High-Resolution DAS Observations of Fault-Zone Structure at SAFOD

Alina Belyalova, Andrew J. Barbour, Stephen H. Hickman, William L. Ellsworth, Martin Karrenbach, & Ettore Biondi

Submitted August 30, 2026, SCEC Contribution #15162, 2026 SCEC Annual Meeting Poster #TBD

Seismic imaging, vertical seismic profiling (VSP), and borehole logging at the San Andreas Fault Observatory at Depth (SAFOD) have characterized the San Andreas Fault Zone at Parkfield, California, as a structurally complex volume featuring sharp lithologic contrasts, secondary fault strands, and a low-velocity damage zone. While these studies successfully mapped the first-order architecture of the fault, resolving finer-scale heterogeneity, wavefield scattering, and localized phase conversions requires denser spatial sampling than conventional borehole seismic arrays provide. To address this observational gap, in March 2026, an approximately 3-km-long distributed acoustic sensing (DAS) cable was deployed in SAFOD to within 0.1 km of the actively deforming fault at depth, enabling continuous recording of local earthquake wavefields with 2 m channel spacing and a 1 kHz sampling rate.

We use this new high-density DAS array to examine how local earthquake wavefields are shaped by fault-zone structure, focusing on events with clear P- and S-wave arrivals across the array. Our analysis focuses on strain-rate variability along the SAFOD borehole, with emphasis on moveout perturbations, scattered arrivals, and depth-dependent amplitude and phase changes. To distinguish structural signatures from source and acquisition effects, we complement these observations with 2D elastic simulations of local earthquakes represented as double-couple moment tensor sources. We then compare synthetic and observed wavefields to separate features driven by borehole geometry and source mechanisms from those reflecting localized fault-zone heterogeneity. Integrating dense DAS observations with forward modeling provides a physical basis for interpreting high-resolution wavefields at SAFOD and supports future full-waveform inversion of the fault-zone volume.

Key Words
SAFOD, San Andreas Fault, DAS

Citation
Belyalova, A., Barbour, A. J., Hickman, S. H., Ellsworth, W. L., Karrenbach, M., & Biondi, E. (2026, 08). High-Resolution DAS Observations of Fault-Zone Structure at SAFOD. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology