Investigation on Fluid Migration Triggered Seismicity in Association with Fault Zone Damage: Case Study for 1999 M7.6 Chi-Chi, Taiwan, Earthquake

Abstract:

Fluid had been considered as a possible factor in triggering earthquakes, but, the evidence in elucidating the behavior and mechanism is still unresolved. Our previous studies in attenuation, noted as 1/ Qs, for the 1999 Chi-Chi earthquake suggest that the fracture zone associated with fault zone could be considered as a fluid reservoir, which possibly yield to some observations/detections of phenomena associated with pre-, co- or post-seismic of a larger earthquake. The sudden changes in attenuation co-seismically with decay following a diffusion process indicated possible high pore-fluid saturation within fractured fault zone from fully to partial saturation. We suspect this process might yield the migration of fluid flow, and thus, related to the occurrence of some aftershocks. Aftershock sequence following a mainshock was often considered to be related to regional tectonic stress and stress triggering of a mainshock. For stress triggering, it refers to the seismicity in the stress increase regime with some lapse time (>one month) for the static stress triggering. In this study, we tried to deviate the possible fluid associated aftershocks by focusing on the stress decrease regime and 1-3-month aftershock sequence to understand their possible association. The migration of fluid flow increases the pore-pressure, which reduce the normal stress, and, thus, yield the co-seismic negative Coulomb’s stress regime to become positive to trigger these fluid flow associated aftershocks. For the study of the Chi-Chi earthquake sequence, from the elapse time versus distance of the seismicity in this regime suggest a fluid flow of about 220m/day. We investigate the possible passage of fluid flow in the regime and understand its transient behavior associated with the fault zone damage and healing process.