There were no large earthquakes in the week of 15-21 January, but there were certainly events which are worthy of comment.
The United States Geological Survey’s real time earthquake map, which shows earthquake of all magnitudes in the US and its territories, and those of at least magnitude 4 (≥M4.0) elsewhere, included 33 tremors of at least M5.0 in a total of 1,280… But for the second week running, the largest of these was M5.8 (this week in Alaska).
The distribution was not unusual, with 24 of the tremors ≥M5.0 in the Pacific, the majority in the west.
This reflects the complexity of the tectonic setting in this region with many plates and microplates in close proximity.
Plate margins are the most common location for earthquakes.
The Week’s Biggest Earthquake: M5.8, Alaska
Earthquakes in Alaska are no big deal. A quick search of the USGS earthquake database shows that the Aleutian Islands, where this week’s largest tremor occurred, experienced over 1200 earthquakes during the week. If the M5.8 which struck on 18 January was the largest of them, it’s unexceptional. (In 2013 there were nine tremors of at least M5.8 along the island chain.)
The reason is the subduction zone between the Pacific and North American plates. The seismically and volcanically-active Aleutian Island chain is the surface expression of where the former descends beneath the latter at a large-scale boundary which is capable of producing tremors far larger than M5.8 and which, indeed, was the location of the second largest earthquake on record (an M9.2 at the eastern end of the zone in 1964) and an associated destructive tsunami.
Although it occurred beneath the sea, this week’s M5.8 was both too small and too deep to generate a tsunami (deeper earthquakes lose energy before they reach the surface). The available evidence of depth and location, with the epicentre north of the island arc, suggests that it was a subduction earthquake associated with movement at or near the plate interface.
M5.0 Quake, Central Asia
Though small, the M5.0 tremor and its M4.6 aftershock are possibly the most interesting events on this week’s earthquake map, located as they are many thousands of kilometres from any active plate boundary — even far north of the diffuse boundary between India and Eurasia.
The tremor occurred in central Asia, roughly 300 km north west of Lake Baikal.
Tectonically, the area consists of an S-shaped rift belt (possibly associated with an underlying hotspot) which occurs within the otherwise stable Mongolian plateau. The rift extends for over 2000km and this week’s earthquake occurred at its the northern end, where the fault trends roughly east-west.
Typically, earthquakes in such zones are caused by normal faulting — i.e. the extensional movement of the two sides of the rifts allows rocks within to drop downwards. Though there’s no detailed information on this week’s earthquake, such a cause would be entirely consistent with the extensional tectonics of the rift.
US Earthquakes: San Andreas
This week, California rocked. An M4.4 on the Pine Rock fault, broadly parallel to the San Andreas fault (which is a conservative boundary between the North American and Pacific plates), generated light to weak shaking which was felt (just) in the southern area of San Francisco.
This section of San Andreas Fault is moving slowly and surely so that earthquakes are by no means unusual, although they tend not to be particularly large: Yeats suggest that the largest earthquake to have occurred in this area was an M6.4 in 1885, which is significantly smaller than earthquakes which have occurred on the ‘locked’ section of the fault, further north and south.
Quakes and Science: Different Margins
Tremors can result from a variety of tectonic settings. This week’s featured earthquakes illustrate three different types of tectonic setting in which earthquakes can occur: convergence (at subduction zones); extensional tectonics in rift settings (which also includes mid-ocean ridges); and transverse margins where faults slide past one another.