Tidal Modulation and Tectonic Implications of Tremors in Taiwan

We present results from 6 years of tremor observations in a continental subduction zone where the continental crust of Eurasia is colliding with the Philippine Sea Plate, forming a collisional orogen. During 2007–2012, 1,893 tremor events with durations ranging from 60 to 2,216 s were identified and located. Spectral analysis of the tremor times reveals several sharp peaks that correlate with the semi annual, semidiurnal, and diurnal constituents, indicating a strong tidal and seasonal modulation. When the tidal stresses and stressing rates are compared with tremor activities, we find that the percent excess number of events increases with amplitude of shear stress and tensile stress but not with shear stressing rate. The percent excess reaches a mean value of 150% for highest shear stress at 3.5 kPa. Such high correlation with tidal shear stress is consistent with the observation from tidal records, where 76–82% of tremors occurred at higher than the median water level at tide gauge stations on the east coast. Spatially, the ambient tremors appear to form a steep, southeast dipping ellipsoidal structure at depths of 15 to 45 km in the southern part of the collisional orogen where the continental crust of Eurasia has been subducted, metamorphosed, and imbricated. We propose a weak fault model for the tremor zone that represents the initiation or reactivation of a brittle-ductile shear zone deep in the crust and reconciles both the steeply dipping and gently dipping fault geometries proposed previously from different studies.