At subduction zones, earthquakes occur from the surface to nearly 700 km depth. At shallow depths (0-60 km), earthquakes occur where brittle rock is strained over time, causing a sudden release of energy that travels away from the source. Deeper than 60 km depth, earthquakes occur where temperature and pressure conditions should prevent brittle fracture, suggesting a different rupture mechanism is responsible for deep earthquakes. Deep earthquakes have been observed to have fewer aftershocks and a larger magnitude differential ΔM between the mainshock and the largest aftershock than shallow earthquakes. To determine whether the observed differences are real or due to observational constraints, we analyze 14 intermediate-depth earthquakes (70-305 km depth) with local magnitude MJ >= 5.7 from the Japan Meteorological Agency (JMA) earthquake catalog. For 7 of these mainshocks, we observe aftershock sequences similar to those for shallow earthquakes: the number of earthquakes increases immediately following the mainshock and decreases over time with a decay rate exponent of ~1. In contrast to shallow aftershock sequences, which have ΔM of ~1, intermediate-depth earthquake aftershock sequences typically have ΔM of 2-3.5. The larger ΔM for intermediate-depth earthquakes would prevent us from observing most aftershocks for the remaining 7 earthquakes and suggests that previous observations have been limited by observational constraints. A larger ∆M also suggests that more stored energy is released in the mainshock with less available for aftershock production. The relatively high mainshock energy release could be caused by slip on a relatively homogeneous fault with few barriers to prevent slip or on a melt-lubricated fault.

Cara M. Baez would like to thank their faculty sponsor Dr. Linda M. Warren for their support of this project.