| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 106 === The southernmost Ryukyu subduction zone, where the Philippine Sea Plate (PSP) subdutcs beneath the Eurasia Plate (EP), is both complex and active, and induces high seismicity offshore eastern Taiwan. Large negative gravity anomaly has been reported in this section of the Ryukyu forearc region, intense crustal deformation was also observed in this forearc region. It is an important issue that how the crustal structures beneath Ryukyu forearc region are deformed due to the subduction-collision process. In this study, we present a re-processed multichannel seismic (MCS) reflection profile, its P-wave velocity-depth structure model and a density model along a large-offset (using a 6-km long streamer) seismic profile MGL0906-22N which runs in NW-SE direction in the forearc region of the southernmost Ryukyu subduction zone. In the western part of the MCS profile, two sedimentary sequences in the Hoping Basin which have been mentioned in previous studies are clearly observed. In the crustal velocity model, the layer boundaries in the upper crust may be defined by two velocity contours of 4.5 km/s and 6.0 km/s, represent the top of the basement and lower crust, respectively. The depth of a deep reflection interface is lying between 25 km to 30 km in our velocity model, we suggest this reflection interface could be the moho of the down-going Philippine Sea Plate. The thicker upper crust is observed in the southeast part of our profile. We suggest that the high subduction obliquity may cause the thicker upper crust. In gravity modeling, the model of gravity modeling profile shows similar variation as the observed gravity anomaly profile. Beneath the accretionary prism, there are some high density material deeper than 25 km.
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