Comparison of the Intraslab Stress Field Before and After the 2011 Tohoku Earthquake

Abstract:

Subducting slabs are subject to static stresses from bending/unbending and slab pull. At strongly coupled margins, a locked interface will result in additional tensional stress downdip in the slab. When an underthrusting earthquake ruptures along the plate boundary, it may temporarily reduce the magnitude of the downdip tensional stresses. Stress field variations may be manifest as changes in the seismicity rate and/or the orientation of the principal stress axes. One of the largest recent underthrusting earthquakes was the 11 March 2011 M_W 9 Tohoku earthquake at the highly-coupled Japan Trench. Japan has high-resolution seismic networks and earthquakes catalogs, and we use NIED earthquake hypocenter and focal mechanism catalog of earthquakes >Mw 3.5 from 1997-2015 to investigate possible temporal and spatial variations in the downdip intraslab stress field related to the Tohoku earthquake. To ensure that we only include intraslab earthquakes, we analyze earthquakes >60 km depth. From 60-200 km depth, the subducting slab has a double seismic zone (DSZ) with predominantly down-dip compressional earthquakes near the slab surface and predominantly down-dip extensional earthquakes 20-30 km deeper. We divide the subducting slab into three spatial bins covering the main rupture area (37.5°－39.5°), the region to the north of the rupture area (39.5°－41.5°), and the region to the south of the rupture area (35.5°－37.5°). For each bin, after separating earthquakes in the two planes of the DSZ, we compute the seismicity rate and invert the focal mechanisms for the principal stress axes. To investigate temporal variations, we separate earthquakes before and after the Tohoku earthquake and use a damped inversion to minimize model complexity and only keep changes required by the data. Following the Tohoku earthquake, for the bin including the rupture area, we observe a small increase in seismicity <70 km depth in the upper plane of the DSZ.