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 (dˇŻAlessio 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 ˇ«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
122ˇă15'W
38ˇă15'N
38ˇă22'30"N
122ˇă22'30"W
0
Mainshock M6
Red Tags
M3
Yellow tags
M2
M ................
................
In order to avoid copyright disputes, this page is only a partial summary.
To fulfill the demand for quickly locating and searching documents.
It is intelligent file search solution for home and business.
Related download
- c caassccaaddiiaa ssuubbdduuccttiioonn zzoonnee eeaarrtthhqquuaakkeess
- regional report labor market risks of a magnitude 7 8 earthquake in
- frequency of earthquakes in california california institute of
- the mw 6 0 24 august 2014 south napa earthquake
- chapter 8 earthquake
- earthquake outlook for the san francisco bay region 2014 2043 usgs
- forecasting california s earthquakes—what can we expect in the next 30
- investigations of the 8 november 1980 earthquake in humboldt usgs
- earthquake ridgecrest california cs3 ed
- earthquake california today just now san gabriel