The 2020 Mw 6.6 Vernadsky transform earthquake sequence: rupture and Coulomb stress changes surrounding an oceanic core complex

Oceanic core complexes (OCCs) are upwards-convex areas of outcropping lower crustal or upper mantle rocks raised to the seabed by normal faults, commonly associated with weak lithologies such as serpentinites. They are common adjacent to transform valleys of slow-spreading ridges. In this paper, we analyse the September 2020, Mw 6.6 strike-slip earthquake and its two-week long aftershock sequence within the Vernadsky transform valley, using recordings from regional seismic stations. The aftershocks occurred in two phases. During the first four days, ~131 events occurred east and northeast of the mainshock, overlapping an adjacent OCC complex along the northern flank of the transform valley. During the following nine days, 20 aftershocks occurred, including a Mw 5.8 event close to the OCC. To refine the locations of aftershocks, the spatial spread of the events (clustering) was refined by relatively relocating 114 events. Modeling of static Coulomb stresses was carried out, based on a mainshock rupture length compatible with the 26-30 km horizontal extent of aftershocks. This revealed that the aftershocks occurred mainly in areas where static Coulomb stresses decreased, not increased. Other researchers have suggested that changes in fault strength can arise from dynamic stresses during major earthquakes, leading to seismicity in areas of decreased Coulomb stress. We explore this idea in the context of OCCs.