Predicting Earthquake Damage

A crack in the Earth runs through a flat landscape
A ruptured fault in Searles Valley, California, after the 2019 Ridgecrest earthquakes. A study of earthquakes led by The University of Texas at Austin found that seismic shockwaves are shaped by jagged faults and the debris wedged between them. Credit: Ben Brooks/USGS

New research from the Jackson School of Geosciences could change the way scientists think about potential damage from earthquakes.

The study examined data from one of the densest seismic arrays ever deployed and found that earthquakes emit their strongest seismic shockwaves in four opposing directions. The effect, which leaves a pattern resembling a four-leaf clover, has been known for decades but never measured in such vivid detail.

Daniel Trugman, an assistant professor in the Department of Geological Sciences, said that the study looked at only one type of seismic shaking caused by very small earthquakes in northern Oklahoma.

“What’s important in these results is that close to the source, we’re seeing a variation in ground motion, and that’s not accounted for in any sort of hazard model,” Trugman said. He added that efforts were already underway to see how the phenomena plays out in California’s big fault systems.

The analysis was published in Geophysical Research Letters and is based on measurements of two dozen small earthquakes recorded by the LArge-n Seismic Survey in Oklahoma (LASSO), an array of 1,829 seismic sensors deployed for 28 days in 2016 to monitor a remote corner of the state measuring 15 by 20 miles.

Two rows of four circles. In the top left circle a blue line and black dots trace a four-leaf clover (double figure 8). The shape becomes progressively less clear until the last one where the black dots are scattered in a ring.
A weak, magnitude 2.03 earthquake measured at different seismic frequencies ranging from 2.50Hz (hertz) to 35Hz. The University of Texas at Austin-led study revealed that a tremor’s low frequency seismic waves travel in a four-leaf clover pattern; above about 15Hz however, the pattern breaks down and seismic waves travel in all directions. The finding could change how we think about potential earthquake damage. Credit: Daniel Trugman, Victor Tsai/AGU

When earthquakes strike, they release a thunderclap of seismic energy at many frequencies, but the actual ground shaking people feel ranges from about 1 hertz to 20 hertz. The study found that low-frequency energy — about 1 to 10 hertz — shot from the fault in four directions but barely registered outside of the four-leaf clover pattern. This is important because buildings are more vulnerable to low-frequency waves. The four-leaf clover pattern was not found for higher frequency waves, which travelled at equal strength in all directions, like ripples in a pond.

Co-author Victor Tsai, a geophysicist at Brown University, said that the reason the ground shook unevenly at different frequencies might have something to do with the complex geometry of earthquake faults and the broken-up material packed between them. This material redirects the energy randomly, but at lower frequencies seismic waves simply bypass the rough geologic mess near the fault, travelling in a nice four-leaf clover pattern just as physics predicts.

This means that on the surface, people might feel the same shaking regardless of where they stood, but buildings — which are sensitive to low-frequency waves — would feel the earthquake much more intensely within the lines of the four-leaf clover pattern.

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