iPlex Lunch spring-2017

Improving back-projection imaging with a slowness-based aftershock calibration approach

Over the last decade, the development of large-scale dense seismic networks has promoted rapid progress in a broad spectrum of seismological sciences. High-frequency seismic waveforms from these large arrays have enabled back-projections (BP), an emerging tool to probe earthquake dynamics. BP is widely used to study large earthquakes, but its derived images are array-dependent. For the same earthquake, different arrays often produce different images, and it is difficult to judge which result should be trusted more. In this talk, I will present a new approach to effectively mitigate the BP uncertainties of large earthquakes based on their aftershocks. I introduced a slowness (ray parameter) error term calibrated by aftershocks to achieve consistency between BPs of different seismic networks. This correction accounts for the P-wave travel time errors in each receiver array, due to approximating 3D earth structure with 1D reference velocity model.

Such technological improvement in earthquake source observations allows me to address the questions of earthquake source dynamics in the case studies of recent large earthquakes. In the 2015 Mw 7.8 Gorkha earthquake, our refined source imaging reveals a narrow unilateral eastward rupture unzipping the lower bottom of the locked portion of the Main Himalaya Thrust. The limited rupture extent indicates that the Gorkha earthquake in Nepal is possibly a medium-size event resulted from heterogeneous loading stress during the inter-seismic period of larger earthquakes. We have also performed a joint seismic and geodetic investigation of the 2016 Mw 7.8 Kaikoura earthquake in New Zealand. We find that the earthquake spans over 100 km through stepping and branching of at least six distinct fault planes. The high-frequency radiations occur mainly on three shallow thrust faults located in the dilatational quadrants of rupture on the Hope-Kekerengu fault, consistent with the unclamping effect predicted by the dynamic Coulomb stress. This study demonstrates the capability of the BP method, enhanced by aftershock calibrations, to describe earthquake rupture kinematics in regions of complex fault systems.