Strongly Depth-Dependent Aftershock Production in Deep Earthquakes

The mechanisms that permit earthquake rupture at the high pressures occurring below 100 km depth remain enigmatic and controversial. The most distinct difference between deep and shallow earthquakes is a scarcity of aftershocks below 100 km. We report strong changes in aftershock productivity with depth. From 100 to 300 km, aftershock productivity normalized to account for the effect of main- shock magnitude is more than an order of magnitude lower than that of shallow events. Between 300 and 550 km, aftershock productivity is particularly low, falling to about one-third the level between 100 and 300 km. Below 550 km, however, aftershock productivity abruptly increases to more than twice as great as that between 100 and 300 km. A similar pattern is seen in alternate measures of aftershock productivity, including the raw numbers of aftershocks, the fraction of mainshocks that have aftershocks, and the fraction of mainshock seismic moment released in aftershocks. The depth dependence of aftershock productivity is consistent with a previously proposed change in earthquake generation mechanism near 300 km depth. Notably, the abrupt increase in aftershock productivity below 550 km corresponds with abrupt decreases in earthquake duration and complexity and sug- gests a major change in the rupture mechanism at that depth, as well. Online Material: Catalog of mainshocks and related aftershocks used in this study.