| Electromagnetic methods such as magnetotellurics (MT) are well suited for imaging the nature of continental faulting on both local and regional scales. These methods are sensitive to both the contrast in resistivity often found across a fault as well as zones of fluids and/or physically altered materials located within an active fault.; High resolution MT studies of the San Andreas fault (SAF) near Hollister, California have imaged a zone of high fluid content flanking the SAF and extending to mid-crustal depths. The spatial relation between this zone and local seismicity suggests that the presence of fluids inhibits seismicity within the upper crust (0–4km). In the region examined, the San Andreas fault acts as a conduit for along-strike fluid flow yet acts as a barrier for fluid flow across the fault. Combined with previous work, these results suggest that the geologic setting of the SAF gives rise to the observed distribution of fluids in and surrounding the fault, as well as the observed along-strike variation in seismicity.; Regional magnetotelluric studies of faulting in the northeast corner of the Tibetan plateau have helped establish the style and extent of faulting in the region. A series of MT profiles crossing the Altyn Tagh fault near its eastern terminus have been analyzed. One of these profiles additionally crosses a sequence of thrust faults which absorb the strain associated with this fault termination. Together with geologic timing information, the extent of underthrust sediments has been used to establish the rates of convergence and uplift of this proto-plateau. Additionally, the Altyn Tagh fault is imaged to be vertical from the surface to mid-crustal depths, and flanked by an extensive conductive zone which suggests that the present-day fault may have activated along a pre-existing suture. |
