In the early evening of July 4, 2019, a magnitude 6.4 earthquake struck the Searles Valley in California’s Mojave Desert, sending aftershocks across southern California. About 34 hours later, on July 5, a magnitude 7.1 earthquake struck the nearby town of Ridgecrest. The shaking was so strong that it was felt by millions in California as well as in nearby areas such as Arizona, Nevada and Baja California, Mexico.

Known collectively as the Ridgecrest earthquakes, these were the strongest earthquakes to hit California in more than two decades. They have caused widespread structural damage, power outages and injuries. The 6.4 magnitude event at Searles Valley was later classified as an aftershock of the M7.1 event at Ridgecrest. Each earthquake was followed by numerous aftershocks.

Researchers were confused by the array of seismic activity. Why did it take 34 hours for the pre-shock to cause the main shock? How did these earthquakes “jump” from one part of the geological fault system to another? Can earthquakes dynamically “talk” to each other?

To answer these questions, a team of seismologists from the UC San Diego Scripps Institution of Oceanography and Ludwig Maximilian University of Munich (LMU) conducted a new study that focused on the link between two major earthquakes occurring along the multi-fault system. The team used a powerful supercomputer combining data-driven and physics-based models to determine the relationship between earthquakes.

Scripps Oceanographic seismologist Alice Gabriel, formerly of LMU, described the study as a former Ph.D. LMU student Taufik Taufikurrahman and several co-authors. Their findings were recently published in the journalism. Nature.

“We used the largest computers available and probably the most advanced algorithms available to try to understand this truly mysterious sequence of earthquakes that occurred in California in 2019,” said Gabriel, now an associate professor at the Scripps Institute for Oceanographic Geophysics and Planetary Physics. “High-performance computing has allowed us to understand the drivers of these major events, which can assist with seismic hazard assessment and preparedness.”

Gabriel said it’s important to understand the dynamics of multiple fault ruptures because these types of earthquakes tend to be stronger than those that occur on a single fault. For example, the twin earthquakes in Turkey and Syria on February 6, 2023 resulted in significant loss of life and significant destruction. This event was characterized by two separate earthquakes that occurred just nine hours apart, both propagating through multiple faults.

During the 2019 Ridgecrest earthquakes, which occurred along a thrust fault system in the eastern California fault zone, both sides of each fault moved predominantly in a horizontal direction with no vertical motion. Minor or secondary faults running at large (about 90 degree) angles to the main fault, a succession of earthquakes along interlocking and previously unknown “antithetic” faults. Debate continues in the seismological community about which fault segments are actively slipping and which conditions favor successive earthquakes.

While the new study presents the first multi-fault model to integrate seismograms, tectonic data, field maps, satellite data, and other space-based geodetic datasets with earthquake physics, previous models of such earthquakes relied solely on data.

“Through the lens of data-rich simulations enhanced by supercomputer capabilities, we unravel the intricacies of multi-level earthquakes, shedding light on the physics governing the dynamics of cascading fractures,” Taufiqurrahman said. Said.

Using the SuperMUC-NG supercomputer at the Leibniz Center for Supercomputing (LRZ) in Germany, researchers discovered that the events at Searles Valley and Ridgecrest were indeed linked. Earthquakes interacted through a statically strong but dynamically weak fault system driven by complex fault geometry and low dynamic friction.

The team’s 3D fracture simulation shows how faults that were considered strong before an earthquake can become very weak when the earthquake’s rapid motion occurs, and explains the dynamics of how multiple faults can rupture together.

“When fault systems are disrupted, we see unexpected interactions. For example, successive earthquakes that can move from segment to segment, or when one earthquake causes the next to move in an unusual path. Gabriel said the earthquake could be much stronger than we expected. “It’s hard to include in the seismic hazard assessment.”

According to the authors, their model has the potential to have a “transformative impact” on the field of seismology by improving seismic hazard assessment in active, often underestimated, multi-fault systems.

“Our findings suggest that similar models can incorporate more physics into seismic hazard assessment and preparation,” said Gabriel. Said. “Using supercomputers and physics, we have deciphered perhaps the most detailed dataset on the complex pattern of earthquake rupture.”