[1] The subducting Pacific plate acts an efficient waveguide for high-frequency signals and often produces anomalously large intensity on the eastern seaboard of northern Japan during deep earthquakes. The waveform records in the region of high intensity show a low-frequency (f < 0.25 Hz) onset for both P and S waves, followed by large, highfrequency (f > 2 Hz) later arrivals with a long coda. This behavior is not explained by a simple subduction zone model comprising a high-velocity plate with low attenuation. From the analysis of observed broadband waveforms and numerical simulation of seismic wave propagation in the Pacific subduction zone we demonstrate that the high-frequency guided waves traveling in the subducting plate arise from the scattering of seismic waves by heterogeneity in plate structure. Our preferred model of the heterogeneity has elongated scatterers parallel to the plate margin described by a von Karmann function with a downdip correlation length of about 10 km and much shorter correlation length of about 0.5 km in thickness. The standard deviation of wave speed fluctuations from the averaged background model is about 2%. This new heterogeneous plate model generates significant scattering of seismic waves with wavelengths shorter than correlation distance in thickness, but low-frequency waves, with long wavelengths, can easy tunnel through such lamina structure. The result is frequency-selective propagation characteristics with a faster low-frequency phase followed by large and high-frequency signals with very long coda. A low-wave speed channel effect from the former oceanic crust at the top of the subducting slab is not necessary to explain the observed dispersed signals and the very long high-frequency coda. Three-dimensional simulations, using the Earth simulator supercomputer for modeling of high-frequency seismic wave propagation in the Pacific subduction zone including plate heterogeneity, clearly demonstrate the scattering waveguide effects for high-frequency seismic waves traveling in the plate. The region of large intensity for the heterogeneous model migrates away from the hypocenter into northern Japan with an elongated zone along the Pacific coast, almost comparable to the observations from deep events in the Pacific plate.Citation: Furumura, T., and B. L. N. Kennett (2005), Subduction zone guided waves and the heterogeneity structure of the subducted plate: Intensity anomalies in northern Japan,

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.

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[1] Long waves are often assumed to model tsunamis, but the wavelength of the initial water height distribution produced by a large submarine earthquake, particularly in the direction perpendicular to the fault strike, is sometimes not much greater than the water depth. The resulting tsunami may have a dispersive character that cannot be simulated based on a conventional long-wave approximation. The 2004 earthquake off Kii Peninsula (M 7.4) on the southern coast of Japan indeed produced a dispersive tsunami that was recorded at two stations located off Shikoku. For the foreshock (M 7.1), on the contrary, a dominant dispersive tsunami was not recognized at these stations. Because dispersive waves show strong directional dependence with respect to the fault strike, the above difference indicates that the strikes of the main shock and the foreshock were different. We conducted a tsunami waveform inversion analysis based on the dispersive tsunami equations to estimate the initial water height distribution of the main shock. The estimated initial water height distribution overlapped with the aftershock region, suggesting that the fault strike was perpendicular to the trough axis, and the total displaced water volume was 1.7-2.0 × 10 9 m 3 . When we used the conventional long-wave approximation, the estimated initial water height distribution extended considerably from the aftershock area, because artificial sources were needed outside the aftershock area to reproduce the observed dispersive waves.

[1] Based on many recent findings such as those for geodetic data from Japan's GEONET nationwide GPS network and geological investigations of a tsunami-inundated Ryujin Lake in Kyushu, we present a revised source rupture model for the great 1707 Hoei earthquake that occurred in the Nankai Trough off southwestern Japan. The source rupture area of the new Hoei earthquake source model extends further, to the Hyuga-nada, more than 70 km beyond the currently accepted location at the westernmost end of Shikoku. Numerical simulation of the tsunami using a new source rupture model for the Hoei earthquake explains the distribution of the very high tsunami observed along the Pacific coast from western Shikoku to Kyushu more consistently. A simulation of the tsunami runup into Ryujin Lake using the onshore tsunami estimated by the new model demonstrates a tsunami inundation process; inflow and outflow speeds affect transport and deposition of sand in the lake and around the channel connecting it to the sea. Tsunamis from the 684 Tenmu, 1361 Shokei, and 1707 Hoei earthquakes deposited sand in Ryujin Lake and around the channel connecting it to the sea, but lesser tsunamis from other earthquakes were unable to reach Ryujin Lake. This irregular behavior suggests that in addition to the regular Nankai Trough earthquake cycle of 100-150 years, there is a hyperearthquake cycle of 300-500 years. These greater earthquakes produce the largest tsunamis from western Shikoku to Kyushu.Citation: Furumura, T., K. Imai, and T. Maeda (2011), A revised tsunami source model for the 1707 Hoei earthquake and simulation of tsunami inundation of Ryujin Lake, Kyushu, Japan,

S U M M A R YThe distortion properties of the apparent S-wave radiation pattern in the high-frequency seismic wavefield of over f > 2 Hz is investigated using a large number of waveform records of the main shock and 29 aftershocks of the Tottori-Ken Seibu, Japan, magnitude (M w ) 6.6 earthquake in 2000. The dense seismic records from the KiK-net strong motion network show a clear four-lobe pattern in the apparent S-wave radiation pattern in the low-frequency wavefield of f < 2 Hz, and shows an almost isotropic distribution in all directions as the frequency increases above 5 Hz. The distortion of the apparent S-wave radiation pattern in the high-frequency wavefield increases as travel distance increases. Therefore, the path effect caused by the scattering of seismic waves due to small-scale heterogeneities in the crust is a major cause of distortion of the radiation pattern. This hypothesis is examined by a 2-D finite-difference method simulation of seismic waves in heterogeneous structure models. The results of simulations clearly demonstrate the collapse of the S-wave front due to seismic wave scattering in the heterogeneous structure. By comparing the observed wavefield and the results of simulations using different sorts of stochastic heterogeneous models, the most preferable model that can explain the observation is described by a von Karman autocorrelation function with correlation distance of a = 3-5 km, order of κ = 0.5 and rms value of ε = 0.07. However, our simple stochastic random heterogeneity model proposed, herein, somewhat overestimates the scattering of low-frequency signals below 2 Hz.

We have developed an open-source software package, Open-source Seismic Wave Propagation Code (OpenSWPC), for parallel numerical simulations of seismic wave propagation in 3D and 2D (P-SV and SH) viscoelastic media based on the finite difference method in local-to-regional scales. This code is equipped with a frequency-independent attenuation model based on the generalized Zener body and an efficient perfectly matched layer for absorbing boundary condition. A hybrid-style programming using OpenMP and the Message Passing Interface (MPI) is adopted for efficient parallel computation. OpenSWPC has wide applicability for seismological studies and great portability to allowing excellent performance from PC clusters to supercomputers. Without modifying the code, users can conduct seismic wave propagation simulations using their own velocity structure models and the necessary source representations by specifying them in an input parameter file. The code has various modes for different types of velocity structure model input and different source representations such as single force, moment tensor and plane-wave incidence, which can easily be selected via the input parameters. Widely used binary data formats, the Network Common Data Form (NetCDF) and the Seismic Analysis Code (SAC) are adopted for the input of the heterogeneous structure model and the outputs of the simulation results, so users can easily handle the input/output datasets. All codes are written in Fortran 2003 and are available with detailed documents in a public repository.

S U M M A R YThe present study investigates the tsunami generation process by using 3-D numerical simulations and the linear potential theory. First, we evaluate the relation between sea-bottom elevation and sea-surface elevation as function of source size L, sea depth H and source duration T, based on 3-D numerical simulations. The surface elevation decreases with increasing sea depth and source duration. The difference between the sea-bottom and the sea-surface elevation appears when the source size is smaller than approximately 10 times the sea depth for a short source duration. The linear potential theory can precisely predict the numerical simulation results. Based on the theory, we can consider the tsunami generation as two spatial lowpass filter processes, in which the cut-off wavenumbers are given by the sea depth and the source duration. The criteria for small source size and short source duration are given as L < 13H and T < L/(8c), respectively, where c is the phase velocity of the tsunami. We then simulate the tsunami generation of the 1896 Sanriku tsunami earthquake, Japan. The simulated sea-surface elevation is significantly different from the sea-bottom elevation, which suggests the need for correction of the sea depth and source duration for the precise evaluation of the initial water-height distribution. To include these effects in 2-D simulations, we can use the impulse response function and add the fractional sea-surface uplift within the time step to the sea surface, for each time step.