Do Creeping Faults Ever Host Large Earthquakes?: An Investigation of Thermal Alteration in the SAFOD Drillcore

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Broader Significance This work will address one of the major questions of fault mechanics and seismic hazard: can significant earthquakes happen on faults that are slowly creeping? This question has garnered more attention after the 2011 Tohoku-oki earthquake in Japan ruptured through a shallow section of the fault that was assumed to only slide stably (rather than as part of an earthquake). The same may be true of the creeping section of the San Andreas Fault, and this project is evaluating the earthquake potential on that section of the fault.Due to friction, when a large earthquake ruptures a fault, the fault surfaces heat up. This project is using the breakdown of organic molecules found in physical rock samples from the San Andreas Fault Observatory at Depth to search for such heating along the San Andreas Fault in an area of slow steady slip. If the results indicate significant pulses of heating, this may indicate significant earthquakes can rupture this region, despite the slow creep, which would mean the seismic hazard is larger than currently thought. This novel approach to detecting heating in faults has potential application to many other questions in rock mechanics and earthquake physics as well.Technical Description This project is determining the maximum temperature rise, and therefore aspects of earthquake history, along the creeping section of the San Andreas Fault within the SAFOD drillcore. Although the fault is currently creeping near tectonic plate rates, it is possible that this section of the fault hosted earthquakes in the past. Evidence of previous earthquakes will aid in understanding the seismic potential of this area of the San Andreas fault and more generally whether creeping faults can host large earthquakes. Past earthquakes are identified by measuring the thermal maturity of organic molecules (biomarkers) within both the active creeping sections as well as areas of relict fault gouges and cataclasites compared to the less deformed protoliths of the fault-zone rocks. Biomarkers are sensitive to temperature rise such as achieved during earthquakes, and because they are not susceptible to retrograde reactions, they preserve the earthquake temperature rise. In addition to the thermal maturity of the fault zone, the chemical kinetics for reactions of biomarkers within the core are determined from laboratory heating and shearing experiments. Well-established kinetics allows for quantitative constraints on temperature history. Because temperature rise during earthquakes is dependent on fault shear strength, slip, and thickness, determining the reaction kinetics is important for understanding earthquake mechanics. Thermal diffusion models coupled to biomarker reaction kinetics will constrain the mechanics for the creeping section of the San Andreas fault.