Puzzling Permeabilities: In Situ Permeability Measurements at the Punchbowl Fault, California

Lisette Ochoa, Noah J. Phillips, & Montserrat De Allende Silva

Submitted September 7, 2025, SCEC Contribution #14500, 2025 SCEC Annual Meeting Poster #TBD

Faults serve as permeable structures through the upper crust due to increased fracture densities within fault damage zones. While many studies have used measurements of fracture density to infer permeabilities of exhumed faults, measurements of in situ permeability are rare. Here, we use in situ measurements of permeability at the Punchbowl Fault to assess its permeability structure. The Punchbowl Fault is an inactive strand of the San Andreas located ~5km southwest of active strands. The fault is composed of a continuous ultracataclasite layer bounded by damaged host rock (Chester and Logan 1986). The damage zone is riddled with fractures whose densities decay logarithmically with distance from the fault core. Measurements were made using a TinyPerm3 on smooth outcrops at various distances from the fault. We made at least 10 measurements at every location for a total of 323 measurements from 17 locations. Permeabilities do not decay logarithmically with distance from the fault core as expected from previous measurements on microfracture densities. We hypothesize that grain size variations may be more important in determining permeability than microfractures within the Punchbowl Formation or that microfractures could have sealed over time through mineral precipitation.

Key Words
Damage Zone; Permeability; Fault Structure

Citation
Ochoa, L., Phillips, N. J., & De Allende Silva, M. (2025, 09). Puzzling Permeabilities: In Situ Permeability Measurements at the Punchbowl Fault, California. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology