Sub-Shear Rupture on Bifurcating Faults: Kinematics and Dynamics of the
2021 Mw7.4 Maduo Earthquake in Qinghai
Abstract
We utilize the slowness-enhanced back projection (SEBP) and joint finite
fault inversion (FFI), which combines body waves, surface waves, and 3D
ground displacements to image the rupture process and slip distribution
of the Mw7.4 Maduo earthquake. The results indicate a 160-km-long
bilateral rupture occurring on a north-dipping fault. The WNW branch
ruptures a length of ~75 km at 2.7 km/s, while the ESE
branch ruptures a length of ~85 km at 3 km/s. Most slip
concentrates above 10 km depth, with several major slip patches up to
5.7 m located on the east segment of the main fault and on the
bifurcated branching fault. The comparison between SEBP and FFI shows
the consistency between large slip pulses and high-frequency sources. We
observe up to 3 m slip with large uncertainty in depth beyond 20 km,
which is abnormal since it requires rupture penetrating into the ductile
layers. We propose that the apparent deep slip could be an artificial
compensation to match the observed moment, which indicates that the
crust in the source region is more rigid than the current understanding.
Or the deep creeping fault is turned into seismic ones by the strain
localization and dynamic weakening. The stress analysis on the forks of
the fault demonstrates that the branching behavior on the eastern fork
could be well explained by the pre-stress inclination, rupture speed,
and branching angle.