Study of Fault-Zone Damage and Heal: 30-Years Retrospective of the 1992 M7.4 Landers Earthquake and Numerical Tests of FZTWs from a Layered Fault Zone

Yong-Gang Li

Submitted September 11, 2022, SCEC Contribution #12373, 2022 SCEC Annual Meeting Poster #052

We review our previous study of fault zone co-seismic damage and post-mainshock healing progressions on the 1992 M7.4 Landers rupture zone in California via observations and 3-D finite-difference modeling of fault-zone trapped waves (FZTWs) generated by explosions and aftershocks and recorded at dense seismic arrays deployed across and along the ruptured faults in repeated experiments. These FZTWs allow us to depict the Landers rupture core zone that is marked by a ~200-m-wide low velocity waveguide (LVWG), likely extending to 5-8 km depth, within which wave velocities are reduced by 25-50% from wall-rock velocities and Q values are 20-50. This LVWG is interpreted as being a damage zone during dynamic rupture in historical earthquakes. Our repeated experiments at Landers show post-seismic healing of the damage zone with recovery of wave velocity by 1.2% between 1994 and 1996, and further by 0.7% between 1996 and 1998, most likely due to the closure of cracks opened in the mainshock. However, the healing process on Landers rupture zones was interrupted by the 1999 M7.1 Hector Mine earthquake that occurred 25 km away and added damage as a temporal reversal of the healing on Landers faults by strong shaking and permanent strain. The healing trend on Landers faults then recovered again. In our recent numeric test of trapping efficiency of a layered LVWG in fault zone, synthetic FZTWs show a package of successive dispersive wavetrains corresponding to multiple layers within the fault zone. Although the dominant FZTWs are produced in the top layer having the lowest wave velocity, the FZTWs with smaller amplitudes in later-coda coming from deeper layers of LVWG could help evaluate the depth extension of fault damage zone. Because a majority of aftershocks occur outside the narrow fault core zone (hundreds of meters in width), FZTWs with large amplitude but shorter duration formed within the upper layers of LVWG could be dominate in massive amount of seismograms (particular by low-pass filtered) recorded at stations within the rupture zone for a large number of off-fault aftershocks. Therefore, it might lead underestimation of the depth extension of fault damage zone.
Contributors to this research include: the late Professor K. Aki (SCEC Founder), J. Vidale, C. Marone, E. Cochran, A. Hasemi, K. Mayeda, D. Adams, D. Bowman, F. Xu, J. McRaney, WHK Lee, T. Burdette, R. White, M. Alvarez and many researchers and graduates from the USGS, PASSCAL and Universities.

Key Words
Landers earthquake, Damage zone, Fault healing, Low-velocity waveguide, Fault-zone trapped wave

Li, Y. (2022, 09). Study of Fault-Zone Damage and Heal: 30-Years Retrospective of the 1992 M7.4 Landers Earthquake and Numerical Tests of FZTWs from a Layered Fault Zone . Poster Presentation at 2022 SCEC Annual Meeting.

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