Earthquakes triggered by the notorious San Andreas Fault in California may have been triggered by an ancient rising lake, a study has found.
The San Andreas Fault is the border section between two massive tectonic plates under the surface of the Earth. It stretches for more than 800 miles through California, past San Francisco and nearly as far south as San Diego.
The study, published in Nature on June 7, found that according to geologic and palaeoseismic data, six major earthquakes on the fault line occurred during times when the prehistoric Lake Cahuilla was rising.
The southern end of the fault lies next to what is now known as the Salton Sea, a remnant of the ancient lake. Lake Cahuilla used to fill periodically over the past thousand years, however, now, the Salton Sea is just a shallow body of water.
"We have found the Southern San Andreas Fault, which poses the largest seismic hazard in all of California, was likely triggered from the filling of an ancient lake called Lake Cahuilla. The lack of this lake since ~1725 might also help explain in part why we have not had a large earthquake on this section of the fault for 300 years," corresponding author of the study and graduate student at the University of California San Diego, Ryley Hill, told Newsweek.
Scientists already know that large bodies of water can stimulate seismic activity, the study said. However, the contribution it has to triggering a huge earthquake is not well studied.
The San Andreas Fault has been causing concern among experts as multiple segments appear to be significantly stressed. For this reason, they fear that a gigantic earthquake could be imminent.
This earthquake could be triggered by a rapid filling of the Salton Sea, according to this new study.
"This section of the San Andreas Fault has accumulated a great deal of tectonic stress. While it is very unlikely that Lake Cahuilla will fill again, there is potential that the Salton Sea, the modern day remnant of Lake Cahuilla, may be filled again," Hill said. "There is evidence to suggest that it might not just be the total weight/size of the lake that can trigger an event but the rate at which the lake fills. So if we were to rapidly increase the filling of the Salton Sea I would be very worried of potentially triggered earthquakes on the [southern fault]."
Water influences fault lines by increasing the pore pressure which reduces the normal stress on the line.
"The pore pressure increases in two ways. The first is the weight of the lake at the surface instantaneously increases the pore pressure at depth. Imagine squeezing a sponge... the pore pressure will increase... it increases so much so that fluid wants to escape out of the sponge... the Earth's crust does not let the fluid escape so easily so the pore pressure rises," Hill said.
"The second is with water seeping into the cracks and pores (diffusing) down the fault to depth, further unclamping the fault. It is a lot like a puck on an air hockey table. With the air on, the puck will slide easily but when the air is off the friction makes the puck difficult to slide. When you slide the puck with your fingers that's like the tectonic stress on the fault. However, the increasing pore pressure is like turning the air on—it pushes out on the two sides of the fault making it easier to slide, thus triggering an earthquake."
The U.S. Geological Survey has previously predicted that it is very likely that some areas across the San Andreas Fault will experience a magnitude 6.7 earthquake in the next 30 years.
There have been previous examples of earthquakes being triggered by water loads. One example is the 2008 Wenchuan earthquake in China, which was linked to the Zipingpu reservoir.
The new study demonstrates how stress changes influenced by hydrological loads could be underestimated.
"The modeling we use is a sophistication that is more true of the realistic behavior of the Earth's crust, but is much more difficult to solve. However, our results indicate that the fluid diffusion and the fluid-solid coupling that occurs is very important to consider," Hill said.
"Without considering these effects you can underestimate the potential stress changes induced by hydrological loads. We have already seen that large man made reservoirs probably triggered devastating earthquakes. There is also precedent that natural loads could be associated with seismicity. Our work shows some of the fundamental mechanisms involved in the process and we are hopeful it will shed new light on these human created and natural instances around the world."
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