Deep postseismic slips that reflect the rheology and the deformation of the seismogenic fault after large earthquakes are usually investigated less than shallow slips because of the difficulty of obtaining direct observations. In this study, we used the seismicity of repeating aftershock sequences (RASs) to improve our understanding of the deep postseismic slips after three large earthquakes in southern Taiwan. To more precisely characterize the evolution of the RASs, we applied a template-matching technique to continuous waveform data for 90 days before and after the mainshocks. We identified 28 RASs that mainly occurred in regions near the mainshock hypocenters with relatively high VP/VS ratios. The deep fault slips estimated from the RASs show that the postseismic slip rates decreased logarithmically with increasing duration time, suggesting that the faults were creeping according to the velocity-strengthening friction law. We inferred that the high hydraulic pressure environment contributed to the fault creep, but the dynamic stress perturbation may have been the main factor affecting the fault creep. The results of this study improve our understanding of the behavior of deep faults and may aid in future seismic hazard assessments in Taiwan.
At present, both Rayleigh and Love waves can be extracted from cross-correlations of ambient noise in ambient noise tomography (ANT). It has been demonstrated that accurate Rayleigh-wave dispersion curves can be extracted from vertical-component noise when noise sources are evenly distributed in space. However, because the rotation of horizontal components alters the distributions of effective sources in horizontal–horizontal cross correlations between a pair of stations, it is still not completely sure if dispersion curves of Rayleigh and Love waves extracted from horizontal-component noise data are accurate for ANT. In this study, through numerical simulations, we demonstrate that surface-wave phase velocities extracted from the horizontal–horizontal cross-correlation functions (CCFs) systematically deviate from structural phase velocity even when noise sources are evenly distributed in space. The deviations of phase velocities quickly increase with decreasing station separations of CCFs when station separations are shorter than three wavelengths. Further analysis on field data recorded at seismic stations in the contiguous United States confirms the existence of the phase-velocity deviations of Rayleigh and Love waves when they are measured from horizontal–horizontal CCFs. Because the deviations become smaller with increasing station separations and are less than 0.25% at station separations longer than three wavelengths, we suggest that it is best to only select those CCFs with station separations longer than three wavelengths in ANT when measuring phase velocities of surface waves from horizontal-component CCFs, such as Love waves from radial–radial CCFs.
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