Vp/Vs distribution in the northern Taiwan area: Implications for the tectonic structures and rock property variations

The ratio of compressional-wave velocity (Vp) to shear-wave velocity (Vs) is often indicative of lithology and rock conditions. In this study, we determine the Vp/Vs characteristics for the different tectonic features in northern Taiwan and quantify the crack density and saturation by using the OB74 method. The result shows that the highest Vp/Vs values are located beneath the Western Foothills at depths between 4 and 21 km. These Vp/Vs highs are simultaneously characterized by low Vp and low Vs, suggesting the presence of fluids. The average crack density and saturation for each depth can reach as high as 25% and 91% at depths of 7 and 11 km, respectively. This distribution could be caused by the presence of less compacted sediments within the foreland basin and the progressive convergence of crustal faults with depth. As the shallow portion of the foreland basin is highly fractured, fluids could infiltrate into deeper parts to create a highly saturated environment. However, when the depth is deeper than 11 km, the high crack density and saturation area migrate from the west to the east of the Shanchiao Fault. This observation may indicate the presence of a preexisting syn-rift basin buried at depth during the collision. In the Ilan Plain, the highest crack density and saturation are observed in its northeastern portion at a depth of 4 km with values of 17.1% and 60.3%, respectively. However, slightly high saturation values spread throughout the plain at a depth of 15 km, which may indicate a possible fluid conduit from a deep, hot rock body to the surface and could be applied toward studies on geothermal fluid activity.