High resolution tomographic images of the crust and upper mantle in and around the area of the 2011 Iwaki earthquake (M 7.0) and the Fukushima nuclear power plant are determined by inverting a large number of high-quality arrival times with both the finite-frequency and ray tomography methods. The Iwaki earthquake and its aftershocks mainly occurred in a boundary zone with strong variations in seismic velocity and Poisson's ratio. Prominent low-velocity and high Poisson's ratio zones are revealed under the Iwaki source area and the Fukushima nuclear power plant, which may reflect fluids released from the dehydration of the subducting Pacific slab under Northeast Japan. The 2011 Tohoku-oki earthquake (Mw 9.0) caused static stress transfer in the overriding Okhotsk plate, resulting in the seismicity in the Iwaki source area that significantly increased immediately following the Tohoku-oki mainshock. Our results suggest that the Iwaki earthquake was triggered by the ascending fluids from the Pacific slab dehydration and the stress variation induced by the Tohoku-oki mainshock. The similar structures under the Iwaki source area and the Fukushima nuclear power plant suggest that the security of the nuclear power plant site should be strengthened to withstand potential large earthquakes in the future
In this study we used a large number of P‐wave arrival times recorded by the Sichuan permanent and portable seismic stations during January 2001 and June 2008 to obtain a fine three‐dimensional P‐wave tomographic model of the crust around the Longmenshan fault zone. The results show that there exist obvious differences between south and north of the Wenchuan mainshock in the seismic structure. North of the Wenchuan mainshock there exist much more heterogeneities, where many aftershocks occurred. These results have significantly improved over the previous results suggesting that the Longmenshan fault zone is only a transition zone between different blocks. The source area of the Wenchuan mainshock is underlain by obvious low velocity anomalies, suggesting that there exist fluids within the Longmenshan fault zone. These fluids may affect the occurrence of the large Wenchuan earthquake. The present results provide significantly seismological evidence for the upward intrusion of the lower crustal flow along the Longmenshan fault zone.
We collected all the available arrival time data recorded by the temporary seismic stations from the projects of PASSCAL, INDEPTH-II, INDEPTH-III, HIMNT, WKL and TIBET 973, and phase reports from the International Seismological Center from January 1990 to February 2004 in Tibet and its surrounding areas. In the tomographic inversion, we used 139021 P-wave arrival times from 9649 teleseismic events recorded by 305 seismic stations. Our study area covers the southern Ganges plain in India to the northern Tarim basin and Qaidam basin in China. Our tomographic images show that the subducting angles of Indian lithospheric mantle are different under different areas, but their front locations are all beneath the Qiangtang terrane. The tomographic image along 88• E shows that the Indian lithospheric mantle is underthrusting northward with a dip angle of about 22• beneath the center of Qiangtang terrane at about 34• N latitude, and its frontier has reached to the deep part of the upper mantle. The tomographic image along a northeasterly profile shows that the Indian mantle underthrust nearly horizontally under Tibet from the Ganges plain to 33• N. Then, the Indian mantle broke off down to the asthenosphere and caused the asthenophere upwelling. A consequence of the asthenophere upwelling was to form a big low-velocity zone beneath the Qiangtang terrane.
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