The San Andreas Fault in California has variable slip behavior along its length, with a stable central section sandwiched between the earthquake-producing northern and southern sections. Because the central San Andreas fault relieves strain through aseismic creep, it is not thought to nucleate earthquakes. What still remains a relatively open question though, is whether earthquakes that nucleate in either the northern or southern locked sections can propagate into or through the central San Andreas fault. Making this distinction has important implications to earthquake hazard in California as the largest earthquake possible along the San Andreas fault is markedly higher in magnitude if the fault can rupture in a single continuous event.
Graduate student Genevieve Coffey has partnered with lab member Stephen Cox to couple organic geochemistry with geochronometry to explore this scenario. Biomarker thermal maturity is used to search for evidence of earthquakes within material from the San Andreas Fault Observatory at Depth (SAFOD), a core drilled through the central San Andreas fault. Biomarkers enable the identification of regions that have experienced the high temperatures (> 500°C) that occur during coseismic slip and allow the delineation of where earthquakes have occurred. We use K-Ar techniques to date these samples revealing a clear distinction in age between background and coseismically heated samples. This allows us to place maximum constraints on when central San Andreas Fault earthquakes occurred. Furthermore, we carried out a series of rapid heating experiments which confirm that at least partial resetting of K-Ar ages occurs under coseismic slip heating conditions and demonstrates the potential of K-Ar as an earthquake dating tool.