The Cape Mendocino Triple Junction and the Amazing Uplift of 1992

This is a historial post from Hiking the California Trail, a 1998/2002 book set by Bob Lorentz and Richard Nichols. Where possible an update has been provided.

At Cape Mendocino, California’s Lost Coast thrusts westward against the driving California Current. The San Andreas Fault Zone trends west from its northernmost onshore extension at Shelter Cove to end as it joins the Mendocino Fracture Zone where the triple-plate junction of the North American, Pacific and Gorda Plates all meet. Directly offshore from the tiny Lost Coast hamlet of Petrolia, Gorda Ridge (flanked by the steep Mendocino Escarpment on the north), an undersea mountain range, sprawls westward for about 50 miles. South of this complex and still only partially understood geologic jumble, the California coast is uplifting from the Pacific. The coastline north of the Triple Junction currently seems to be sinking into the ocean.

Along the Lost Coast, radiocarbon dating shows that the land uplifted a remarkable 66 feet in the past 6000 years. If this still seems like a snail’s pace, compare it to the more typical 80 or 100 feet of uplift occurring over 100,000 years elsewhere on the California coast. The Lost Coast apparently iS rising from the Pacific Ocean I to 14 times faster than most of the California coast!

The rapid uplift along the Lost Coast seemed to be obscure academic information until April 25, 1992. Then the Ferndale earthquake, 6.9 on the Richter scale, shook up the Humboldt County coast. Lost Coast locals were not thinking about coastal uplift as they extinguished the quake-caused fire that destroyed Petrolia’s only store, secured their homes against aftershocks, or surveyed the remaining edge of the quarter acre of land that dropped into the Pacific west of town.

About two weeks after the temblor, however, someone in Petrolia noticed a powerful stench wafting into town on the otherwise typical coastal breeze. When locals went to the shoreline to investigate, they found that nearly half of the biologically rich tidal zone sat high and dry above the surf with its once abundant tidal plants and animals rotting in the spring sun, creating an awful stink. After this news got around, dozens of scientists descended on the Lost Coast to study the disturbed tidal zone. Geologists measured the uplift near Devil’s Gate south of Cape Mendocino at nearly three feet, all in the few seconds of the April 1992 quake. Marine biologists first surveyed the damage, then studied how long it would take various tidal critters to repopulate the tidal zone. Not only was the event a scientific gold mine, but it brought home just how quickly the Lost Coast is being created from the ocean floor.

Originally Published in Hiking the California Coastal Trail: Guide to Walking the Golden State's Beaches and Bluff from Border to Border - Volume One: Oregon to Monterey (2nd Edition) by Bob Lorentzen and Richard Nichols
Category -
For trail section -