Subduction Earthquakes in Nicaragua and Greece: 12-18 June 2013

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US Earthquakes: Patterns of Seismicity

Earthquakes across the US give clues to its geological structure. Image credit: USGS

Earthquakes across the US give clues to its geological structure. Image credit: USGS

With no outstanding tremors to report, the week provides an opportunity to look at the structure of the US and its relationship to seismic activity. In very general terms, the east of the US is largely tectonically stable and earthquakes which occur there tend to be small and shallow, along faults which are far distant from continental margins.

In the west of the continent, however, the pattern is different. As the associated map shows, the west coast is dominated by the major strike-slip fault zone of the San Andreas and, further north, the Cascadia subduction zone.

Western North America is geologically the product of a series of separate slivers of crust which have accreted on to the stable continent over time, causing a complex situation where both extensional and compressional forces are at play. The result is a pattern of earthquakes at the edges of these blocks.

Complex Tectonics

Of course, the geology of two such complicated regions as the Mediterranean or the western United States can’t be clearly outlined in just a few paragraphs. Both are complex collision zones in different settings and with many different forces at play (in Wyoming the situation is further complicated by the presence of an underlying hotspot at Yellowstone). But in both cases the outcome is an extensive area of regular, and sometimes large, earth tremors.

Sources

USGS. Historic earthquakes list. Accessed 18 June 2013.

USGS. Real time earthquake map. (2013). Accessed 18 June 2013.

Yeats, R. Active Faults of the World. (2012). Cambridge University Press.

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