M6.9 California Earthquake: Largest Tremor for Seven Years Strikes off Cape Mendocino

An earthquake of magnitude 6.9 (M6.9) which struck off the coast of north California on 10 March is the largest to hit the region since an M7.2 which occurred in the same area seven years ago.

Early reports recorded by the United States Geological Survey suggest that residents felt the quake as far south as San Francisco but at the time of writing there’s no indication of any damage.

M6.9 Ferndale Earthquake: Location and Tectonic Setting

The west coast of North America marks a continental margin. In both north and south, the North American plate marches alongside the Pacific plate and the plate margin is conservative, with the plates sliding past one another.

Between California’s Cape Mendocino and a point to the north of Vancouver Island, however, the remnant of an ancient plate (the Juan de Fuca plate) is moving against the North American continent and being forced beneath it – an area known to seismologists as the Cascadia subduction zone.

The earthquake occurred to the south of this zone, close to the Mendocino triple junction – a point where the plate boundaries between the North American and Pacific plates, the Juan de Fuca and Pacific plates (both conservative boundaries) and the the North American and Juan de Fuca plate (subduction zone) meet.

With conflicting stresses resulting from different directions of movement, there are high levels of deformation and fault maps show fold and thrust belts parallel to the coast both on and off shore.

Data published by the USGS at this early stage are limited, but the shallow depth of the earthquake (around 16.6km) suggests that it was this internal deformation that caused the tremor, rather than the collision between the two continents.

This is supported by the absence of any tsunami, indicating that there was little or no vertical displacement.

Historic Earthquakes in Cascadia and the Mendocino Fault Zone

This suggestion of the probable cause of the tremor is supported by a look at the distribution of large earthquakes in the region. A map drawn from the USGS database of historic earthquake shows that large tremors (≥M6.0) in the region since 1900 cluster around the triple junction and the southern and eastern margins of the Juan de Fuca plate.

The leading edge of the plate (the subducting part) shows a clear absence of large tremors, indicating that the subduction zone is presently inactive, other because it is aseismic or because it is ‘locked’ (in other words, strain continues to build).

The map shows clearly how significant today’s tremor was in terms of size; it is the eighth largest recorded in the area since 1900. The largest, at M7.3, occurred in 1980 and on average the region has experienced a tremor of M6.0 or greater around every three years.

Earthquakes, Fault Margins, and Tremor Damage

Earthquakes at this type of margin and of this nature are typically not damaging. They lack a vertical component to the movement, so tremors of this nature are unlikely to generate tsunamis – and they are also located at distance from major population centres. Other tremors can have much greater impact, however: In 1989, an earthquake of the same magnitude struck the San Andreas Fault Zone. That tremor, at Loma Prieta in California, killed 63 people and caused extensive damage.