Stress drops for intermediate-depth intraslab earthquakes beneath Hokkaido, northern Japan

Abstract:

Spatial variations in the stress drop for 1726 intermediate-depth intraslab earthquakes in the subducting Pacific plate beneath Hokkaido were examined, using precisely relocated hypocenters, the corner frequencies of events, and detailed determined geometry of the upper interface of the Pacific plate. The analysis results show that median stress drop for intraslab earthquakes generally increases with an increase in depth from 10 to 157 Mpa at depths of 70–300 km. Median stress drops for events in the oceanic crust decrease (9.9–6.8 MPa) at depths of 70–120 km and increase (6.8–17 MPa) at depths of 120– 170 km, whereas median stress drop for events in the oceanic mantle decrease (21.6–14.0 MPa) at depths of 70–170 km, where the geometry of the Pacific plate is well determined. The increase in stress drop with depth in the oceanic crust at depths of 120–170 km can be explained by a lithofacies change (increases in velocity and density and a decrease in the water content) due to the phase change with dehydration in the oceanic crust. At depths of 70–110 km, the decrease in the median stress drop in the oceanic crust would also be explained by that the temperature-induced rigidity decrease would be larger than that of the rigidity increase caused by lithofacies change and water content. Stress drops for events in the oceanic mantle were larger than those for events in the oceanic crust at depths of 70–120 km. Differences in both the rigidity of　the rock types and in the rupture mechanisms for events between the oceanic crust and mantle could be causes for the stress drop differences within a slab. These analysis results can help clarify the nature of intraslab earthquakes and provide information useful for the prediction of strong motion associated with earthquakes in the slab at intermediate depths.