| Summary: | 碩士 === 國立臺灣師範大學 === 地球科學系 === 104 === In the seismogenic zone, ordinary earthquakes experience rapid slip on a fault with a few ~ tens of second durations. While below the seismogenic zone, slow rupture propagation and/or low slip rate can be also taking place with a wide range of duration from minutes to days. This class of slow-slip events, characterized by noise-like, long-lasted signals with consistent arrival at various stations is called tremors. It is believed to play an important role in the assessment of regional seismic hazard and plate boundary processes. Unlike subduction zones and transform faults where the ambient tremors occur on a known fault plane, the tremors in Taiwan are located in a place where no active faults have been identified. To better understand the possible generation mechanism, carefully examining different detection schemes for a more complete tremor catalog is necessary.
In this study we adopt the identification scheme similar to Ide et al. (2015) but applied slightly different techniques: (1) Higher waveform cross-correlation coefficient (>0.6) (2) careful visual inspection for excluding local earthquakes and short-lasted event (duration < 60 s) (3) Signal to noise ratio higher than 1.2 and lower than 30 (4) No spatio-temporal clustering technique used. We also develop an approach to systematically determine the duration of tremor events. As a result, 1893 tremor events with duration ranging from 60 s to 2216 s are found during the period of 2007-2012. They are mainly located underneath southern Central Range, forming a NS striking and SE-dipping ellipsoid structure at a depth of 15-45 km. The up-dip extension of this tremor structure reaches an aseismic zone under the western flank of Central Range at shallow depth, where is an area characterized by high heat flow, low Vp and Vs anomaly. Power spectrum analysis of tremors reveals several sharp peaks that are consistent with that from tidal data, indicating strong tidal modulation. The most significant annual periodicity is also found in ground water and strainmeter data, indicating an influence of Sun on hydro-geological activity. Tremor activity exists a clear tidal modulation of the annual, semidiurnal, and diurnal constituents, ~67% tremors happened while the observation of tide level higher than average, ~83% tremors happened while tide level rise. This phenomenon indicates solid earth and ocean tide may influence tremor activity. With this tidal-tremor correlation, we found a best response structure of tidal induced shear stress: (strike,dip,rake)=(60°,40°,90°), which is similar with both ellipsoid structure and the mechanism of very-low frequency event in previous study.
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