Tidal Modulation and Tectonic Implications of Tremors in Taiwan

Abstract

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.

Evaluating the Association Between Tectonic Tremors and Earthquakes in Taiwan From 7 Years Catalogs

“…The ambient tremors in this study were detected by seven broadband stations in Broadband Array in Taiwan for Seismology (BATS) and 11 short‐period stations from the Central Weather Bureau Seismic Network (CWBSN) in a 6‐year period (2007–2012; Chen et al, ). Combined with BATS, a temporary array deployed on the southern Central Range also allows us to obtain a short‐term tremor catalog from 1 January to 16 September 2016.…”

“…In Figure , both window length reveals a clear annual variation in tremors, with two peaks coinciding to earthquake clustering in 2008 and 2010 (corresponding to the 2008 Taoyuan and the 2010 Jiashian mainshocks). The annual variation was previously interpreted by a strong tidal and seasonal modulation in Taiwan (Chen et al, ; Ide et al, ). To diminish the effect of tidal triggering that caused annual acceleration in tremor activity, we aim at removing the seasonal trend from tremor catalogs.…”

“…The fault orientation for tremor events was considered as a low‐angle thrust (strike = N54°E, dip = 13°, rake = 120°), derived from a moment tensor inversion based on very low frequency events (Ide et al, ). Using a more complete tremor catalog, Chen et al () obtained a slightly different fault orientation (strike = N60°E, dip = 40°, rake = 90°), which also reveals a strong correlation with the tides. Since we used the same tremor catalog as Chen et al (), the fault orientation of strike = N60°E, dip = 40°, rake = 90° is chosen for our static Coulomb stress computation.…”

“…Using a more complete tremor catalog, Chen et al () obtained a slightly different fault orientation (strike = N60°E, dip = 40°, rake = 90°), which also reveals a strong correlation with the tides. Since we used the same tremor catalog as Chen et al (), the fault orientation of strike = N60°E, dip = 40°, rake = 90° is chosen for our static Coulomb stress computation.…”

“…Static Coulomb stress change caused by the (a) 2008 Taoyuan, (b) 2010 Jiashian, (c) 2012 Wutai, and (d) 2016 Meinong events on tremor source, along the cross‐section A‐B (shown in inset of a). Here we assume a receiver depth of 30 km (horizontal dashed line) and fault plane (red line) corresponding to the best correlation with tidal stress by Chen et al ().…”