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

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The week's earthquakes, 20-26 February. Image credit: USGS

The week’s earthquakes, 20-26 February. Image credit: USGS

This week’s earthquake map showed no outstanding event in terms of magnitude (the largest recorded on the USGS real time earthquake map was just magnitude 6.1 (M6.1) and tremors were relatively spread out in geographic terms, with no outstanding cluster of events, other than the on-going aftershocks from the 6 February M8.0 quake in the Solomon Islands (seven tremors of M5.0-M5.9 made it still the highest concentration of earthquake activity this week).

The M6.1 in Argentina (see below) was the only tremor to exceed M6.0, although there were 34 recorded earthquakes of M5.0-M5.9 and 121 of M4.0-M4.9.

The Week’s Largest Earthquake: M6.1 in Argentina

At M6.1, the tremor that occurred to the east of the Andes on 22 February is not major. Though it  appears from the map to have occurred in a relatively stable continental interior, it is in fact related to the subduction of the Nazca plate beneath the South American plate, a contact which occurs at the Earth’s surface several hundred kilometres to the east.

It’s possible that an event of this magnitude might have been caused by normal faulting associated with mountain building. In this case, however, the depth of the tremor (it occurred 585km below the point at which it was recorded on the surface) is a clue to a subduction origin. The angle of subduction is relatively low, explaining how such a tremor might occur at a significant distance from the main subduction zone.

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