Well now. It isn’t that long since I was talking about statistical variation in a week when there was a (seemingly abnormally) high level of seismic activity. The week of 30 April- May 2016 both redressed the balance (there was relatively little significant activity) and illustrated the point (because this is not particularly unusual either).
Just one earthquake of at least magnitude 6 (≥M6.0) appeared on the United States Geological Survey’s real time earthquake map. The map, which (broadly) includes all tremors in the US and its territories and those of ≥M4.0 elsewhere, showed 23 events of ≥M5.0 and 101 of ≥M4.0 in its total of just over 1500. That’s at the quiet end of what we’d expect — but still normal.
Distribution was normal, too. All of the larger earthquakes (≥M4.5) were at or near the margins of the planet’s tectonic plates.
The Week’s Largest Earthquake: M6.0, Vanuatu
There’s nothing new here, either — just another in the ongoing series of earthquakes at the margin between the Pacific and Australian plates. This week’s tremor, an M6.0, is part of a series slightly further south than the earlier tremors but they’re close enough in time and space to be connected (though I wouldn’t bet my house on it).
I didn’t post a digest last week, for reasons beyond my control, but if I had I would have been talking about the largest in the series so far — an M7.0, in the same southern section of the plate margin as this week’s.
The USGS has pinpointed that one as “a result of thrust faulting on or near the boundary between the Australia and Pacific plates”. This week’s M6.0, in a similar location but very shallow (less than 5km), is probably also a result of thrust faulting as the two plates come together.
Taking all the recent tremors together, it’s clear that the focus of seismic activity has shifted southwards. You can choose to view them as two separate clusters or as a continuing series. Personally I’m inclined to the latter. Either way, I suspect there may be more activity to come on this section of the trench in the near future.
M5.3 Tremor, Gulf of Aden
Somewhere under the province of Afar, in Ethiopia, there’s a rising plume of magma which has lifted the crust into a dome. Under increasing tension, the crust must, eventually, split. You can see it on any topographical map — three arms of a rift system radiating outwards from that dome; the Red Sea, the Gulf of Aden and the (on land) East African Rift.
Both the Gulf of Aden and the Red Sea are widening as ocean ridges develop — the early stages in the growth of new oceans. And extensional movement, like compressional movement, places stress on faults and causes earthquakes. This week’s M5.3, between Yemen ad the Horn of Africa, is caused by just such processes.
It isn’t that big and it isn’t that significant. But I always like a reminder that the shape pf the oceans and the continents as we see them is very much a work in progress.
US Earthquakes: Waiting For the Big One
I observed, when I was commenting on the plethora of larger earthquakes a couple of weeks ago, that the fact that there hadn’t been a large seismic event in that week didn’t necessarily mean that there would be one imminently.
I also remarked that it didn’t mean there wouldn’t be. We all know the California is waiting for the Big One. The absence of large earthquakes on sections of the San Andreas Fault over many decades and even centuries, means that strain continues to build and will eventually have to be released. It was slightly alarming, therefore, to read of a study presented at a conference on earthquakes by seismologist Thomas Jordan.
“The springs on the San Andreas system have been wound very, very tight. And the southern San Andreas fault, in particular, looks like it’s locked, loaded and ready to go,” he’s quoted as saying. That’s something to worry about — and to prepare for.
Oceans Opening, Oceans Closing
Continents move, are created, consumed, recycled and reconfigured. If a new ocean is created, as it is being in the Gulf Aden and the Red Sea, somewhere another must be being destroyed. The Atlantic Ocean is growing; the Pacific is being consumed. It’s why there are so many subduction zones around the world’s (current) largest ocean.
“When the Lord closes a door,” says the Mother Superior in The Sound of Music, “somewhere He opens a window.” It may or may not be true of doors and windows — but it’s certainly true of oceans.