ALBERT

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 ( ), with a smaller standard deviation of intensity variations based on a standard decay function of intensity versus hypocentral distance.

Description: The spatial and temporal variations of coda attenuation ( ) were studied in the source region of the 2011 Off the Pacific Coast of Tohoku, Japan ( M w  9) earthquake. The 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 JMA 3~6.5) in the region recorded in a period from January 2005 to August 2011, including pre- and postseismic period. Our estimates of the spatially averaged value, 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 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 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 after the 2011 event in the source rupture region.

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. Online Material: Mpeg movies of fault slip, seismic-wave propagation, seafloor and ground displacement, and tsunamic propagation.

Description: The robustness of broadband ground-motion simulation can promote seismic-hazard assessment. A broadband ground-motion simulation technique called "the recipe" is used in the scenario earthquake shaking maps of the National Seismic Hazard Maps for Japan. The recipe represents a fault rupture based on a multiple asperity model referred to as the characterized source model. Broadband ground-motion time histories on the engineering bedrock are computed by a hybrid approach of the 3D finite-difference method and the stochastic Green’s function method for the long- (〉1 s) and short-period (〈1 s) ranges, respectively, using a 3D velocity structure model. The ground motion on the ground surface is computed using the 1D site response of the surface soil layers. Because the need for ground-motion simulations of scenario earthquakes is increasing, it is important to validate the method from seismological and engineering perspectives. This study presents a validation of the recipe using velocity waveforms, peak ground velocity (PGV), seismic intensity, and pseudoacceleration response spectra. The validation scheme follows the framework of the Southern California Earthquake Center Broadband Platform. We selected two M w  6.6 crustal earthquakes that occurred in Japan as the targets of this study: the 2000 Tottori and the 2004 Chuetsu (mid-Niigata) earthquakes. The validation results are satisfactory except for those in the shortest-period range (0.01–0.1 s) at large hypocentral distances (〉70 km); such conditions are outside of the target range of the recipe. Simulations using a 1D velocity structure model were also examined. The simulation results for the 1D and 3D velocity structure models indicated that the 3D velocity structure models are important in reproducing PGV and the later phases with long duration, especially on deep sediment sites.

Description: The 3D wave propagation simulation is suitable for the long-period ground-motion hazard analysis, because the realistic seismic source and velocity structure models are applicable and ground motion at any point can be obtained. However, large-scale simulations for various scenarios are needed to adopt uncertainty due to the source model of a large earthquake. Here we show our approach to overcome a problem of heavy computational expense. We improve computation performance of the 3D finite-difference method by using discontinuous grids and by adapting to multigraphics processing units technique. Then, we simulate long-period ground motion of many scenarios of a hypothetical megathrust earthquake in the Nankai trough. We use the characterized source models based on the "recipe" in the simulation. Finally, we apply the simulation results based on about 400 scenarios to a technical framework of the probabilistic seismic-hazard analysis. The hazard maps of velocity response spectra for a given conditional exceedance probability show that the long-period ground motion is more significant with respect to the underground structure than is the distance from the source area.

Description: Love- and Rayleigh-wave microseisms with a dominant period of about 20 s have been clearly detected in Japan. Because their arrival direction was almost the same in any part of the Japanese islands, they are expected to have propagated as plane waves from a very distant source. We estimated the source area of the microseisms by multiple array analysis of data from the Hi-net, high-sensitivity accelerometers in Japan and the broadband seismic array at Gräfenberg, Germany. The source was distributed to be in the North Atlantic and migrated from north of the British Isles to the Norwegian Sea in a period of one day. This migration coincided with the propagation of ocean swells. Our results indicate that ocean swells in the North Atlantic can be a major source of seismic noise with a period of 20 s in Japan. Our analysis also suggests that the source areas of the Love and Rayleigh waves were different. It is hoped that such observations will be helpful in understanding the excitation mechanisms of microseisms.