Resolving the Deformation Style and Slip Behavior of the Castle Mountain Fault, South-Central Alaska

The Castle Mountain fault (CMF) is a major active fault in south-central Alaska that poses a significant seismic hazard to the Anchorage and Matanuska-Susitna Valley urban areas. Previous studies of the CMF have reached conflicting conclusions regarding its kinematics, slip behavior, and earthquake rupture history. Earlier paleoseismic, geomorphic, and geodetic studies suggested that the CMF is predominately right-lateral with slip rate values ranging from 0.07 - 3.0 mm/yr, while more recent work suggests that the CMF accommodates reverse dip-slip motion of <0.3 mm/yr (based on the long-term bedrock rate). Early studies were constrained by limited methodologies and data, such as low-resolution topographic maps. In this study, we apply modern geomorphic and geophysical methods at several sites along the CMF to reassess interpretations of its slip sense and better constrain the number and timing of past earthquake ruptures. We have completed geomorphic mapping using high-resolution digital elevation models (DEMs) and collected two electrical resistivity tomography (ERT) profiles at one of two designated study sites. The well-defined CMF scarp resolved in lidar DEMs allows precise placement of ERT profiles across the fault. The two profiles spanned 80 meters across the fault scarp. ERT probes measured resistivity at 5m-spacing for a deeper profile and 2m-spacing for a more detailed profile closer to the surface. Relative fault displacements along strike of the CMF will be analyzed and measured using statistical analyses of scarp heights.  Preliminary results indicate that the ERT profiles can distinguish different geologic units and fault features such as fault planes, fracture zones, and stratigraphic offsets that have strong lateral resistivity contrasts. Based on geomorphic features observed in the DEMs, our preliminary findings suggest that past earthquakes on the CMF involved predominantly reverse slip. These features include hanging-wall-grabens, south-facing scarps, folded surfaces, and left-stepping en echelon scarps superimposed on the larger scarp. To better define the slip-rate history and geometry of the CMF, we plan to collect additional ERT profiles across the scarp where it displaces various fluvial terraces. We will also describe sediment cores and soil profiles. Samples from the cores and profiles will be collected for optically stimulated luminescence and radiocarbon dating. Our results will be compared with previous interpretations and observations in the field to help resolve long-standing discrepancies in interpretations of CMF behavior and improve regional seismic hazard assessments.