Geologists have often thought that this recovery period was a smooth, continuous process. But MIT research published recently in Science has found evidence that while healing occurs quickly at shallow depths - roughly above 10 km - deeper depths recover more slowly, if at all.
"If you were to look before and after in the shallow crust, you wouldn't see any permanent change. But there's this very permanent change that persists in the mid-crust," says Jared Bryan, a graduate student in the MIT Department of Earth, Atmospheric and Planetary Sciences (EAPS) and lead author on the paper.
The paper's other authors include EAPS Professor William Frank and Pascal Audet from the University of Ottawa.
"One person's signal is another person's noise," says Bryan. They also used general ambient noise from sources like ocean waves and traffic that are also picked up by seismometers. Then, using a technique called a receiver function, they were able to see the speed of the waves as they traveled and how it changed due to conditions in the Earth such as rock density and porosity, much in the same way we use sonar to see how acoustic waves change when they interact with objects. With all this information, they were able to construct basic maps of the Earth around the Ridgecrest fault zone before and after the sequence.
What they found was that the shallow crust, extending about 10 km into the Earth, recovered over the course of a few months. In contrast, deeper depths in the mid-crust didn't experience immediate damage, but rather changed over the same timescale as shallow depths recovered.
"What was surprising is that the healing in the shallow crust was so quick, and then you have this complementary accumulation occurring, not at the time of the earthquake, but instead over the post-seismic phase," says Bryan.
What remains unclear is the timescales at which deeper depths recover, if at all. The paper presents two possible scenarios to explain why that might be: one in which the deep crust recovers over a much longer timescale than they observed, or one where it never recovers at all.
"Either of those are not what we expected," says Frank. "And both of them are interesting."
Further research will require more observations to build out a more detailed picture to see at what depth the change becomes more pronounced. In addition, Bryan wants to look at other areas, such as more mature faults that experience higher levels of seismic activity, to see if it changes the results.
"We'll let you know in 1,000 years whether it's recovered," says Bryan.
Research Report:"Crustal stresses and damage evolve throughout the seismic cycle of the Ridgecrest fault zone"
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Department of Earth, Atmospheric and Planetary Sciences
Tectonic Science and News
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