# 2024 Cape Mendocino earthquake

On December 5, 2024, a M$_w$ 7.0 earthquake struck off the coast of Humboldt County, California, at 10:44 a.m. PST. It was felt in the state's northern regions and in the Central Valley. The earthquake prompted the National Weather Service to issue a tsunami warning which was canceled soon after.

## Tectonic setting

Near Cape Mendocino, the Mendocino triple junction is an area of active seismicity where three tectonic plates come together. The Mendocino Fracture Zone (also known as the Mendocino Fault east of the Gorda Ridge) is a transform fault that separates the Pacific and Gorda plates. To the south, the relative motion between the Pacific plate and North American plate is accommodated by the San Andreas Fault, and to the north, the Gorda plate is converging with the North American plate at the Cascadia subduction zone. Earthquakes within the Gorda plate are the result of north–south compression at the Mendocino Fault.

## Earthquake

The earthquake's epicenter was located offshore, 63 km (39 mi) west of Petrolia and struck at a focal depth of 10 km (6.2 mi). A similarly-sized earthquake of identical mechanism occurred in the area in 1980. According to Harold Tobin, director of the Pacific Northwest Seismic Network, the earthquake occurred along the Mendocino Fracture Zone at the triple junction of three tectonic plates. The rupture initiated along a steeply dipping fault striking either east-southeast or north-northeast, according to the focal mechanism solution. A rupture on the Mendocino Fracture Zone would correspond with the former fault orientation. It is the largest earthquake to strike the state of California since the 2019 Ridgecrest earthquakes, and the strongest to strike the northwestern coast of the state since 2005. People as far away as Phoenix, Arizona, and Vancouver, Canada, reported shaking. By December 10, 308 aftershocks exceeding M$_w$ 2.0 were recorded, with the strongest measuring M$_L$ 4.7.

## Damage

Over 10,000 customers in Humboldt County were left without power following the earthquake. Some homes were shifted from their foundations in the Eel River Valley region. In Ferndale, a volunteer firefighter said damage to homes was limited. In Rio Dell, a road was cracked and a gas leak occurred in a middle school. Minor damage also occurred in Fortuna. Many stores incurred damage from fallen merchandise and goods.

## Response

A tsunami warning was issued by the National Weather Service (NWS) for the northern California and Oregon coast. In San Francisco, Bay Area Rapid Transit ceased all services on the Transbay Tube. The San Francisco Zoo was closed, with guests being evacuated and animals secured. An estimated 5.3 million people in California were affected by the tsunami warning. The warning was canceled by the NWS at 11:54 PST. During the tsunami warning period, some people along the beaches of San Francisco and schools in Pacifica and Arcata were evacuated.
California governor Gavin Newsom said the state's emergency offices were "actively responding" to the situation. Mike McGuire, the President pro tempore of the California State Senate, wrote on X (formerly Twitter) that the state was "sending immediate assistance" to Humboldt County and Del Norte County "to assist with emergency operations". A few hours after the earthquake, Newsom declared a state of emergency in Del Norte, Humboldt and Mendocino counties in order to support the emergency response.