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M6.0 – 6km NW of American Canyon, California

Summary

The Mw6.0 earthquake (depth 11km) occurred August 24th 2014 8km SSW of Napa, California.

More info here:

http://www.eqclearinghouse.org/2014-08-24-south-napa/category/general-info/

http://earthquake.usgs.gov/earthquakes/eventpage/nc72282711#summary

Comparative ShakeMaps for the Mw 6.0 South Napa earthquake and the 2013 Mw 6.6 Grassmere (NZ) earthquake

(info compiled by C. Holden with ShakeMaps provided by N. Horspool (input fault geometry from I. Hamling) (GNS Science)  and USGS, with help from Quincy Ma (University of Auckland))

The 2014 Mw 6.0 South Napa earthquake and the 2013 Mw 6.6 Grassmere earthquakes share similarities in:

  • magnitude sizes and depths (both events being rather shallow and of moderate magnitudes),
  • focal mechanisms (strike-slip earthquakes)
  • High recorded peak ground accelerations: 70%g 6km from the Grassmere epicentre and 63%g 8km from the South Napa epicentre
  • and affected regions (provincial and wine country regions).

The following figures provide comparative information on how shaking characteristics such as Instrumental Intensity (figure 1), Peak Ground Acceleration (figure 2) and 1s period spectral acceleration (figure 3) impacted the Napa (a) and Marlborough (b) regions.

 

Fig 1a- Instrumental Intensity ShakeMap for M6.0 South Napa earthquake (Source USGS)

Fig 1b- Instrumental intensity ShakeMap for the 2013 M6.6 Grassmere earthquake (source N. Horspool GNS Science) - see details below

Fig 2a- Peak Acceleration (%g) ShakeMap for the M6.0 South Napa earthquake (source USGS)

Fig 2b- Peak Acceleration (%g) ShakeMap for the 2013 M6.6 Grassmere earthquake (source N. Horspool GNS Science) - see details below

Fig 3a- 1sec Spectral Acceleration (%g) ShakeMap for the M6.o South Napa earthquake (source USGS)

Fig 3b- 1sec Spectral Acceleration (%g) ShakeMap for the 2013 M6.6 Grassmere earthquake (source N. Horspool GNS Science) - see details below

(Side by side comparison as a pdf)

Shakemaps for the Mw 6.0 SouthNapa earthquake have been produced by USGS and are accessible here:

http://earthquake.usgs.gov/earthquakes/eventpage/nc72282711#shakemap

Shakemaps for the Mw 6.6 Grassmere earthquake have been produced by Nick Horspool (GNS Science):

the “ShakeMap” is showing the shaking intensity as determined by GeoNet instruments and modelled against ground conditions (Horspool, Ristau and Salichon, GNS Science). This map is preliminary and people are advised to use actual recorded data if possible. The map is based on the ground motion prediction equation of Bradley (2013). The Vs30 grid has a spatial resolution of 100 m and is based on an assigned average velocity for 8 soil classes which was developed by Nick Perrin and Dave Heron based on geological maps of NZ (QMAP data, GNS Science). ShakeMap takes observed ground motions, and uses the ground motion prediction equation and Vs30 grid to interpolate predicted ground motions (PGA, PGV, MMI) onto a uniform grid.