Multi-Mode Ambient Noise Double Beamforming Tomography with a Dense Linear Array: Revealing Accretionary Wedge Architecture across Central Taiwan

Abstract

Taiwan, one of the most active orogenic belts in the world, undergoes orogenic processes that can be elucidated by the doubly-vergent wedge model, explaining the extensive island-wide geological deformation. To provide a clearer depiction of its cross-island orogenic architecture, we apply ambient noise tomography across an east-west linear seismic array in central Taiwan, constructing the first high-resolution 2-D shear velocity model of the upper crust in the region. We observe robust fundamental- and higher-mode Rayleigh waves, with the latter being mainly present in the western Coastal Plain. We develop a multi-mode double beamforming method to determine local phase velocities across the array between 2 and 5-sec periods. For each location, we jointly invert all available fundamental- and higher-mode phase velocities using a Bayesian-based inversion method to obtain a 1-D model. All 1-D models are then combined to form a final 2-D model from the surface to ∼10 km depth. Our newly developed 2-D model clearly delineates major structural boundaries and fault geometries across central Taiwan, thereby corroborating the previously proposed pro-wedge and retro-wedge models while offering insight into regional seismic hazards.