Spatial Correlation between Crustal Strength and Relocated Seismicity in the Taiwan

Abstract

High-resolution 3-D crustal Vp and Vs structures in the Taiwan region are validated from the similarities between the observed and synthetic P- and S-wave station corrections using JHD method, from the modeling of anomalous Pn waves observed in eastern Taiwan, and from the spatial coherence between the observed gravity data and calculated gravity data from the 3-D velocity structure. All hypocenters in the local earthquake catalog are then reliably relocated using the resultant 3-D Vp and Vs model. Distribution of the relocated hypocenters reveals significant improvement in the depth and epicenter determinations and exhibits clustering features showing excellent spatial correlation with many known and unknown active faults. It is apparent that the north-south trending Central Range in the Taiwan region is characterized by a high velocity, brittle and seismically active upper crust (~15 km) and a lower velocity, ductile, and aseismic mid-to-lower crust (15~55 km). Although seismicity can be closely correlated to the active crustal deformation, the crustal strengths beneath active mountain belts and alongmajor faults have been notoriously difficult to constrain. Since 3-D crustal density distribution can be estimated from the 3-D velocity structure and from the inversion and modeling of the observed gravity data, it is essential to use these recently available velocity and density information to estimate crustal strength from spatial distribution of elastic constants of the crust. Spatial correction between crustal strength and corresponding seismicity can thus be explored.