SCEC2024 Poster #039: Numerical Investigations of Waveguide Effects for Multi-Layer Fault Damage Zones at Seismogenic Depths

Group A, Poster #039, Seismology

Numerical Investigations of Waveguide Effects for Multi-Layer Fault Damage Zones at Seismogenic Depths

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Poster Presentation

2024 SCEC Annual Meeting, Poster #039, SCEC Contribution #13851 VIEW PDF

Actual fault zones composed by damaged rocks with varying lithology at depths might form a multiple layered low-velocity waveguide (LVWG). 3-D finite-difference synthetic fault-zone trapped waves (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 o...ut 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 the deep on-fault events when we analyze the recorded waveform data to accurately extract the depth extension of a realistic fault damage zone. These numerical tests of waveguide effects for a depth-dependent multi-layer LVGW are consistent with our previous observations and modeling of FZTWs at rupture zones of Landers, Hector Mine, Parkfield, and show the complexity in analysis and interpretation of FZTW data to document the fault damage zones at seismogenic depths. The geometry and material properties of fault damage zones at seismogenic depths will influence rupture propagation, source parameters and the upper bound frequency “fmax” of seismic waves that can be radiated from a fault zone in sake for the strong ground motion prediction (Aki, 1979; Papageorgiou and Aki, 1983). Ferry et al. (2024) found the depth dependence of coseismic off-fault damage and damage formation markedly reduces rupture velocity and delays for a narrow damage zone across the entire seismogenic zone.
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