Coseismic Slip and Early Afterslip of the m6.0 August 24, 2014 South Napa, California, Earthquake

Date: 4/25/2019

Time: 06:00 PM

Room: Fifth Avenue

We employ strong motion seismic acceleration recordings (low-pass filtered at 1.5 Hz) combined with static offsets estimated from GPS, InSAR, alignment-array and mobile laser scanning observations in order to derive a kinematic coseismic slip and afterslip model of the M6.0 August 24, 2014 South Napa earthquake. This earthquake ruptured a ~13 km long portion of the West Napa fault with predominantly right-lateral strike slip. In the kinematic seismic slip inversions, we prescribe a priori a range of local rupture velocities, allowing for arbitrary shape of the local source-time function but bounding the effective rise time as measured by the second temporal moment of slip. We also couple the coseismic slip and afterslip distributions by requiring both distributions to involve predominantly right-lateral strike-slip motion with positive amplitude, with the net static slip being the sum of the two. We consider two candidate fault geometries: that of Wei et al. (2015) involving two steeply east-dipping planes, and that of Dreger et al. (2015) involving one steeply west-dipping plane. The resulting coseismic slip distribution involves up to ~1.2 m slip on a dominant shallow asperity about 10 km north of the hypocenter, associated with relatively long rise times, and on deeper asperities on the southern part of the rupture, associated with much shorter rise times. These high-slip zones define a unilateral rupture along a narrow slip zone that emanated updip and northward from the hypocenter to the maximum slip area. Afterslip of up to 1 m is concentrated along the southern part of the rupture at depths <~5 km, consistent with surface observations of afterslip. Seismic moments associated with coseismic slip and afterslip are 1.11x10¹⁸N m (Mw 6.00) and 3.25x10¹⁷N m, respectively.

Presenting Author: Fred F. Pollitz

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