On 25 April 2015, a large earthquake of Mw 7.8 occurred along the Main Himalayan Thrust fault in central Nepal. It was caused by a collision of the Indian Plate beneath the Eurasian Plate. The epicenter was near the Gorkha region, 80 km northwest of Kathmandu, and the rupture propagated toward east from the epicentral region passing through the sediment-filled Kathmandu Valley. This event resulted in over 8000 fatalities, mostly in Kathmandu and the adjacent districts. We succeeded in observing strong ground motions at our four observation sites (one rock site and three sedimentary sites) in the Kathmandu Valley during this devastating earthquake. While the observed peak ground acceleration values were smaller than the predicted ones that were derived from the use of a ground motion prediction equation, the observed peak ground velocity values were slightly larger than the predicted ones. The ground velocities observed at the rock site (KTP) showed a simple velocity pulse, resulting in monotonic-step displacements associated with the permanent tectonic offset. The vertical ground velocities observed at the sedimentary sites had the same pulse motions that were observed at the rock site. In contrast, the horizontal ground velocities as well as accelerations observed at three sedimentary sites showed long duration with conspicuous long-period oscillations, due to the valley response. The horizontal valley response was characterized by large amplification (about 10) and prolonged oscillations. However, the predominant period and envelope shape of their oscillations differed from site to site, indicating a complicated basin structure. Finally, on the basis of the velocity response spectra, we show that the horizontal long-period oscillations on the sedimentary sites had enough destructive power to damage high-rise buildings with natural periods of 3 to 5 s.

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The geometry of the slab as well as the stress inside the slab near the junction of the Kurile and the northern Honshu arcs, that is, the Hokkaido corner, are studied based on the seismicity and the focal mechanism solutions of mantle earthquakes. Focal mechanism solutions vary depending on the focal depth and the local strike of the slab. The distribution of mantle earthquakes and the variation of the mechanism solutions reveal slab contortion on the Kurile side of the Hokkaido corner. The relative motion on the fault plane is estimated based on the analysis of source processes of mantle earthquakes. The sense of the earthquake slip motion is consistent with the sense of the slab contortion. This may indicate that the slab contortion, suggesting a plastic deformation, results partly from the earthquake slip motion.The Kurile and the northern Honshu arcs are accompanied by an inclined seismic zone, the structure of which is simple and which forms a descending lithosphere, that is, a slab. The dip angle of the slab in each of these arcs is, however, different. The intersection of the Kurile and the Japan trenches which meet at an oblique angle is concave toward the oceanic side. ISACKS and MOLNAR (1971) have discussed how in such a situation the slab was either contorted or disrupted. Contortion and disruption of the slab near the junction of the Kurile and the northern Honshu arcs, that is, the Hokkaido corner have been investigated on the basis of distribution and focal mechanism solutions of mantle earthquakes. They have suggested that these mechanism solutions seem to be related to stresses inside one of the segments rather than to stresses attributable to the presumed contortion or disruption. Most recently, STAUDER and MUALCHIN (1976) have studied focal mechanism solutions of mantle earthquakes near the same junction and have suggested the effects of the slab contortion upon mechanism solutions. Although reliable solutions were determined from long-period data in these two studies, only 341