South Napa earthquake linked to summer groundwater dip | 6/13/2018 | Staff
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A summertime expansion in the Earth's crust caused by changes in groundwater may have triggered the magnitude-6.0 earthquake in California's wine country in 2014, according to a new study.

The August 24, 2014 South Napa quake was the largest earthquake to shake the San Francisco Bay Area since the magnitude-6.9 Loma Prieta quake in 1989. It was also the first earthquake in the region since 1906 to break through to the surface, leaving buckled asphalt and cracked soil in vineyards along the length of the fault.

Earthquake - Kilometers - Miles - San - Pablo

The earthquake originated 11.1 kilometers (6.7 miles) under the San Pablo Bay marshes along the West Napa Fault. Falling bricks, debris and household objects killed two people and injured close to 300, and the earthquake caused roughly half a billion dollars of damage.

The Earth's surface is a collage of thin, rigid plates floating on a hot, liquid interior. Friction prevents the plates from moving smoothly past each other and stress builds up in the rocks as the plates pull and push on each other. Earthquakes occur when pieces of the Earth's crust slide past each other to release this stress.

Study - Journal - Geophysical - Research - Solid

In the new study in the Journal of Geophysical Research: Solid Earth, a journal of the American Geophysical Union researchers report the discovery of an expansion occurring in Earth's crust every summer at the site of the South Napa earthquake, adding to the stress on the fault. Their analysis suggests seasonally receding groundwater under the Napa and Sonoma valleys, which flank the fault, causes the summertime expansion.

The study's authors believe the added seasonal stress could have been the final straw that caused the fault to fail in 2014.

Accumulation - Stress - Earth - Plate - Tectonics

"You have an overall, long-term accumulation of stress in the Earth, that happens from standard plate tectonics forces. Then you have a seasonal component," said Meredith Kraner, a graduate student...
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