This week, like last week, the planet seems to be making up for its few week’s rest with a significant amount of shaking.
The United States Geological Survey’s real time earthquake map (which records tremors of all magnitudes in the United States and its territories and those of at least magnitude 4 elsewhere) included two tremors of at least magnitude 7 (≥M7.0), four of ≥M6.0, 45 of ≥M5.0 and 126 of ≥M4.0.
The probable explanation for this blossoming of seismic activity lies in the two largest tremors of the week which, along with their foreshocks and aftershocks, between them accounted for over half of earthquakes ≥M5.0.
Taking these out, the distribution of medium and larger tremors was more or less as expected, focussed along the boundaries of the Earth’s major tectonic plates.
The Week’s Biggest Earthquake: M7.5, New Britain
It’s by no means unusual for the largest earthquake of the week to occur along the congested margin between the Pacific and Australian plates. The irregular nature of the boundary, and its reversals in direction of subduction and complex motions, along with the smaller slivers of crust trapped between the two larger plates, are a recipe for earthquakes.
This week’s M7.5 tremor off the archipelago of New Britain appears to have been the result of subduction of the Solomon microplate beneath the South Bismarck microplate — in other words, a classic subduction-zone earthquake. The magnitude, depth and location of the tremor all contributed to the generation of a small tsunami, although little damage occurred as a result.
Still in the Western Pacific: M7.1, Solomon Islands
Just two days after the New Britain tremor, a second earthquake, this time of M7.1, occurred around 300km further along the boundary of the Solomon plate, but this time where it marches with the Pacific plate.
This time, as the USGS earthquake summary notes, it wasn’t a subduction earthquake but “the result of normal faulting within the Australia plate, a few tens of kilometers to the southwest of the plate boundary where Australia begins its subduction beneath the Pacific plate at the New Britain Trench.” (To avoid confusion, note that the USGS regards the microplates as part of the Australian plate, presumably to keep a complex situation simple).
This second tremor begs a much-asked question: did the first earthquake trigger the second? The key is the distance. The USGS, looking at the relationship between large tremors occurring within a short time of one another, is clear: “Over long distances, the answer is no. Even the Earth’s rocky crust is not rigid enough to transfer stress efficiently over thousands of miles.”
But these two tremors were relatively close together, along the northern edge the same plate. “There is evidence,” continues the USGS, “to suggest that earthquakes in one area can trigger seismic activity within a few hundred miles.”
Given the immense amount of energy released from the larger New Britain earthquake, it does seem possible that there may be a relationship between the two.
US Earthquakes: Michigan
Mississippi, Texas and Tennessee all got in on the act this week, along with the usual suspects, but the earthquake that caught the eye was the M4.2 in southern Michigan. It’s an area where earthquakes are rare and the largest on record is just M4.6 (in 1947). Though it’s not verified, this week’s is likely to be one of the largest felt in the state.
Of course the first question, given the rash of anthropogenic (caused by human activity) earthquakes elsewhere, is whether humans had a hand in this one, too. There’s wastewater injection in Michigan but very little in the immediate area of the quake, suggesting a natural cause — a supposition borne out by seismologists quoted online.
Earthquakes: Two Interesting Points
This week’s featured earthquakes are interesting because they illustrate two things. Firstly, nature, as well as humans, can produce earthquakes where we don’t expect them. And secondly, significant earthquakes can have physical repercussions, apart from their aftershocks, elsewhere.