Tectonically Driven Along-Strike Variations in Subsurface Shear-Wave Velocity in Eastern Taiwan Revealed by Ambient Noise Tomography Using Dense Arrays Data

Abstract

Eastern Taiwan sits in a plate suture zone where the Luzon volcanic arc, carried by the Philippine Sea plate, collides northwestward with the Eurasian continental margin, producing a series of geological formations and predominantly north-northeast–south-southwest-trending active faults. Seismic studies and surface geological investigations have continually improved our understanding and revealed variations across the elongated eastern Taiwan. However, the uppermost crustal structure remains poorly constrained, leaving a gap in integrating surface geology with the underlying tectonic architecture. In this study, we sequentially deployed five temporary arrays between 2016 and 2019. These arrays together are composed of a total of 411 seismic stations, covering the entirety of eastern Taiwan. We conducted ambient noise tomography to obtain the 3D shear-wave velocity structure of the upper 6 km. We observe a distinct north–south (N–S) difference in the low-velocity zone characteristics, where in the south the anomalies are nearly twice as thick as those in the north and exhibit near-surface velocities lower than 1.0 km/s, which we speculate to arise from three factors: (1) the Longitudinal Valley fault, footwall for which sustained subsidence and transported fluvial sediments to greater depths; (2) rich fluids and gases within the footwall of the Longitudinal Valley fault; and (3) the presence of the sedimentary Lichi Formation. We also infer that the Lichi Formation may extend farther northward to near 23.5° N, underlying the surface geology units. In addition, fault-zone shear-wave velocities present a nearly consistent pattern in the north, where the velocity contrast across the fault is subtle. In contrast, in the south, faults can either perfectly match the velocity contours or be entirely perpendicular to them. The observed N–S differences link well with other geophysical, geological, and geodetic constraints, bridging surface processes with deeper tectonic activities.