Argentina, the South-West Pacific and Alaska: Earthquakes 20-26 February

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Earthquakes at the boundary between the Australian and Pacific plates. Image credit: USGS

Fiji, Tonga and Samoa: a Trio of Earthquakes

The south-western Pacific again saw seismic activity this week. Some way east of the Solomon Islands a sharp ‘u-shaped’ bend is obvious in the boundary between Pacific and Australian plates. This week, three earthquakes of at least M5.0 occurred in this area and, although not individually significant, the trio nevertheless serve to draw our attention to their tectonic setting.

This region is characterised by contrasting – and competing – plate motions. To the east is the Tonga Trench, a classic subduction zone backed by a line of volcanos and a short length of ridge where the eastwards-moving Pacific plate is forced beneath the Australian plate. This contrasts with the subduction direction of the Solomon Islands further west, where the direction of subduction is reversed.

Caught between these two, lies an area where the boundary is not well-defined and, indeed, is not typically marked on geological maps, although the broad line of it can be traced on the map (shown) along an aseismic fracture zone (heavily faulted terrain). Inevitably, however, with such strong and contrasting motions, earthquakes will occur – and frequently do.

The Week’s Largest Earthquake in the U. S.

Earthquakes are so common in Alaska that this weekly report normally focuses on smaller tremors in other areas: but this week, we take a closer look at the United States’ largest earthquake – M5.4 in the Aleutian Islands. The Aleutians form a typical subduction zone with the Pacific plate subduction beneath the North American plate.

Both of these plates are, at this point, of oceanic crust: the subduction, melting and eruption of subducted rock creates new continental crust which has its expression on the surface in chains of volcanic islands. Such arcs are typified by extensive earthquake activity along the subduction zone. This week’s larges US earthquake took place in the subducting plate and, at M5.4, was small for the area. The subduction of the Pacific plate in Alaska is capable of generating earthquakes many times larger – and, indeed, this region is the source for some of the world’s largest tremors.

Sources

USGS. Real time earthquake map. (2013). Accessed February 26, 2013.

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

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