Remember the Big One in 1992?
Where were you in April 1992? If you were in Humboldt County, you probably remember that there was a whole lot of shaking going on.
On April 25, 1992, at 11:06 on a sunny Saturday morning, the Cape Mendocino earthquake suddenly unleashed land-thrusting, wall-busting motion. Registering a magnitude of 7.2, within 18 hours it was followed by two major post-midnight, aftershocks, one at 12:41 a.m. and another at 4:18 a.m. Sunday. Each registered a magnitude of about 6.5 to 6.6.
All told, the sequence caused nearly $70 million in damage and more than 380 injuries along the North Coast, but no fatalities. Ten days later, President George H.W. Bush declared the region a major disaster area. It is the only time an earthquake has ever earned that designation in Humboldt County.
To mark the date, the City of Eureka and the Redwood Coast Tsunami Work Group—an organization of emergency and earthquake professionals from Del Norte, Humboldt and Mendocino counties—will host a “remembrance” gathering Saturday, April 29, at the Wharfinger Building in Eureka. Doors open at 6 p.m. and the program begins at 7 p.m.
Along with offering the public opportunities to share memories of the earthquakes, view displays and hear music, the event will include presentations about the underlying geology and how it rearranged the landscape, the legacy of enhanced preparedness, and major advances in the science of earthquakes and tsunamis. Free preparedness materials will be available, and individuals will be able to sign up for Humboldt County’s new emergency notification system and try out a new tsunami-zone app. For details, visit http://earthquake.cascadiageo.org and http://facebook.com/1992earthquake.
According to Lori Dengler, an emeritus professor of geology at Humboldt State University and expert in tsunami and earthquake hazards, most Humboldt County earthquakes originate under the ocean on strike-slip faults, where the land moves horizontally.
“The ’92 quake was different,” she said. “It was centered onshore, three miles east-northeast of Petrolia at a depth of six miles beneath the surface. The fault that caused it tilts to the east like a ramp, and the land above it was thrust up and over the land beneath. This caused a 15-mile-long stretch of the coast near the mouth of the Mattole River to rise as much as five feet, killing the intertidal marine life and changing the coast line.”
The movement uplifted a 40-square-mile area just offshore, producing a small tsunami, she said. A tide gauge in Humboldt Bay recorded an 8-inch surge less than a half-hour after the earthquake. Later the Crescent City tide gauge, about 100 miles away, detected a nearly 24-inch wave. Gauges in Hawaii and Monterey, Calif., also detected it. Eyewitnesses on local beaches reported a surge of about 3 feet high.
As it coincided with low tide, the tsunami caused no flooding.However, for geologists and emergency planners, it underscored the potential for “near-source” tsunamis to create damage quickly in areas near the epicenter. Ultimately, it prompted Congressional hearings that lead to the establishment of the National Tsunami Hazard Mitigation Program, and it led to the creation of the Redwood Coast Tsunami Working Group.###Background information on the Cape Mendocino earthquakes
Earthquakes are relatively common in Humboldt County. North Coast California and the adjacent offshore area is the single most seismically active region of the contiguous 48 states, accounting for 46% of all of the earthquake energy released. Fifty-five earthquakes since 1900 have been strong enough to cause damage. (fig.1 North Coast earthquake timeline)
The Cape Mendocino earthquakes occurred within the Mendocino triple junction region, where three plates of the earth’s surface, the Pacific, the North American and the Gorda plate meet. It is also the meeting point of three great fault systems: the San Andreas to the south, the Mendocino to the west and the Cascadia to the north.The April 25th mainshock was centered on shore, three miles ENE of Petrolia at a depth of six miles beneath the surface. The fault that caused the earthquake tilted to the east like a ramp and the land above it was thrust up and over the land beneath. The fault was close to the southern end of the Cascadia subduction zone, the convergent plate boundary between the offshore Gorda and Juan de Fuca plates and the North American plate.This thrust faulting caused a 15-mile long stretch of the coast near the mouth of the Mattole River to rise as much as five feet, killing the intertidal marine life and changing the coast line. A team of geologists, biologists and HSU students measured the uplift by recording the distance between the dead organisms and the surviving ones.The earthquake produced the strongest ground shaking ever recorded in a California earthquake, registering over twice the acceleration of gravity on the nearest strong motion instrument at Cape Mendocino, strong enough to bounce a Caterpillar D-9 tractor trailer into the air and land it a foot away without leaving a track in the mud it was mired in.
The same faulting that raised the coast uplifted a 40 square mile area just offshore as well, producing a tsunami. Because the uplift was relatively modest, the tsunami was as well. The nearest tide gauge inside Humboldt Bay recorded an eight inch surge and the Crescent City tide gauge, about 100 miles away detected a nearly two foot wave. An eyewitnesses at College Cove near Trinidad estimated the tsunami was about three feet high. It was also recorded as far away as Monterey, Port Orford and Hawaii.
The tsunami coincided with low tide and caused no flooding. But it was important for several reasons. It was the first potentially damaging “near source” North Coast tsunami ever recorded. The first tide gauge was installed in Crescent City’s harbor in 1933 and since then, 38 tsunamis have been detected. Before 1992, they had all had been triggered by earthquakes more than 2500 miles away. The tsunami warning system for the US West Coast was designed primarily for these distant tsunamis, where there are four or more hours before the first wave arrives. This gives emergency managers time to issue warnings and to organize a coordinated evacuation response. The 1992 tsunami arrived in Humboldt Bay less than a half hour after the earthquake. On the beaches in Southern Humboldt County, the time was even less.
The 1992 earthquake occurred at a time when a relatively small group of geologists and seismologists were becoming aware of the hazards posed by the Cascadia subduction zone, the 700-mile long convergent plate boundary extending from Cape Mendocino to Vancouver Island, Canada. Paleotsunami deposits, Native American oral history, written records form Japan and geophysical modeling all suggested earthquakes perhaps as large as magnitude 9 had occurred in the past. But there was very little awareness in the larger earthquake preparedness community and among legislators and decision makers.
The 92 quake jolted emergency planners and earthquake professionals throughout the country. The thrust fault that produced the 1992 quake was very close to the inferred location of the Cascadia subduction zone. It was a mini-version of what researchers expected a bigger Cascadia quake to cause – thrust faulting, strong shaking, coastal deformation and a tsunami. The ground had barely stopped shaking when ramifications began to emerge.
Don Hull, head of the Oregon geology agency DOGAMI, contacted Oregon Senator Mark Hatfield, the chair of the Senate Appropriations Committee at the time and used the 1992 quake to convince Hatfield that the US was woefully unprepared for a larger Cascadia tsunami. In 1994, Hatfield convened the first ever hearings to mention the Cascadia subduction zone in Congress. The hearings led to the establishment of the National Tsunami Hazard Mitigation Program. It was a modest program for the first years, funded by a $2.3 million a year earmark, but it initiated a program of tsunami modeling in the five Pacific states, began the deployment of deep sea instruments to measure tsunamis and led to changes in tsunami warning center operations.
California’s geology agency obtained FEMA funding to compile a study of a larger Cascadia earthquake in Humboldt and Del Norte County. It was the argest earthquake they had ever studied, the first in a predominately rural area, and the first and only one to include a tsunami. The tsunami modeling was primitive by today’s standards but it opened eyes to planners and managers that surges could arrive only minutes after an earthquake. It was the release of this scenario in 1995 that led to the creation of the Redwood Coast Tsunami Work Group (RCTWG), an organization of emergency and earthquake professionals from Del Norte, Humboldt and Mendocino County to develop messages and programs to address the Cascadia threat that continues to this day.