Observation and Scaling of Microevents from TCDP Borehole Seismometers

Yen-Yu Lin

Abstract

Microearthquakes with magnitude down to -0.5 were detected by the TCDP BHS. A temporary seismic array with 10 short period seismometers around the TCDP drill site was also installed to incorporate with the TCDP BHS for the precise locations of the microearthquakes. A location software (MIMO) (Oye and Roth, 2003) was used to determine P- and S-wave onset times, incidence and azimuth angles for the locations of the microearthquakes. Regardless of the large co-seismic slip of 12 m at the drill site during the 1999 Chi-Chi earthquake, our preliminary studies show very less seismicity near the drill site from the TCDP BHS recording. The micro earthquakes clustered at a depth of 8-10 km, where the 30 degree dipping of the Chelungpu thrust fault becomes flat to a decollement of the Taiwan fold-and-thrust tectonic structure. As a continuous GPS survey did not observe post-slip at the large slip region, and as no seismicity was observed near the drill site, we suggest that the thrust belt above the decollement during the interseismic period might be locked. A Fluid Injection Test (FIT), pumping high pressure fluid into hole B and C with hole A as observation well was carried out at the TCDP boreholes in November 2006, and January, March and April, 2007. The observation from TCDP BHS, for the time period of Nov., 2006 to Feb., 2007, did not show significant correlation of the FIT related seismicity, despite the distinct observations on the arrival of gas and chemical monitoring through FIT. This discrepancy in the physical and chemical observations to the FIT is an odd. However, more conclusions can be made only if the complete data set has been sorted. In addition to the FIT related observation, we also try to understand the scaling of the small earthquakes in stress drops and seismic moment. The source parameters of 89 micro-events were examined from the source spectra using Brune ω-2 model for a constant Q model. The scaling of the magnitude to the Brune stress drop is a significant positive correlation. However, there has been a debate that this positive relationship might be biased for without Q correction. We had observed five clusters propagation events showing similar waveforms. We will further use the small event in the clusters as the empirical Green’s function to remove the path effect to validate the source scaling studies.