Tamara Jeppson, USGS

Fracture healing is a critical component of the earthquake cycle, induced seismicity, and enhanced geothermal systems. Accordingly, there is significant interest in understanding the process of healing, in terms of both recovery of strength and evolution of fluid transport properties. We performed laboratory slide-hold-slide and flow-through experiments, at temperatures from 22 to 200 ˚C, to examine effects of fracture reactivation and quasi-static loading on the evolution of frictional strength and fluid transport properties of simulated fractures in Westerly granite. Consistent with previous studies, the rate of long-term decay of hydraulic transmissivity is positively correlated with temperature. Periods of sliding on the fracture surface result in transient increases in the transmissivity, due to shear dilation, as is expected for Coulomb materials. These transients are superimposed on the long-term decay. When sliding ceases and a new hold period commences, there is a rapid reduction in transmissivity and return to the long-term rate of transmissivity decay. However, the rate at which this rapid reduction occurs is inversely proportional to temperature, in contrast to the long-term decay rates and the expected behavior for processes like subcritical crack growth and indentation creep. Generally, a logarithmic increase in strength recovery with hold duration is observed, at a rate of 0.2 MPa per e-fold increase in hold duration. However, at 200 °C, there are clear indications that the fracture surface is weakening for hold durations ≥ 5,000 s. The weakening is consistent with previous rate-stepping tests by Blanpied et al. (1998) on Westerly granite using a similar experiment configuration. However, this weakening behavior has not been observed in many previous frictional studies at hydrothermal conditions, reflecting the importance of considering fluid and rock composition and experiment configuration when comparing experiment results to field observations and incorporating experimental data into numerical models.

Short Biography
Tamara Jeppson did her undergrad at Utah State University working with Dr. Jim Evans looking at geophysical signatures of deformation and alteration in rocks from the SAFOD borehole. She then when on to do a Master’s and PhD at University of Wisconsin-Madison working with Dr. Harold Tobin doing experiments to characterize the elastic properties of plate boundary fault zones. In 2017, after finishing her PhD she received an NSF GeoPRISMS postdoc fellowship to work with Dr. Hiroko Kitajima at Texas A&M University on a project examining the deformation behavior of subduction zone sediments. Then in 2019, she went on to do a postdoc at the USGS with Dr. David Lockner in the Experimental Rock Physics Laboratory. In December she started a permanent position as a research geophysicist at the USGS Earthquake Science Center in Menlo Park, CA. At the moment, her research is really focused on experiments examining fault healing but more generally her research interests include experimental rock deformation, physical properties of fault zones, and mechanics of earthquakes.