The number of earthquakes in the world follows its own pattern; but in general it’s reasonable to expect that most weeks will see a tremor of at least magnitude 6 (≥M6.0) somewhere on the planet.
This week is unusual in that the largest recorded on the United States Geological Survey’s real time earthquake map (which includes earthquakes of all magnitudes in the US and its territories and those of ≥M4.0 elsewhere) were just M5.7 – one of them in Alaska and a second in the south eastern Pacific.
To complete the numbers: There were 21 tremors of ≥M5.0; 108 of ≥M4.0; 256 of ≥M2.5; and 1665 of all magnitudes.
Overall these numbers don’t vary hugely from what might be expected and the distribution of tremors – around the Pacific, in the eastern Indian Ocean and along the southern margins of Eurasia – is pretty much normal too.
But a week like this does offer the opportunity to look at the context of some of the minor tremors which might otherwise be overshadowed by those of larger magnitude.
The Week’s Biggest Earthquake – M5.7 Easter Island
Jointly with an event in Alaska (considered below) the largest earthquake of the week, the M5.7 which occurred in the Pacific is of interest because it’s unusual (though not exceptional) for the largest event in any given week to occur away from a subduction zone – and the location of this particular tremor was an ocean ridge.
Fracturing and faulting occurs at all three types of continental margin (constructive, destructive and conservative) as well as along ancient faults within continents. Ocean ridges are areas where new crust is created as the plates are forced apart and earthquakes occur as a result. Most tremors occur not along the ridges themselves but along the faulted areas (fracture zones) which offset them and accommodate differential movements. This is the case in this week’s tremor, where the Nazca and Pacific plates are mocking apart along the East Pacific Rise.
The Arabian Sea Triple Junction
More noteworthy tremors occurred at another constructive margin, this time in the Arabian Sea. Here, three plates come together at a point called a triple junction – in this case between the Indian, Arabian and African plates.
Four earthquakes occurred this week on the westernmost arm of this junction, between the latter two plates.
Although the earthquakes are small, they demonstrate that constructive margins can produce regular tremors. They also illustrate an earlier stage in the development of a tectonic cycle – because in this section of the boundary, the ridge is driving apart two continental plates – creating a new ocean.
The week’s four earthquakes in this area – between M4.5 and M4.9 – represent just a tiny fraction of the energy required to separate continents.
US Earthquakes: Alaska
Alaska produced the joint largest earthquake of the week, at M5.7, with a tremor which occurred in what might be termed the ‘elbow’ of Alaska, where the Pacific plate moves against the North American plate.
This is a complicated region, with the northern margin broadly delineated by the Aleutian Trench, where the plates are in direct opposition, and the eastern by a series of transform faults, where the plates slide past one another.
The Aleutian Trench is the source of major earthquakes (in excess of M8.0) and is the source of the second largest tremor in the historic record, the M9.2 Great Alaskan Earthquake of 1964. A look at a fault map of the area, however, shows that this week’s Alaskan earthquake was the product of lateral movement rather than subduction.
But it’s worth noting that the faults which accommodate the complex stresses generated by conflicting direction of movement provide frequent seismic activity throughout the state of Alaska.
Last Thoughts: Constructive Margins
Generally speaking, earthquakes which occur at constructive margins are smaller than those at destructive ones, largely because of the structure of the margin and its ability to accommodate strain. But they are seismically active and structurally interesting – so always worth a look.