SCEC2021 Plenary Talk, Earthquake Geology

Shallow Creep Along the Central San Andreas Fault from Differential Lidar Topography

Chelsea P. Scott, Ramon Arrowsmith, Stephen DeLong, Michael P. Bunds, Nathan A. Toke, & Manoochehr Shirzaei

Oral Presentation

2021 SCEC Annual Meeting, SCEC Contribution #11144
Topographic differencing reveals horizontal and vertical deformation of the Earth’s surface due to fault motion. This technique has been applied to large surface rupturing earthquakes where the along- and near-fault measurements show how the shallow fault zone deforms coseismically. We extend these differencing methods to the central San Andreas Fault and present spatially dense creep rates along the 140-km-long Creeping fault section. These measurements indicate along-strike and depth-dependent variability in the creep rate and the degree of localization of deformation along the fault trace. Mapping the creeping trace of the fault reveals the relationship between the actively creeping trace and the longer-term deformation recorded in the fault zone geomorphology. Our approach demonstrates a viable method for the use of repeat lidar following a large earthquake along the San Andreas Fault or elsewhere that would provide a fully resolved and 3D slip budget not possible from orthophoto or InSAR analyses.

Our results show a decade-long average creep rate of less than 10 mm/yr towards the ends of the section and approximately 30 mm/yr along much of the central 100 kilometers of the creeping fault. The creep rate decreases linearly from a maximum rate of ~40 mm/yr near Slack Canyon to 8 mm/yr at Parkfield, where the fault is partially creeping as it transitions to a fully locked segment farther to the southeast. By spatially resolving creep at a 20-m measurement spacing over an aperture of at least 1 km and comparing these rates to creepmeter rates, we find that 20-50% of the deformation occurs beyond a 30 m aperture. This amount of off-fault deformation exceeds observations for a coseismic fault rupture, possibly due to a slip-rate dependence on the localization of deformation. At Mustang Ridge along the central Creeping section, field and lidar-based mapping indicated that en echelon faults along 10 km of the fault zone appeared to accommodate this stepover, but the lidar differencing results indicate that the active creep is accommodated on a subset of these faults that span just 4 km of the fault zone. These results highlight the importance of topographic differencing for locating the actively creeping fault and for assessing fault displacement hazard. We will discuss future work including a lidar-InSAR creep inversion to constrain depth-dependent creep near Parkfield.