M6.7 Earthquake Shakes Alaska, 28 May 2015 (But California is Still There)

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Location of the M6.7 Alaska earthquake of 28 May 2015

Location of the M6.7 Alaska earthquake of 28 May 2015. Image by USGS.

Okay, so it wasn’t the M9.8 that wild rumours on the Internet had said it would be…

And it wasn’t in California as some people were expecting…

But at least the Earth gave us something to talk about on the evening of 28 May – even if it was two orders of magnitude smaller than the predicted apocalyptic earthquake which kept some people awake.

What we got was a magnitude 6.7 earthquake (revised down from initial estimates of M7.0) in Alaska,

M6.7 (7.0) Quake:The Tectonic Setting of the 28 May Alaska Earthquake

You don’t have to be a seismologist to take a brave guess that Alaska is going to endure regular, and sometimes large, earthquakes. The northwards-moving Pacific plate is subducting beneath the North American plate along the Aleutian Trench at speeds of up to 78mm pr year. In the west the margin between the two plates is lateral; they slide past one another.

Look at the map. At some point the direction of movement has to change and that change comes in the bend roughly west south west of Anchorage. The massive forces at work here crumple the crust up into mountain ranges such as the Alaska Range in the north and the Chugach Mountains further south as the collision shortens and uplifts the crust.

North of the eastern part of the subduction zone, this thickened crust is heavily fractured and major fault zones run broadly parallel to the plate boundary. The situation is further complicated by the existence of a sliver of crust, the Yakutat microplate, which also being subducted beneath North America. Between them, these factors mean that the tectonic setting in the Gulf of Alaska is far more complicated than it is further west.

In the absence of any detailed information, the location and depth of the most recent earthquake, when compared with existing fault maps, suggest that it is probably not the direct result of subduction but is instead the result of movement along the Border Ranges Fault Zone.

Earthquakes in Alaska

Alaska has experienced many large earthquakes

Alaska has experienced many large earthquakes – these are those of at least M7.0 since 2015. Image by USGS.

There is no question that Alaska is earthquake territory. A quick search of the United States Geological Survey’s real time earthquake map and earthquake archive throws up over 50 earthquakes of at least M7.0 in the last century alone, with six of them at least M8.0 and one (of which more later) in excess of M9.0.

Less than a year ago, in June 2014, the Aleutian Islands were shaken by a tremor of M7.9; while in 2001 another tremor of the same magnitude caused extensive damage in central Alaska, although thanks to the remoteness of the area and scarcity of population, just one person was reported injured.

And in just the 30 days up to and including the most recent tremor, Alaska experienced over 220 earthquakes in excess of M2.5; while the preceding week saw almost 300 of all magnitudes — most of them concentrated in the inland fault zones rather than along the subduction zone.

The Big One: The Prince William Sound Earthquake of 1964

The 1964 Alaska earthquake caused extensive damage

The 1964 Alaska earthquake caused extensive damage. Image by USGS.

The forces at play off and beneath Alaska are great — great enough to have triggered the second largest earthquake ever recorded at a magnitude of >9.2 (some sources have it slightly smaller). The scale of the 1964 tremor was enormous. Yeats notes that the tremor ruptured a fault zone along 850km and caused displacement as high as 11.3m (though in most places the movement was much smaller, of the order of 2.3m).

The earthquake killed 131 people in Alaska, Oregon and California — an astonishingly low number, given its magnitude — largely as a result of tsunamis generated by the earthquake and by local landslides.

The area affected was enormous: the Alaska Earthquake Information Center summarises it as follows:

The area where there was significant damage covered about 130,000 square kilometers. The area in which it was felt was about 1,300,000 square kilometers (all of Alaska, parts of Canada, and south to Washington). The four minute duration of shaking triggered many landslides and avalanches. Major structural damage occurred in many of the major cities in Alaska. The damage totalled 300-400 million dollars (1964 dollars)”.

Alaska, Earthquakes, and Planetary Alignment

Alaska will go on having earthquakes, regardless of how the planets line up, purely because it’s a part of the world where compression of tectonic plates cause enormous local and regional stresses. It’s fortunate that it’s so sparsely populated that even major earthquakes cause relatively little damage.

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