napa, street view, earthquake damage
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The earthquake in Napa caused a half-billion dollars worth of damage.

On Aug. 24, 2014, an earthquake shook Northern California’s Napa-Sonoma Valley. It was the largest in the San Francisco Bay Area in 25 years and caused damage that cost half a billion dollars. Though earthquakes happen all the time in California, it has been suggested that there is such a thing as earthquake weather. One scientist proved that’s true.

Meredith Kraner, a geophysicist from the University of Nevada, examined seismic recordings from the region around the Napa quake. She discovered a pattern of expansion and contraction in the Earth’s crust. A study that describes this finding in the Journal of Geophysical Research, Kraner and her colleagues also explore whether seasonal fluctuations in local aquifers might explain that cycle of expansion and contraction, a phenomenon that could have triggered the earthquake itself.

How Earthquakes Happen

Earthquakes occur when Earth’s slow-moving tectonic plates suddenly slip against one another, usually due to a build up of stress that destabilizes the two. “It’s just like breaking a stick,” Kraner said. “If you pull on it and pull on it, it’ll reach breaking point eventually.”

The fluctuations in the Earth’s crust  suggested that there may have been an added seasonal element that triggered this process. Earthquake weather? She identified this pattern by collecting data from Earthscope. It is a vast network of high-precision GPS sensors spread across the earthquake-prone western United States. Since 2005, these sensors have been recording millimeter-scale shifts in the Earth’s crust. There is a huge and detailed data set. Using this information, Kraner was able to precisely map out the expansion and contraction in the crust around the earthquake zone.

Earthquake Weather?

“The crust is in extension during the late summer, and it’s in contraction during the winter. You see that happening every single year in that location,” Kraner said. “Our theory is that this seasonal component provided the final straw to this earthquake occurring.”

How is this unusual seasonal cycle linked to the quake? After the team ruled out several other factors, “the only thing we could think of it being related to was some sort of local aquifer system,” Kraner said. To test that idea, Kraner used satellite data of the Napa-Sonoma Valley. This revealed noticeable seasonal changes in the ground elevation over the areas where aquifer basins exist.

Groundwater Depletion

It’s known that increases and declines in groundwater levels can cause the elevation at the Earth’s surface to rise and fall. If groundwater levels do recede in summer due to low water availability, that would cause the land above to subside. As this happens, it would pull on the crust, contracting it horizontally and stretching the land surrounding the aquifer.

If aquifer basins on either side of the fault line did recede, this would also cause the land to contract horizontally on either side of the fault. (GPS recordings in the data showed a 3 millimeter spread across the landscape in summer.) Like loosening a clamp, that would release the stabilizing pressure on the fault. It would make it easier for plates to slide against each other and trigger an earthquake.

On top of these aquifers are the California wineries that pump up a lot of water. That dependence on groundwater increases during the dry summer months and during a drought. Groundwater levels also fluctuate naturally, driven by rains, evaporation and other natural phenomena.

This is the first study to identify this localized pattern of expansion and contraction in the Earth’s crust. Kraner’s research does reveal that seasonal stress may be one of many factors helping to trigger earthquakes. Understanding this seasonal element might one day help scientists create richer and more-accurate forecasts for earthquakes.

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