M5.7 Earthquake off Cape Mendocino, California: 28 January 2015

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M5.7 California earthquake

How close are you to the location of the January 28 M5.7 California earthquake? Image by USGS.

An earthquake with a magnitude of 5.7 (M5.7) struck off the coast of northern California in the afternoon of 28 January 2015.

The shallow tremor (which occurred at a depth of 17km, around 40km from the town of Ferndale) caused minor shaking along the coast but at the time of writing there were no reports of damage or injuries – and, despite the offshore location, the magnitude of this quake was too small to generate a tsunami.

January 28 California Earthquake: Tectonic Setting

California is tectonically interesting, and the north of the state particularly so. Most laymen are aware that the state is split along its length by the San Andreas fault zone, along which the Pacific and North American tectonic plates are sliding past one another. This transform fault zone is the source of many of California’s earthquakes – certainly those which make the news.

The San Andreas fault leaves the land and reaches the sea just off Cape Mendocino, in the north of the state. Here the tectonic setting undergoes a significant transition, as the Pacific plate gives way to the Juan de Fuca plate, remnant of the much larger Farallon plate which has already been largely subducted beneath the North American continent.

The eastern edge of this plate forms a major subduction zone capable of producing large and potentially devastating earthquakes.

Mendocino Fault Zone

Geology and structure of the Mendocino Fault Zone. Image by J.B. Barnes, J.D. Cecil, and K.A. Cyr

Seismologists subdivide the Juan de Fuca plate into smaller microplates and the earthquake occurred at the junction of one of these, the Gorda plate, and North America.

Although there is as yet no available information on the earthquake, the tectonic history, location and depth suggest that this was not a subduction earthquake but was probably the result of movement along the so-called Mendocino Fracture Zone.

Seismic History of the Mendocino Fracture Zone

California seismicity.

Seismicity in California, 1973-2012 illustrates the point that the Mendocino Fracture Zone is highly seismically active. Image by USGS.

Despite the popular association of seismic activity in California with San Andreas, the northern part of the state, and the Mendocino Fracture Zone in particular, in the words of seismologist Robert Yeats, “include the most seismically active part of California.

This is evident from the map of seismicity in the state (the MFZ is indicated by the cluster of red dots in the top left of the map).

Yeats notes that “the north coastal region of California accounted for about 25% of the seismic energy released in California in a 50-year period’. That’s a lot of energy — but the Mendocino earthquakes often pass unnoticed simply because they tend to occur offshore and aren’t widely felt.

In addition, these quakes aren’t tsunamigenic — they’re too small and associated with normal or transform faulting.

Since 1900, the area around the MFZ has experienced over 30 earthquakes of at least M6 – larger than the most recent event. Six of these have been M7.0 or bigger (the largest recorded was M7.3 in 1922) of which four have occurred since 1980.

California Earthquake: Much Ado About Nothing?

Large earthquakes are always interesting and often potentially damaging. But tremors on the Mendocino Fracture Zone are much less of a likely hazard to Californians (and the residents of other states) than those which occur along either the San Andreas or Cascadia fault zones — despite their frequency and their magnitude.

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