[S-CG59] Oceanic plate as inputs to subduction zone: evolution process of the oceanic plate

[SCG59-06] Stress drops of intraslab earthquakes at intermediate depth beneath Tohoku, northeastern Japan

*Saeko Kita¹, Takahiro Shiina², Heidi Houston³, Katsuhiko Shiomi⁴ (1.IISEE, Building Research Institute, Japan, 2.Geological Survey of Japan, National institute of Advanced Industrial Science and Technology, 3.University of Southern California, 4.National Research Institute for Earth Science and Disaster Resilience)

Keywords:Oceanic Plate, Intraslab earthquake, Stress drop, rigidity

Stress drops for ~4500 small magnitude (2.0<M<5.0) intraslab earthquakes at intermediate depths were estimated beneath Tohoku. In the estimation of the stress drops, a S-coda-wave spectral ratio method to estimate earthquake corner frequencies was applied to waveforms of events at depths of 50 to 200 km. Detailed velocity values for the oceanic crust (OC) were adopted from previous observational studies [e.g. Shiina et al. 2013]. The median stress drops of intermedia-depth intraslab earthquakes were estimated in the range of 4.8 to 13.1 MPa for OC and 7.2 to 14.7MPa for the oceanic mantle (OM). Similar to the results in Hokkaido [Kita and Katsumata 2015], the median stress drops in the OC are about half those in the oceanic mantle (OM). This result may reflect different occurrence mechanisms and/or rigidity between OC and OM. The median stress drops in the OC decrease from depths of 70 to 120 km but switch to increase from 120 to 170 km. These results suggest that the rigidity in the OC decreases and then increases with depth due to combined effects of dehydration associated with eclogite formation and the increasing temperature with depth. These depth variations are consistent with results of the previous study beneath Hokkaido, and we also find that the median stress drops in the oceanic plate beneath Tohoku are generally smaller than those beneath Hokkaido. A previous imaging study for the seismic structure beneath off-Tohoku and off-Hokkaido near-trench regions [e.g. Fujie et al. 2018] and b-value analyses of intraslab earthquakes [Kita and Ferrand 2018] indicate that the near-trench region of the oceanic plate off-Tohoku is more hydrated than that off Hokkaido. Considering these observational results from previous studies, our stress drop results suggest that differences in the degree of hydration of the oceanic plate in the near-trench regions could produce the different behaviors of in-slab stress drops at intermediate-depth observed in Tohoku and Hokkaido. We also examine the time change of stress drops in the oceanic plate after the 2011 M9 Tohoku event, finding that the median stress drops for the OC at depths of 70 to 120 km after the event tend to be slightly larger than the stress drops prior to it. This increase is consistent with an inferred increase in along-dip compressive stress level in the intermediate-depth OC due to the M9 [Delbridge et al. 2024]. Thus, intermediate-depth intraslab earthquakes may reveal interactions between slab stresses and earthquake source properties.