ALBERT

Description: The dense networks of strong-ground-motion instruments in Japan (K-NET and KiK-net) provide a means of direct visualization of regional wave propagation during large earthquakes. For the 2000 Tottori-ken Seibu earthquake (M (sub w) 6.6) in western Japan, snapshots of ground motion, derived directly from interpolation of a large number of array observations, demonstrate clearly the nature of the source radiation and the character of the seismic wave field propagating to regional distances. In western Japan the wave field from the earthquake is characterized, in most parts, by the dominance of high-frequency (0.2-5 Hz) Lg waves on three-component acceleration records and longer-period (T approximately 10 sec) fundamental-mode Love waves in tangential displacement. The presence of strong lateral variations in the crust and upper-mantle structures, such as the low-velocity superficial layer and the high-velocity Philippine Sea plate with shallow subduction into the mantle, impose significant modifications on the regional wave field. Further insight into the nature of the seismic wave field is gained by comparison of the observations with numerical simulation for a 3D model, including sedimentary basins by an embedded submesh. A multigrid, parallel computation using a hybrid Pseudospectral/Finite Difference Method allows the inclusion of a realistic model of the source process for the 2000 Tottori-Ken Seibu earthquake. There is good agreement in the dominant feature of the regional wave field propagating through the complex structure of western Japan. However, the differences between the observations and the computer simulations indicates the need for further refinement of the source and structural models. The inclusion of amplification effects for the population centers in sedimentary basins means that the modeling procedure is already suitable for estimating the main pattern of ground motion for future earthquake scenarios.

Description: The seismic waves from subduction zone earthquakes are significantly affected by the presence of 3D variation in crust and upper-mantle structure around the source area. These heterogeneous structures also profoundly modify the character of seismic waves as they propagate from the source area to regional distances. This is illustrated by studying shallow, interplate earthquakes along the Mexican subduction zone, and deeper, inslab, normal-faulting earthquakes in the subducted Cocos plate beneath Mexican mainland. The strong-motion recordings of these earthquakes are used to evaluate the character of wave propagation along the path between the source region and Mexico City. We compare the wavefield from two large earthquakes of different source type. During the shallow (H = 17 km), interplate, 1995 Guerrero earthquake (M (sub w) 7.3), the Lg phase is the most prominent feature at regional distances of about 150 to a few hundred kilometers from the source. The presence of a lateral velocity gradient in the crust, caused by the subduction of the Cocos plate, enhances the Lg-wave amplitude, which is then amplified further in the Mexican volcanic belt by amplification in the low-velocity volcanic rocks. Both effects lead to very large ground motions along the path from the coast to the Mexican inland, in the frequency band from 0.2 to 4 Hz. However, for the deeper (H = 40 km), inslab, normal-faulting, 1999 Oaxaca earthquake (M (sub w) 7.5), the amplitude of the Lg phase is too small to produce the abnormal wave propagation, and the direct S wave and its multiple SmS reflections between the free-surface and Moho show a simple attenuation with increasing distance. We compare these observations with numerical simulations of seismic-wave propagation using the Fourier spectral method. The results provide a key to the understanding of seismic-wave field generated by shallow interplate and deeper inslab earthquakes in a realistic 3D heterogeneous structure.

Description: The high level of seismicity and dense network of short-period stations in Japan allows a detailed characterization of regional phase propagation. There are substantial differences depending on the location of the source and stations. Despite the structural complexity, Lg is clearly seen in many parts of Japan. Lg is particularly well developed in the western part of the main island, Honshu, and will propagates through some volcanic zones with loss of high-frequency components. There are also zones of Lg blockage in northeastern Honshu and for subduction zone events with significant oceanic paths for which the mantle phases Pn and Sn are particularly clear. The oceanic region in the Sea of Japan blocks Lg propagation paths for events in mainland Asia at many stations, but there are clear Lg propagation corridors through Korea to stations in the west (Kyushu, western Honshu) and in the north into Hokkaido. The Lg phase carries substantial energy for large events, and the differences in efficiency of propagation influences the intensity of ground shaking. Thus the 1995 Hyogo-ken Nanbu (Kobe) earthquake shows intensity contours extended to the west in the region of efficient Lg-wave propagation. The Rg phase and the fundamental mode of Love waves are very significant for the shallow 2000 Tottori-ken Seibu event, and the effect of the combination of Lg with these slightly lower frequency surface waves would help to explain the discrepancy between the Japan Meterorological Agency magnitude from regional stations (Mj 7.3) and the moment magnitude from distant observations (Mw 6.6).

Description: A recent article (Tejeda-Jacome and Chavez-Garcia, 2007) showed that significant differences in seismic ground-motion attenuation perpendicular to the Pacific coast exist between Guerrero and Colima, in western Mexico. For hypocentral distances larger than 100 km, larger ground motions are predicted using a model derived from local data in the northern section of the subduction zone (Colima) relative to ground-motion models derived using data from the southern section (Guerrero). In this article we consider two possible explanations for the differences. The first possible explanation is differences in the geometry of the subducting slab between Colima and Guerrero. The second is the varying shallow structure of the overriding plate due to the presence of the trans-Mexican volcanic belt (TMVB), oblique to the Middle American trench. We use 2D numerical modeling for P-SV waves to investigate which of the two factors can explain the differences in the observed attenuation. Our results show that it is the presence of the TMVB, closer to the coast in Colima, that better explains the differences.

Description: Unusually large (〉5 cm) and prolonged shaking associated with long- period ground motions at periods of about 7 sec were observed in central Tokyo during the M (sub w) 6.6 Niigata-ken Chuetsu earthquake of 23 October 2004. The long- period ground motions caused significant resonance in high-rise buildings of about 70 floors in height. Thus, it is an urgent matter to understand the development and amplification properties of long-period ground motions in Tokyo associated with large earthquakes. In this study, we use numerous waveform records from 585 stations in a nationwide accelerometer network (K-NET, KiK-net) and 495 intensity meters in the area around Tokyo. The data reveal that the long-period ground motion is characterized in most part by a surface, Rayleigh wave generated at the northern edge of Kanto basin, and the surface wave is developed as propagating through a thick cover of sediments (〉3000-4000 m) that overlies rigid bedrock. To complement the observational data, we conducted a large-scale computer simulation of seismic-wave propagation by employing the Earth Simulator supercomputer with a detailed source-slip model and a high-resolution 3D sedimentary structural model of central Japan. The results of the computer simulation demonstrate that the anomalously prolonged ground shaking of the long-period signal recorded in the center of Tokyo occurred because of the stagnation of seismic energy resulting from the multipathing and focusing of Rayleigh waves toward the bottom of the Kanto basin from surrounding mountain regions with interaction to the 3D basin structure.

Description: The 2011 Tohoku earthquake (M 9.0) caused significant hazards including strong ground motion, coseismic ground deformation, and tsunamis. For a deeper understanding of these phenomena, we conducted a large-scale, parallel computer simulation by means of the finite-difference method (FDM) with newly developed tsunami-coupled equations of motion. This method is based on the equations of motion for an elastic medium, treating seawater as elastic material having zero S-wave velocity. Tsunami waves are included by taking into account gravity and their equilibrium with the pressure gradient. An effective method of FDM simulation using the state-of-the-art massively parallelized K computer is also studied. With the use of a 3D layered velocity structure that includes topography and bathymetry, basement structure, Moho depth, and plate boundaries, and use of the appropriate source-rupture model based on joint inversion of seismic waves and geodetic observations, the simulated seismic waves demonstrate anomalous seismic-wave propagation by the thick 3D basin structure inland. We also found that the sea column acts as a strong absorber of seismic waves for shallow up-dip slip. On the ocean side, coseismic deformation and tsunamis are dynamically generated in this simulation. Tsunami attacks on the subsided Pacific coast are simulated without any additional assumptions. We also modeled the tsunami and seismic records of ocean-bottom pressure gauges above the fault. Because sea-bottom tsunami and seismic observation is powerful but very complex, recording both seismic and tsunami waves, the use of these coupled numerical simulations has great potential for resolving complex source fault rupture processes in a 3D heterogeneous structure.

Description: The spatial and temporal variations of coda attenuation (Formula ) were studied in the source region of the 2011 Off the Pacific Coast of Tohoku, Japan (M (sub w) 9) earthquake. The Formula values were determined from the amplitude decay rate of the S-wave coda in narrower overlapping frequency bands in the range f=1.0-24 Hz, based on a single isotropic scattering model for more than 400 earthquakes (M (sub JMA) 3 approximately 6.5) in the region recorded in a period from January 2005 to August 2011, including pre- and postseismic period. Our estimates of the spatially averaged Formula value, Formula in f=1.0-24 Hz, is almost stable with small variations (〈10%) during the preseismic period, which is in good agreement with that estimated by former studies in the area of northeast Japan. We found that the value of Formula increases by about 10%-16% after the 2011 Tohoku event in f=1.25-3.5 Hz in some stations of northern Japan, which is confirmed by a statistical t-test at 99.9% confidence level. The change in Formula is spatially limited to the rupture zone, while other paths remain nearly unaffected, suggesting local changes of scattering properties in the vicinity of the Tohoku-Oki source volume. This change may be attributed to an increase in the density of open microcracks in the mainshock source volume (such as due to increase in stress induced by the 2011 event) and probably the fluid content in fractures in the rocks. A model of heterogeneity due to coseismically opened cracks (dominant scale length of a=0.6-1.8 km in f=1.25-3.5 Hz) enhanced by increased stress change possibly controls the increased Formula after the 2011 event in the source rupture region.

Description: We have estimated broadband (f=0.5-8 Hz) site amplification factors at 1800 sites in Japan based on the coda normalization method using 4001 seismograms obtained from 48 events. The results show that, in the low-frequency band (f=0.5-1 Hz), the site amplification factor varies over a wide range from -4 to 24 dB, with larger site amplification factors (〉8 dB) in such major basins as Kushiro, Rumoi, Sapporo, Aomori, Sendai, Niigata, Tokyo, Toyama, Nagoya, Osaka, Oita, and Kagoshima. On the other hand, the site amplification factor in the high-frequency band (f=4-8 Hz) varies over a relatively narrow range from 0 to 18 dB at each station and has no clear correlation with the surface geology of each station. We then examined the contribution of the site amplification at each station to the anomalous distribution of the intensity pattern during large earthquakes by correcting the observed ground accelerations at each K-NET and KiK-net station with their corresponding estimated site amplification factors. The corrected pattern of intensity shows simple concentric isoseismal intensity contours around the hypocenter, indicating the applicability of our estimates to strong motion predictions. We confirmed that our estimates are superior to those derived from geology, such as those based on the regression of the averaged S-wave velocity from the surface to a depth of 30 m (V (sub S30) ), with a smaller standard deviation of intensity variations based on a standard decay function of intensity versus hypocentral distance.