Fault geometry of the 2004 off the Kii peninsula earthquake inferred from offshore pressure waveforms (original) (raw)
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Earth, Planets and Space, 2011
The 2011 Tohoku Earthquake caused a devastating tsunami along the shoreline from the Tohoku to Kanto districts. Although many of the tide gauge stations along the Tohoku coast were saturated or damaged due to the tsunami, two cabled ocean-bottom tsunami sensors installed off Kamaishi successfully recorded the tsunami waveform just above the source rupture area. The records indicated a characteristic two-stage tsunami development sequence: a smoothly increasing tsunami amplitude from 0 to 2 m during the first 800 s from the earthquake origin time, and a short-period impulsive tsunami with a peak of more than 5 m in the following 200 s. Such observations strongly suggest the lack of any sea floor upheaval at the stations during the earthquake, and the occurrence of an extremely large slip in the shallow portion of the subducting Pacific Plate near the trench axis. The source model derived from the offshore tsunami records indicates that a very large slip of 57 m occurred off Miyagi near the trench axis, south of the rupture area of the 1896 Meiji Sanriku tsunami earthquake, and was the major source of the highly destructive tsunami that subsequently developed.
Physics of the Earth and Planetary Interiors, 1986
The fault model of the 1940 Shakotan-oki, Japan, earthquake of Aug. 1, 1940 is reexamined on the basis of magnitude, aftershock distribution and tsunami data. From a careful examination of the S-P time distribution of aftershocks and comparison of the tide gauge records with numerical simulation of tsunami, the fault area is estimated to be 100 km x 35 km, the slip 1.5 m and the seismic moment 2.4 x 1027 dyn cm. The fault parameters estimated in this study are significantly different from those of Fukao and Furumoto and do not support their conclusion that this event has a very long duration and is a tsunami earthquake. The source process time is estimated to be 30 s, which is normal for an earthquake of this size. Fault parameters of six earthquakes along the eastern margin of the Japan Sea including the Shakotan-oki event are compiled and compared with those of inter-plate earthquakes in the Pacific Ocean. Different relations from well-known scaling laws of the inter-plate shocks are found for the earthquakes in the Japan Sea. Dip angles, aspect ratios of the fault, and the average stress drop are larger in the Japan Sea than those of the Pacific events, although the seismic moment release per unit fault length is the same. The differences can be interpreted in terms of a recent plate boundary model in which the young, immature boundary between the North American and the Eurasian plates lies at the eastern margin of the Japan Sea. They also partially reflect the different excitation of the tsunami in the Japan Sea and the Pacific Ocean.
Physics of the Earth and Planetary Interiors, 1985
The source process of the Japan Sea earthquake of May 26, 1983 is studied by using the long-period surface waves and tsunamis. The moment tensor inversion of Rayleigh waves which consider the lateral heterogeneity of the Earth is developed and applied to the IDA records of this event. The moment tensor solution and first-motion data indicate that the mechanism is dip-slip and the seismic moment is 7.6 x 1027 dyne-cm. The fault is estimated to be 120 km in length. 40 km in width, and dips eastward with a dip angle of 30°from the aftershock distribution. Tsunami simulation for an actual topography is made to restrain the slip on the fault. The aftershock area and the tsunami records suggest that the fault is divided into two segments. The northern part strikes NNW and slips 4 m, while the southern part strikes NNE and slips 5 m. The fault geometry, the aspect ratio, and the stress drop of this event are similar to those of the 1964 Niigata earthquake, but differ from those of the earthquakes in the Pacific coast of Japan.
Earth, Planets and Space, 2011
Finite-source rupture models for the great 11 March 2011 off the Pacific coast of Tohoku (M w 9.0) Earthquake obtained by inversions of seismic waves and geodetic observations are used to reconstruct deep-water tsunami recordings from DART buoys near Japan. One model is from least-squares inversion of teleseismic P waves, and another from iterative least-squares search-based joint inversion of teleseismic P waves, short-arc Rayleigh wave relative source time functions, and high-rate GPS observations from northern Honshu. These rupture model inversions impose similar kinematic constraints on the rupture growth, and both have concentrations of slip of up to 42 m up-dip from the hypocenter, with substantial slip extending to the trench. Tsunami surface elevations were computed using the model NEOWAVE, which includes a vertical momentum equation and a non-hydrostatic pressure term in the nonlinear shallow-water equations to account for the time-history of seafloor deformation and propagation of weakly dispersive tsunami waves. Kinematic seafloor deformations were computed using the Okada solutions for the rupture models. Good matches to the tsunami arrival times and waveforms are achieved for the DART recordings for models with slip extending all the way to the trench, whereas shifting fault slip toward the coast degrades the predictions.
Earth, Planets and Space, 2005
Tsunamis from the 2004 off the Kii Peninsula earthquakes (M 7.1 and 7.4) were recorded on offshore tsunami gauges, a GPS tsunami gauge and eight bottom-pressure gauges, as well as coastal tide gauges located south of Honshu and Shikoku. The maximum amplitudes on the GPS and bottom-pressure gauges were several to ten cm, while those on tide gauges were up to 0.9 m. We first computed tsunami waveforms from the earthquake source models proposed Yamanaka (2004) and Yagi (2004) from seismic waveform analysis, and compared them with the observed waveforms. For the first event (foreshock), both models produce similar waveforms with the observations. For the second event (mainshock), the waveforms computed from the Yamanaka model is closer to the observed waveforms, but there are still discrepancies between the observed and computed waveforms. We then performed tsunami waveform inversions to estimate the water height distributions in the source area. The foreshock source is ≈1600 km2 with t...
Journal of Physics of the Earth, 1987
A method for estimating fault heterogeneity by an inversion of tsunami waveforms is presented. The ocean bottom bathymetry, by which the velocity of tsunami wave is determined, is more accurately known than the seismic velocity structure, so that the effect on the propagation path can be precisely evaluated by means of numerical computation. Since the propagation velocity of tsunami is much smaller than any kind of seismic waves or rupture velocity, only a final slip distribution on a fault can be estimated. The amount of slip on each segment of the fault is obtained by an inversion of the observed tsunami waveforms by using the numerically computed waveforms from each segment as the Green's function. Several numerical experiments are carried out to examine the spatial resolution of the fault heterogeneity by the present method. It is clarified that the size of the segment needs to be more than eight times the grid size used for the computation of the Green's function, indicating that accurate bathymetric data and large computation are required to get a fine picture of the heterogeneous fault motion. Simulation of the inversion shows that the slip distribution on the fault can be estimated stably. The effect of tide gauge response is the only unknown parameter contained in tsunami records besides the source information, so it is necessary to investigate it before the present method is applied to actual tsunami records.
2004
Tsunamis from the 2004 off the Kii Peninsula earthquakes (M 7.1 and 7.4) were recorded on offshore tsunami gauges, a GPS tsunami gauge and eight bottom-pressure gauges, as well as coastal tide gauges located south of Honshu and Shikoku. The maximum amplitudes on the GPS and bottom-pressure gauges were several to ten cm, while those on tide gauges were up to 0.9 m. We first computed tsunami waveforms from the earthquake source models proposed Yamanaka (2004) and Yagi (2004) from seismic waveform analysis, and compared them with the observed waveforms. For the first event (foreshock), both models produce similar waveforms with the observations. For the second event (mainshock), the waveforms computed from the Yamanaka model is closer to the observed waveforms, but there are still discrepancies between the observed and computed waveforms. We then performed tsunami waveform inversions to estimate the water height distributions in the source area. The foreshock source is ∼1600 km2 with t...
Potential tsunamigenic faults of the 2011 off the Pacific coast of Tohoku Earthquake
Earth, Planets and Space, 2011
Faults related to the tsunamigenic 2011 Tohoku-Oki Earthquake (M w 9.0) were investigated by using multichannel seismic reflection data acquired in 1999 and submersible seafloor observations from 2008. The location of the fault system interpreted in the seismic reflection profile is distributed around the area with largest slip and tsunami induction of the 2011 event. Cold-seep communities along the trace of the branch reverse fault and a high scarp associated with the trace of a normal fault suggest current activity on these faults. We interpret the fault system in the seismic profile as a shallow extension of the seismogenic fault that may have contributed to the resulting huge tsunami.
Tsunami generation of the 1993 Hokkaido Nansei-Oki earthquake
Pure and Applied Geophysics PAGEOPH, 1995
Heterogeneous fault motion of the 1993 Hokkaido Nansei-Oki earthquake is studied by using seismic, geodetic and tsunami data, and the tsunami generation from the fault model is examined. Seismological analyses indicate that the focal mechanism of the first 10 s, when about a third of the total moment was released, is different from the overall focal mechanism. A joint inversion of geodetic data on Okushiri Island and the tide gauge records in Japan and Korea indicates that the largest slip, about 6 m, occurred in 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 subfault, which is dipping to the east, is about 2 m, while the slips on the southern subfaults, which are steeply dipping to the west, are more than 3 m. Tsunami heights around Okushiri Island are calculated from the heterogeneous fault model using different grid sizes. Computation on the smaller grids produces larger tsunami heights that are closer to the observed tsunami runup heights. Tsunami propagation in the nearly closed Japan Sea is examined as the free oscillation of the Japan Sea. The excitation of the free oscillation by this earthquake is smaller than that by the 1964 Niigata or 1983 Japan Sea earthquake. , show that the maximum tsunami runup height was 32 m on Okushiri Island and that the entire island subsided by 5-80 cm.
2016
obtained by inversions of seismic waves and geodetic observations are used to reconstruct deep-water tsunami recordings from DART buoys near Japan. One model is from least-squares inversion of teleseismic P waves, and another from iterative least-squares search-based joint inversion of teleseismic P waves, short-arc Rayleigh wave relative source time functions, and high-rate GPS observations from northern Honshu. These rupture model inversions impose similar kinematic constraints on the rupture growth, and both have concentrations of slip of up to 42 m up-dip from the hypocenter, with substantial slip extending to the trench. Tsunami surface elevations were computed using the model NEOWAVE, which includes a vertical momentum equation and a non-hydrostatic pressure term in the nonlinear shallow-water equations to account for the time-history of seafloor deformation and propagation of weakly dispersive tsunami waves. Kinematic seafloor deformations were computed using the Okada soluti...