Laurie Dingler | The 1992 earthquake triggered a tsunami – The Times Standard

The earthquakes, which occurred on April 25, 1992, caused more than $60 million in property damage. The biggest losses were the destruction of the Scotia Shopping Center from fires caused by midnight aftershocks, road slides and landslides. More than a hundred homes in the Ferndale and Rio del borough suffered foundation damage when old structures slipped from post-curb foundations or walls collapsed.

All the damage was caused by the powerful tremor, the strongest tremor ever recorded in a California earthquake. The accelerations on the apparatus at Cape Mendocino exceeded twice the value of gravity, which is strong enough to bounce objects in the air. A caterpillar tractor near Petrolia had been sitting idle during the winter months and was bogged down in mud up to the axle covers. Rebound in the air with acceleration, descending to rest one foot, leaving no scratch in the mud.

Thirty years later, the most enduring legacy of the 92nd earthquake has not been acceleration or damage. It was not the two major aftershocks or the complexity of the triple junction area revealed by the earthquake sequence. It wasn’t the strange green and blue lights that many reported seeing at night after the big aftershocks. It was something that did no harm and wasn’t noticed until days later. The main earthquake caused a modest tsunami.

He hadn’t heard of a tsunami in California in 30 years. Likely to be called tsunamis, many of us know something about Crescent City and the people who died in the tsunami caused by the 1964 Alaska earthquake. The Crescent City Harbor sustained some damage in 1957 and 1960 and a Santa Cruz resident died in 1946 from an earthquake Aleutian tsunami. We’ve come to realize that large earthquakes far away from us on the edge of the Pacific Ocean can send waves in our direction. We expected tsunami warning centers to give us at least four hours’ warning that waves were on their way.

As I was counting how long the shaking lasted that Saturday morning in 1992, the tsunami never crossed my mind. I followed the literature on the Cascadia subduction zone and was familiar with the paper published in Science in 1989 by Brian Atwater documenting tsunami sand deposits in Washington State associated with past great earthquakes, but I didn’t correlate it with what I was experiencing and was pretty sure that the shaking did not It lasts long enough to be the kind of earthquake Brian was writing about. There were no post-quake tsunami alerts or bulletins and I wasn’t aware of any discussion of tsunamis until weeks later.

The first thought that perhaps not all was calm in the ocean on that day were reports over the next few days from ancient residents that the coast was permanently stuck at low tide. Word got to me about a week later, when the stench of dead tidal creatures confirmed that the ancients knew what they were talking about. It sparked great interest and Bob Rasmussen in the Humboldt Department of Biology collaborated with Gary Carver in geology to map the elevation.

They developed a method for locating the top of Dead Sea Urchins and other tidal organisms and measuring the distance to the top of the still-living creatures and found that the coast rose by five feet near the Matul Estuary and the raised area extended more than 10 miles.

Coastal elevation means that the sea area has also risen and that means a tsunami has occurred. Without my knowledge, the California geologist at the time, Jim Davis was more curious about the tsunami than I was. About two days after the earthquake, he called Eddie Bernard at the Pacific Marine Environment Laboratory (PMEL) of the National Oceanic and Atmospheric Administration (NOAA) and asked if there was a tsunami produced. Eddie spent time as a team scientist at the Pacific Tsunami Warning Center and was busy building a tsunami research program at PMEL.

NOAA has operated a tide gauge at Crescent City since 1933 and the North Spit on the Samoa Peninsula since the late 1960s. Nothing was online in 1992, so querying tide gauges meant finding paper records. Eddy found that tsunamis were not only recorded on the northern coast but also on five tidal gauges from the coast of central California to southern Oregon and into Hawaii. These scales have recorded many tsunamis in the past, but what was so special in ’92 was that surges arrived just 26 minutes after the earthquake in Humboldt Bay and 47 minutes after in Crescent City. This was the first local or nearby tsunami ever recorded on the northern California coast.

The tsunami was not large, about three feet from trough to peak in Crescent City. This coincided with low tide and is generally not observed. A group on the beach at College Cove near Trinidad reported seeing an unusual surge that may have approached a six-foot peak to trough just in time when the tsunami was supposed to have arrived. But altitude isn’t the only way tsunamis cause damage. Strong tsunamis can damage and kill beaches and port areas, and in hindsight we all agree there should have been official notifications.

But of course, we were all warned. Mother Nature shook the earth and most of us realized that the shaking went on for a long time. I, and most other people, didn’t realize that prolonged shaking is a natural warning of a possible tsunami on the way.

In the weeks and months following the earthquake, recognition of the danger of a nearby tsunami changed. California has begun studying what a larger earthquake and tsunami might do to the area. That’s when I first met Eddie Bernard. PMEL was responsible for tsunami modeling, and worked on the intensity. Senators from Oregon, California, Hawaii and Alaska urged careful scrutiny of the national tsunami preparedness that eventually led to the National Tsunami Risk Mitigation Program. I would have gotten to know Eddie a lot better when he was running the show, and I was a California representative in the early years.

If I was at the beach on a beautiful April Saturday in 1992, what should I have done when I felt the earthquake? Head to the higher ground. This could be a backup for the path you came on or to a nearby high dune. Then I would stay there until I received official notification of the safe place. In 1992, that notice could have come quickly—within tens of minutes. If the earthquake is larger, it may take several hours. But I know I would have thought of a “tsunami” because I was counting how long the shaking lasted.

Laurie Dingler is Cal Poly Humboldt Professor Emeritus of Geology and an expert in tsunami and earthquake hazards. Questions or comments about this column, or want a free copy of Preparedness magazine “Living on Shaken Earth”? Leave a message at 707-826-6019 or email [email protected].

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