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Speaker：陳卉瑄 (國立台灣師範大學 地球科學系 教授)

Title：Tidal modulation and tectonic implications of tremors in Taiwan.

Time：3/29 (Thu) 10:30

Location：2nd floor large lecture hall of IONTU
 

Abstract

Here, we present results from 6 years of tremor observations in a continental subduction zone where continental crust of Eurasia is colliding the Philippine Sea Plate, forming a collisional orogen. During the period of 2007-2012, 1893 tremor events with durations ranging from 60 s to 2216 s were identified. Spectral analysis of tremor times reveals several sharp peaks that were correlated with annual, semidiurnal, and diurnal constituents, indicating a strong tidal and seasonal modulation. When compared with the records from tide gauges on both the east and west coasts of Taiwan, 67.93% and 81.86% of tremors coincided with higher-than-average sea level and increasing sea level, respectively; the correspondence changes to 69.42% and 87.54% if only long duration tremors were considered. This suggests strong tidal modulation occurred with tremors of longer duration and under higher tidal stressing rates. Spatially, the ambient tremors appear to form a steep, southeast-dipping ellipsoidal structure 15 to 45 km deep in the southern part of the collisional orogen where continental crust of Eurasia has been subducted. The steep dip suggests a crustal-scale ramp that imbricates the crust in this area. This section of the orogen is also characterized by high heat flow, low Vp and Vs values, high Vp/Vs ratios and relatively fast surface and rock uplift rates. We propose that the zone of tremors represents the initiation of a brittle-ductile shear zone deep in the crust, possibly near the crust-mantle boundary.