A BIOMARKER ALTERATION HISTORY OF EARTHQUAKES ON THE CENTRAL SAN ANDREAS FAULT, CALIFORNIA (Invited Presentation)

Characterizing earthquake behavior along creeping faults is an important component in understanding the earthquake cycle, particularly so when slip behavior is variable along a fault. Creeping faults are considered stable and are not thought to nucleate large earthquakes. However, we do not understand whether creeping sections fail dynamically when loaded by an earthquake from a seismogenic portion of a fault. Furthermore, what is the longevity of a creeping fault, does slip behavior change over time? In this work, we address these issues by diving into the seismic history of the central San Andreas fault (SAF), California.

The SAF consists of two earthquake-producing locked sections sandwiching the central SAF, which creeps at near plate rate. We measure biomarker thermal maturity in samples from the San Andreas Fault Observatory at Depth (SAFOD) to search for evidence of past earthquakes. Biomarker structure is systematically altered during heating and can be used to quantify temperature rise. We measure the amount of alteration that has occurred in SAFOD and find that a 4 m wide patch has hosted many large earthquakes.

Along with biomarker thermal maturity we utilize K/Ar dating of illite to investigate the timing of seismicity. K/Ar dating reveals younger ages within the heated patch. Furthermore, we conducted rapid heating experiments that show thermal resetting occurs under the heating conditions measured at SAFOD. Therefore, we interpret the measured ages as maximum earthquake ages implying that the central SAF experienced earthquakes less than 16.2–3.3 Myr ago.

Our findings suggest that either earthquakes can propagate into the central SAF or that creeping sections are short-lived features. Regardless of which scenario is true, we show that the central SAF has hosted earthquakes in the past and may do so again. This highlights the important role of these creeping faults in the earthquake cycle and their potential impact on the seismic hazard of a region.