Ah, summer (in the northern hemisphere) at least — a time when much of the population steps down a gear and life’s rhythms change. And in the week of 30 June-6 July 2016, the Earth seems to have calmed down a little, too, with one of the less seismically active weeks I can remember.
The United States Geological Survey’s real time earthquake map doesn’t have many tasty tectonic snippets for us this week. The map, which (broadly speaking) includes tremors of all magnitudes in the US and its territories and those of art least magnitude 4 (≥M4.0) elsewhere, is relatively quiet.
There were no earthquakes in excess of M6.0 and the largest on the map, at M5.9, was deep in the middle of the South Atlantic and must surely have passed unnoticed. Out of the 1619 recorded in total, 242 ≥M2.5, 107 ≥M4.0, and just 21 ≥M5.0.
The Week’s Largest Earthquake: The Middle of Nowhere
I think this may be the first time that I’ve failed to produce a map for the week’s largest earthquake, in the far south of the Pacific. It isn’t that I couldn’t produce one (the USGS has its usual array of graphics) but the best I could offer would just be a single star on plain blue and you might easily mistake that for the flag of a country that’s just left Europe. But let’s not go there — that was last week’s running gag.
In terms of isolation, this week’s largest earthquake, and M5.9, was a cracker. The nearest land is a shade under 2,000 km distant (Antarctica) and the nearest inhabited land, New Zealand, is almost 3,500 km. The USGS page includes a tell-us-if-you-felt-it form but something tells me they aren’t expecting anyone to fill it in.
We do know that the earthquake occurred at a shallow depth (10m) on or close to the ridge dividing the Pacific and Antarctic tectonic plates. Earthquakes in these settings are typically fairly small but there are usually a few of between M4.5 and M6.0 on the map each week; they can be larger. They are caused by extensional movement, as the plates move apart.
M5.8 Tremor, Vanuatu
Some way further north in the Pacific Ocean, in a contrasting tectonic situation, an earthquake occurred in an area regular readers will be familiar with. The complicated margin between the Pacific and Australian plates produces earthquakes in excess of M4.0 several times a week, and this week the largest tremor was in the island chain of Vanuatu.
At this point in the plate margin, the Australian plate subducts northeastwards beneath the Pacific plate along the New Hebrides Trench. This is an area which, as the seismologist Robert Yeats notes, is “frequently struck by earthquakes and tsunamis”. This week’s event didn’t generate a tsunami; it was too small and its epicentre was on land.
Unlike the largest earthquake, this one does appear to have been felt. The USGS shake-up implies that up to a quarter of a million people may have felt weak to moderate shaking.
US Earthquakes: Offshore Oregon
A very small earthquake in a populated area can be felt by many hundreds of thousands — an M2.9 in California, for example, was reported by residents of San Diego. But the biggest earthquake in the contiguous USA, an M4.9 off the coast of Oregon, appears, like the largest of the week, to have passed unnoticed.
Like the larger earthquake, this baby’s crying woke no-one. It occurred along the Blanco Fracture Zone, almost 400km offshore, and was again the result of consecutive tectonics, with new crust forcing plates apart.
Last Thoughts: Did Anyone At All Feel Anything This Week?
I said at the beginning that it was quiet, and the earthquakes featured rather illustrate the relationship between earthquakes and earthquake hazard. If there is no-one living nearby, and an earthquake doesn’t create a tsunami, then even the larger seismic events can pass unnoticed.
To get a measure of how remote the M5.9 in the South Pacifc actually was, compare it with the Oregon earthquake — much larger, true, but ten times as far from inhabited land.
An earthquake is only hazardous if people live near enough to feel its impacts.