UC Berkeley scientists discuss link between weather cycles, seismic activity

BERKELEY, Calif. (KGO) -- Flooding is an immediate and obvious danger from our wet winter storms. There's also the potential for another kind of danger -- earthquakes.

All the rain and snow are sitting on our fault lines. It's heavy and it's more than we've had in years.

Pretty much every Bay Area city has far exceeded its normal amount of rain for the season, which started in October.

Gauges at the Sonoma County Airport have recorded nearly double the usual amount of rainfall. Even San Jose is at 137 percent of normal. As of Monday, the Sierra snow pack is at 188 percent of normal.

Scientists have long wondered if there is a connection between weather and earthquakes and now they're starting to get some answers.

"We get more seismicity during the summer months and when there is an unloading of the crust," UC Berkeley Seismology Lab's Chris Johnson said.

UC Berkeley seismologists say it's not necessarily heavy rainfall but our seasonal fluctuations that they're finding are linked to increased earthquake activity.

"We don't think there's an impact on bigger faults. What we've been focusing on is our smaller earthquakes, magnitude-2 or magnitude-3," Johnson said.

All the water and especially the heavy snow up in the Sierra Nevada essentially press down on the Earth's crust. When things dry out and melt in the summer, the crust rises by 5 to 10 millimeters over Northern California, triggering quakes near the surface.

Scientists at the USGS says the runoff from the heavy rainfall can take months or even years to get down to major fault lines, like the San Andreas, that are 2 to 10 miles deep.

"It's relatively easy for the water to percolate through the top layers of the Earth's crust, but as you get deeper and deeper, the permeability or ability of the water to seep down diminishes," USGS geophysicist Art McGarr said.

So scientists say the weather does trigger small surface temblors during the summer but so far, no evidence that rain or drought contribute to major earthquakes.