Did I miss any important details about the 1992 quakes in my last column? Of course I did, and it was intentional. Space is always a limitation, but the main reason was my own limited awareness at the time. I made no mention of a tsunami because on April 25, 1992, I wasn’t connecting a 7.2 earthquake to a potential tsunami. And I wasn’t thinking about land-level changes either.

It took about a week before I heard murmurs drifting about the department that the coastline near the mouth of the Mattole had changed in the earthquake. It looked like low tide all the time, and some people could now wade in knee-deep water, harvesting abalone. Petrolia old-timers noticed the change immediately, but it took about a week of decaying intertidal marine life for the rest of us to believe accounts that the coastal sea-level had abruptly “dropped.”

By the time I visited the area almost three weeks later, a full-scale research project had been launched. Gary Carver in the Geology Department had teamed up with Bob Rassmussen in Biology to map out the uplift, roping a number of students, former students, and colleagues in both disciplines into the effort.

One of those was Thomas Dunklin, who lived in Petrolia at the time (and still does). Thomas’ geology background and knowledge of the triple junction area made him the perfect field assistant for post-earthquake geologic studies, and he was quickly pulled into the coastal uplift project.

Measuring relative sea level is easy if you have a tide gauge. There are no tide gauges near Petrolia, so how can scientists measure the elevation change? There were no previously established benchmarks in the area of interest. The team came up with a unique method, relying on the marine life itself and measuring the “vertical extent of mortality,” the distance between the dead organisms and where they had survived.

More than a dozen organisms were included in the study, but sea urchins proved particularly useful. They are plentiful on our coast and hollow out small pockets on the rocks to anchor themselves. Thomas and others spent weeks measuring the distance between the highest uplifted “dead” dimple marking where a sea urchin had once lived, and the top of the still living sea urchin zone.

The result was a detailed graph of how the coast had bulged upwards as a result of the 1992 fault slip. Uplift could be detected over 15 miles of coastline between Cape Mendocino and Punta Gorda. The largest uplift was just over 4.5 feet in the center of the profile, gradually diminishing to both the north and south.

This uplift shouldn’t have surprised me. Geomorphologists studying the Cape Mendocino coastal area have identified at least eight other uplifted former beach surfaces. We call them marine terraces, a flat surface that marks the former surf abrasion platform. If you stand on Mattole Road and look down on Singley Flat, you can see these surfaces clearly etched in the landscape. If you don’t want to drive, view Thomas Dunklin’s Mendocino triple junction video for a virtual tour of the spot (https://www.youtube.com/watch?v=qhDdzHae4Rc).

Each of these uplifted terraces marks a previous earthquake in the past 10,000-year time window. Some of them show a similar amount of uplift as 1992, suggesting earthquakes in the magnitude 7 range. But others are more extensive, indicating larger quakes. Of course, it’s not just the coast that goes up in these quakes. The King Range is one of the most rapidly uplifting areas on the West Coast and repeating large earthquakes are a major contributing factor.

The 1992 earthquake fault slip did not stop at the coast. While the rupture began on land beneath Petrolia, it grew upwards and to the west, creating a slanting fault plane roughly 10 miles wide and 15 miles long. The fault dipped up to the west and possibly broke the sea floor surface at the continental shelf. We will never know because no offshore reconnaissance was done after the earthquake.

We do know that all of the rock mass above the fault was shoved up and to the west several feet during the earthquake. That means a 10-mile-long patch of the sea floor displaced the ocean above it, and the uplift produced a tsunami. I didn’t know anything about the tsunami for several weeks. No tsunami warning had been issued, and tide gauges weren’t online in those days.

Tsunami protocols were different in 1992, and the local tsunami threat was not well-recognized. The initial magnitude of 6.9 put it below the threshold to issue an alert and local emergency officials were not in favor of making any mention of it. If the earthquake occurred today, a tsunami warning would have been issued, and we would see the Dec. 5, 2024, scenario play out but amid much more damage.

It took the prodding of then California State Geologist Jim Davis to query NOAA about a possible tsunami. Pulling up the tide gauge records showed not only a tsunami in Humboldt County, but it was well-recorded on other instruments from the central California coast to Port Orford and in Hawaii. It was just over 1.5 feet high at Crescent City and had it occurred at a higher tide, could have impacted harbors and low areas.

We also had no robust post-tsunami field survey system in place in 1992. The first International Tsunami Survey Team would be deployed about four months later to study a tsunami in Nicaragua. Since then, groups of national and international scientists have quickly mobilized to study the physical characteristics and impacts of moderate to major tsunami events soon after they happen.

By the time we were aware of the 1992 tsunami, wave action would have erased any physical traces. We put out a call for people to report any observations of unusual water level activity on April 25, 1992. A group at College Cove near Trinidad reported a sudden water level rise of about three feet and several surfers described unexpected turbulence.

The 1992 earthquake is a story of coseismic (during the earthquake) uplift. A paper was published this week in the Proceedings of the National Academy of Sciences on coseismic subsidence during great Cascadia earthquakes. The paper, headed by Tina Dura of Virginia Tech, examines the potential of sudden lowering of the land surface associated with earthquakes in the upper magnitude 8 to 9 range in the Cascadia region from Cape Mendocino to Vancouver Island, Canada.

How can earthquakes uplift one coastline, like the Cape Mendocino area in 1992 and cause other areas to drop? It all depends on the type of earthquake faulting, where you are relative to the earthquake fault slip, and how much strain had been stored before the quake. I’ve had the opportunity to see both uplift and subsidence in post-earthquake/tsunami field investigations. For earthquakes like Indonesia 2004, Chile 2010, and Japan, the primary signature is the land dropping down.

This week’s Cascadia subsidence study estimates that the coastal land elevation could drop between 1.5 feet and 6 feet at coastal sites from Humboldt County to southern British Columbia in our next great earthquake. We’ve paid a lot of attention to ground shaking and tsunami hazards associated with great earthquakes and not enough attention to ground level changes. Check out next week’s column for a closer look at the implications of the study for the North Coast.

Lori Dengler is an emeritus professor of geology at Cal Poly Humboldt and an expert in tsunami and earthquake hazards.