Low-Levels of Earthquake Actvity in the Week of 21-27 November

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Earthquakes of at least M5.0, 21-27 November 2012. Image: credit USGS

The week of 21-27 November 2012 was relatively quiet, with just 27 of magnitude 5 or greater (≥M5.0) following a fortnight of significant seismic activity.

This week’s quakes were concentrated in the western Pacific and south of Sumatra. It is particularly interesting to note the almost total absence of earthquakes of ≥M5.0 along the eastern rim of the Pacific plate – leaving us in the unusual position that the largest tremor recorded in the US was an M3.6 in the tectonically-stable continental interior, in Illinois.

The Week’s Largest Earthquake: the Andean Margin

The largest earthquake recorded this week was an M5.9 just on the coast of Chile, accompanied by a cluster of foreshocks and aftershocks.

Here, oceanic crust (the Nazca Plate) collides with continental crust (the South American Plate): the former, being colder and denser, is subducted beneath the lighter continental crust. The result is continental uplift and a zone not only of earthquakes but of volcanoes, as the subducted crust comes under immense pressure and melts, with the molten rock rising upwards to form the chain of volcanoes which extends not just along the South American margin but also further north, forming mountain chains from the Andes to the Rockies.

China Earthquake: Continental Collision

Following from last week’s chain of tremors in the east of the Himalayan region an earthquake in China, resulting from the same general collisional mechanism, clearly illustrates the diffuse nature of this type of collision when compared with the more narrow tremors of subduction zones. This event occurred in a region dominated by lateral (strike-slip) faults which, the USGS notes, accommodate a significant amount of the compressive force generated by the northwards-moving Indian plate as it collides with Eurasia.

M4.7 in Western Brazil

Seismic activity in Brazil and around, 1990-present. Image: credit USGS

Also worth a closer look is the M4.7 quake which occurred to the east of the Andes in Brazil. Major earthquakes are rare in the country: the USGS historic earthquake listing includes just two of greater than M6.0, both of which had their epicenters further to the west.

Although theoretically, Brazilian earthquakes have a similar tectonic setting to the Chilean earthquakes (driven by subduction of the Nazca Plate below South America) the location of this particular tremor is a long way from the subduction zone itself.

The USGS notes that deep earthquakes do occur in the subducting Nazca Plate and this is borne out by a seismic map of the country which shows most earthquakes to be deep, and following an expected pattern in a narrow belt to the east of the Andes. The most recent earthquake, however, was shallow in depth (just 19km) making it more likely to be a product of movement along shallow faults, probably as a result of uplift.

Lessons From Low-Level Activity

The week’s earthquakes demonstrate that even areas of high seismic activity do not record a constant level of earthquakes. Further,we learn that earth movements aren’t just related to the usually-dominate subduction zones but also occur as a result of mountain-building and at shallow depths in settings where even a relatively small earthquake may be considered unusual.

Sources

USGS. Real time earthquake map. (2012). Accessed November 27, 2012.

USGS. M4.7 – 49km N of Ariquemes, Brazil. (2012). Accessed November 27, 2012.

USGS. M5.5 – 296km SSE of Turpan, China. (2012).  AccessedNovember 27, 2012.

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