dc.description.abstract | Microearthquakes with magnitude down to 0.3 were detected by the Taiwan Chelungpu fault Drilling Project Borehole Seismometers (TCDPBHS). Despite the large co-seismic slip of 12 m at the drill site during the 1999 Chi-Chi earthquake, our studies show little seismicity near the TCDPBHS drill site 6 years after the Chi-Chi main shock. The microearthquakes clustered at a depth of 9-12 km, where the Chelungpu thrust fault turns from a 30-degree dipping into the horizontal decollement of the Taiwan fold-and-thrust tectonic structure. This observation suggests that the thrust belt above the decollement is locked during this interseismic period. A cross-correlation (CC) was made to the identified microeartqhuakes, 287 clusters were discovered for CC > 0.8. These clusters are mostly burst-type, as which occur in a short time period. The examination of the interseismic time interval within the clusters reveals two significant peaks in time intervals, as if 10^-1~10^0 hr (minutes to hour) and 10^3.5 hrs (year). For the similarity in waveforms, we observed unique earthquake clusters, which have near constant P- and S-wave durations regardless the magnitudes of events within the clusters.
Further studies on source scaling from the investigation of source parameters of 242 microearthquakes, we used SH-wave spectra by fitting ω^2-shaped Brune source spectra with a frequency-independent Q model. We find that the static stress drop increases significantly with increasing seismic moment, and also the similar positive feature for the apparent stress scaling with seismic moment. To avoid the contamination from attenuation, we further analysis the data for events within clusters to remove the path effect of the events in the clusters. The Empirical Green’s function (EGF) and Futterman Q correction methods are utilized. The derived source time function, Tw, from the both methods showed similar feature as the Tw does not follow the earthquake self-similarity scaling of Tw ∝ M0^3. The results obtained a nearly constant Tw with moment. Our observations provide a direct evidence of an earthquake non-self-similarity behavior for events ranging from Mw 0.0 - 2.0 within the cluster.
To explore the potential mechanisms for rapid event recurrence (sec-hour), we developed a model of repeating earthquakes based on rate-and-state friction. In the model, several small patches governed by steady-state velocity-weakening friction are located in a close proximity to each other and surrounded by a larger velocity-strengthening region with a background loading slip rate. Our modeling results indicate that the rapid triggering does not occur in the long-term response with typical lab parameters. However, a model with stronger barriers and slip law of state variable evolution matches the observations, suggesting high heterogeneity of the fault zone. In another way, we develop a model with pore pressure variation indicating fluid flows. The modeling results suggest high pore pressure change ( >50% normal stress) with artificial time drifting may also trigger the rapid triggered seismic clusters. | en_US |