Earthquakes in Oregon, Colombia and New Zealand: 8-14 August 2013

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Earthquakes of at least M4.5 814 August. Image credit: USGS

Earthquakes of at least M4.5 814 August. Image credit: USGS

The United States Geological Survey’s real time earthquake map recorded a total of 1589 earthquakes worldwide between the 8-14 August 2013.

This number is by no means a complete record, as the map shows only tremors above magnitude 4 (≥M4.0) except for the US and its territories, for which all tremors are shown.

The majority of larger tremors (≥M5.0) occurred, as is usual, along the seismically active tectonic margins and around the Pacific Ocean – the so-called Pacific Ring of Fire.

The Week’s Largest Earthquake: M6.6, Colombia

The largest earthquake of the week was an M6.6 event which occurred just off the country’s Pacific coast on 13 August. Despite the offshore location, the tremor was too small to pose a significant tsunami risk: no tsunami occurred and no warning was broadcast. Although shaking in the nearby coastal areas was intense, no damage or injuries had been reported at the time of writing.

The earthquake occurred at the northern end of the Nazca plate, which is currently being subducted beneath the South American plate. The situation is complicated by the juxtaposition of this margin with two other plates, the Cocos and Caribbean. The epicentre of the earthquake was in the descending plate, suggesting that if was caused by intra-plate deformation rather than movement along the subduction plane – another reason why the risk of a tsunami might be reduced.

M6.0 North of New Zealand – the Kermadec Trench

The subduction zone to the north-east of New Zealand was also the location for an earthquake – this time an M6.0 which struck off L’Esperance Rock. Here the Pacific plate is being subducted beneath the Australian plate, resulting in a long zone defined by the Tonga Trench in the north and the Kermadec Trench in the south.

In this case the earthquake provides a neat counterpoint to the Colombia tremor: its epicentre occurred in the over-riding plate and this, combined with the depth and distance from the trench itself suggest that this was probably a result of slippage at or near the interface between the two plates, although without further detail it’s impossible to be certain.

Earthquakes in the US: M4.5 off the coast of Oregon

The Gorda Ridge and its relationship to the Cascadia subduction zone. Image credit: USGS

The Gorda Ridge and its relationship to the Cascadia subduction zone. Image credit: USGS

Like the previous two, the M4.5 tremor which occurred off the coast of Oregon was associated with the Pacific Rim of Fire. In this case, however, although it occurred offshore and near a subduction zone, it is very clearly associated with fracturing rather than subduction. The tectonic setting involves the subduction of a relatively small plate, the Juan da Fuca plate (thought to be the remnants of a larger one) beneath the North American plate.

Although the leading (subducting) edge is characterised by a major subduction zone (Cascadia), the boundary to the west is a constructive boundary. Between the two, tensions have generated extensive fracturing and offsetting along the boundary: these are the source of many significant, though not major, earth tremors and one of them, the Blanco Fracture Zone, was the source of this week’s M4.5.

Final Words: Subduction Zones?

A glance at the map would seem to suggest that all three earthquakes considered above are subduction zone earthquakes. In fact only that off New Zealand can be said to be so. The others, though unquestionably associated with subduction, are probably related to either intra-plate faulting or conservative movement near, but not directly as a result of, such zones.

Sources

United States Geological Survey Real-time earthquake map. Accessed 14 August 2013

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