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The southern Okinawa Trough has been widely considered an important area with hydrothermal activity. Several active hydrothermal fields have been reported, especially around the Yonaguni Knoll IV. In this study, we collected marine sediment cores around the Yonaguni Knoll IV by using the R/V Ocean Research 1. Core sites with and without gas disturbance were selected based on the single-beam bathymetry (Chirp) by using the onboard echo sounder system. For the sites away from gas disturbance, which are generally considered showing the background situation in the southern Okinawa Trough, variations of the magnetic susceptibility in these cores are relatively stable with values about between 20 × 10-5 and 60 × 10-5 SI. As for the sites with clear gas-features detected by the Chirp sonar, the signature with dramatically changing magnetic susceptibility is observed in the cores. In general, gasfeatures are considered linking to the hydrothermal activity in the southern Okinawa Trough. Magnetic signature with such abruptly changing magnetic susceptibility could also be considered being caused by authigenic iron sulfides associated with the hydrothermal alternation. Therefore, the rock magnetic anomaly could be suggested as an indicator revealing the hydrothermal activity in the southern Okinawa Trough.

The Okinawa Trough (OT) is widely recognized as an area with extensive volcanism and hydrothermal activity. However, finding new hydrothermal vent sites in this region remains a technical challenge due to their small sizes. A deep-towed magnetic survey can acquire short-wavelength signals, which can provide useful information on the shallow subsurface structures of hydrothermal systems. In 2017, a deep-towed magnetic survey was conducted in selected area of the southern OT. After data processing and removing the regional magnetic field, crustal magnetization map was obtained by assuming a magnetized layer with a constant thickness of 500 m whose top surface is the seafloor. In addition, onboard echo sounder signals (EK-60) suggest the presence of gas plumes emanating from the seafloor within the water column. Some of these emanating gas plumes are concentrated in several small areas that correlate well with magnetization lows in the study region. The deep-towed magnetic survey results and other geochemical/geophysical observation results suggest that the Geolin Mounds (GLM) site and site Penglai Fault Zone (PFZ) are the highly potential hydrothermal areas in the southern OT.

The westernmost Okinawa Trough back-arc basin is located to the north of the Ryukyu islands and is situated above the northward dipping Ryukyu subducted slab. In the northern continental margin of the Okinawa Trough, the continental slope between the Keelung Valley and the Mein-Hua Submarine Canyon shows a steep angle and future slope failures are expected. The question is how slope failures will proceed? A sudden deep-seated slump or landslide would probably cause local tsunami and hit northern coast of Taiwan. To understand the probable submarine landslides, we conducted multi-channel seismic reflection, sub-bottom profilers, and multi-beam bathymetry surveys off NE Taiwan. Two general trends of shallow crustal faults are observed. The NE-SW trending faults generally follow the main structural trend of the Taiwan mountain belt. These faults are products of inversion tectonics of reverse faults from the former collisional thrust faults to post-collisional normal faults. Another trend of roughly E-W faults is consistent with the current N-S extension of the southern Okinawa Trough. The fault offsets in the eastern portion of the study area are more pronounced. No obvious basal surface of sliding is found beneath the continental margin. We conclude that the movement of the submarine landslides in the Keelung Shelf off northeastern Taiwan could be in a spread type. The submarine landslides mainly occur in the continental slope area and it is more obvious in the east than in the west of the Keelung Shelf.

Methane emissions out of the seabed could seriously affect Earthâs climate and are usually associated with the dissociation of gas hydrates stored in marine sediments on the continental margins. Spatially, gas emissions out of the seafloor are not evenly distributed in continental margins. Gas emissions out of the seabed generally occur through submarine mud volcanoes and gas seeps. To understand the seabed gas emissions off SW Taiwan, we investigate the distributions of active submarine mud volcanoes, gas seeps, and gas plumes off SW Taiwan. We examine all of the available sub-bottom profiler and EK echo sounder data. We identified 19 submarine mud volcanoes, 220 gas seeps, and 295 gas plumes. The gas emissions are generally distributed at the crests of mud diapiric ridges. Most of the active mud volcanoes and gas seeps cluster at the KASMVG (Kaoping submarine mud volcanoes group) area. We speculate that the intensive mud volcanism and gas seepage at the KASMVG area are ascribed to submarine channel erosion along the continental slope base. The erosion causes a deep V-shaped channel and a steep BSR (Bottom-Simulating Reflector) slope curve across the continental margin. The upward migration rate of free gas beneath the BSR is thus increased and intensifies mud volcanism and gas seepage at the KASMVG area. The gas seeps can reduce the slope stability and generate small-scale slides. The development of mud volcanoes in an area could effectively disturb the seabed morphology so that large-scale submarine landslides cannot easily happen.

In the fall of 2004, four profiles of a total length of about 80 km and 24 recovered Ocean Bottom Seismometers (OBSs) were acquired along offshore southwestern Taiwan. These profiles were oriented in E-W and N-S directions to map the velocity structure of sediments by modeling the vertical component of OBS data. The thickness of sedimentary layers ranges from a few hundred meters to about 2 km, with the largest thickness being found beneath ridges. The most striking feature in the velocity models along E-W profiles is the relatively high velocity wedges, extending from the seafloor to about 2 km in depth; these could be interpreted as a series of westward up thrust faults. The feature of relatively high P-wave velocity could be considered a result of local shall owing of the base of the gas hydrate stability zone, caused by ascending flows of deep fluids passing through the fault zone. In the N-S profiles, a dome-like high velocity structure was observed and could be associated with the phenomenon of hydrate/gas phase boundary fluctuation. After erosion, the sub-seafloor temperature gradient redistributes and results in the hydrate/gas phase boundary moving downward. The results obtained in this study show that tectonic control is an important factor in determining bottom-simulating reflector (BSR) distributions offshore of southwestern Taiwan.