Imaging an active normal fault in alluvium by high-resolution magnetic and electromagnetic surveys

High-resolution geophysical investigations can improve paleoseismic studies and the characterization of fault zones. Here, forward modeling of detailed magnetic data indicates that 60-70 in of vertical separation occurred on the South Crater Flat fault (SCFF), Nevada, since the eruption of Pliocene basalt approximately 3.75 Ma. The average throw rate on the SCFF, therefore, is 0.017 +/- 0.003 mm/year, and the average slip rate, assuming fault plane dips of 55degrees-65degrees, is 0.02 +/- 0.003 mm/year during this interval. These values are approximately one order of magnitude greater than Quaternary slip rates derived from trench studies of alluvial stratigraphy across the fault and are in agreement with slip rates derived for the same period on nearby faults. In addition, these surveys reveal several synthetic and antithetic normal faults west of the SCFF and a decrease in vertical displacement on the SCFF from north to the south. A 140- to 200-m-wide, 1.5- to 2.5-milliSiemens (mS)/m electrical conductivity anomaly parallels the fault zone in the alluvial wedge of the hanging wall. This anomaly correlates with active drainageways and reflects lateral variations in allostratigraphy caused by Quaternary slip on the SCFF. These combined results indicate that displacement has been distributed across several faults and that the long-term average slip rate (Pliocene to present) is higher than Quaternary slip rates derived from trench studies alone. Thus, these geophysical methods provide an areal and temporal perspective on the fault zone that can augment traditional fault characterization techniques.