SCEC Award Number 15183 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title In Situ Fault Zone Hydrogeological Structure Measured at a Southern Californian Site
Investigator(s)
Name Organization
Emily Brodsky University of California, Santa Cruz
Other Participants Vincent Allegre (postdoc)
SCEC Priorities 3b, 4a SCEC Groups FARM, Seismology, Geology
Report Due Date 03/15/2016 Date Report Submitted 05/04/2016
Project Abstract
 In situ measurements of hydrogeological properties within fault zones are few, in part because of the challenges of measuring the scale-dependent quantities in fractured rock reservoirs. This work aims to fill the gap by utilizing a combination of tidal, barometric and seismic response analyses on pressure head time-series from the Santa Susana Field Laboratory in Southern California, where an environmental remediation site provides abundant data. The techniques sample different effective volumes and so allow us to investigate the scale-dependent structure near fault zones. Permeability and specific storage were inverted from tidal response at 14 locations, and specific storage was also computed from barometric efficiencies at 10 locations. In addition, we computed hydro-seismic transfer functions after nine local earthquakes and teleseismic events. We found that permeability computed from tidal response is quite homogeneous over the site regardless of the presence of moderate to large faults. The tidal responses also showed higher variability of specific storage inside the fault zones suggesting that fault damage zone generates a storage architecture. We also see an increased sensitivity to seismic waves for at least one well inside the fault core. The result suggests possible sensitivity to compliance contrasts in the fault core, which have been historically difficult to measure at the field scale.
Intellectual Merit We have made in situ measurements of fault zone permeability and storage at unprecedented resolution. Fluid flow inside fault zone is appealed to in most mechanistic studies of fault physics, but the properties governing its transport are seldom measured in situ.
Broader Impacts The project has helped foster a dialog between hydrogeologists and seismologists working in Southern California. It has also provided partial support to two postdoctoral scholars, one of which has been launched on a faculty position at the University of Bourdeaux (Allegre) and the other who will shortly begin a Miller Fellowship at Berkeley (Xue). Both have been cross-trained in an unusual hybrid of hydrogeological and seismological skills.
Exemplary Figure Figure 2. (a) Vertical ground velocity recorded at PASC station after the Mw = 6.9 Chiapas earthquake that occurred on July 7th 2014. The red line shows the envelope (instantaneous amplitude) of the seismic signal. (b) Water pressure response recorded in the well RD-38B at the same time with the envelope. (c) Transfer function between the seismic and hydraulic recordings.
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