29 August 2016
By Brendan Bane
A handful of faults lining the border of California and Nevada may be near the point of rupture, according to a new study assessing earthquakes in the region as far back as 1,400 years ago.
Scientists report in a new study that earthquakes in a fault network east of the Sierra Nevada Mountains are not random, but are likely triggered from stress bestowed by past earthquakes.
This same type of stress has built up in six faults near Death Valley, California, and Reno, Nevada, according to the new research.
These faults could be the next among their network to generate a moderate to major earthquake, according to the new study published in Tectonics, a journal of the American Geophysical Union. Rupture on these faults is not imminent and the new study does not specify when an earthquake might occur.
“The spatial distribution of earthquakes in this region is not a random process,” said Alessandro Verdecchia, a geologist at the Ludwig Maximilian University of Munich, Germany, and lead author of the new study. “If we model these stress changes, we can see if a fault may be prone, perhaps ready, to produce an earthquake.”
The Pacific and North American plates meet to form a series of faults along the western edge of the United States. In some sections, like along the San Andreas Fault, the plates’ boundaries move past each other quickly and in easily discerned patterns. Faults like these have been studied extensively and the earthquakes they produce can be tracked to specific locations. These plate boundaries are referred to as localized because movement is confined to a small area near the plate boundary.
East of the San Andreas Fault, however, another section of the plates’ boundaries slips past each other in a very different way. There, along the California-Nevada border, is a network of many smaller, slower-moving faults. Unlike localized plate boundaries, movement along this section of the plates’ boundaries is diffused over a wider area. As a consequence, this fault network produces earthquakes occurring much farther apart in time and space. Because they interact in complex ways and less is known about them, these faults are generally more difficult to study, according to Verdecchia.
Looking at a map of the region’s past earthquakes, Verdecchia suspected there may be a pattern. Previous studies looked closely at earthquakes along the California-Nevada border, but many focused on a smaller timescale or fewer events. The new study, however, assessed all of the region’s tremors from 587 CE to 1954, a period comprising nearly 1,400 years of geological history.
Using data from previous studies, the study’s authors built a three-dimensional model of the fault network’s geometry. They used historical measurements to track how the region’s earthquakes had interacted over the centuries. The authors found most of the events shared a link: Coulomb stress.
Coulomb stress refers to the transfer of stress along a fault. To picture Coulomb stress, imagine rubbing the heels of two cleated shoes together. Stress accumulates where the spikes meet and unloads when they finally push past each other.
Stress is unloaded similarly during earthquakes. But that stress doesn’t disappear — it moves to and accumulates in new places by traveling along a fault. Some of the stress is transferred instantaneously, like during an earthquake. Other stress is transferred much more slowly, usually over a long period following an earthquake. To study where previous earthquakes have struck and figure out when and where they might rupture again, the study’s authors said it’s important to consider both types of Coulomb stress.
Verdecchia and his coauthors used their model to understand where Coulomb stress traveled between earthquakes. They found more than 80 percent of the earthquakes in the region happened in areas where the increase in stress was high, leading them to conclude that faults in the region with high stress increases could be the next to produce earthquakes.
While the majority of the faults in the study network have accumulated Coulomb stress, six of those faults have reached roughly the average stress level of 30 bars needed to trigger a moderate to major earthquake, according to the new study. Because those active faults haven’t produced an earthquake in more than 1,000 years, Verdecchia suspects they may be the next to fail, though their failure is not necessarily imminent nor does the new study predict when an earthquake might occur.
“If we can say a fault has accumulated 30 bars, then its number is very close to an average stress drop for a large earthquake,” Verdecchia said. “That means that fault may be ready to produce a large earthquake.”
Most of the next-to-fail faults, like the Black, White and Hunter Mountain fault zones, are in or near Death Valley, California. There are no densely populated areas nearby. But tourist destinations like Bishop, Big Pine and Mammoth lakes, which harbor a few thousand people, do stand near the fault zones, according to Verdecchia.
Further north, the Pyramid and Honey Lake fault zones stand closer to Reno, Nevada, which is home to more than 200,000 people. Honey Lake is roughly 96 kilometers (60 miles) from Reno, while Pyramid Lake is less than 64 kilometers (40 miles) away.
Verdecchia said the same method used in the new study has been used to examine faults in other areas of the world, though only in regions with detailed records of past earthquakes. He warns earthquake prediction in general is a distant enterprise, saying far too many uncertainties, like the lingering mysteries of earthquake mechanics, remain.
“The real challenge now is to try to minimize those uncertainties,” he said.
-Brendan Bane is an intern at AGU’s public information department. Follow him on Twitter @brendan_bane.