Temporal changes of seismic velocity and anisotropy in the shallow crust induced by the 1999 October 22 M6.4 Chia-Yi, Taiwan earthquake

Abstract

Observing temporal changes of material properties in the upper crust has been a long-sought goal within the geophysics community for many decades. A better quantification of temporal changes in the near-surface layers associated with nearby large earthquakes is crucial for improved understandings of rock rheology in the shallow crust, better characterization of site response, and accurate predictions of strong ground motions for future large earthquakes. We systematically analyse temporal changes of seismic velocity and anisotropy in the shallow crust in southwest Taiwan using local earthquakes recorded at a 200-m-deep borehole station CHY. This station is located directly above the hypocentre of the 1999 October 22, M6.4 Chia-Yi earthquake. Three-component seismograms recorded at this station show clear direct (upgoing) and surface-reflected (downgoing) P and S waves, and S-wave splitting signals. We use the time delays between the upgoing and downgoing waves in the autocorrelation functions as the proxies for the two-way traveltimes in the top 200 m. The S-wave traveltimes measured in two horizontal components increase by ∼1-2 per cent at the time of Chia-Yi main shock, and followed by a logarithmic recovery, while the temporal changes of S-wave splitting and P-wave traveltimes are less than 1 per cent and are not statistically significant. We obtain similar results by grouping earthquakes into clusters according to their locations and waveform similarities. This suggests that the observed temporal changes are not very sensitive to the seismic ray paths below CHY, but are mostly controlled by the variation of material properties in the top 200 m of the crust. We propose that strong ground motions of the Chia-Yi main shock cause transient openings of fluid-filled microcracks and increases the porosity in the near-surface layers, followed by logarithemic recovery processes. No clear change in the delay times of the shear wave anisotropy is found following the main shock, suggesting that cracks opened during the earthquake do not have a preferred orientation. Our results also show a gradual increase of time delays for both the fast and slow S waves in the 7 yr prior to the Chia-Yi main shock. Such changes might be caused by variations of water table, sediment packing or other surficial processes. © 2009 The Authors Journal compilation © 2009 RAS.