The Mw6.0 24 August 2014 South Napa earthquake

Seismological Research Letters
By: , and 



The Mw 6.0 South Napa earthquake, which occurred at 10:20 UTC 24 August 2014 was the largest earthquake to strike the greater San Francisco Bay area since the Mw 6.9 1989 Loma Prieta earthquake. The rupture from this right‐lateral earthquake propagated mostly unilaterally to the north and up‐dip, directing the strongest shaking toward the city of Napa, where peak ground accelerations (PGAs) between 45%g and 61%g were recorded and modified Mercalli intensities (MMIs) of VII–VIII were reported. Tectonic surface rupture with dextral slip of up to 46 cm was observed on a 12.5 km long segment, some of which was along a previously mapped strand of the West Napa fault system, although the rupture extended to the north of the mapped Quaternary strand. Modeling of seismic and geodetic data suggests an average coseismic slip of 50 cm, with a maximum slip of about 1 m at depths of 10–11 km. We observed up to 35 cm of afterslip along the surface trace in the week following the mainshock, primarily along the southern half of the surface rupture that experienced relatively little coseismic offset. Relocation of the sparse aftershock sequence suggests en echelon southwest‐ and northeast‐dipping fault planes, reflective of the complex fault geometry in this region. The Napa basin and historic and late Holocene alluvial flood deposits in downtown Napa amplified the ground motions there. Few ground failures were mapped, reflecting the dry season (as well as a persistent drought that had lowered the groundwater table) and the short duration of strong shaking in the epicentral area.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title The Mw6.0 24 August 2014 South Napa earthquake
Series title Seismological Research Letters
DOI 10.1785/0220150004
Volume 86
Issue 2A
Year Published 2015
Language English
Publisher Seismological Society of America
Contributing office(s) Earthquake Science Center
Description 18 p.
First page 309
Last page 326
Country United States
State California
Other Geospatial Napa Valley
Online Only (Y/N) N
Additional Online Files (Y/N) N
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