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Study: 2014 Napa quake may be linked to groundwater…


Research suggests the magnitude 6.0 earthquake that rocked California wine country in 2014 may have been caused by an expansion of Earth’s crust because of seasonally receding groundwater under the Napa and Sonoma valleys.

The vineyard-filled valleys flank the West Napa Fault, which produced the quake that killed one person, injured several hundred and caused more than $500 million in losses.

The study recently published in the American Geophysical Union’s Journal of Geophysical Research: Solid Earth suggests land between the valleys is stretched each summer as groundwater levels fall beneath the valleys and the ground in the valleys sinks and contracts.

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Bricks and debris surround the Alexandria Square building in Napa, California following Sunday's earthquake that has left 15 to 16 buildings no longer inhabitable after the 6.0 tremor

Bricks and debris surround the Alexandria Square building in Napa, California following the earthquake that left 15 to 16 buildings no longer inhabitable after the 6.0 tremor

THE NAPA QUAKE 

The early morning Napa quake on Aug. 24, was the largest to hit the San Francisco Bay Area since the magnitude 6.9 Loma Prieta earthquake of Oct. 17, 1989.

It left 8 miles (12.8 kilometers) of surface rupture and damaged many historical masonry buildings and older residences, according to the U.S. Geological Survey.

 

The amount of the horizontal stretching measured is tiny – about 3 millimeters (0.12 inch) – but enough to stress faults, according to the researchers.

‘We think it’s more of a localized effect, something related to the groundwater system. 

‘We don’t know if it is groundwater pumping specifically, or something related to how the natural aquifer system works, or a combination,’ said lead author Meredith Kraner, formerly of the Department of Geosciences at Stony Brook University in New York and now with the University of Nevada, Reno.

Co-authors were William E. Holt of Stony Brook University and Adrian A. Borsa of the Scripps Institution of Oceanography at the University of California, San Diego.

The early morning Napa quake on Aug. 24, was the largest to hit the San Francisco Bay Area since the magnitude 6.9 Loma Prieta earthquake of Oct. 17, 1989.

It left 8 miles (12.8 kilometers) of surface rupture and damaged many historical masonry buildings and older residences, according to the U.S. Geological Survey.

Skater Gabe Haugen (R) of Napa took advantage of a buckled street after a 6.1 magnitude earthquake hit the San Francisco Bay Area, in Napa, California, USA, 24 August 2014.   More than 70 people were sent to hospital with injuries and power outages darkened multiple cities in northern California after a 6.1-magnitude earthquake struck early on 24 August. The United States Geological Survey (USGS) said the earthquake struck at 3:20 am (1020 GMT) at a depth of 10.8 kilometres. It was located nine kilometres south-west of the Napa wine region, and 81 kilometres north of San Francisco.

Skater Gabe Haugen (R) of Napa took advantage of a buckled street after a 6.1 magnitude earthquake hit the San Francisco Bay Area, in Napa, California, USA, 24 August 2014. More than 70 people were sent to hospital with injuries and power outages darkened multiple cities in northern California after a 6.1-magnitude earthquake struck early on 24 August. The United States Geological Survey (USGS) said the earthquake struck at 3:20 am (1020 GMT) at a depth of 10.8 kilometres. It was located nine kilometres south-west of the Napa wine region, and 81 kilometres north of San Francisco.

Experts have used a radical new satellites to capture unique psychedelic images of the ruptures in the Earth’s crust caused by the Napa earthquake.

Radar images from the UK’s Sentinel-1A satellite have been used to map the biggest earthquake that has shaken northern California in 25 years.

The images reveal the rupture is larger than previously thought.

Yngvar Larsen from Norway’s Northern Research Institute and Petar Marinkovic from PPO.labs in the Netherlands processed this new interferogram from two images: one that Sentinel-1A acquired on 7 August, the day the satellite reached its operational orbit, and another captured on 31 August. 

By processing two Sentinel-1A images, which were acquired on 7 August and 31 August 2014 over this wine-producing region, an interferogram was generated. The two round shapes around Napa valley, which are visible in the central part of the image, show how the ground moved during the quake.

By processing two Sentinel-1A images, which were acquired on 7 August and 31 August 2014 over this wine-producing region, an interferogram was generated. The two round shapes around Napa valley, which are visible in the central part of the image, show how the ground moved during the quake.

It clearly confirms that part of the West Napa Fault system was responsible for the 6.0 earthquake that rocked California’s wine-producing region.  

However, the fault had not been identified as being particularly hazardous prior to the quake that hit on 24 August.

Importantly, the extent of the ground deformation in the interferogram shows that the fault slip continues further north than the extent of the rupture mapped at the surface.

Sharp lines in the interferogram show minor movements on other faults, such as the part of the West Napa Fault system that crosses Napa airport.

The maximum deformation is more than 10 cm, and an area of about 30×30 km was affected significantly. 

 

 



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