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The acoustic basement map of the East China Sea, established by the Shanghai Offshore Petroleum Bureau with all available industry seismic data, shows the existence of a 30-km-wide, 10-km-deep basin, that we named the Ho Basin. The Ho Basin belongs to a series of elongated deep basins extending over 600 km east of the Taiwan-Sinzi Ridge and flanked to the East by a ridge named the Longwan Ridge in its northern part. This new system of basin and ridge was probably formed during middle Miocene, sometimes in between rifting episodes occurring in the Taipei Basin and Okinawa Trough. It complements the already defined system of five belts of backarc basins and associated arc volcanic ridges in the East China Sea, which are progressively younger from the Mainland China shoreline (late Cretaceous/early Tertiary) to the Okinawa Trough (Present). In order to determine the crustal thickness beneath the East China Sea continental shelf, we used a power spectrum method to calculate the depth of the top (Zt) and the centroid (Zo) of the magnetic basement by fitting a straight line through the high-and low-wave number portions of the power spectrum, respectively. Then, the depth of the base (Zb) is estimated from Zt and Zo. After optimizing the size of the data squares, we demonstrate that, except for basins more than 10 km deep, Zt corresponds to the basement depths and Zb, the depth of the Curie point, to the Moho depth. As wide-angle reflection and refraction data are scarce in the East China Sea, this method provides a way to characterize the crustal thickness of the East China Sea and to compute the theoretical heat flow values.

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[1] In July 2005, about 6 months after the 2004 great Sumatra-Andaman earthquake, 20 ocean bottom seismometers were deployed in the northern Sumatra area. More than 1000 events were identified during the 12 day recording period. After relocation, the seismicity shows different patterns on each side of the Lower Splay Fault. East of this feature and beneath the Aceh basin and fore arc, the deep earthquakes outline the subduction megathrust fault. West of it, the aftershocks distribution is strongly influenced by the active N-S oceanic fracture zones of the subducted plate. Two N-S trending clusters of 10-to 50-kmdeep earthquakes observed below the lower wedge are interpreted as reactivated oceanic fracture zones. The postseismic activity suggests a significant influence of the N-S active fracture zones of the oceanic plate on the toe of the wedge explaining the dextral wrenching of wedge thrusts in the morphology. The megathrust fault does not extend to the trench but outcrops east of these active oceanic fractures and seems to merge upward along the Lower Splay Fault where shallow earthquakes are observed.

During a passive seismic experiment in the Okinawa Trough the shots of two reflection profiles were recorded by ocean bottom seismometers (OBS). Both profiles include 3 ocean-bottom instruments, are about 65 km in length and located in the axial portion of the southwestern Okinawa Trough. Processing of the reflection seismic data images recent deformation of the sedimentary units. Forward modelling of the wide-angle data on both profiles reveals a 1-2 km thick sedimentary infill overlying an acoustic basement characterised by seismic velocities between 3.2 and 3.5 km/s. Crustal thickness could only be modelled on one profile and was determined to be around 10 km, thickening towards the Ryukyu Arc in the south. Gravity modelling was used to additionally constrain both profiles especially the deep structure of Profile 1.

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Ocean‐bottom seismometers (OBSs) commonly record short‐duration events (SDEs) that could be described by all of these characteristics: (i) duration <1 s, (ii) one single‐wave train with no identified P nor S wave arrivals, and (iii) a dominant frequency usually between 4 and 30 Hz. In many areas, SDEs have been associated with gas‐ or fluid‐related processes near cold seeps or hydrothermal vents, although fish bumps, instrumental, or current‐generated noise have been proposed as possible sources. In order to address some remaining issues, this study presents results from in situ and laboratory experiments combined with observations from two contrasting areas, the Sea of Marmara (Turkey) and the Chilean subduction zone. The in situ experiment was conducted at the European Multidisciplinary Seafloor and water column Observatory‐Molène submarine observatory (near Brest, France) and consisted in continuously monitoring two OBSs with a camera. The images revealed that no fish regularly bumped into the instruments. Laboratory experiments aimed at reproducing SDEs' waveforms by injecting air or water in a tank filled by sand and seawater and monitored with an OBS. Injecting air in the sediments produced waveforms very similar to the observed SDEs, while injecting air in the water column did not, constraining the source of SDEs in the seafloor sediments. Finally, the systematic analysis of two real data sets revealed that it is possible to discriminate gas‐related SDEs from biological or sea state‐related noise from simple source parameters, such as the temporal mode of occurrence, the back azimuth, and the dominant frequency.