Detect water-saturation degrees within the fault zone during co-seismic damage and post-seismic heal
Yong-Gang LiSubmitted September 7, 2025, SCEC Contribution #14609, 2025 SCEC Annual Meeting Poster #TBD
Our repeated experiments to record seismic waves from explosions and earthquakes at dense linear arrays deployed across rupture zones of the 1992 M7.4 Landers, 1999 M7.2 Hector Mine and 2004 M6 Parkfield earthquakes revealed fault co-seismic damage and post-seismic heal progression. These observations suggest that a fault damaged in the earthquake may regain strength rapidly in the early stage of the interseismic period, but take a long time to fully recover the strength for the next earthquake on it. We consider that the “crack dilatancy” mechanisms associated with the earthquake are likely to operate for fault damage and healing. Co-seismic deformation might lead to crack opening either by elevated pore fluid pressure or stress drop. After the earthquake, crack healing, fluid diffusion and post-seismic deformations cause the cracks to close again with a logarithmic recovery rate. In this study, we measured the ratio of traveltime decreases for P to S waves recorded for the same shot-receiver pairs in repeated experiments to detect variations in water-saturation degree of cracks within the rupture zone during co-seismic damage and post-seismic heal. According to equations for the elastic moduli of the medium with penny-shaped cracks, the ratio of dtp/dts is ~1.22 for dry cracks, and ~0.27 for water-saturated cracks in a Poisson solid. In our study area where the rock has an anomalous Poisson’s ratio of 0.33, dtp/dts is 1.64 for dry cracks and 0.17 for water-saturated cracks. At Parkfield, we measured the dtp/dts to be 0.57 within the SAF rupture zone and ~0.65 in the surrounding rocks between two shots before and after the M6 earthquake, indicating that cracks within the SAF are wetter than those out of the zone. Water may be coming up from depth along the highly fractured fault-zone acting as a fluid conduit. The mean value of dtp/dts ratio (~0.72) observed at the Hector Mine rupture zone indicate that cracks within the fault zone were partially water filled after the 1999 earthquake. The dtp/dts ratio within the Landers rupture zone decreased from 0.75 between 2 and 4 years to 0.65 between 4 and 6 years after the 1992 M7.4 earthquake and further decreased to 0.55 until 2001, suggesting that cracks within fault damage zone became apparently more fluid saturated with time after the mainshock. These observations suggest that monitoring the temporal variations in dtp/dts ratio within the fault zone may play a role of precursor of pending earthquake on it.
Key Words
Rupture zone, Fault damage and heal, Crack dilatation, Water Saturation, dtp/dts ratio, Earthquake precursor.
Citation
Li, Y. (2025, 09). Detect water-saturation degrees within the fault zone during co-seismic damage and post-seismic heal. Poster Presentation at 2025 SCEC Annual Meeting.
Related Projects & Working Groups
Seismology
