Biomarker thermal maturity at seismic timescales in high-velocity rotary shear experiments

Hannah S. Rabinowitz, Heather M. Savage, Elena Spagnuolo, & Giulio Di Toro

Published August 25, 2016, SCEC Contribution #6370, 2016 SCEC Annual Meeting Poster #026

Studies of past seismicity on sampled fault structures often rely on evidence of frictional heating on slip surfaces. Estimates of temperature rise on a fault provide constraints on the size of previous earthquakes hosted on the fault, allowing for a more thorough understanding of the earthquake history of seismically active regions of the world. One method that has been developed to estimate temperature rise on faults is biomarker thermal maturity, a tool that has previously been used to understand the production of hydrocarbons through burial heating over millions of years. The application of this method to paleoseismicity is based on the fact that the organic material in sediments will change when heated even over short timescales, as long as temperature increase is large. This concept has been demonstrated in multiple faults around the world (Pasagshak Point Megathrust, AK; Punchbowl Fault, CA; Japan Trench) and kinetics of thermal maturation have been established experimentally for several biomarkers (methylphenanthrenes, alkenones, n-alkanes). However, the experimental range to date has been limited to times over ~20 min. While these short-duration experiments provide evidence that biomarkers thermally mature at time-scales significantly shorter than those relevant to hydrocarbon formation, the kinetics still require extrapolation to time-scales relevant to seismic slip. Here, we present high velocity rotary shear experiments conducted on SHIVA (Slow to HIgh Velocity Apparatus) at INGV in Rome. Samples of Woodford shale were sheared at slip velocities of 1 m/s for a range of displacements in order to test whether methylphenanthrenes thermally mature at times between 1 s and 1 min, yielding temperatures up to ~300 ˚C (as measured from thermal couples at the edge of the slipping zone). Thin sections show that slip localizes to a ~100 µm zone in all experiments that achieve earthquake speeds; one experiment run at 1 mm/s did not show localization. We observe that methylphenanthrene thermal maturity positively correlates with frictional work and power density. We compare the experimental thermal maturity change to that expected from extrapolation of the established kinetics to the appropriate time/temperatures conditions. These experiments provide confirmation that biomarker thermal maturity can be used to map and quantify earthquake temperature rise in fault zones.

Key Words
fault heating, biomarker thermal maturity

Citation
Rabinowitz, H. S., Savage, H. M., Spagnuolo, E., & Di Toro, G. (2016, 08). Biomarker thermal maturity at seismic timescales in high-velocity rotary shear experiments. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)