How do we know and earthquake happened if we didnt feel it?

These events are especially challenging to explain because of their unstable but sluggish nature. The fault does not slide steadily but instead, sliding periodically, accelerates, yet never reaches the point where it sends out seismic waves large enough for humans to detect.

Despite their stealthy nature, slow slip events can add up. In an ice stream in Antarctica, the slow slip events occur twice daily, last 30 minutes and are equivalent to magnitude 7.0 earthquakes, Dunham said.

Researchers think changes in friction explain how quickly rock on either side of the fault slips. With that in mind, they assumed slow slip events started as earthquakes, with a type of friction known as rate-weakening that makes sliding fundamentally unstable. But many laboratory friction experiments contradicted that idea. Instead, they had found that rocks from slow slip regions display a more stable kind of friction known as rate-strengthening, widely thought to produce stable sliding. The new computer simulations resolved this inconsistency by showing how slow slip can arise with contrary-seeming rate-strengthening friction.

"A handful of studies had shown that there are ways to destabilize rate-strengthening friction. However, until our paper, no one had realized that if you simulated these instabilities, they actually turn into slow slip, they don't turn into earthquakes," according to lead author Elias Heimisson, a doctoral candidate at Stanford Earth. "We also identified a new mechanism for generating slow slip instabilities."

Explanation:

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