Alfred Kroeber recorded several Yurok stories that include earthquakes. A few can be found here. Some stories involve earthquakes as a moderating force between humans and non-humans (Woge, pronounced wah-gay), and between humans and Sa:al, or evil forces. In one of the referenced stories, evil forces were attempting to rid the world of humans but thunder told earthquake he could save the earth and humans by shaking the earth. And so he did.

This unattributed story tells how earthquake and thunder created the ocean.

As to what natives thought of earthquakes, it seems that they were supernatural, but had many qualities of humans in that they could travel and converse with other forces. The Yurok considered many natural phenomena and inanimate objects as having human qualities, as did many other NW groups. As a general reference Kroeber's Handbook of California Indians is pretty hard to beat.