A hypothesis of the obliquely subducted Gagua Ridge below Nanao Basin based on uplifted structures in Ryukyu forearc

Abstract

Abstract 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.

Keywords

• Forearc basin
• Gagua Ridge
• Long-offset multi-channel seismic
• Ocean-bottom seismometers
• Ryukyu Trench