The 3-D velocity and density structure of NE Taiwan inferred from seismic and gravity data

Taiwan lies at the convergent boundary between the Eurasian Plate and the Philippine Sea Plate. In northeastern Taiwan, at the Ryukyu subduction zone, the Philippine Sea Plate subducts northward beneath the Eurasian Plate. This tectonic interaction has formed a series of geological features, including the Ryukyu Trench, the Ryukyu Arc, and the extensional Okinawa Trough. This study area is significantly influenced by both the extension of the trough and the plates subduction. As a result of these tectonic processes, this region experiences notable igneous activity, such as the formation of Kueishan Island and the Tatun Volcano Group. Additionally, extensional activities related to plate rifting are observed beneath the Ilan Plain.

In this study, we apply the Sequential Inversion method, utilizing seismic data provided by the Central Weather Bureau. The analysis applies Double-Difference Tomography to construct a three-dimensional velocity model of the study region. The initial model is based on the 3-D velocity structure proposed by Su et al. (2019). To address the limited resolution of seismic waves in the shallow subsurface, gravity data are integrated to enhance the resolution of shallow structures. A velocity-density conversion formula is used to transform the velocity model into a corresponding density model for gravity inversion. The resulting density model is subsequently converted back into a velocity model, completing the first iteration of the sequential inversion process.

Through multiple iterations of sequential inversion incorporating both gravity and seismic datasets, final velocity and density models are obtained that align well with observed travel-time and gravity data. These results demonstrate a significant improvement in the resolution of shallow structures, with residuals for both velocity and density models exhibiting stable convergence. In the shallow subsurface, low P-wave velocity (Vp) values are associated with the extensional basin of the Ilan Plain, whereas higher Vp values further south correspond to the topographic relief of the northern Central Range. Furthermore, Vp/Vs ratios are utilized to infer rock properties, providing insights into magmatic intrusions in the Ilan region and the geothermal heat source of the Chingshui area.