Tomography of the 2011 Iwaki earthquake (M 7.0) and Fukushima nuclear power plant area

P Tong,D Yang,D Zhao

doi:10.5194/se-3-43-2012

Abstract

Abstract. 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.

Highlights

The subduction of the Pacific plate beneath the Okhotsk plate causes intense seismicity in the Northeastern (NE) Japan arc
About 11-km long coseismic surface ruptures were recognized along the Idosawa fault, which are interpreted as a surface manifestation of the fault reactivation associated with the normal faulting event
This normal-faulting earthquake is in contrast to the compressional stress regime in NE Japan and may reflect enhanced extensional stress on the overriding block induced by the Tohoku-oki mainshock (Ishiyama et al, 2011)

Summary

Introduction

The subduction of the Pacific plate beneath the Okhotsk plate causes intense seismicity in the Northeastern (NE) Japan arc. The great Tohoku-oki earthquake (Mw 9.0) occurred on 11 March 2011 in the NE Japan forearc region and it was the largest recorded earthquake ever to hit Japan (Fig. 1a) It has caused large variations in stress field near the source zone and in regions far away from the epicenter, and so the seismic activity in the crust of the overriding plate west of the source area has increased significantly after the Tohokuoki mainshock that ruptured the megathrust zone beneath the Pacific Ocean (Okada et al, 2011). About 11-km long coseismic surface ruptures were recognized along the Idosawa fault, which are interpreted as a surface manifestation of the fault reactivation associated with the normal faulting event (http://en.wikipedia.org/wiki/April 2011 Fukushima earthquake) This normal-faulting earthquake is in contrast to the compressional stress regime in NE Japan and may reflect enhanced extensional stress on the overriding block induced by the Tohoku-oki mainshock (Ishiyama et al, 2011). The results will provide important information on the FNPP site security, which will prove useful for reviewing seismic safety of the existing nuclear plants and other nuclear facilities on the Japan Islands

Data and method

Resolution and tomographic results

Findings

Discussion and conclusions