The Mw 6.0 24 August 2014 South Napa Earthquake

The M w 6.0 24 August 2014 South Napa

Earthquake

by Thomas M. Brocher, Annemarie S. Baltay, Jeanne L. Hardebeck, Fred

F. Pollitz, Jessica R. Murray, Andrea L. Llenos, David P. Schwartz,

James Luke Blair, Daniel J. Ponti, James J. Lienkaemper, Victoria E.

Langenheim, Timothy E. Dawson, Kenneth W. Hudnut, David R. Shelly,

Douglas S. Dreger, John Boatwright, Brad T. Aagaard, David J. Wald,

Richard M. Allen, William D. Barnhart, Keith L. Knudsen, Benjamin A.

Brooks, and Katherine M. Scharer

INTRODUCTION

The M w 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 M w 6.9 1989 Loma

Prieta earthquake. The rupture from this right-lateral earthquake propagated mostly unilaterally to the north and updip, 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¨CVIII 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¨C11 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 northeastdipping 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.

TECTONIC SETTING OF THE SOUTH NAPA

EARTHQUAKE

The South Napa fault rupture lies within an 80 km wide set of

major north-northwest-trending faults of the San Andreas

fault system, forming the boundary between the Pacific and

doi: 10.1785/0220150004

North American tectonic plates (Jennings, 1994; Fig. 1).

The West Napa fault system is a relatively minor strike-slip

fault in the Contra Costa shear zone, which transfers slip between the Northern Calaveras, West Napa, and Concord faults

(Unruh and Kelson, 2002a,b; Kelson et al., 2004, 2005; Wesling and Hanson, 2008; Brossy et al., 2010). Block modeling of

Global Positioning System (GPS) data estimated a slip rate of

4:0 ! 3:0 mm=yr on the West Napa fault (d¡¯Alessio et al.,

2005). In the Uniform California Earthquake Rupture Forecast (UCERF 3) model, the entire Contra Costa shear zone

was assigned a maximum slip rate of 1 mm=yr (Field et al.,

2013, appendix C, 18 pp). The earthquake was located near

the eastern shore of San Pablo Bay, midway between two major

active fault systems: the Hayward¨CRodgers Creek fault system

12 km to the west and the Concord¨CGreen Valley fault system

13 km to the east. The earthquake epicenter lies 1.7 km west of

the main mapped surface trace of the West Napa fault system

and close to the surface traces of the lesser known Carneros and

Franklin faults (Graymer et al., 2002). Although several faults

are mapped in the vicinity of the earthquake, only the West

Napa fault system is known to have displaced Holocene-age

sediments (Wesling and Hanson, 2008).

The West Napa fault system forms the western margin of a

basin that underlies much of Napa Valley. The basin, filled with

Cenozoic sedimentary and volcanic deposits, is ¡«2 km deep

beneath the city of Napa. The South Napa earthquake occurred where the prominent east-facing gravity and magnetic

gradients associated with the main bedrock strand of the West

Napa fault system diminish and are replaced by prominent

west-facing gravity gradients that mark the eastern margin

of the San Pablo Bay basin and the Carneros and Franklin

faults (Langenheim et al., 2006). The 2000 M w 4.9 Yountville

earthquake was attributed to the main bedrock strand of the

West Napa fault system 20 km north-northwest of the South

Napa epicenter (Langenheim et al., 2006). Modeling of potential-field data, coupled with aftershock locations, indicates a

Seismological Research Letters

Volume 86, Number 2A

March/April 2015

309

122¡ã15'W

38¡ã15'N

38¡ã22'30"N

122¡ã22'30"W

0

Mainshock M6

Red Tags

M3

Yellow tags

M2

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