Three-dimensional P- and S-wave velocity structures beneath Japan
ABSTRACT We determined the three-dimensional Vp and Vs structures beneath Japan by applying seismic tomography to a large number of arrival times recorded at temporary stations in the Japan Sea and the Pacific Ocean, as well as those at permanent stations on the Japan Islands. As a result, we obtained more precise seismic images than previous studies. In the crust and the uppermost mantle, southwestern Honshu exhibited weaker heterogeneity than the other areas in Japan, corresponding to the distribution of active volcanoes. Stripe-like heterogeneities exist in the subducting Pacific slab. Relatively low-velocity zones correspond to low-seismicity areas in the Pacific slab, suggesting that the slab is possibly torn or thin around the areas. The fact that nonvolcanic deep tremors associated with the subducting Philippine Sea slab beneath Shikoku, Kii, and Tokai do not occur in zones of high Vp, high Vs, and low Vp/Vs ratio may reflect the existence of fluids generated by the dehydration processes of the slab. Prominent and wide low Vp and Vs zones exist beneath central Honshu at the depth range of 30–60 km, where the volcanic front related to the subducting Pacific plate is located and seismicity around the Philippine Sea plate is very low. This condition may exist because magma genesis processes related to the subducting Pacific plate activate the same processes around the Philippine Sea plate.
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ABSTRACT: Large crustal earthquakes (magnitude 5.7-8.0, depth 0-20 km) during a period of 115 years from 1885 to 1999 in Japan occurred in or around zones of low seismic velocity revealed by seismic tomography. The low-velocity zones may represent weak sections of the seismogenic crust. The crustal weakening is considered to be closely related to the subduction of the oceanic Pacific and Philippine Sea plates in this region. Along the volcanic front and in back-arc areas the crustal weakening may be caused by active volcanoes and magma chambers. In the forearc areas, fluids are detected in the earthquake source zones, which may have contributed to the crustal weakening and the rupture nucleation. The fluids may be related to the dehydration of the subducting Philippine Sea slab. These results indicate that large crustal earthquakes do not strike anywhere, but only anomalous areas that may be detected with geophysical methods.Journal of Geophysical Research 01/2000; 105:13579-13594. · 3.17 Impact Factor
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ABSTRACT: I invert GPS displacement rates in the Tokai district to investigate strain accumulation associated with the anticipated Tokai earthquake. I apply a representation of interseismic strain accumulation at subduction zones by Savage  and estimate a distribution of virtual normal fault slip (back-slip) on the plate boundary surface. Estimated back-slip takes its maximum value of 30-40mm/year under the Sea of Enshu. This value is nearly equal to the relative plate motion there, implying almost complete coupling. Back-slip is smaller under Suruga Bay, and its direction is different from the relative motion between the Eurasian and the Philippine Sea plates. Localized deformation at the northern tip of the Philippine Sea plate may be responsible for this discrepancy, partitioning tectonic strain due to the plate convergence into faults on both sides of the Izu peninsula and also distributed strain inside the peninsula.Geophysical Research Letters - GEOPHYS RES LETT. 01/1999; 26(15):2315-2318.
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ABSTRACT: In 1986, observations of microearthquakes were conducted with the use of five ocean-bottom seismometers (OBSs) on the continental slope east of the Kushiro Submarine Canyon (KSC) off Hokkaido Island, Japan. The observation area is located about 50-100 km landward of the southern Kuril Trench beneath which the Pacific Plate is subducted toward Hokkaido Island. The OBS data were analysed together with land data on Hokkaido Island to determine the precise earthquake distribution and the geometry of the subducted plate.The OBS data revealed for the first time the seaward seismicity east of KSC. High and shallow seismicity is concentrated in an area between the coast line of Hokkaido Island and the 2000 m isobath. A region between the 2000 and 4000 m isobaths corresponds to the source area of an interplate earthquake occurring in 1973 (the Nemuro Hanto-Oki earthquake, M= 7.4). The OBS array detected more than 10 events in this region. Most of them, however, are classified as earthquakes occurring within the subducted plate, and the remarkable aftershock activity of the 1973 event is not recognized. The seismic activity is extremely low between the 4000 and 6000 m isobaths as is commonly seen in subduction zones. The seismicity is also very low beneath the trench, in contrast with the adjacent trench areas.In a vertical section taken perpendicular to the trench axis, most of the located events form a seismic plane dipping landward. The dip-angle of the plane is 20d̀-25d̀ beneath the continental slope, while 35d̀-40d̀ under Hokkaido Island. The change in dip angle occurs 150 km landward of the trench axis. This seismic plane is located 30-40 km below the plate boundary defined from the aftershock distribution of the Nemuro Hanto-Oki event. The seismicity on the plate boundary is correlated with the geometry of the subducted plate. The high activity occurs in a restricted region where the subducted plate shows a remarkable bend.The seismic activity obtained is different from those in the adjacent regions along the southern Kuril Trench. In particular, the regional difference in seismicity is remarkable between the western and eastern sides of KSC. Our result suggests that the subduction zone along the Kuril Trench is divided into several tectonic blocks and KSC is one of their boundaries.Geophysical Journal International 04/2007; 105(3):693 - 711. · 2.85 Impact Factor