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In order to understand gas hydrate related seafloor features in the near shore area off SW Taiwan, a deep-towed sidescan sonar and sub-bottom profiler survey was conducted in 2007. Three profiles of high-resolution sub-bottom profiler reveal the existence of five gas seeps (G96, GS1, GS2, GS3 and GS4) and one pockmark (PM) in the study area. Gas seeps and pockmark PM are shown in lines A and C, while no gas venting feature is observed along line B. This is the first time that a gas-hydrate related pockmark structure has been imaged off SW Taiwan. The relatively high backscatter intensity in our sidescan sonar images indicates the existence of authigenic carbonates or chemosynthetic communities on the seafloor. More than 2,000 seafloor photos obtained by a deep-towed camera (TowCam) system confirm the relatively high backscatter intensity of sidescan sonar images related to bacteria mats and authigenic carbonates formation at gas seep G96 and pockmark PM areas. Water column gas flares are observed in sidescan sonar images along lines A and C. Likewise, EK500 echo sounder images display the gas plumes above gas seep G96, pockmark PM and gas seep GS1; the gas plumes heights reach about 150, 100 and 20 m from seafloor, respectively. Based on multichannel seismic reflection (MCS) profiles, an anticline structure trending NNE-SSW is found beneath gas seep G96, pockmark PM and gas seep GS2. It implies that the gas venting features are related to the anticline structure. A thermal fluid may migrate from the anticline structure to the ridge crest, then rises up to the seafloor along faults or fissures. The seafloor characteristics indicate that the gas seep G96 area may be in a transitional stage from the first to second stage of a gas seep self-sealing process, while the pockmark PM area is from the second to final stage. In the pockmark PM area, gas venting is observed at eastern flank but not at the bottom while authigenic carbonates are present underneath the pockmark. It implies that the fluid migration pathways could have been clogged by carbonates at the bottom and the current pathway has shifted to the eastern flank of the pockmark during the gas seep self-sealing process. [ABSTRACT FROM AUTHOR]

The western termination of the Okinawa Trough is a tectonically complex area in the Ryukyu trench-arc-backarc system. It is a transitional zone between two tectonic regimes. In the eastern part, tectonics with E-W trending normal faulting focal mechanisms indicates an active N-S rifting of the Okinawa Trough. In the western part, tectonics with ENE-WSW left-lateral strike-slip faulting is associated with the general trend of the Taiwan orogenic belt. There is a seismicity gap in between; however, the structure of this transitional zone is not very clear. Based on high-resolution bathymetric data, we find a NE-SW trending structure located between those two tectonic regimes or major seismic zones. This structure could reveal key tectonics of the transitional zone. In order to understand the characteristic of the transitional zone, we have collected several high-resolution seismic reflection profiles, roughly perpendicular to the linear structure offshore Ilan Plain. This structure reveals the tectonic transition from normal faulting in the east to strike-slip faulting in the west. [ABSTRACT FROM AUTHOR]

Abstract: In April 1771, a subduction earthquake generated a great tsunami that struck the south Ryukyu islands and killed ∼12,000 people, whereas its mechanism is still enigmatic (Nakata and Kawana, 1995; Nakamura, 2006; Matsumoto et al., 2009). In this paper, we show its probable source on a mega-splay fault system existing along the southern Ryukyu forearc. Analyses of deep multi-channel seismic reflection profiles indicate that the mega-splay fault system is rising from the summit of a ∼1km high ridge situated at a ∼5° landward dipping plate interface. An outer ridge marks the seafloor outcrop of the splay fault system and separates the landward inner wedge and the oceanward outer wedge. The inner wedge is uplifting and exhibits widespread normal faulting while the outer wedge shows folded structures. The mega-splay fault system is parallel to the Ryukyu Trench east of 125.5°E and is estimated to be ∼450km long. The origin of this south Ryukyu mega-splay fault system is ascribed to a resistant subduction of the elevated transverse ridges associated with the subducting portion of the trench-parallel Luzon–Okinawa Fracture Zone. In contrast, no similar splay fault is found west of 125.5°E where the oblique subduction has produced large shear zones along the south Ryukyu forearc. We infer that a thrust earthquake linked to the mega-splay fault system is responsible for the south Ryukyu tsunami. However, another possible scenario of generating a large tsunami affecting the south Ryukyu islands is that the subducted ridge in the western end of the mega-splay fault system nucleated a large earthquake and simultaneously triggered the ∼100km long E–W trending strike-slip fault west of 125.5°E and induced a southward-dipping tsunami-genic subsidence. In any case, after a quiescence of ∼241yr, a large earthquake and tsunami is anticipated in the south Ryukyu forearc in the near future. [Copyright &y& Elsevier]