Shaking, Shocking, and Disappearing

Groundwater Earthquakes
UC Berkeley researchers, publishing last week in Nature, determined that small earthquakes on the San Andreas Fault may be affected by groundwater depletion. Over the past 150 years, around 160 km³ (40 cubic miles)of groundwater in California’s Central Valley has been lost through pumping, irrigation, and evapotranspiration. This change in load on the underlying rock has notable impacts on crustal motion and seismic activity.

The scientists monitored GPS measurements that revealed that the regions surrounding the San Joaquin Valley are rising by up to 1–3 mm per year. This rise is sufficient to alter stresses on the adjacent San Andreas Fault, which may explain some of the annual modulation of seismicity observed in this area.

Our Turn?
A study published this week in the Bulletin of the Seismological Society of America warns that the San Francisco Bay Area is due for a large earthquake very soon. Looking at earthquakes of the last century and paleoseismic data, researchers led by David Schwartz of the U.S. Geological Survey found a cyclical pattern to seismic activity in the region. Schwartz told Nature News, “The idea is that stress builds up, is released and builds up again.” He says it could be time for another release.

Let’s just hope it doesn’t hit before the Bay Bridge rods and bolts are are fixed. The Chronicle reported last week that the defects could spell trouble in a large quake.

Lightning and the Solar Wind
Could the Sun be responsible for lightning here on Earth? A new study in Environmental Research Letters finds a link between increased thunderstorm activity on Earth and streams of high-energy particles accelerated by the solar wind. Studying the lightning rates across Europe, scientists witnessed a substantial and significant increase for up to 40 days after the arrival of high-speed solar winds, which can travel at more than a million miles per hour, into Earth's atmosphere.

“Cosmic rays, tiny particles from across the Universe accelerated to close to the speed of light by exploding stars, have been thought to play a part in thundery weather down on Earth,” explains lead author Chris Scott of the University of Reading. “But our work provides new evidence that similar, if lower energy, particles created by our own Sun also affect lightning. As the Sun rotates every 27 days these high-speed streams of particles wash past our planet with predictable regularity. Such information could prove useful when producing long-range weather forecasts.”

Disappearing Tule Fog?
Tule fog in California’s Central Valley is different from the fog we see here in the San Francisco Bay.  Tule fog is denser and occurs in the winter, while the fog that hits the coast usually occurs with warmer weather. The fog has caused major issues for highway safety, but a study published last week in Geophysical Research Letters reports that tule fog could be disappearing.

The fog has been in decline over the past several decades, and is affected by droughts and high rains. “Generally, when conditions are too dry or too wet, we get less fog,” says lead author Dennis Baldocchi of UC Berkeley. “If we’re in a drought, there isn’t enough moisture to condense in the air. During wet years, we need the rain to stop so that the fog can form.”

The tule fog helps chill the air in the valley and without it, warmer air could mean bad news for fruit and nut trees. Baldocchi, whose father grew almonds and walnuts in Antioch and Oakley, says, “The trees need this dormant time to rest so that they can later develop buds, flowers and fruit during the growing season. An insufficient rest period impairs the ability of farmers to achieve high quality fruit yields.”

Image: Cablekid/Wikipedia