In search of earthquakes, a biomarker thermal maturity investigation into the seismic potential of the central San Andreas Fault

Genevieve L. Coffey

Published August 14, 2018, SCEC Contribution #8489, 2018 SCEC Annual Meeting Poster #270

Understanding the seismogenic potential of a fault is important in the assessment of seismic hazard in a region. We know that faults can exhibit a range of seismic behaviors from slow aseismic creep to fast earthquake slip. While earthquakes are not thought to nucleate in creeping regions, it is not well understood whether an earthquake nucleating elsewhere, with sufficient momentum, can propagate through one of these stable regions. This problem is of particular relevance to the San Andreas Fault, where the style of deformation varies from locked and seismogenic in the northern and southern sections, to active aseismic creep in the central section. Determining whether earthquakes can rupture through the stable, central San Andreas fault has significant implications to earthquake hazard in California, as the maximum magnitude event possible on the fault is markedly higher if the whole fault can rupture during a single event versus when rupture is restricted to the northern and southern sections.

We address this by searching for evidence of coseismic heating within material collected from the central San Andreas Fault. During coseismic slip, frictional heating along a fault can lead to the generation of very high temperatures, which means we can use evidence of temperature rise to identify where coseismic slip has occurred and also to extract earthquake properties like shear stress and displacement. Biomarkers (organic molecules produced by living organisms) are useful paleothermometers as their structure is systematically during heating to achieve more stable configurations with increasing temperature. Here we present preliminary biomarker thermal maturity results from the San Andreas Fault Observatory at Depth (SAFOD). These show that a 3 m wide region of high thermal maturity occurs along the fault (3193 – 3196 m), adjacent to the southern deforming zone. We apply these thermal maturity results to model temperature rise across the fault, which is a function of shear stress, displacement, and earthquake duration and is modulated by thickness of the slipping layer. Because earthquakes are known to localize along very narrow layers, our heating signal likely represents multiple earthquakes that have ruptured the creeping section at SAFOD.

Key Words
SAFOD, earthquakes, frictional heating, faults

Coffey, G. L. (2018, 08). In search of earthquakes, a biomarker thermal maturity investigation into the seismic potential of the central San Andreas Fault . Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)