From: Roger Greenway, ENN
Published August 26, 2014 07:05 AM

The Magnitude 6.0 NAPA earthquake

A strong earthquake hit the Napa valley yesterday.

The US Geological Survey has provided a detailed summary of the quake and the possibilities of both strong aftershocks and the remote potential of an aftershock stronger than the quake (Magnitiude 6.0).

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The earthquake occurred within a 70-km-wide (44 miles) set of major faults of the San Andreas Fault system that forms the boundary between the Pacific and North American tectonic plates. The persistent northwestward movement of the Pacific plate relative to North America primarily causes right-lateral slip across the major faults, but also causes deformation between the major faults. The ongoing complex deformation field is revealed by modern geodetic surveys and earthquake patterns as well as the regional geologic structure.

The earthquake was located north of San Pablo Bay between two major active fault systems: the Hayward-Rodgers Creek Fault system on the west and the Concord-Green Valley Fault system on the east. The earthquake occurred near the well-known West Napa Fault, and the less well known Carneros-Franklin Faults, which juxtapose different suites of rocks. Although there are several faults in the region of the earthquake, only the West Napa Fault is known to have displaced Holocene-age sediment - which is positive evidence of surface fault rupture in the last 11,000 years. 

Historically, in this region shaking sufficient to seriously damage structures at Mare Island occurred during the M6.8 1868 Hayward Fault earthquake, the M7.8 1906 San Andreas Fault earthquake, and particularly during the M6.3 1898 Mare Island earthquake. The 1898 earthquake may have occurred about 20 km (12 miles) to the northwest on the southern Rodgers Creek Fault. Even larger nearby events than the 1898 earthquake can be expected in the future. In addition, the epicentral region of this earthquake is depicted on the USGS National Seismic Hazard Maps to have a high probability of strong shaking in the future.

Wine cellar in Napa Valley image via Shutterstock.

Read more at USGS.

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