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Ecology and Conservation

Research Report

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Abstract
Dongsha atoll is situated at the continental shelf area of the northern margin of the South China Sea (SCS) which is composed of coral reef with about 25km in diameter. The evolution of Dongsha atoll and its tectonics is still poorly understood mainly due to limited observations for deeper structure beneath it. Recent studies have shown the existence of a high-velocity lower crust which may associate with volcanic-intrusion activity at Dongsha region. In this study, we apply the marine geophysical experiments to investigate the shallowest structure and deeper velocity structure beneath the Dongsha atoll. There are two main objectives in this study. One is using 4 ocean-bottom seismographs (OBS) to collect a 120-180 days-long ambient noise data and teleseismic data. The 4 OBS plus one BATS-station on Dongsha island formed a seismic array with the aperture around 16km. We use the continuous recording of vertical components to investigate the velocity structure down to 10km deep. The results show fundamental mode Rayleigh waves with the dominant period of 0.8-5 sec and relatively slow speed of 1-2 km/s. With these robust dispersion measurements, we inverted for the 1-D shear-wave velocity structure beneath the Dongsha lagoon. The result shows the low-velocity layer exists within 1km deep which might imply the thickness of the sediment layer. Due to lacking high signal-to-noise teleseismic data recorded at OBSs, we analyzed 18-months seismic data from Dongsha Island station to estimate the depth of crustal seismic discontinuity beneath the Dongsha Island. Our results from the receiver functions analysis show the depth of the crustal velocity discontinuity beneath Dongsha Island is around 9km which might reveal the top of the high velocity lower crust. The other objective is using sub-bottom profiling (SBP) system to recognize the shallow seabed feature. We compare the acoustic signals from the seabed and high-resolution images taken by the underwater camera to characterize the feature on the seabed. Three types of the acoustic signals from SBP are recognized for the different seabed feature.

Key words: Dongsha atoll, velocity structure, ocean bottom seismograph, sub-bottom profiler

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