Seismogenesis of the 2021 Mw 7.1 earthquake sequence near the northeastern Japan revealed by double-difference seismic tomography

Recently, a Mw 7.1 earthquake struck the Pacific coast of northeastern Japan on 2021/02/13. To investigate the cause of this earthquake and its aftershocks, we applied a Vp/Vs model-consistency constrained double-difference (DD) seismic tomography method to the earthquake arrival time data from the dense seismic networks in Japan to image the velocity structures at the hypocenter areas. Compared to the conventional DD tomography, the new method can determine the Vp/Vs model with high resolution and precision. The reliability of the seismic tomography results is testified by checkerboard resolution test. The velocity profiles show that the oceanic crust and uppermost part of the oceanic mantle are characterized by low Vs and high Vp/Vs due to hydration reaction, while the lower oceanic mantle is associated with high Vs and low Vp/Vs anomalies that are likely caused by serpentine dehydration. The mainshock of the Mw 7.1 earthquake sequence is located at the boundary between the low velocity layer and underlying high velocity zone. The aftershocks are mainly distributed within the low velocity layer. The rupture plane inferred from the aftershock distribution and mainshock focal mechanism solution has penetrated ∼10 km into the slab and extended ∼50 km along a direction nearly parallel to the trench. These observations have suggested that the Mw 7.1 earthquake sequence occurs as a result of reactivation of a pre-existing fault likely created at the outer-rise before subduction. The association of the mainshock with the velocity anomaly edge suggests that this earthquake sequence is probably caused by dehydration of the mantle serpentines, which increases the pore pressure on the fault. Besides, the variation of rock mechanics at the hypocenter area of the mainshock also facilitates the nucleation of the whole earthquake sequence.