Numerical Investigations and Observations of Waveguide Effects for Multi-layer Fault Damage Zones at Seismogenic Depths

Yong-Gang Li

Published May 1, 2025, SCEC Contribution #14321

Actual fault zones composed by damaged rocks with varying lithology at depths form a multiple layered low-velocity waveguide (LVWG). Our recent numerical tests of waveguide effects for a depth-dependent multi-layer LVGW are consistent with the previous observations and modeling of fault-zone trapped waves (FZTWs) at rupture zones of Landers, Hector Mine, and Parkfield earthquakes in California, and show the complexity in analysis and interpretation of FZTW data to accurately extract the fault damage zones at seismogenic depths. 3-D finite-difference synthetic FZTWs in terms of a multi-layer LVWG show multiple amplitude peaks in the long post-S coda duration when the surface receiver and the deep source are both located within the LVWG. The early portion of FZTWs with larger amplitude peak at lower dominant frequency is produced within the top layer of the LVWG having slower velocity, while the late portion of FZTWs with smaller amplitude peaks at higher frequencies arises from deeper layers of the LVWG having faster velocities. When the source is put out of the LVWG, as long as the source is lower than the bottom of upper layers of the LVWG, FZTWs with large amplitudes could be produced as waves enter the upper layers. However, these FZTWs show shorter post-S durations and smaller ratios of PSSP (the post-S duration time of FZTW to the P-to-S arrival time) than those when the source at the same depth within the LVWG. It might lead to underestimate the depth extension of a real fault damage zone, because large numbers of earthquakes occur out of the narrow fault damage zone with the width of several tens to hundreds of meters and the FZTWs with large amplitudes but short durations produced within upper layers of the LVWG by them would be dominant in seismograms recorded at the fault zone. Therefore, we should carefully identify the FZTWs from these deep on-fault events when we analyze the recorded waveform data to accurately extract the depth extension of a realistic fault damage zone.

Key Words
Fault-zone trapped waves (FZTWs), Multi-layer low-velocity waveguide (LVWG), Post-S duration time of FZTW to P-to-S arrival time (PSSP) ratio, Finite-difference simulation, Fault damage zone at seismogenic depth

Citation
Li, Y. (2025). Numerical Investigations and Observations of Waveguide Effects for Multi-layer Fault Damage Zones at Seismogenic Depths. In Li, Y., , , , & (Eds.), Scientific Investigation of Continental Earthquakes and Relevant Studies, (, pp. 153-186) , Singapore: Springer Nature & Higher Education Press https://doi.org/10.1007/978-981-96-2826-1_10.

Related Projects & Working Groups
Seismology