Lori Dengler | Earthquake Mystery: Where Did the 1954 Earthquake Happen? – Times Standard

The North Coast is a volatile place. The U.S. Geological Survey catalog lists 28 earthquakes of magnitude 6 or greater in coastal areas of Northern California and the surrounding marine area since 1950, far more than the rest of the state. Eight of those were in the triple junction area 25 miles off Cape Mendocino; 13 were on the Gorda Plate off the coasts of Humboldt and Del Norte counties; and six were Mendocino fault earthquakes, on the plate boundary that marks the southern edge of the Gorda Plate.

There is one mystery. On Monday, December 21, 1954, at 11:56 a.m., a magnitude 6.5 earthquake struck Humboldt County. The magnitude is probably a good enough value to be accurately estimated from distant seismic stations. The epicenter can be pinpointed somewhere between Dinsmore, Eureka, and Arcata, but the exact location and depth of the quake remain a mystery.

Why is the earthquake of nearly 70 years ago significant? Because it is the only quake in the catalogue recorded by instruments that may have been shallow enough to be located on one of the many shallow faults that are mapped on our maps.

There are few places on Earth with such a complex layering of potential seismic sources as the North Coast. And what we see on the surface doesn’t reflect what’s behind most of the earthquakes we experience. We can blame the subduction zone and the triple junction for this crazy paradox of faults.

Explaining what's going on requires imagination and three-dimensional visualization. Subduction zones are nature's ultimate recycling centers where one plate (the Earth's outer layer) is pulled by gravity beneath another plate. The Cascadia subduction zone, stretching from Cape Mendocino to Vancouver Island in Canada, is one such place.

Subduction zones not only produce earthquakes, they also compress and wrinkle the Earth's surface. This deformation is most noticeable near the leading edge of a subduction zone where the upper plate is thin. The amount of pressure is enormous. It causes the rocks to bend into gentle folds. Rocks do not bend easily, and the folds eventually break as one plate is pushed over another.

We call this deformed area the fold and thrust belt. In most subduction zones, this area is offshore, hidden under the ocean. Humboldt County is unique because many of these folds and faults are exposed on land. The Big Lagoon and Arcata Bay are areas where gentle folds form where the center slopes downward. The opposite type of fold that pushed the land up passes under Eureka, creating the islands and the narrower part of the bay.

There are plenty of faults visible on the surface as well. You can drive over two strands of the Feckle Hill Fault on G and H Streets in Arcata. Cranberry trees stand at the top of the larger slope. A more subtle feature cuts through Jacoby's Depot. The Little Salmon Fault runs through the College of the Redwoods campus and there are a number of parallel fault strands in the exposed Mad River Fault Zone at Blue Lake, McKinleyville and Trinidad.

All of these faults trend northwestward, parallel to river and mountain ranges. They are considered active (evidence of ruptures during the past 10,000 years). However, none of the earthquakes of magnitude 6 or greater on the north coast during the past 75 years have occurred on these mapped surface faults, except perhaps in 1954.

There have been at least ten different attempts to pinpoint the location of the 1954 earthquake, and they all differ from one another. Pinpointing the location of earthquakes is a triangulation problem. You carefully note down the arrival times of seismic waves at each seismic station and find the best place to fit all the data. It may seem straightforward, but there are challenges.

There were only two continuously operating seismic stations in Humboldt County in 1954, one in the basement of the campus in Founders Hall and an old (even by 1954 standards) smoke drum recorder in the Ferndale Fire Hall. The strong-moving (vibration-activated) seismographs in Eureka and Ferndale were also turned on by the event. Several distant seismic stations also recorded this earthquake, but the nearby stations are the most useful for constraining location and depth.

Timing is of great importance in seismology. The most beautiful seismic record is useless if you don't have a good clock. Nowadays, we use GPS signals from satellites to synchronize the local clock. In 1954, a radio signal was sent, including the GMT code on the record twice a day.

Seismic waves travel at incredible speeds of miles per second. Inaccurate readings of seismic signals and timing corrections will result in errors of tens of miles. Even the thickness of a pencil can cause problems. It is essential that we constantly “catch the seismic wave”—the place where we notice the first seismic wave arrives.

The speed of earthquakes depends on the type of rock. Subduction has been churning up the rocks of the North Coast for more than a hundred million years. Developing accurate “community velocity models,” a three-dimensional picture of the regional structure and velocities of earthquakes, remains a research priority today. The models used in 1954 were simplistic and used a broad brush to paint all of California in roughly the same colors.

Three years ago, we began talking about revisiting the 1954 earthquake. The first step was to collect the original recordings, digitize the signals, and apply the latest technology to relay the event and analyze the source mechanism. Peggy Hellwig, who recently retired from the Berkeley Earthquake Network, led the effort, uncovering many of the original paper records from the Berkeley archives.

Our group has narrowed down the possibilities, but we still don't have confirmation of the depth. Depth is the key to knowing whether 1954 lies on a shallow surface fault or on a deeper fault buried beneath the subduction zone interface. You may be able to help solve this problem.

This is a call for stories about the 1954 earthquake. Your story can tell us a lot about how strong the shaking was and how quickly it weakened away from the source. Shallow earthquakes tend to be very strong at the epicenter, but the strength quickly fades with distance. Deep earthquakes lose shaking strength gradually with distance.

Even small observations can be helpful. The trout fisherman on the Mad River who noticed the current briefly flowing backwards or the girl who found herself unable to ride a bike in Eureka provide important data points. You may not remember anything about 1954, but do your parents? Did anyone in your family leave a diary?

I'm interested in any memories, but with a special appeal to rural areas like Maple Creek, Davis Creek, Angel Ranch, and Upper Fickle Hill. These are sparsely populated areas, but with long-lived families who may remember something. Traditionally, shake maps rely on damage to built structures to indicate the strongest ground movements. This puts rural areas at a disadvantage.

You can submit your stories to me in several ways. Note the phone line and email address below. If you prefer to talk face-to-face, leave me a message about how to get in touch. You can also enter information on the USGS's “Did you feel it?” page for this earthquake at https://earthquake.usgs.gov/earthquakes/eventpage/cdmg19541221195629000/tellus

Note that the survey form is long, and you can skip any questions that are not relevant to the topic. I promise to share what we learned in a future column.

Lori Dengler is a professor emeritus of geology at California Polytechnic University Humboldt and an expert on tsunami and earthquake hazards. Have questions or comments about this column, or would you like a free copy of the Living on Shaky Ground preparedness magazine? Leave a message at 707-826-6019 or email us at [email protected].

Originally published: July 27, 2024 at 10:08 am

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