Damage from earthquakes at deep depths occurs long after the initial activity MIT News

Earthquakes often bring to mind images of destruction, the earth splitting apart and changing the landscape. But after an earthquake, the surrounding area goes through a period of post-earthquake deformation, where the areas that did not break face new stresses due to the sudden change in the surroundings. Once he adapts to this new stress, he reaches a state of recovery.

Geologists have often believed that this recovery period was a smooth, continuous process. But MIT research recently published in the journal Science found evidence that while healing occurs quickly at shallow depths — beyond about 10 kilometers — deeper depths recover more slowly, if at all.

“If you look at the shallow crust before and after, you don't see any permanent change,” says Jared Bryan, a graduate student in MIT's Department of Earth, Atmospheric, and Planetary Sciences (EAPS) and lead author on the paper. “But there is this very permanent change that continues in the middle crust.”

Other authors of this paper include Professors William Frank and Pascal Audet from the University of Ottawa.

Everything except earthquakes

In order to piece together a full understanding of how the crust behaves before, during and after earthquake sequences, researchers looked at seismic data from the 2019 Ridgecrest earthquakes in California. The immature fault zone experienced the state's largest earthquake in 20 years, and tens of thousands of aftershocks over the following year. They then removed the seismic data generated by the sequence and looked only at waves generated by other seismic activity around the world to see how their paths through the Earth changed before and after the sequence.

“One person's signal is another person's noise,” Brian says. They also used general ambient noise from sources such as ocean waves and traffic, which are also picked up by seismometers. Then, using a technique called the receiver function, they were able to see the speed of the waves as they traveled and how they changed due to conditions in the ground such as rock density and porosity, in the same way we use sonar to see how sound waves change when they interact with objects. Using all this information, they were able to create basic maps of the land around the Ridgecrest Fault Zone before and after the sequence.

What they found was that the shallow crust, extending about 10 kilometers into the Earth, recovered over the course of a few months. In contrast, the deeper depths in the middle crust did not suffer immediate damage, but rather changed on the same time scale as the shallower depths recovered.

“What was surprising was that the healing in the shallow crust was so fast, and then this complementary accretion occurred, not at the time of the earthquake, but instead during the post-earthquake phase,” Brian says.

Balancing the energy budget

Understanding how recovery occurs at different depths is crucial to determining how energy is expended during different parts of the seismic process, which includes activities such as releasing energy in waves, creating new fractures, or elastically storing energy in the surroundings. Collectively, this is known as the energy budget, and is a useful tool for understanding how damage accumulates and recovers over time.

What remains unclear are the timescales on which the abyssal depths will recover, if at all. The paper presents two possible scenarios to explain why this is: one where the deep crust recovers over a much longer period of time than they observed, or one where it does not recover at all.

“Neither of them were what we expected,” Frank says. “And they're both interesting.”

Further research will require more observations to build a more detailed picture to see how deep the change becomes more apparent. In addition, Brian wants to look at other areas, such as more mature faults that experience higher levels of seismic activity, to see if that changes the results.

“We'll tell you in 1,000 years whether it's been restored or not,” Brian says.

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