NORMAL FAULTING GEOMETRY IN THE GREAT BASIN: HOW WELL DO WE UNDERSTAND IT AND WHAT DOES IT MEAN FOR TECTONICS AND SEISMIC HAZARD?

Tectonic geomorphology is an important tool for paleoseismic analysis. Although it generally cannot yield quantitative information on paleoearthquake magnitudes or recurrence intervals, it is useful in identifying faults that are highly active and should be targeted for further investigation. If ages of surfaces that are offset by the fault can be determined, long-term slip rates can be calculated, providing further information on fault activity. In order to perform this calculation, fault geometry (i.e., dip angle and orientation) must be measured or assumed. Often information on fault dip is lacking and a default value of 60º is frequently assumed. We have examined normal fault exposures along the Sierra Nevada frontal fault in the Owens Valley. Previous interpretation of the tectonic geomorphology has indicated that these faults are at most moderately active. Slip rates inferred from dated offset surfaces range from 0.2 to 0.5 mm/yr. We have used a variety of methods to quantify the dips of these faults in outcrop. Our data show that most of the dips are less than 45º, the mode is about 35º, and some are 25º or less. The low apparent slip rates were calculated assuming fault dip of 60º. If the measured dips are used instead, the slip rates approximately double. If the differences in geomorphology produced by low-angle faults are taken into account, these much higher slip rates are in fact consistent with the observed tectonic geomorphology. Failure to correctly identify fault dip can result in significant underestimation of both seismic hazard and rates of tectonic strain.