Scientists simulate earthquake swarms – Temblor.net

Computer simulations show that fluid flow can cause earthquakes.

Written by Akash Kharita, Indian Institute of Technology, Rorky, expert on scientific writing in Tumblr (seismo_akash)

Quotation: Kharita, A. , 2021, Scientists Simulate Earthquake Swarms, Temblor, http://doi.org/10.32858/temblor.178

In August of 2020, a swarm of earthquakes hit the southern tip of the Salton Sea, California. The swarm consists of at least 240 earthquakes with the largest recorded magnitude 4.9, according to the US Geological Survey.

Such swarms, which usually consist of frequent low-strength earthquakes at the same approximate location, may last for a few days, months, or even years. Seismologists have long suspected that fluids moving along faults cause swarms, but what exactly? How long will the swarm last? What controls the rate of earthquakes in a particular swarm? These questions have long puzzled seismologists.

Salton earthquake swarm in August 2020.

Although they may not have all the answers yet, researchers from Stanford University have used the most recent computer simulations to explore the relationship between fluids injected into a system failure – either by natural or synthetic means – and rates of earthquake occurrence. In their study published in Nature Communications, they found that the ebb and flow of fluid pressure around the fault plays a major role in whether that fault will erupt in an earthquake.

under pressure

“Earthquakes in many swarms migrate outward or in one direction, a behavior that is conceptually attributable to moving fluids,” says Eric Dunham, associate professor at Stanford University and co-author of the paper.

In such a model, any type of fluid – natural groundwater or industrial wastewater – can become trapped along a fault. As the amount of fluid in an enclosed space increases, the pressure exerted by the fluid in the trap also increases, says Robert Fiesca, associate professor of civil engineering at Tufts University, who was not involved in the study. Ultimately, the fluid pressure reaches a limit value, and a metered valve opens, allowing the fluid to flow out. This marks the start of the swarm. The newly released fluid moves along the fault (or faults), triggering a series of small earthquakes as the fluid gets stuck, accumulates, and released repeatedly – like valves that open and close sequentially, according to Viesca. This fluid migration, and the associated earthquake sequence, continues as long as high pressures can push the fluid through the pore spaces. Once the fluid pressure has decreased sufficiently, the swarm will dissolve, often abruptly as it may start.

To explore this “fault valve” model, Weiqiang Zhu, a doctoral student in geophysics at Stanford University and colleagues designed a fault simulation that predicts the relationship between the frequency of earthquakes during a given swarm and the rate of fluid flow along faults. Their computer simulation simulates the San Andreas Fault, California, which breaks through the Earth’s crust vertically. The team observed how typical fluid pressure changed over time and found that when it increases, more earthquakes occur.

This typical finding suggests that in practice, by simply tracking the movement of earthquakes, scientists can “better understand how fluids move inside the Earth’s interior, and how pressure changes,” says Dunham.

The Salton Sea region has hosted many earthquake swarms over the past decade. Credit: Diklion (CC BY-SA 4.0)

Future possibilities

“One of our most important findings is how constant injection of fluids, either from a natural source or through a syringe through a well, can intermittently lead to pressure pulses traveling through the fault,” says Dunham.

Viesca says the study could be expanded to include artificial sources of fluid injection, such as sewage disposal. “It is not entirely clear how this injection interacts with defects in depth, but it has been known to significantly increase earthquake rates, for example, in Oklahoma, an area historically unknown for its earthquakes,” he says.

Comments from Fiesca Dunham and colleagues foreshadow next steps. “We have not explored what happens when liquids are injected at much higher rates, such as during the disposal of wastewater from oil and gas processes or during geothermal energy production,” he says. “Our group is currently working on a solution to this problem.”

References

Zhu, Weiqiang, Kali L. Allison, Eric M. Dunham, and Yuyun Yang. “Valve malfunctions and evolution of pore pressure in simulating seismic and seismic slip sequences.” Nature Communications 11, no. 1 (2020): 1–11. DOI: 10.1038 / s41467-020-18598-z

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