The Pacific Northwest Seismic Network has detected more than 4,500 tremors over the past two weeks deep beneath the Olympic Peninsula and southern Vancouver Island and from another swarm stretching from Eugene to the Siskiyou Mountains.
It’s a phenomenon called “episodic tremor and slip” or “slow slip.” Humans can’t feel it, but seismologists have discovered this happens along Cascadia’s tectonic plate boundary on a regular basis — every 14 months or so in the case of Puget Sound and on a different cycle in Oregon and northern California.
(Source: https://www.opb.org/news/article/slow-slip-tremors-oregon-washington-california-cascadia-earthquake/)
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