Lori Dengler | An earthquake enigma: Where was the 1954 earthquake located?

The North Coast is a shaky place. The USGS catalog lists 28 earthquakes of magnitude 6 or larger in the coastal areas of North Coast California and the adjacent offshore area since 1950, outstripping the rest of the state by far. Eight were centered in the triple junction region within 25 miles of Cape Mendocino; 13 were in the Gorda plate offshore of Humboldt and Del Norte counties; six were Mendocino fault quakes, on the plate boundary that marks the southern edge of the Gorda plate.

There is one enigma. On Monday, Dec. 21, 1954, at 11:56 a.

m. a magnitude 6.5 earthquake rocked Humboldt County.

The magnitude value is probably pretty good as it can be accurately estimated from seismic stations far away. We can constrain the epicenter to somewhere between Dinsmore, Eureka, and Arcata, but exactly where and how deep is still a mystery. Why is an earthquake that occurred nearly 70 years ago of any interest? Because it is the only earthquake in the instrumental catalog that may have been shallow enough to have been located on one of our many mapped surface faults.

There are few places on the planet with such a complex layering of potential seismic sources than the North Coast. What we see on the surface doesn’t reflect what is causing most of our earthquakes. We can blame the subduction zone and the triple junction for our crazy juxtaposition of faults.

Explaining what is going on requires imagination and 3D visualization. Subduction zones are nature’s ultimate recycling cent.