Precise relocation of low-frequency earthquakes in Northeast Japan: new insight into arc magma and fluids

High-resolution (similar to 10-20 km) P-and S-wave velocity (Vp, Vs) tomography of the crust and uppermost mantle is determined to relocate precisely a large number of low-frequency earthquakes (LFEs) which occurred in Hokkaido and Tohoku during 2002-2016. The LFEs and seismic tomography are combined to study the arc magma and fluids in the study region. We divide the 4036 LFEs in Hokkaido and 4946 LFEs in Tohoku into 43 groups. Most of the LFEs are located in or around low-Vp, low-Vs and high Poisson's ratio anomalies beneath active arc volcanoes, which indicate the existence of abundant fluids and magmatic activities in the crust and uppermost mantle beneath the volcanoes. Our results also reveal the influence of large crustal earthquakes on the spatial and temporal distributions of the LFEs. Many of the LFEs occurred at edges of the low-Vp and low-Vs zones within similar to 15 km of the active volcanoes, indicating transportation and/or cooling of the arc magmas.