Using 3D modeling and machine learning, scientists claim they have solved the mystery of the tiny earthquakes that rumbled daily under Cahuilla, California, from early 2016 to late 2019 - nearly four years.

Some kind of natural fluid, such as water or liquid carbon dioxide, is likely to be the culprit. As a new study points out, it undoubtedly breached a barrier in the underground rock, altering the pressure and friction equilibrium along the fault line, and contributing to a long sequence of minor tremors.

The study can be seen in the Science journal.

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The Cahuilla Swarm's fault creep

The techniques deployed here could prove vital in understanding and predicting future earthquakes. Like the one that happened near Cahuilla, both major quakes and smaller swarms add up to tens of thousands of individual occurrences.

Around 22,000 mysterious earthquakes rumbled Southern California since 2016. The behavior has confounded scientists for years.

"We used to think of faults more in terms of two dimensions: like giant cracks extending into the earth," says geophysicist Zachary Ross, from the California Institute of Technology (Caltech).

"What we're learning is that you really need to understand the fault in three dimensions to get a clear picture of why earthquake swarms occur."

Ross and his colleagues used neural networks to examine thousands of seismic events, ranging from 0.7 to 4.4 in magnitude. The analysis revealed a deep, narrow 3D fault line extending down approximately 8 kilometers (5 miles).

The model showed an underground reservoir's likely presence. It was initially cut off from the fault zone before it leaked through and triggered the tremors. Spotting this was only made possible by high-resolution simulation of the team.

If the work is completed, it sheds light when viewed in profile on a narrow, 50-meter long fault zone with steep curves. It was those curves, Ross says, that revealed the explanation for seismic activity in the area for many years.

The study says the leading potential explanation for the swarms was groundwater circulation or a slow slippage on an active fault, called fault creep.

"The detail here is incredible," seismologist Elizabeth Vanacore from the University of Puerto Rico at Mayagüez, who wasn't involved in the research, told Maya Wei-Haas at National Geographic.

"This type of work is cutting edge and really where the science is going."

Swarms like the one studied here typically don't contain any major quakes. They are also much less predictable than the big earthquakes, which usually begin with a major shock followed by slowly declining aftershocks.

Slow quakes

What made this particular swarm fascinating is it lasted too long -- more like a swarm in slow motion. Other swarms are likely to last days, weeks, or months. The scientists say it is now winding down, probably because the fluid has reached an impermeable barrier.

The next step is to test the simulation technique further afield, across southern California and elsewhere - it could be that further earthquake swarms are responsible for natural fluid injections, which seismologists would study using this methodology.

"These observations bring us closer to providing concrete explanations for how and why earthquake swarms start, grow, and terminate," says Ross.

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