We evaluated the long‐period site response of peak ground velocities (PGVs) and peak ground displacements (PGDs) in the 5–30 s period range at 198 K‐NET, KiK‐net, and Japan Meteorological Agency strong‐motion stations in northeastern Japan. Long‐period site responses were estimated empirically based on the ratio of observed ground motions on free surfaces to the values predicted by ground‐motion prediction equations (GMPEs) on bedrock in which the shear‐wave velocity is ≥2000 m/s. Our results show large site amplifications generally dominate at basin stations, whereas site deamplifications generally dominate at mountain stations. The long‐period site response factors of PGVs and PGDs have ranges of 0.6–3.6 and 0.4–3.1, respectively, for which the factors of PGVs are larger than those of PGDs by an average factor of ∼1.3. Long‐period site responses were used to correct the observed strong ground motions of eight earthquakes. The long‐period site‐corrected data fit better with GMPEs that were inferred from smaller standard errors and show a better correlation with sediment thickness, and thus may contribute to reducing the variability of seismic‐hazard assessment.
We propose a recipe to predict strong ground motions from scenario earthquakes which are caused by active faults. From recent developments in waveform inversion analysis for estimating rupture processes during large earthquakes, we have understood that strong ground motion is relevant to slip heterogeneity rather than total moment on the fault plane. The source model is characterized by three kinds of parameters, which we call : outer fault parameters, inner fault parameters, and extra parameters. The outer fault parameters are parameters characterizing the entire source area such as total fault length, fault width, and seismic moment. The total fault length (L) is related to the grouping of active faults, i.e. the sum of the fault segments. The fault width (W) is related to the thickness of the seismogenic zones. The total fault area S (=LW) follows the self-similar scaling relation with the seismic moment (M0) for moderate-size crustal earthquakes and departs from the self-similar model for very large crustal earthquakes. The locations of the fault segments are estimated from the geological and geomorphological surveys of the active faults and/or the monitoring of seismic activity. The inner fault parameters are parameters characterizing fault heterogeneity inside the fault area. Asperities are defined as regions that exhibit large slip relative to the average slip on the fault area. The relationship between combined area of asperities and seismic moment M0 satisfies the self-similar scaling relation. The number of asperities is related to segmentation of active faults. The rake angles of slips on the asperities should be estimated from the geological survey and/or geodetic measurements. The extra fault parameters are related to the propagation pattern of rupture within the source area. Rupture nucleation and termination are related to the geometrical patterns of the active-fault segments. The recipe proposed here is to construct the procedure for characterizing those inner, outer, and extra parameters for scenario earthquakes. Then, we have confirmed that the scaling relations for the inner fault parameters as well as the outer fault parameters are valid for characterizing earthquake sources and calculating ground motions from recent large earthquakes, such as the 1995 Kobe (Japan) earthquake, the 1999 Kocaeli (Turkey) earthquake, and the 1999 Chi-Chi (Taiwan) earthquake. We have also examined the recipe for estimating strong ground motion during the 1948 Fukui (Japan) earthquake. The simulated ground motions clearly explain the damage distribution in the Fukui basin.
The Japan islands are in a complex tectonic setting with various subducting plates, and most of urban areas are located over large-scale sedimentary basins. Since the sediments filling basins amplify ground motions and their velocity structures complicate the propagation of seismic waves, it is important for the prediction of strong ground motion and seismic hazard to determine the three-dimensional (3-D) velocity structures of these urban basins. This importance motivated various organizations to carry out extensive geophysical experiments and geological investigations, and velocity structure models are being constructed all over Japan. We have already proposed a standard procedure for modeling a regional 3-D velocity structure in Japan, simultaneously and sequentially using various kinds of datasets such as the extensive refraction/ reflection experiments, gravity surveys, surface geology, borehole logging data, microtremor surveys, and earthquake ground motion records. We applied the procedure to the Tokyo metropolitan area for constructing a reference 3-D velocity structure model. As this application confirmed the validity of the standard procedure, it is then applied to the central, eastern, and western parts of Japan in 2006 to 2008, to construct the Japan Integrated Velocity Structure Model. The velocity structure and source modelings have been dramatically improved after the 1995 Kobe earthquake with the advance of ground motion simulations based on substantial data of strong motion observation. The quality of broadband ground motion simulations enable us to predict realistic strong ground motions. The National Seismic Hazard Maps released in 2005 as a result of comprehensive research on long-term evaluation of earthquake occurrence and strong ground motion prediction are expected to be proved useful in earthquake disaster mitigation and scientific outreach to the public. It is also quite important for the simulation of long-period ground motion and its seismic hazard to validate the three-dimensional (3-D) velocity structure for the whole Japan islands. Since we are threatened by future earthquakes whose probabilities of occurrence are estimated to be high in long-term evaluation, long-period ground motions from future Tokai, Tonankai, Nankai, and Miyagi-oki earthquakes and their response spectra will be computed by using this Japan Integrated Velocity Structure Model, and combined into the Long-Period Ground Motion Hazard Maps in 2009 and later by the Headquarters for Earthquake Research Promotion. These hazard maps for the megathrust earthquakes may be proved to be touchstones of strong motion seismology.
The 2007 Chuetsu-oki, Japan, earthquake is the world’s first major earthquake upon a source fault that extends beneath a nuclear power plant and is also characterized by difficulty determining the source fault plane. Centroid Moment Tensor solutions indicate an M w 6.6 reverse-faulting crustal earthquake with conjugate fault planes dipping to the northwest and southeast. Early results of aftershock locations suggest that either northwest-dipping plane or southeast-dipping plane can be the source fault plane of this earthquake. We carried out source inversions and empirical Green’s function simulations of observed seismograms; however, they resulted in similar waveform residuals for the two fault planes. We then determined the relative locations of earthquake asperities to the hypocenter using travel-time differences of strong-motion pulses and relocated the aftershocks observed by ocean bottom seismometers deployed in the source region. These results imply that slips mainly occurred on the southeast-dipping fault plane. This implication was later confirmed by results of reflection surveys. During the earthquake, the Kashiwazaki-Kariwa nuclear power plant experienced stronger ground motions than those anticipated at the time of design. The ground motions consist of three seismic pulses that correspond to three asperities. The first and second pulses arose from rupture propagation to the plant, while the compact asperity on the distant southeast-dipping fault plane and its S -wave radiation pattern are responsible for the significant third pulse.
[1] Highly similar seismograms commonly observed along transform or convergent plate boundaries indicate that repeated small to moderate earthquakes occur within a single area. The results of source-process inversions for seismic events within the northeastern Japan subduction zone suggest that large earthquakes and their component asperities also exhibit this repeating nature under certain conditions. However, sets of similar seismograms that might provide direct evidence of the repeated rupture of large-earthquake asperities have not been observed. This paper analyzes two offshore earthquakes near Miyagi Prefecture, Japan, the most recent occurring in 2005 with a JMA magnitude (M) of 7.2 in the source region of the 1978 earthquake (M7.4). We demonstrate the similarity in waveforms from the seismograms recorded during the 1978 and 2005 earthquakes. The early portions of the 2005 seismograms resemble the 1978 seismograms, suggesting that the asperities located close to the 1978 hypocenter ruptured again during the 2005 event. The seismogram similarities provide the first direct evidence for the repeated rupture of asperities during successive large earthquakes. The results of our waveform inversions further indicate that the two asperities of the 2005 event largely coincide with the southern two asperities of the 1978 event. These repeating asperities are recorded in the early portions of the 2005 seismograms with 60% of the amplitude recorded in the 1978 seismograms. The characteristic behavior of the asperities supports the slip-predictable recurrence model of earthquake rupture rather than the time-predictable recurrence model.
We estimate the variance in ground motions related to repeated large earthquakes occurring on the same fault segment with similar magnitudes. We find eight earthquake pairs for which suitable strong‐motion records exist. Two are crustal strike‐slip earthquakes from California and six are subduction zone earthquakes from Japan. We consider only large earthquakes and deal with frequencies greater than the earthquake corner frequency, so the variability that is considered here is related to smaller scale differences in the rupture process, particularly on the part of the fault nearest the station. We find that the variance of the 5% damped spectral accelerations of these pairs, termed ![Graphic][1] , averages to about 45% and 80% of τ 2 for the crustal and subduction zone earthquakes, respectively, in which τ 2 is the contribution of source variability to the total variability of ground motion estimated by some recent ground‐motion prediction equations. We suggest that ![Graphic][2] is lower than τ 2, for the frequencies at which ![Graphic][3] is estimated, because it depends primarily on only local physical properties of a fault that are the same in repeated earthquakes. We therefore suggest that at sites where the hazard is controlled by a single rerupturing source, one could potentially use a between‐event variance that is smaller than τ 2 in seismic‐hazard calculations. Thus, these results may help to resolve the inconsistencies that are now present between the national hazard maps and some precariously balanced rocks in southern California.
[1]: /embed/inline-graphic-1.gif
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Shallow S-wave velocity V S profiles were estimated for 26 temporary strong motion observation sites surrounding the epicenters of a sequence of the 2016 Kumamoto earthquake. The microtremor array method was used to gather the dispersion characteristics of Rayleigh waves. V S profiles were obtained by inverting the dispersion curves for each site and those of three permanent strong motion stations that recorded the sequence of seismic events. The shallow V S profiles near two of the permanent strong motion stations in the town of Mashiki were almost identical. However, the V S profiles at other stations varied. The V S profiles were found to have the common feature of the uppermost low-velocity layer being widely distributed from Mashiki to the village of Minami-Aso, and it was especially thick in the areas that suffered heavy damage. This low-velocity layer was a major contributor to the site amplification. The horizontal-to-vertical spectral ratios of the microtremors indicate that both the shallow soil and deep sedimentary layers may control the site response characteristics over a broad frequency range.
