M5.0 Tremor off California: Tectonics of the North America-Pacific Margin

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Location of the M5.0 California earthquake of 5 March 2014. Image credit: USGS

Location of the M5.0 California earthquake of 5 March 2014. Image credit: USGS

The magnitude 5 (M5.0) earthquake which struck off the coast of California in the early hours of 5 March UTC (4 March, local time) was unusual in that it occurred at some distance from the major fault zones of the San Andreas fault system.

This quake, at a depth of just 2km and around 250km from Coronado, was by no means significant in world terms, but it was one of the larger tremors to hit the region recently.

California Earthquake: Tectonic Setting

Earthquakes in California are associated, in the public mind at least, with the San Andreas Fault zone. This zone marks the southern part of the boundary between the Pacific and North American tectonic plates where the Pacific plate, to the east, is moving northwards relative to its continental neighbour.

Dominated by the San Andreas , the region also includes a number of other broadly parallel faults, both large and small, extending from the Gulf of California to the coast close to Manchester Beach.

When continents move, huge forces are at play. In these circumstances, it’s unsurprising that ground deformation — and associated faulting — occurs along a much broader zone than that marked by the surface expression of the faults. A glance at fault maps of southern California show extensive offshore faulting reaching as far as the edge of the continental shelf. The location and depth of the March 5 tremor suggest that it occurred as a result of shallow faulting at the outer margins of the continental fault zone.

The Pacific-North American Plate Boundary

Major earth tremors (at least M7.0) along the Pacific margin, 1900-present. Image credit: USGS

Major earth tremors (at least M7.0) along the Pacific margin, 1900-present. Image credit: USGS

The San Andreas Fault Zone is only part of the highly seismically active North American-Pacific plate boundary. Leaving aside the complex fault geology of California, Nevada and the Rockies and looking only at the narrow plate boundary shows how potentially significant it is.

The northern section (from Alaska to around Vancouver) is a conservative margin. Southwards, the nature of the boundary changes and the section between Vancouver and northern California is marked by a subduction zone, where one plate (the Pacific) is forced beneath another (the North American) while to the south the nature changes again and the San Andreas is a conservative margin.

The entire length of the western seaboard of the north American continent, then, is vulnerable to major earthquakes, the size of which render the most recent insignificant. A map of tremors of at least M7.0 from 1900 onwards shows that the northern and southern margins have regularly experienced major tremors.

Cascadia Subduction Zone: Some of the Largest Tremors Ever Recorded

Interestingly, the notable gap in this map occurs along the Cascadia subduction zone (CSZ). While this may seem to suggest that major earthquakes are unlikely to occur, this is not the case.

In 1700, the region experienced one of the largest tremors ever recorded (M9.0). Current scientific thinking is that this part of the margin is ‘locked’ rather than inactive – that is, that the strain continues to build without being released.

In the words of the Pacific Northwest Seismic Network: “The CSZ has produced magnitude 9.0 or greater earthquakes in the past, and undoubtedly will in the future” – potentially a serious concern for residents and authorities alike.

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