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Several slope failures are observed near the deformation front on the frontal ridges of the northern Cascadia accretionary margin off Vancouver Island. The cause for these events is not clear, although several lines of evidence indicate a possible connection between the occurrence of gas hydrate and submarine landslide features. The presence of gas hydrate is indicated by a prominent bottom-simulating reflector (BSR), at a depth of ∼265–275 m beneath the seafloor (mbsf), as interpreted from vertical-incidence and wide-angle seismic data beneath the ridge crests of the frontal ridges. For one slide, informally called Slipstream Slide, the velocity structure inferred from tomography analyses shows anomalous high velocities values of about 2.0 km s−1 at shallow depths of 100 mbsf. The estimated depth of the glide plane (100 ± 10 m) closely matches the depth of these shallow high velocities. In contrast, at a frontal ridge slide just to the northwest (informally called Orca Slide), the glide plane occurs at the same depth as the current BSR. Our new results indicate that the glide plane of the Slipstream slope failure is associated with the contrast between sediments strengthened by gas hydrate and overlying sediments where little or no hydrate is present. In contrast, the glide plane of Orca Slide is between sediment strengthened by hydrate underlain by sediments beneath the gas hydrate stability zone, possibly containing free gas. Additionally, a set of margin perpendicular normal faults are imaged from seafloor down to BSR depth at both frontal ridges. As inferred from the multibeam bathymetry, the estimated volume of the material lost during the slope failure at Slipstream Slide is about 0.33 km3, and ∼0.24 km3 of this volume is present as debris material on the ocean basin floor. The 20 per cent difference is likely due to more widely distributed fine sediments not easily detectable as bathymetric anomalies. These volume estimates on the Cascadia margin are approaching the mass failure volume for other slides that have generated large tsunamis—for example 1–3 km3 for a 1998 Papua New Guinea slide. [ABSTRACT FROM PUBLISHER]

To improve the resolution in imaging the 3-D VP and VP/ VS structures in the Taiwan region, especially offshore eastern Taiwan, we combine the arrival times from eleven ocean bottom seismometers (OBSs) and from seismic stations of the Japan Meteorological Agency (JMA) with those from the permanent stations of Taiwan Central Weather Bureau Seismic Network (CWBSN) and Taiwan Strong Motion Instrumentation Program (TSMIP). By doing so, we have obtained a new 3-D tomographic model for Taiwan and its surrounding regions with a better resolution, especially in the area offshore northeastern Taiwan. We also used this new tomography model to relocate the hypocentres of the earthquakes in northeastern Taiwan and determined the focal mechanisms of relatively large events. Our tomography results indicate that in the region northeast of Taiwan, the subducting oceanic Philippine Sea Plate beneath the Eurasian continental lithosphere is characterized by a high VP layer, surrounded by lower VP areas. This P-wave velocity characteristics of the subducting slab provides a better constraint on the geometry of the subduction interface, especially in its shallower portion. In the hangingwall (Eurasian) block above the subduction interface, a vertically elongated high VP/ VS body appears to originate from the interface at depths between 100 and 140 km. We suggest that it represents the partially melted materials that are related to the magmatic activity of the Ryukyu volcanic arc. [ABSTRACT FROM AUTHOR]