This week, the United States Geological Survey’s real time earthquake map recorded a total of 1,397 tremors (of all magnitudes in the US and its territories and at least magnitude 4 elsewhere).
Of these just one was greater than magnitude 6 (≥M6.0) while there were 25 ≥M5.0 and 99 ≥M4.0.
Although as usual the tremors were concentrated along the destructive boundaries of the Earth’s tectonic plates, there was also a scattering of tremors along the ocean ridges, with seismic events in the Atlantic, Pacific and Indian oceans.
The Week’s Largest Earthquake: M6.1 Alaska
On the anniversary of one of Alaska’s largest earthquakes (an M7.3 which struck in 1905) it’s appropriate that the largest earthquake of the week should occur in Alaska’s Aleutian island chain. The Aleutians are a subduction zone, where the Pacific plate subducts beneath the North American plate, creating a highly volcanic and seismically active zone. Occurring at a depth of around 265 km some way to the north of the boundary, this week’s tremor is typical of an earthquake at the interface between the two plates.
The earthquake magnitude scale is logarithmic; the 1905 earthquake, at M7.3, was just 1.2 points larger than the most recent in this respect but would have appeared six times larger on a seismometer and released 63 times as much energy.
The fact that the Aleutian island subduction zone is capable of producing so-called ‘megathrust’ earthquakes many times larger even than that of 1905 is indicative of the incredible power of tectonic movements.
The Carlsberg Ridge Earthquake Cluster
Although subduction zones and conservative boundaries (where plates slide laterally past one another) are the locations for most major earthquakes, mid ocean ridges are also seismically active. At these margins, upwelling magma from the mantle reaches the surface, forcing plates apart along major ridge systems and, as it rises, extending and fracturing the crust. Earthquakes, as well as submarine volcanic activity, result.
This week, a cluster of eight earthquakes ≥M4.5 struck along the Carlsberg Ridge in the northern Indian Ocean. The largest of these was M5.6, making it the second largest of the week. While this is not large in the wider range of earthquake magnitudes it is significant for an ocean ridge. The largest known tremor in such a location was also in the Indian Ocean, in 1942: initially estimated at M8.3, in reality it was probably considerably smaller.
US Earthquakes: Patterns of Seismic Activity
This week there were no major tremors across the contiguous United States. It is, however, worth having a look at the overall patterns of seismic activity.
The map shows how earth tremors define the boundaries of tectonic plates and minor units such as the blocks of the Basin and Range provinces in the western US – but it also shows a scattering of activity further east, which demonstrates that even apparently stable continental interiors are not immune to movement along ancient buried faults.
Seismic Activity at Ocean Ridges
The cluster of tremors along the Carlsberg Ridge is not the only activity at constructive boundaries this week. The Atlantic Ocean (both north and central) and Pacific (East Pacific Rise) also produced earthquakes of at least M4.5, making an unusually high contribution to the pattern of tremors, although the overall impact of such earthquakes is small.