Coseismic Slip of the 2014 Mw 6.1 Napa, California Earthquake Revealed by Sentinel-1A InSAR

LI Yongsheng1*, Feng Wanpeng2,3, ZHANG Jingfa1*,4, LI Zhenhong4 1 Key Laboratory of Crustal Dynamics, Institute of Crustal Dynamics, China Earthquake Administration; 2 Institute of Geophysics, China Earthquake Administration; 3 School of Geographical and Earth Sciences, University of Glasgow; 4 COMET,School of Civil Engineering and Geosciences, Newcastle University; * yongsheng2009@gmail.com

On 24 August 2014, an Mw 6.1 earthquake occurred in the Napa Valley shook a large area of northern San Francisco Bay, California, USA. An interferometric pair acquired from the newly launched ESA’s Sentinel-1A satellite was used to map the coseismic displacements, and then combined with GPS coseismic displacements to determine its fault geometry and slip distribution. The Sentinel-1A interferogram shows that this earthquake produced significant ground displacements with maximum uplift and subsidence of 10 cm in the satellite light of sight (LOS). The best-fit slip model in the joint inversion suggests that the major coseismic slip is concentrated on a right-lateral fault with a strike of 344° ,a dip of 80° and an average rake angle of -146.5°. The maximum slip of ~1.1 m at a depth of ~4 km, suggesting that shallow slip deficit happened during the mainshock. The accumulative seismic moment is up to 1.5x1018 N.m, equivalent to a magnitude of Mw 6.1. The seismic energy estimated using InSAR and GPS is less than that obtained by InSAR only, which might be due to considerable postseismic displacements included in the Sentinel-1A coseismic interferogram. The fault is believed to be part of the West Napa fault system, but little attenetion has been paid before the quake.

Summary

Tectonic background of the 2004 Napa Earthquake overlaid on topographic relief.

Red Lines represent the main faults in this region, whilst white line means the

surface fault rupture which was mapped by the filed works. Black dash lines

means the spatial coverage of the Sentinel-1A images.

(a) Original Sentinel-1A coseismic interferogram of the 24th Aug. 2014 Napa earthquake, which is generated by ROI_PAC

(b) LOS coseismic displacements of the Napa earthquake. White lines denote surface rupture trace.

(a) Comparison of modelled surface GPS horizontal deformation determined by different datasets.

(b) Comparison of modelled LOS surface deformation along the profile A-B in the Fig a.

Red points mean InSAR observations, whilst grey,blue and green lines denote modelled results

from the InSAR-Only ,GPS-Only and InSAR-GPS joint slip models respectively.

Comparison of slip models determined by different datasets. (a) InSAR based slip model; (c) the GPS only based slip model and (e) both data determined slip model. (b), (d) and (f) show the accumulative seismic moment distribution along the depth

for models (a), (c) and (e).

Source Locations

Depth(km) Focal Mechanisms Fault Dimensions

Magnitude Lon(°) Lat(°) Strike(°) Dip(°) Rake(°) Length(km) Width(km)

GCMT a -122.40 38.32 12 338 78 -174.0 - - 6.1

USGSb -122.35 38.30 5 341 80 -176 - - 6.05

This study

-122.33 38.29 5 344 81 -176.2 8.5 0.5 6.1c

-122.33 38.29 4 344 80 -146.5e 24 20 6.1d

Optimal fault geometric parameters determined by inverting the Sentinal-1A coseismic interferogram and GPS

Original, modelled interferograms and residuals for the Sentinel-1A coseismic interferometric pair.

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ID: 1398

Reference:LI Yongsheng,Feng Wanpeng, ZHANG Jingfa, Coseismic Slip of the 2014 Mw 6.1 Napa, California Earthquake Revealed by Sentinel-1A InSAR, Chinese Journal Of Geophysics, In press.

Position Time Series of the GPS Stations. Blue dashed line means the earthquake occurrence time, whilst red dashed line means the acquisition time of the postseismic Sentinel-1A image. NS displacement time series and (b) EW displacement time series

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