Lowry focused much of his work on the Mexican events. "Down in southern Mexico we find that these things happen, very regularly, around January-February," he said.

One idea is that it's triggered by a yearly shift in the weight of the oceans, caused by climate fluctuations and called the "pole tide." That has a period similar to the Puget Sound changes, but not to the other patterns.

Changes in the weight of the atmosphere or water puts stress on the earth. This force can shift because of yearly patterns like the pole tide, or seasonal changes in rain or snowfall.

In southern Mexico, the ground water rises and falls by half a yard over the year, because of rain and runoff. "Half a yard of water is about half a ton per square yard," he said.

"It's a lot of weight. It's a lot of mass that you're changing at the surface every year."

While this change seems massive, it's little when compared with the force generated by the sliding of seismic plates. It may be only 10 percent, or even just 1 percent, of the yearly energy of plate movement.

According to Lowry, the sheer weight of the seasonal variations isn't enough to cause silent earthquakes. Instead, that force is propagated down deep into the fault, to a place where the zone is sensitive to vibrations.

"Many physical systems have a frequency at which a tiny little change can cause a big response," he said.

Lowry mentioned the Tacoma Narrows Bridge, which infamously collapsed in 1940, because wind caused it to vibrate at just the wrong frequency. As the canyon winds jiggled it back and forth, it twisted and turned and flew apart.

Fault slips also have a resonance frequency. When water or atmospheric fluctuations cause the system to resonate, the result is a silent earthquake. The changes affect the faults at a particularly sensitive location.

Even though the changes may be small, Lowry added, "they're the right frequency to produce these resonances."

Resonance explains why the slow slips have a regularity, and difference in resonance from fault to fault shows why they don't all repeat in the same periods.

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