The 2008 Iwate-Miyagi earthquake (M7.2) and arc volcanism: Insight from irregular-grid tomography

To better understand the seismotectonics and arc magmatism in the Northeast Japan subduction zone, we studied high-resolution 3-D P-and S-wave velocity structures of the crust and upper mantle under this region using a large number of high-quality arrival-time data from the local shallow and intermediate-depth earthquakes recorded by the high-sensitivity seismic network installed on the Japan Islands. We adopt an irregular-grid parameterization to conduct the tomographic inversion in order to extract more useful information from the data used. Our results show that the mainshock of the 2008 Iwate-Miyagi earthquake (M7.2) was located in a boundary zone where seismic velocity changed dramatically, indicating that the crustal medium in the source area is very heterogeneous. A pronounced low-velocity anomaly is revealed in the lower crust and uppermost mantle under the mainshock hypocenter, which may reflect the arc magma and fluids ascending from the upper-mantle wedge released by the dehydration process of the subducting Pacific slab. The fluids and arc magma may have influenced the nucleation and rupture process of the large crustal earthquakes.