Complex Dynamic Rupture of an Mw5.8 Intermediate-Depth Earthquake in the Hellenic Slab

Abstract

Earthquakes in the Hellenic slab, traced by tomography down to 1,200 km, occur at depths <200 km with enigmatic origin and dynamics. We study a 2014 Mw5.8 left-lateral strike-slip earthquake originating at a depth of similar to 90 km under arc-parallel compression, featuring a small implosive component of the moment tensor. We use well-recorded aftershock as an empirical Green's function to infer reliable apparent source time functions up to 2 Hz. They exhibit multiple peaks with pronounced directivity and serve as input data to infer kinematic and dynamic rupture evolution in Bayesian source inversion. Our finite-fault kinematic and dynamic source models consistently reveal episodic behavior with two prominent asperities, short rupture duration, unilateral propagation, and locally high rupture speed and stress drop. We speculate the event was triggered and driven by dehydration and stress transfer associated with shear deformation and rock compaction. Plain Language Summary Intermediate-depth earthquakes (60-300 km) occur in subduction zones and their origin is often attributed to dehydration processes as the subducted lithospheric plate drags hydrated rocks from the Earth's surface downwards. Several competing hypotheses related to mineral dehydration or thermal processes explain the earthquakes at these depths yet need to be verified through earthquake parameters such as stress drop, rupture speed, or energy budget. However, traditional seismological analyses usually extract only average values, leaving the possible rupture complexity widely unexplored. In this study, we derive a rupture model of a 90 km deep Mw5.8 earthquake in the western part of the Hellenic subduction zone. The model is constrained by two independent methods using seismic recordings of the mainshock and an aftershock substituting the wave propagation between the source and stations. We find that the rupture started in a highly prestressed concentrated patch with fast velocity. Then it slowed down, but in the final stage, it broke another patch with a stress drop as high as 60 MPa, a value seldom found for crustal events. Deciphering the individual rupture episodes helps to distinguish the generating mechanism, indicating that the earthquake was triggered by dehydration processes related to rock compaction and stress transfer.