What causes seismic earthquakes?
On the morning of July 4, 2019, a series of very small earthquakes began to overturn in the Mojave Desert, near the town of Ridgecrest in Southern California. Geologists take measurements of a fault rupture the day after a 7.1-magnitude earthquake occurred near Ridgecrest, California (Image source: Ben Brooks, USGS)  No one knew at the time that these were earthquakes that would soon be followed by two of the largest earthquakes to hit California in more than 20 years. An earthquake measuring 6.4 on the Richter scale hit the area on July 4, throwing bottles off store shelves, smashing windows and water pipes, slashing a highway and prompting evacuations. After two days and hundreds of small earthquakes, a 7.1-magnitude earthquake (about 11 times stronger) hit a different fault in the same area, leaving behind fires, leaking water and gas lines, cracking buildings and roads and forcing the area to evacuate. A naval weapons facility larger than Rhode Island. The US Geological Survey reports that nearly half of the largest earthquakes recorded in California have been preceded by earthquakes. Worldwide, there is a 6 percent chance that any given earthquake will turn into a foreboding tremor followed by a larger earthquake within three days, although this probability diminishes with increasing time from the initial event. But earthquakes are not recognized until it is too late. A frontal shock is simply an earthquake followed by a larger earthquake  the main shock, explained Paul Segal, professor of geophysics in the School of Earth, Energy and Environmental Sciences at Stanford University (Stanford Earth). Usually, for a previous earthquake to be considered a foreboding, seismologists also look for the epicenter in the same general area as the main shock  at a distance of no more than a few times the length of the fault section that moved during the main shock. Because shocks precede larger earthquakes, increasing in frequency as the main shock approaches, they have long presented a tantalizing possibility to warn of potentially devastating earthquakes. If we can somehow determine that a particular earthquake was a precursor before the main shock arrived, that could be very helpful, Segal said. Tremors arise from fatal errors  For decades, scientists have sought to understand the physical processes that lead to foreboding tremors, and why some earthquake sequences trigger them while others do not. One leading theory proposes that the earthquakes are caused by the acceleration of slip motions along the fault. This motion, known as seismic slip, causes small earthquakes as it extends across larger fault regions and accelerates. The alternative idea is a series of small earthquakes without seismic slip, said Segal, professor of geophysics Cecil H and Ida M. Green. Earthquakes redistribute stress within the Earth. In areas of increased stress, this can cause further earthquakes. This increased cascade eventually triggers an event that grows into the main shock.  Recent research by Segal and seismologist Camila Catania in the Journal of Geophysical Research: Solid Earth suggests that earthquakes are actually driven by feedback between the two. We found that both mechanisms  seismic slip and stress redistribution by small earthquakes  play a role during the foreboding sequence, and they reinforce each other, said Catania, an assistant professor in MITs Department of Earth, Atmospheric and Planetary Sciences who worked on the research as a research scientist. At Stanford University, complex fault engineering allows simultaneous seismic slip and avalanche. Like many natural systems, faults are fractal, which means they are irregular on all scales. When the two sides of the rough fault slide apart, some parts of the fault are pressed together, forming stuck patches, Catania explained. The earthquakes represent the rupture of these stuck patches. These ruptures then increase the pressure in the surrounding areas, gradually cleansing the fault and causing an acceleration of the seismic slip. Faster seismic slip, in turn, compresses nearby seismic patches and leads to more tremors. These positive feedbacks cause an expansion of the slip zone and increased frequency of prognostic tremors, leading to the main shock.  According to Cattania, this means that fault roughness is the primary cause of the feedback between frontal tremors and seismic slip. She said, Since all faults are coarse, seismic and seismic slip likely coexist during most prognostic tremor sequences. The proposed classical mechanisms for shockeither driven by seismic slip or entirely seismic cascadeare all part of the process, but they are not mutually exclusive.  Earthquake sequence simulation  Evidence for the idea of earthquakes resulting from a combination of accelerated seismic slip and series of small earthquakes comes from computer modeling. Cattania and Segall built numerical simulations of seismic cycles on approximate faults, with geometry consistent with data from the field and lab. The teams simulations showed the acceleration of the tremors leading up to the main shock, consistent with patterns seen in data averages from real-world shocks  a testament to the models accuracy. In the future, it is possible that the model predictions will be used to assemble new insights into the prognostic tremor sequences from existing datasets and to reveal missing or previously misinterpreted seismic slips. Our results provide a physical explanation for the earthquakes, Catania said. But there is still a lot of work to be done before this knowledge can be translated into forecasting.  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