Comparison of the rupture history of the southern San Andreas fault with empirical data on fault displacement and rupture length

Katherine M. Scharer

Published August 8, 2016, SCEC Contribution #6555, 2016 SCEC Annual Meeting Poster #121

Offset geomorphic features provide estimates of slip used to derive paleoearthquake moment magnitude based on empirical relationships from historic ruptures (e.g., Wells and Coppersmith, 1994; Wesnousky, 2008). Obtaining these measurements is difficult, as they are typically offset channels that must be serially excavated and radiocarbon dated. Consequently, these measurements are sparse. Along the well-studied 300-km-long combined Mojave, Big Bend, and Carrizo sections of the southern San Andreas fault, for example, only five locations have such data. In this study, I combine data from these locations with hundreds of undated lidar-based measurements of offset along the same sections of the fault published by Zielke et al. (2012) to develop a model of paleorupture length and displacement along strike. To construct the model, I applied the 1200-year-long rupture history in Scharer et al. (2016), which identifies the minimum length of individual paleoruptures based on dated paleoearthquakes at seven sites along the same sections of the San Andreas fault. Each dated paleoearthquake is pinned to the correlative, dated offset measurement and then extended along strike through clusters of lidar-based (undated) offset measurements. The ruptures are straightforward to model where there are dated measurements or where lidar-based displacements are well separated, such as along the southern Big Bend section. Regions with denser lidar-based displacements, such the central Carrizo section, are more subjective to model, and will be highlighted here. I also present alternative models that accommodate uncertainty in event correlation due to radiocarbon dating limitations, but these do not change the overall conclusions. In the preferred model, the displacement curves have variable offset along strike and often have one fairly short stretch with a peak displacement, visually similar to the variability in historic ruptures on strike slip faults. The longest permissible rupture occurred ca. 1550 A.D. and has a 100 km section with over 5 m of displacement, similar to the historic Fort Tejon rupture of 1857 (Zielke et al., 2012). Shorter ruptures 75 to 100 km long have smaller average displacements, less than 3m. This approach is robust in that it honors multiple large datasets and up-weights better constrained data such as dated offset measurements and high-quality lidar-based measurements, while de-emphasizing lower quality lidar measurements.

Key Words
paleoseismology, earthquake magnitude, rupture length

Citation
Scharer, K. M. (2016, 08). Comparison of the rupture history of the southern San Andreas fault with empirical data on fault displacement and rupture length. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Southern San Andreas Fault Evaluation (SoSAFE)