Monday, March 6, 2017

California Floods To Trigger "The Big One"? - Geologists Warn Of Quake Risks From Snowpack, Rising Reservoirs

SOURCE

For years geologists have warned that Southern California is overdue for "The Big One", a massive 8.0 or greater earthquake that would undoubtedly cause unprecedented death and destruction in several heavily populated urban centers sprinkled along the San Andreas Fault line.

While predicting earthquakes remains an uncertain science, there has been concern in recent years among experts that the San Andreas fault may be close to a new, major ruction if only by virtue of the length of time since it happened last, when the southern portion of the fault was struck by a 7.9 shaker all the way back in 1857.
Since then the tectonic plates that meet at the fault have been continuously on the move at a rate of about 2 inches per year. That means that over 159 years there has been a shift of 26 feet as the Pacific plate moves in a northwesterly direction against the American continental plate. Every additional inch creates additional pressures on the rocks beneath the earth’s surface that builds and builds until it eventually snaps.
San Andreas

Now, as the Los Angeles Times points out, the recent flooding in California has prompted some scientists to raise concerns over whether or not Californians are at a greater risk of being struck by an imminent quake. According to geologists, flooding can cause earthquakes in one of two ways: i) the sheer weight of rising reservoirs and snowpack causes tectonic plates to shift and/or ii) increasing pressure created from the refilling of underground water basins pushes plates apart, therefore reducing friction and allowing the earth's crust to shift.

There are two ways a reservoir can cause an earthquake. A rapid filling or emptying of a lake can change the weight pushing on a fault, which can make an earthquake more likely, said Bill Leith, acting associate director on natural hazards at the USGS.

“Especially for a reservoir as large as Oroville, it’s a huge weight on the crust that’s basically being pulled up and down on an annual cycle. So it wouldn’t be surprising if there were earthquakes associated with that,” Leith said. “The rapid filling, I just think it increases the risk. … I would expect that a rapid rise or a rapid fall in the water level would be much more likely to trigger earthquakes.”

The second way a reservoir can cause an earthquake is from added pressure. Water trickling deep into the earth can increase pressure underground that makes it easier for faults to move, according to seismologist Lucy Jones.

“The pressure in the water sort of pushes the fault back apart,” Jones said. The added water pressure underground essentially unclamps a fault — like loosening a vice that keeps two blocks of rocks stuck together — in a way that makes it easier for the earth to move.

It can take years for water to filter down into the deep crevices of the earth and add fluid pressure around the fault, Leith said. The dramatic fluctuation in reservoir levels in 1975 may have been the trigger point for the earthquake that year.

As proof of the added risk from flooding, geologists point to a series of earthquakes that occurred around Lake Oroville in the mid-70s after an urgent dam repair required a rapid draining and subsequent refilling of the reservoir.

Lake Oroville had been filled before. What made the winter of 1974-75 unusual was that water levels had to be reduced to their lowest level since the reservoir was first filled to repair intakes to the hydroelectric power plant.

Months later, there was an unprecedented refilling of the lake that ended in June 1975.

Then the earthquakes started.

Instead of fewer than five earthquakes a month in a zone within 25 miles from the dam, as had been the case for the previous year, June and July suddenly saw more than 10 earthquakes each month.

Then the largest earthquake in the sequence hit: a magnitude 5.7 on Aug. 1, 1975. It was strong enough to crack plaster and walls in Oroville and was felt as far away as San Francisco and Sacramento, where the Capitol’s dome suffered minor damage. The 50-person staff of Treasurer Jesse Unruh was ordered to evacuate its office.

That August became a banner month for earthquakes around Oroville Dam — more than 3,000 temblors were recorded, before fading to more than 700 a month later and over 100 by October.

Of course, in reality water levels at Lake Oroville fluctuate substantially pretty much every year with snow melt generally filling the reservoir from February through July followed by reductions as water is shipped to the drier southern regions of California. That said, it's difficult to argue that this year's recharge was fairly unprecedented in its volume and speed.