Uncorking Shallow Slip and the Slip History of the 2014 South Napa Earthquake

Abstract:

Shallow fault slip (< ~1km) during and immediately following earthquakes is poorly understood, largely because of challenges measuring deformation near a surface rupture. The need for better measurement is further motivated by an apparent deficit of shallow slip in regional source models of strike-slip earthquakes and by the suggestion that near-surface frictional heterogeneity over spatial scales of 100s of meters can control shallow fault slip. Here, we use a nascent mobile laser scanning technique to quantify with unprecedented detail the coseismic surface rupture and rapid post-seismic deformation from the 24 August, 2014 M6.0 South Napa earthquake. We infer shallow fault slip and find that both co- and post-seismic slip at depths of ~3-25 m significantly exceeds traditional measurements of surface displacements. There is no deficit in shallow slip: near-surface slip values are greater than maximum reported co-seismic fault slip values at depth. By ~ 1 month, afterslip along the southern portion of the fault accounted for as much shallow slip potency as the shallow co-seismic rupture on the northern portion. Further, we show that the afterslipping portion of the fault cuts across a ~3000 m thick sedimentary basin whereas the co-seismically ruptured portion does not. A rate and state friction model is consistent with the basin thickness, afterslip, and rock-sample mechanical measurements and strongly suggests that near-surface frictional heterogeneity controlled the distribution of coseismic and post-seismic shallow slip. In the future, we suggest that combining existing basin thickness data with active fault maps could provide more precise estimates of where surface rupture and/or afterslip may occur, both before, and in rapid response to, damaging earthquakes.