ESTIMATED PALEOEARTHQUAKE MAGNITUDES OF NORMAL FAULTS IN SUMMER LAKE BASIN, OREGON

The Summer Lake basin in south-central Oregon lies within the north-western Basin and Range (NWBR), an active low-strain rate region with no historic large earthquakes. However, high-resolution topographic maps derived from lidar data highlight fault scarps along all of the major faults in the basin. Detailed mapping and topographic analysis of fault scarps of the Winter Ridge, Ana River, Slide Mountain, and Thousand Springs faults can constrain the magnitude of paleoearthquakes on these faults. Thanks to Ray Weldon and his students—both undergraduate and graduate—the paleoseismicity of the Ana River and Winter Ridge faults have been documented through trenching, providing estimates of paleoearthquake ages and an additional line of evidence for our magnitude estimates.

We mapped fault scarps on a 0.5-m resolution DEM derived from airborne lidar. Along mapped scarps, we pulled topographic profiles where the far-field slopes of the hangingwall and footwall were close to the same and where erosion subsequent to surface offset was minimal. We determined fault scarp height, scarp slope, and the distance of each profile along the fault and used these properties to calculate a range of possible paleoearthquake magnitudes (minimum, maximum, and average). We assumed a fault dip of 80°, since rock units in both the hangingwall and the footwall are near-horizontal. Using Wells and Coppersmith (1994) and Hemphill-Haley and Weldon (1999) equations, the estimates of the most recent earthquake on the Winter Ridge fault ranged from M6.9 to M8.1, on the Ana River fault from M6.7 to M7.9, and on the Slide Mountain from M7.0 to M8.2. These estimated magnitudes closely match other published work. For the Thousand Springs fault, the extent is poorly defined thus we have made estimates based on the shortest length (8 km) and longest potential length (32 km). Based on these length estimates, the magnitude range is much larger: M6.6 to M7.8.

These preliminary data suggest that the Thousand Springs fault has had large earthquake events like the other faults in the basin, but no studies have yet identified or dated these earthquakes. Our future work will focus on uncovering the earthquake history of the Thousand Springs fault to understand its contribution to extension and the seismic hazard in the Summer Lake Basin.