At the southernmost part of the Ryukyu subduction zone, six long-offset multi-channel seismic profiles were collected across three forearc basins and the southern Ryukyu accretionary prism during the TAIGER experiment in 2009 and the TAICRUST project in 1995. These profiles were reprocessed to generate pre-stack depth migration (PSDM) sections. In addition, two velocity-interface models were obtained by reanalyzing active source data recorded from 28 ocean-bottom seismometers during the same TAIGER experiment, in consideration of the PSDM sections and previous tomography models. Due to the northwest convergence of the Philippine Sea Plate (PSP), it is suggested that the Gagua Ridge may have been obliquely subducting northwestward beneath the Ryukyu prism and below the Nanao Basin. The PSDM sections and the velocity-interface models indicate the subducted Gagua Ridge causing the uplift of the sedimentary basement and the lower crustal structure below the Nanao Basin. The sedimentary and crustal structures near ~ 122.5° E beneath the Nanao Basin were also uplifted where shallow earthquakes had occurred by the oblique subduction of the Gagua Ridge depicted in the isopach map of the crust. Furthermore, a recent earthquake (Mw ~ 6.0) occurred near the northeast coast of Taiwan in 2018 at a depth of approximately 12 km below the Hsincheng Ridge. Our study suggests that this earthquake was caused by a thrust fault near the décollement, which might have been formed by the subduction of the PSP.

Characterizing properties of marine subsurface sediment helps with siting for offshore infrastructure. Shear‐wave velocity (Vs) provides information on the geotechnical properties of the seabed. We present our initial efforts to obtain a detailed two‐dimensional model of Vs for a large‐offset multi‐channel seismic (MCS) transect collected in shallow waters across the Taiwan Strait using surface waves excited by a large volume airgun. We derived the dispersion curves of the Scholte waves along the 37.5‐km‐long transect using the phase‐shift method and then conducted multimodal inversion to obtain a Vs model down to a depth of 150 m. To estimate the dynamic Poisson's ratio across the transect, we combined the Vs model with a compressional wave velocity model derived from the traditional MCS semblance velocity analysis. Lastly, we approximated the seismic attenuation of the profile. Our results show a large lateral variation in shear‐wave velocity. In the north, a low‐velocity zone with shear‐wave velocities of about 150 m/s was identified, while in the south, the shear‐wave velocity was found to be 300 m/s. With synthetic data, several sensitivity tests were performed to derive optimal parameters for offshore large‐offset streamer data. We particularly focused on the depth of the streamer and source and the water depth in combination with different seabed properties. Our results show that we can robustly derive the shear‐wave velocity, along with the Poisson's ratio, using large‐offset streamer data elsewhere based on the criteria we have tested using field and synthetic data sets.

At the southernmost part of the Ryukyu subduction zone, data recorded by 28 ocean-bottom seismometers with active sources during the TAiwan Integrated GEodynamics Research (TAIGER) experiment in 2009 were re-analyzed. Additionally, six long-offset multi-channel seismic profiles across three forearc basins and the Yaeyama accretionary prism north of the southernmost Ryukyu Trench, obtained by the TAIGER experiment in 2009 and the TAICRUST project in 1995, were processed to generate pre-stack depth migration (PSDM) sections. Due to the NW convergence of the Philippine Sea Plate (PSP), the Gagua Ridge may have been obliquely subducting northwestward beneath the Yaeyama accretionary prism and below the Nanao Basin. The PSDM sections and the velocity-interface models show that the Yaeyama accretionary prism near the trench, the sedimentary basement and the lower crustal structure below the Nanao Basin were uplifted by the tip of the subducted Guagua Ridge. Furthermore, the sedimentary and crustal structures near ~ 122.5°E beneath the Nanao Basin were shown a north-trending uplift where shallow earthquakes had been occurred by the oblique subduction of the Gagua Ridge depicted in the isopach map of the crust. Moreover, a recent earthquake (Mw ~ 6.0) near the NE coast of Taiwan in 2018 at a depth of approximately 12 km below the Hsincheng Ridge was caused due to a thrust fault near the décollement that might have been formed by subduction of the PSP.