These diagrams are your key to unlocking the secrets of our local seismic activity. They show fault boundaries between rigid rock plates moving over warmer, weaker rocks below. For example, the San Andreas Fault is the boundary between the Pacific and American plates. The Pacific Plate extends from here to Japan, and the American Plate from here to the Mid-Atlantic Ridge (on which Iceland sits). The Gorda Plate is our young local plate.

These three plates meet at the Cape Mendocino Triple Junction. From that junction, the San Andreas Fault runs south to Mexico; the off-shore Cascadia Mega Thrust Fault runs north to Canada; and the Mendocino Fault runs west to the Gorda Ridge. Along that 300 km-long submarine ridge, new oceanic crust is being created from magma, allowing the Gorda Plate to spread eastward away from the Pacific plate, and then to be thrust downward under the American Plate along the Cascadia Mega Thrust Fault.

Arrows show the directions of movement of the Gorda and American plates relative to the Pacific Plate, at speeds similar to the rate your fingernails grow — 4 cm per year. Oakland was near the Triple Junction 10 million years ago.

Earthquakes occur when faults slip suddenly to release regional strains that accumulate slowly as a consequence of inexorable plate motions.

Cape Mendocino is the most active seismic zone in the U.S., but the Cascadia Mega Thrust is our most dangerous fault. On Jan. 26, 1700 it shook our coast violently and produced a tsunami recorded in Japan. Native American oral history recalls local catastrophes resulting from that earthquake.

“Subduction” is the diving down of one plate beneath another. Mount Shasta is one of the many Cascade Range volcanoes, stretching to Canada, spawned from subducted oceanic crust. Ponder all this and you will begin to comprehend the 20th century “Plate Tectonic” revolution that continues to enlighten all things geologic.

Don Garlick is a geology professor retired from HSU. He invites any questions relating to North Coast science, and if he cannot answer it he will find an expert who can. E-mail dorsgarlick@yahoo.com. He thanks Bob McPherson and Mark Hemphill-Haley for their input this week. They could tell you that his description of local tectonics is somewhat over-simplified.