Earthquake dynamics-from macroscopic to microscopic: examples learnt from the 1999 Chi-Chi earthquake in Taiwan

Determining the seismic fracture energy during an earthquake and understanding the associated creation and development of a fault zone requires a combination of both seismological and geological field data. The analysis of earthquake kinematics from seismic waveform data, which we call macroscopic seismology, can help us understand the temporal and spatial slip distribution, and furthermore, the possible associated hazard pattern. The actual thickness of the zone that slips during the rupture of a large earthquake is not known and is a key seismological parameter in understanding energy dissipation, rupture processes and seismic efficiency. This analysis requires the investigation of fault gouge related to an earthquake, and we call it microscopic seismology. The overall understanding of earthquake dynamics requires the integration of macroscopic with microscopic studies. The 1999 magnitude-7.7 earthquake in Chi-Chi, Taiwan, produced a large slip (8 to 12 m) at or near the surface, which was accessible to borehole drilling and provided a rare opportunity to sample a large slip fault. The Taiwan Chelungpu-fault drilling project retrieved samples from the fresh slip zone associated with the 1999 Chi-Chi earthquake. The surface fracture energy estimated from grain sizes in the gouge zone of the fault was directly compared to the seismic fracture energy determined from near-field seismic data. The chemical and physical characteristics of the retrieved core helped us give the constraints on the faulting dynamics as thermal pressurization, which yielded the large slip and slip velocity with less acceleration observed at the northern portion of the fault. The opportunity of linking earthquake studies from the macroscopic to the microscopic is rare and unique. The 2008 Wenchuan earthquake is another possible earthquake for this integrated seismology study.