ALBERT

Description: A thick accretionary wedge with low seismic velocity overlying a subducting plate is an important factor affecting the amplitudes and durations of long-period ground motions for great offshore earthquakes. We performed a series of 3D long-period ground-motion simulations to obtain a better understanding of the effects of the accretionary wedge along the Nankai trough, Japan. The simulation results demonstrate that the accretionary wedge has the effect of decreasing the peak amplitude and velocity response but amplifying and elongating later phases of long-period ground motions in the land area. These effects depend significantly on the focal depth and are pronounced for shallow seismic sources. The amplified and elongated later phases originate mainly from prominent basin-induced surface waves excited near the trough axis. We also identified hard sediments with intermediate S -wave velocity in the accretionary wedge as a key layer that enhances the propagation of long-period ground motions from offshore to onshore. In the case of the Nankai trough, the hard sediments allow long-period ground motions developed inside the accretionary wedge to be efficiently transmitted outward in the longer period than ~10 s. Comparisons between the observed and simulated records suggest that for more accurate predictions of long-period ground motions, velocity structure models of not only the accretionary wedge but also the surrounding upper crust should be validated and revised using seafloor observations.

Description: We developed long-period (5–30 s) ground-motion prediction equations (GMPEs) for peak ground velocities (PGVs) and peak ground displacements (PGDs) for crustal, interplate, and intraplate earthquakes. We used strong-motion data from KiK-net downhole stations located in layers with shear-wave velocities equal to or greater than 2000 m/s. The data set consisted of 20 earthquakes of 6≤ M w ≤9.1 that occurred in and around Japan, including the 2011 Tohoku earthquake. Two-stage regression analyses were performed to derive the long-period GMPEs. We fitted the data with bilinear regression lines bending at M w  7.5, although additional factors such as focal depth and earthquake type were found to enhance the fitting with the observed data. The developed equations indicated that long-period PGVs and PGDs are larger for crustal earthquakes than for interplate and intraplate earthquakes. The attenuation coefficients indicated that long-period PGVs and PGDs increase with increasing depth. We estimated the moment magnitude by fitting the observed PGVs and PGDs in the 5–30 s period range with the long-period GMPEs. We obtained estimates of the magnitudes of 23 earthquakes recorded by KiK-net downhole accelerometers, and the results were consistent with the moment magnitudes obtained from the Global Centroid Moment Tensor project. The described method was proven useful for estimating the moment magnitude of great earthquakes, offering the potential for rapid estimation of moment magnitude if information from the source area is available.

Description: 〈span〉〈div〉Abstract〈/div〉Near‐fault broadband ground‐motion simulations of the 2016 Mw 6.4 Meinong, Taiwan, earthquake are carried out using the stochastic finite‐fault modeling method with the frequency‐dependent 〈span〉S〈/span〉‐wave radiation pattern. We simulate broadband ground motions that recorded large velocity pulses, with dominant periods of about 2 s, in both east–west (EW) and north–south (NS) components in the forward rupture direction using two hybrid approaches: a hybrid stochastic‐analytical approach and a hybrid stochastic‐deterministic approach. We also simulate broadband ground motions that did not record large‐velocity pulses using a pure stochastic method. The simulated ground motions using the hybrid stochastic‐analytical approach reproduce the observed large EW‐ and NS‐component velocity pulses, and the simulated spectral accelerations show good overall fitting with the observation data for periods shorter than 3 s. However, the EW and NS velocity amplitudes are underestimated because of the limited availability of velocity structure models for western Taiwan even though the simulated ground motions using the hybrid stochastic‐deterministic approach reproduce velocity phases very similar to the observation data. The peak ground accelerations (PGAs) and spectral acceleration values of the simulated ground motions obtained using the pure stochastic method fit well with the observation data. By comparing the ground‐motion prediction equations developed for shallow crustal earthquakes in Taiwan with the observed and simulated PGAs and spectral accelerations, we find that the prediction models without a directivity correction term underestimate spectral accelerations for periods around 1 s and longer for stations that recorded large velocity pulses near the main rupture area. Finally, we simulate strong ground motions at two collapsed building sites in the city of Tainan where ground motions were not observed.〈/span〉

Description: 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.

Description: The 1923 Kanto earthquake occurred along the Sagami trough (central Japan), causing severe damage in the Tokyo metropolitan area. This study was able to characterize the source process for this event using geodetic, teleseismic, and strong-motion data. The Kanto region is located above a large-scale sedimentary basin. Therefore, 3D Green’s functions and a curved fault modeling the subduction interface geometry were used to account for 3D complex wave propagation inside the basin. The later phases of the 3D Green’s functions with long paths inside the basin had large amplitude and long duration, primarily because of amplification effects caused by the lateral heterogeneity of sedimentary layers. However, 3D static displacements were large mainly because of amplification effects caused by the presence of thick soft sedimentary layers in the basin. The geodetic inversions with 3D and 1D Green’s functions had smaller seismic moments than half-space ones. This suggests that wave amplifications caused by large-scale sedimentary basins exert significant effects on seismic moment determinations. The joint inversion with 3D Green’s functions showed two large slip areas with a maximum slip of 〉6 m and an estimated total seismic moment of about 4.1 x 10 20 N·m ( M w  7.7). Strong motions were recovered very well, even for the later phases, by slips in shallow areas. Furthermore, by comparing various source processes inverted by two fault models of 70 or 280 subfaults, 3D or 1D Green’s functions, and a curved or planar fault model, we verified these assumptions based on the inversion results.

Description: 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 , averages to about 45% and 80% of 2 for the crustal and subduction zone earthquakes, respectively, in which 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 is lower than 2 , for the frequencies at which 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 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.