T173 Where Does Earthquake Physics Meet Earthquake Geology?

Tuesday, 2 November 2010
8:00 AM - 12:00 PM
Lobby F

USING ORGANIC MOLECULES IN FAULT ROCKS TO INVESTIGATE FRICTIONAL HEATING

Paper number: T173-9
Program: Technical Sessions
Time: 10:10 AM – 10:25 AM

Authors

Abstract

Determining the absolute stress on faults during slip is one of the major goals of earthquake physics as this information is necessary for full mechanical modeling of the rupture process. One indicator of absolute stress is the total energy dissipated as heat through frictional resistance. The heat results in a temperature rise on the fault that is potentially measurable and interpretable as an indicator of the absolute stress. Despite the relatively straightforward theoretical effect, fault heating has been difficult to constrain in nature. Here we explore a new approach to determining the temperature rise on an exhumed fault by using the thermal maturation of organic molecules. This type of indicator is distinct from previous ones in its extreme sensitivity to peak temperature and its insensitivity to mechanical deformation.

We use the thermal maturity of polycyclic aromatic hydrocarbons to measure the integrated heating history of a well-studied fault zone, the Punchbowl Fault of Southern California. We measured ratios of less thermally stable to more thermally stable isomers of methylphenanthrenes, terphenyls, phenylnapthalenes, and phenylphenanthrenes which have been used extensively in petroleum research to quantify thermal maturity of sedimentary sequences. Samples were collected for approximately 1 kilometer along strike in both the ultracataclasites within the fault core as well as in the damaged Punchbowl Formation up to approximately 50 m from the fault.

All samples had the same thermal maturity, indicating minimal differential heating on the fault. This lack of heat signature could result from several different fault zone properties: 1) the coefficient of friction along the fault was low, 2) the active slipping zone during earthquakes was thick, and/or 3) the 44 km of slip along the fault was accommodated by small earthquakes or aseismic creep.