Structural Relationship between Rupture Zones in a Sequence of Earthquakes Viewed by Fault-Zone Trapped Waves

Yong-Gang Li, J-R Su, T-C Chen, P. Wu, Gregory P. De Pascale, M. Quigley, & D. Gravely

Published August 13, 2019, SCEC Contribution #9539, 2019 SCEC Annual Meeting Poster #180

We used fault-zone trapped waves (FZTW) to image rupture zones of the 20018 M8 Wenchuan and 2013 M7 Lushan earthquakes occurred on the Longmenshan Fault (LSF) in Sichuan, China, and rupture zones of the 2010 M7.1 Darfield and 2011 M6.3 Christchurch earthquakes occurred on the Greendale-Port Hills faults (GF-PHF) in New Zealand. Our aim is to study the structural relationship between rupture zones in a sequence of earthquakes and the fault interaction. We analyzed the recorded FZTWs to characterize the spatial extension and rock damage magnitude of ruptured and un-ruptured fault segments. 3-D finite-difference simulations of FZTWs generated by Wenchuan aftershocks show a ~200-m wide low-velocity waveguide along the LSF composed by severely damaged fault rocks, within which seismic velocities are reduced by up to ~55%. We interpret it to be a break-down zone during dynamic rupture in Wenchuan earthquake. At the southernmost LSF, FZTWs generated by Lushan aftershocks infer a low-velocity waveguide composed by severely damaged fault rocks at depth with 40-50% velocity reduction. There is a ~45-km “slip gap” between Lushan and Wenchuan rupture zones; nevertheless, the FZTWs generated by either Lushan or Wenchuan aftershocks were recorded at stations close to both of them. Modeling results from FZTWs shows fault continuity of the LSF at depth, but the rock damage magnitude and velocity reduction within ruptured faults are greater than those within the un-ruptured fault in “slip gap”. We consider that Wenchuan and Lushan earthquakes occurred in a sequence on the LSF, and therefore the Lushan event could be called the largest aftershock of the Wenchuan earthquake. Similarly, we recorded FZTWs generated by Darfield and Christchurch aftershocks at seismic arrays deployed across the GF-PHF, showing that a continuous low-velocity waveguide extends along them through a ~10-km “slip gap” between-in, but with the greater velocity reduction within the GF-PHF rupture zones than un-ruptured faults in the “gap”. 3-D finite-difference simulations of FZTWs show that velocities within ~200-m wide rupture zones are reduced by 35-55%. The GF and PHF likely connect through a weak portion of the waveguide formed by rocks with milder damage in the “gap”. Structurally, Darfield and Christchurch earthquakes consist of a sequence of earthquakes occurring on the same fault system. Could be the late event called an aftershock?

Key Words
Wenchuan and Lushan earthquakes, Darfield and Christchurch earthquakes, earthquake sequence, rupture zones, fault continuity, fault-zone trapped waves, waveguide effect

Citation
Li, Y., Su, J., Chen, T., Wu, P., De Pascale, G. P., Quigley, M., & Gravely, D. (2019, 08). Structural Relationship between Rupture Zones in a Sequence of Earthquakes Viewed by Fault-Zone Trapped Waves . Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)