High-frequency rupture related to the complex fault system and variable slip motions, insights from hybrid backprojection image of the Mw 7.9 2008 Wenchuan, China, earthquake

Abstract:

Complex-fault system containing bending, step-overs, and multiple fault segments can bring about the heterogeneous distribution of stress and strength during dynamic rupture propagation, and contribute to an irregularity of rupture propagation. Theoretical studies show that the irregularity of rupture propagation generates high-frequency (HF) waves. Thus tracking HF radiation sources at high-resolution is a key to understand earthquake source physics especially in complex-fault systems.

The 2008 Wenchuan, China earthquake occurred along multiple fault segments consist of steps and sub-parallel faults. Kinematic waveform inversions from low frequency waveforms have detected several sub-events and changes in slip motion from thrust to right-lateral motion. Backprojection (BP) of HF (~1 Hz) waveforms have also reported several sub-events of HF bursts at the devastating damaged area, while the detailed role of HF radiation related to the fault geometry or variations in slip motions has not been well documented.

Here we tracked the spatiotemporal distributions of HF (0.5–2.0 Hz) sources during the 2008 Wenchuan earthquake by using a hybrid backprojection (HBP) method. The HBP method extracts signals of HF radiation by cross-correlating observed waveforms and Green’s functions including P-phase and depth phases (pP, and sP). Thus HF radiation depth and variations in focal mechanisms (fault geometry and slip motions) can be resolved by the HBP method. We used 52 teleseismic waveforms observed at GSN stations, and constructed the fault geometry with nine fault segments based on field measurements of surface rupture and focal mechanisms of aftershocks.

Rupture initiates at the hypocenter and propagates unilaterally toward northeast. Variations in rake angles among the HF radiation are generally similar to that obtained by slip inversion, indicating that the HBP method can also be used as a tool to grasp the variable slip motions. Strong HF radiations distribute around the boundary of fault segments (around the Xiaoyudong fault and steps at the middle of the Beichuan fault), indicating that the HF bursts may result from the perturbations of rupture velocity and/or slip-rate at the fault discontinuities that work as geometrical barriers.