Fault morphology controls rupture size: Evidence from the 2015 Gorkha earthquake sequence in Nepal

Abstract:

The rupture characteristics of subduction and collision zone earthquakes are known to be controlled by the chemical and thermal boundaries of the plate interface, and structural features such as subducting seamount and fracture zones. In this picture, the role of fault morphology in controlling rupture dimensions remains poorly understood. Here, we present a striking example of how morphological structure regulated the blind 2015 Mw 7.8 Gorkha earthquake sequence in Nepal.

To unravel where exactly the slip occurred, we reconstructed the plate interface of the Main Himalayan Thrust (MHT). The MHT is composed of the steep Main Front Thrust (MFT), followed by two shallow decollements separated by a ramp, and ends with another steep fault reaching ~30 km depth.

Based on our newly reconstructed fault geometry, we invert the slip of the earthquake sequence through a combination of multi-track InSAR and GPS data. We find that the slip was distributed on the second shallow decollement, had an elongated shape, and was clearly confined by the ramp. Our along-strike slip extent matches that of the Kathmandu klippe, indicating that fault morphology segments the slip along both strike and dip. This confinement of the rupture area limited the size of the earthquake. Our results indicate that fault morphology can control the size of megathrust ruptures, and thus can provide important information for seismic hazard analyses.