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In my last diary (“Oklahoma, Where It’s Complicated Deep Beneath The Plains”)   I noted the wide variety of anthropogenic triggers to some seismicity. That diary was specifically about what is going on under central Oklahoma. I wrote I wanted to return to the subject, because the subject of anthropogenically triggered /induced seismicity is broad and the oil and gas industry are not the only ones who are responsible.

A new paper was published in Nature that is very interesting. Using long-term GPS readings a research group has determined that in central California, the Coast Ranges and Sierra Nevada rise and fall out of phase with the seasons, and the cause is likely due to long-term groundwater depletion for agriculture.

A new paper was published in Nature that is very interesting. Using long-term GPS readings a research group has determined that in central California, the Coast Ranges and Sierra Nevada rise and fall out of phase with the seasons, and the cause is likely due to long-term groundwater depletion for agriculture.  

Anyone who has been to California’s Central Valley or lives there knows it’s sinking. The reason why is the same reason it’s the nation’s Garden Spot: agriculture. The need for water in a semi-arid climate has caused the valley floor to drop some nine meters (29 feet) in places. This subsidence is damaging in itself. It cracks roads, pipelines, the California aqueduct, and other infrastructure. Some areas have subsided below sea level, forcing the need for levee systems that are poorly maintained. It’s a disaster waiting to happen. Subsidence can be bad enough as it is. Subsidence coupled with sea-level rise, as Norfolk, VA is discovering, is a slow-motion disaster just waiting for that cascading failure to make it a total catastrophe.

At the same time, researchers couldn’t quite understand why the mountains were rising so fast. Tectonic models didn’t quite add up. It’s true that the motions along the plate boundary are  accommodated (“dealt with” ) not just at the San Andreas but all the way back into Utah, but even that couldn’t explain what was seen in long-term GPS monitoring.

And at the same time, it’s been known for some time that there’s a hydrological component to seismicity in areas of the country. Sometimes this can cause land to “bulge” or deform in an uplift. These uplifts are generally very small and can only be seen in the minute changes indicated by permanent GPS stations. In Southern California, a large area of land was uplifted by winter rains in 2005, although at the time it caused a mild panic that The Big One was just about to happen. Deformation in the form of uplift is considered to be one of the signs that a massive earthquake may occur. When the uplift “deflated”, so did the quake fears.

In central California, researchers have noted that along the Parkfield segment of the San Andreas, there’s a seasonal component to microquakes along the fault. When it rains, especially when it rains a lot, the weight of the water changes the stress conditions below ground. Same with the accumulation of snow and ice. When the water runs off, or the snow melts, the stress conditions in the crust change again. I’m simplifying a great deal here as I did with Oklahoma, but remember our clock analogy. It’s useful here as it was there.  

The clock will represent the stress that builds up in the upper crust of the Earth, everywhere. This stress acts on all faults that cross through the crust and along the Earth's surface. Every fault has a clock. Sometimes, several clocks, as many faults act in segments. One segment's clock may "tick" at a different rate than others. The fault-clocks of the earth all tick at different time-scales compared to others. When the clocks reach midnight, that means the shear stress has overcome the fault's resistance to slip. An earthquake occurs.
Please keep in mind that I'm greatly simplifying some very complex physics, and I'm probably not doing it the proper justice.
Let’s also remember what we learned about fluids and faults too.
It isn't that the fluids are lubricating the faults. That's not what is happening. Instead the fluid injection and its pressures change the stress conditions that are acting on any possible subsurface faults. Because all of the continents have an overall stress and strain field due to their motions relative to each other, a fault that is oriented in a favorable fashion may find its clock advanced significantly forward if the local stress and strain field is changed.
Before we go further, let’s take a trip to Spain.

On May 11, 2011, a group of extremely  shallow earthquakes shook the Murcia region in southeast Spain. The largest was Mw 5.1 and this earthquake, despite its moderate size and perhaps because of its very shallow depth, was extremely destructive to the town of Lorca. Nine people were killed, and three hundred were injured.

Another paper in Nature suggests that long-term groundwater depletion may have contributed to the stress changes along the active Alhama de Murcia Fault to cause it to fail. The groundwater depletion was over a period of several decades. The region around Lorca is agriculturally rich; the water table is said to have dropped some 250 meters since the early 1960s. In short, the clock of the Alhama de Murcia may have been wound forward. However because the Alhama de Murcia and its related faults are active, and the region is on a complicated plate boundary, it may be a coincidence. The discussion and research are still ongoing.

Back to California!

The Coast Ranges and the Sierra Nevada rise in the late summer and early autumn. This is because the snows have melted and the water has run off. The valley floor also rises in spring in response to runoff from the mountains. The runoff refills aquifers to some extent. The signal of both is seen in GPS stations scattered around California and Nevada.

In the late summer and autumn, the level of microseismicity on the San Andreas rises. This region is the transition region between two segments: the one that is “creeping” near Parkfield and the locked segment responsible for the 1857 earthquake that is estimated to have been Mw7.8. Both segments have a seasonal component to their microseismicity. The reason why are likely changes in groundwater.

Now remember that the valley floor has subsided in places as much as nine meters since the 1860s. Some 160 cubic kilometers of water has been removed from the region ---this is roughly the capacity of Lake Tahoe, or to put it another way, it could cover the entire land area of California to a depth of 14 inches. The weight of all that water removed has caused the underlying crust to bounce up. This is called isostatic rebound, and it has raised the Sierra Nevada.  The surrounding mountains of the San Joaquin Valley may have risen as much as 6 inches since the year 1860, far faster than they otherwise would have given the plate boundary convergence rate and erosion rates. 30 cubic kilometers of water alone have been removed for agriculture since 2003. This uplifted the Sierra by 10 milimeters (2/5ths of an inch.)

Over the past 150 years, around 40 trillion gallons of groundwater in California's Central Valley has been lost through pumping, irrigation and evapotranspiration. That's roughly equal to all the water in Lake Tahoe, the volume of which can cover the entire state of California in 14 inches of water.

"This massive withdrawal of water has relieved pressure on the Earth's crust, which is now rebounding upwards in response," Blewitt said. "This is counter-intuitive to most people, even geologists, who tend to only think that water withdrawal causes subsidence, which is only true in the sediments of the valley from which the water is withdrawn. With the weight of the groundwater missing, the hard-rock crust under the valley is actually rising too."

The stress changes (remember our clock!) could be enough to wind an already critical fault (and not necessarily the San Andreas) to fail, causing an earthquake. A review of the earthquake catalogue in central California suggests that there’s been a gradual increase of the rate of earthquake occurrence in the region, possibly related to anthropogenic use of groundwater.

I hope to keep exploring this topic as time allows. The links between human activities and the processes of the earth are fascinating, and perhaps quite dangerous.

The paper can be downloaded at Dr. Roland Burgmann’s site (it's the first one at the top--Amos et al.)

Originally posted to SciTech on Mon Jun 02, 2014 at 03:26 PM PDT.

Also republished by California politics, Central Valley Kossacks, and Community Spotlight.

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