Rheological contrast traps and controls MW 6.8 Michilla (northern Chile) intraslab aftershocks

Abstract

The factors controlling the extent and distribution of aftershocks in intermediate-depth earthquakes remain poorly understood, largely due to the scarcity of dense seismological networks and high-resolution seismic catalogs. Between 2013 and 2015 CE, a temporary dense seismic network installed in northern Chile allowed the detailed monitoring of a peculiar, long-lasting aftershock sequence confined within the rupture zone of the 16 December 2007 MW 6.8 Michilla intermediate-depth earthquake, located within the subducting oceanic plate. This unique dataset allowed us to compile a high-resolution seismic catalog and to analyze the possible factors that influence this type of rupture. Our results show that seismicity was trapped between the upper and lower oceanic crust, correlating strongly with expected dehydration reactions in basaltic and gabbroic rheologies. In addition, aftershocks were distributed across a complex multi-fault system, which can remain active for an extended period through the same dehydration process. These observations provide new constraints on the mechanics of intermediate-depth rupture and highlight the role of dehydration-driven processes in controlling their seismic behavior, with implications for seismic hazard in subduction zones.