Microfractures within the fault damage zone record the history of fault activity

For faults without sedimentary covers, there is no robust method to obtain paleoseismic data that is crucial for the prediction of future damaging earthquake events. Sudden failure along faults during earthquakes induces off-fault damage surrounding such basement faults. We showed that the Quaternary-active fault has the damage zone characterized by a fracture density that decays exponentially with distance from the fault for both healed and open microfractures. In contrast, Quaternary-inactive faults contain only healed microfracture damage zone. Cross-cutting relationships between microfractures and a minimum healing temperature of similar to 100 degrees C suggest that healed microfractures formed before, and at deeper levels, than did unhealed microfractures. The damage zone defined by open microfractures reflects the recent fault movement during exhumation, associated with erosion and regional uplift, from the maximum depth at which microfractures may remain unhealed. Microfracture analysis can therefore be used to examine the history of basement fault activity.