Earthquakes may obey certain scientific rules, but they aren’t bound by numbers.
After a few weeks of relative seismic quiet, the planet burst into activity with significant earthquakes and their aftershocks in both the north and south Pacific Ocean.
Between them, these two events accounted for 28 out of 53 tremors of at least magnitude 5 (≥M5.0) recorded on the United States Geological Survey’s real time earthquake map for the past seven days.
Without these two series of tremors, the world went quietly around its business with the main focus of activity, as usual, around the boundaries of the Earth’s tectonic plates.
There was a flurry of tremors (≥M5.0) along the west coast of South America and in the western Pacific and eastern Indian Oceans, with a smaller number of tremors on mid-ocean ridges.
The Week’s Biggest Earthquake: M7.9, Alaska
Initially recorded as being M8.0, the week’s largest tremor occurred off the Aleutian Islands in Alaska, triggering a tsunami warning that “widespread dangerous coastal flooding accompanied by powerful currents are possible and may continue for many hours after tsunami arrival” in certain areas relatively close to the epicentre.
Authorities subsequently cancelled the warning – although a series of small waves, with a maximum height of 0.7 feet (around 8 inches) did eventually reach the shore locally.
Alaska Quake: Why No Tsunami?
Seismologists work on the assumption that an offshore earthquake with vertical movement is capable of generating a tsunami if it has a magnitude (roughly speaking) of 7 or more. At M7.9, this week’s tremor was theoretically capable of generating a larger tsunami than it did. So why not in this case?
The tremor appears to have been a classic subduction earthquake, occurring at the interface between the Pacific plate and the North American plate where the former subducts beneath the latter. This quake, and the more significant of its aftershocks, occurred at a depth of around 100-110 km.
In deeper earthquakes much of the energy released is lost before it reaches the surface – so the likelihood of a major tsunami is reduced.
M6.9 and Aftershocks, Raoul Island, New Zealand
The second largest tremor of the week, the M6.9 quake which occurred around 650km north of New Zealand, illustrates different aspects of subduction zone earthquakes.
The overall tectonic setting here is similar to that of Alaska; again, the Pacific plate is subducting, this time beneath the Australian plate. With its magnitude of M6.9 (initially given as M7.2) and a depth of just 20km, the earthquake might have been expected to trigger a tsunami; but it didn’t.
Although detailed information isn’t available, the location of the tremor (roughly 100km to the west of the actual boundary in the over-riding plate) suggests a different mechanism to the Alaskan event.
It’s more likely that the cause was not vertical movement along the plate interface, but deformation and faulting within the over-riding plate. Although this is obviously associated with subduction it doesn’t seem to have involved vertical movement; and so did not generate a tsunami.
US Earthquakes: Oklahoma
Alaska clearly dominated the world, not just the US in seismic terms this week. But it’s worth keeping a watching brief on Oklahoma, where earthquake activity continues – and this week, with two tremors in excess of M4.0, even looks to be strengthening.
Subduction Zones and Earthquakes
The two major earthquakes which occurred this week illustrate that even within the same setting – in this case a subduction zone – different types of earthquakes may occur. Neither a shallow intraplate earthquake nor a deep plate interface event generated a significant tsunami. On another day, a tremor the same magnitude as Alaska’s, at a shallower depth, might have very different results.