanriku Tsunami Earthquake estimated from Tsunami Numerical Modeling

The June 15, 1896 Sanriku earthquake generated devastating tsunamis with the maximum run-up of 25 m and caused the worst tsunami disaster in the history of Japan, despite its moderate surface wave magnitude (Ms=7.2) and weak seismic intensity. This is a typical tsunami earthquake, which generates anomalously larger tsunamis than expected from its seismic waves. Previously proposed mechanisms of tsunami earthquakes include submarine slumping and slow rupture in the accretionary wedge or in the subducted sediments. In this paper, we estimate the fault parameters of the 1896 tsunami earthquake by numerically computing the tsunami and comparing the waveforms with those recorded at three tide gauge stations in Japan. The result indicates that the tsunami source is very close to the Japan trench and the fault strike is parallel to the trench axis. The fault width is about 50 km, suggesting that the tsunami earthquake is a slow rupture in the subducted sediments beneath the accretionary wedge.

Detailed coseismic slip distribution of the 1944 Tonankai earthquake estimated from tsunami waveforms

Coseismic slip distribution on the fault plane of the 1944 Tonankai earthquake is estimated from inversion of tsunami waveforms. Three improvements from a previous study [Satake, 1993] are made. These are: (1) smaller subfaults are used to resolve detailed slip distribution; (2) the sub faults fit better to the plate interface geometry; and (3) finer and more accurate bathymetry data is used. The inversion result shows that a maximum slip of about 3 m occurred on the plate interface off Shima peninsula. The total seismic moment is estimated to be 2.0 × 1021 Nm (Mw 8.2). The result confirms that the 1944 Tonankai earthquake did not rupture the plate interface beneath the Tokai region and supports the existence of the seismic gap in the Tokai region suggested by Ishibashi [1981]. The slip of about 1.5 m on the plate interface beneath Atsumi peninsula, northeast of the large slip area, is necessary to explain the observed tsunami waveforms, although no seismic moment release was estimated from strong motion data by Kikuchi et al. [1999]. This may suggest that the rupture beneath Atsumi peninsula was slow.

Sediment effect on tsunami generation of the 1896 Sanriku Tsunami Earthquake

The 1896 Sanriku earthquake was one of the most devastating tsunami earthquakes, which generated an anomalously larger tsunami than expected from its seismic waves. Previous studies indicate that the earthquake occurred beneath the accretionary wedge near the trench axis. It was pointed out recently that sediments near a toe of an inner trench slope with a large horizontal movement due to the earthquake might have caused an additional uplift. In this paper, the effect of the additional uplift to tsunami generation of the 1896 Sanriku tsunami earthquake is quantified. We estimate the slip of the earthquake by numerically computing tsunamis and comparing their waveforms with those recorded at three tide gauges. The estimated slip for the model without the additional uplift is 10.4 m, and those with the additional uplift are 5.9-6.7 m. This indicates that the additional uplift of the sediments near the trench has a large effect on the tsunami generation.

Near-field tsunami forecasting from cabled ocean bottom pressure data

[1] We propose a method for near-field tsunami forecasting from data acquired by cabled offshore ocean bottom tsunami meters (OBTMs) in real time. We first invert tsunami waveforms recorded at OBTMs to estimate the spatial distribution of initial sea-surface displacements in the tsunami source region without making any assumptions about fault geometry and earthquake magnitude. Then, we synthesize the coastal tsunami waveforms from the estimated sea-surface displacement distribution. To improve the reliability of the tsunami forecasting, we use updated OBTM data to repeat the forecast calculation at 1-min intervals. We tested our method by simulating the 1896 Sanriku tsunami earthquake, which caused a devastating tsunami with maximum runup height of 38 m along the Pacific coast of northeastern Japan. Instead of real OBTM records, proxies were used. The simulation demonstrated that our method provided accurate estimations of coastal arrival times and amplitudes of the first peak of the tsunami more than 20 min before the maximum amplitude wave reached the coastal site nearest to the source. We also applied the method to real data of a small tsunami that was caused by a local earthquake and successfully forecasted the tsunami at coastal tide stations. We found that accuracy of our estimated coastal tsunami amplitudes can be affected by the spatial relationship between the tsunami source and the offshore observation stations. Our numerical simulation showed that even more accurate tsunami amplitude forecasts would be achieved by deployment of additional offshore stations separated by a distance comparable to the trench-parallel length of the tsunami source.

Coseismic slip distribution of the 1946 Nankai earthquake and aseismic slips caused by the earthquake

Coseismic slip distribution on the fault plane of the 1946 Nankai earthquake (Mw 8.3) was estimated from inversion of tsunami waveforms. The following three improvements from the previous study (Satake, 1993) were made. (1) Larger number of smaller subfaults is used; (2) the subfaults fit better to the slab geometry; and (3) more detailed bathymetry data are used. The inversion result shows that the agreement between observed and synthetic waveforms is greatly improved from the previous study. In the western half of the source region off Shikoku, a large slip of about 6 m occurred near the down-dip end of the locked zone. The slip on the up-dip or shallow part was very small, indicating a weak seismic coupling in that region. In the eastern half of the source region off Kii peninsula, a large slip of about 3 m extended over the entire locked zone. Large slips on the splay faults in the upper plate estimated from geodetic data (Sagiya and Thatcher, 1999) were not required to explain the tsunami waveforms, suggesting that the large slips were aseismic. Two slip distributions on the down-dip end of the plate interface, one from geodetic data and the other from tsunami waveforms, agree well except for slip beneath Cape Muroto in Shikoku. This suggests that aseismic slip also occurred on the plate interface beneath Cape Muroto.

Rupture process of the 2004 great Sumatra-Andaman earthquake estimated from tsunami waveforms

Rupture process of the 2004 Sumatra-Andaman earthquake is estimated using tsunami waveforms observed at tide gauges and the coseismic vertical deformation observed along the coast. The average rupture speed of the 2004 Sumatra-Andaman earthquake is estimated to be 1.7 km/s from tsunami waveform analysis. The rupture extends about 1200 km toward north-northwest along the Andaman trough. The largest slip of 23 m is estimated on the plate interface off the northwest coast in the Aceh province in Sumatra. Another large slip of 21 m is also estimated on the plate interface beneath the north of Simeulue Island in Indonesia. The other large slip of 10–15 m is estimated on the plate interface near Little Andaman and Car Nicobar Inlands. The total seismic moment is calculated to be 7.2 × 1022 Nm (Mw 9.2) which is similar to the other studies using seismic waves (Park et al., 2005; Ammon et al., 2005).

The 2004 Indian Ocean tsunami: Tsunami source model from satellite altimetry

Satellite altimetry measurements of sea surface heights for the first-time captured the Indian Ocean tsunami generated from the December 2004 great Sumatra earthquake. Analysis of the sea surface height profile suggests that the tsunami source, or the seafloor deformation, of the great earthquake propagated to the north at an extremely slow speed of less than 1 km/sec on average for the entire 1300-km-long segment along the northern Sumatra-Nicobar-Andaman Trench. The extremely slow propagation speed produces a very long duration of tens minutes, longer than earthquake source duration estimated (480–500 sec) from short-period P-wave radiation. The satellite altimetry data requires a total seismic moment of 9.86 × 1022 Nm (Mw=9.3). This estimate is approximately 2.5 times larger than the value from long-period surface wave analysis but nearly the same as that from the ultra-long-period normal mode study. The maximum amount of slip (∼30 m) is identified in an offshore region closest to the northern most part of Sumatra where the largest tsunami run-up heights were observed.

Slip distribution of the 2003 Tokachi-oki earthquake estimated from tsunami waveform inversion

The slip distribution of the 2003 Tokachi-oki earthquake is estimated from the 11 tsunami waveforms recorded at 9 tide gauges in the southern Hokkaido and eastern Tohoku coasts and two ocean bottom tsunami-meters (pressure gauges) off Kamaishi, Tohoku. The largest slip of 4.3 m is estimated on the subfault located off Hiroo. A large slip of 2.1 m is also estimated on the subfault located near Kushiro. The total seismic moment of the 2003 Tokachi-oki earthquake is 1.0 × 1021 Nm. The slip distribution estimated from the tsunami waveform inversion is similar to the slip distribution deduced by Yamanaka and Kikuchi (2003) from the inversion of the teleseismic body waves. The rupture area of the 2003 Tokachi-oki earthquake is similar to the western part of the rupture area of the 1952 Tokachi-oki earthquake estimated by Hirata et al. (2003).

Total analysis of the 1993 Hokkaido Nansei-oki earthquake using seismic wave, tsunami, and geodetic data

The fault geometry and slip distribution of the Hokkaido Nansei-oki, Japan, earthquake of July 12, 1993 are estimated using seismic wave, tsunami, and geodetic data. The Moment Tensor Rate Function inversion from P waves shows one nodal plane shallowly dipping to the west and the other nodal plane steeply dipping to the east. The best depth is estimated as 10–15 km. The source time history consists of an initial pulse with a duration of 10 s and moment release of 2 × 1020 Nm, followed by a complex rupture for at least 40 s. The Centroid Moment Tensor (CMT) solution shows one nodal plane shallowly dipping to the east and the other steeply dipping to the west. The overall seismic moment is estimated as 5.5 × 1020 Nm (Mw 7.8). The joint inversion of geodetic data on Okushiri Island and tsunami waveforms in Japan and Korea shows that the largest slip, about 6 m, occurred at a small area just south of the epicenter. This corresponds to the initial rupture on a fault plane dipping shallowly to the west. The slip on the northernmost fault, dipping to the east, is about 2 m. The slips on the southern faults, dipping steeply to the west, are more than 3 m. Total seismic moment of 4.9 × 1020 Nm, estimated from the slip distribution, is similar to the estimate from CMT inversion.