The week of 19-25 January, 2017, produced a couple of interesting, not to say distressing, seismic events.
In the western Pacific, a major earthquake caused damage, though (thankfully) early warnings of a potentially damaging tsunami were later withdrawn, and in Italy, a couple of intermediate-sized earthquakes are thought to have triggered a deadly avalanche.
These are individual events in what was, in terms of numbers, an ordinary week. The United States Geological Survey’s real time earthquake map, which records earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included just two events of (≥M6.0) and 25 of at least M5.0 in the total of just under 1300.
As usual, most of the larger earthquakes were exactly where we’d expect them to be — at or near the margins of the planet’s major tectonic plates, with a significant concentration in the western Pacific.
The Week’s Biggest Earthquake: M7.9, Papua New Guinea
The USGS gives the location of this week’s M7.9 (initially recorded as M8.0) as Papua New Guinea, but in actual fact it occurred beneath the northern Solomon Islands. M7.9 is a large earthquake in anyone’s thinking, given that we can expect to see, on average, one earthquake of M8.0 or larger in a year.
As anyone who reads this digest on a regular basis will know, large earthquakes are common in this part of the globe. The collision between the Pacific and Australian plates is a twisted and complex one, and in parts it’s further complicated by the existence of smaller slivers of crust (microplates) trapped between the two major plates.
This is the situation in and around the Solomons and Papua New Guinea. Happily (for me, at least) the earthquake was large enough for the USGS to produce a summary and they explain it very simply: “the January 22nd event occurred along the boundary between the Solomon Sea microplate and the Pacific plate. The Solomon Sea microplate moves slightly faster and more northeasterly with respect to the Pacific plate (and South Bismarck and Manus microplates) than does the Australia plate due to sea-floor spreading in the Woodlark Basin to the southeast of the January 22nd earthquake.”
This is a classic subduction zone, but the depth and location of the earthquake imply that it wasn’t caused by movement at the plate boundary but by deformation further into the crust. Earthquakes of this magnitude are more than capable of causing a tsunami but none occurred — the earthquake was deep and its epicentre on land, with key to countering tsunamigenesis.
M5.7 and M5.6 Tremors, Central Italy
Shortly after I hit the ‘submit’ button for last week’s digest, two earthquakes of intermediate size — M5.7 and M5.6 — occurred in central Italy. Like the Western Pacific, the Mediterranean is a geological and tectonic mishmash, caused by the collision of Africa and Eurasia.
Again the USGS manages to explain it for me: “The evolution of this system has caused the expression of all different tectonic styles acting at the same time in a broad region surrounding Italy and the central Mediterranean.”
In Italy, the dominant tectonic style is extension, resulting in normal faulting as one block of rock slips downward relative to another. Shallow earthquakes of this nature are a fact of life in Italy, but the historic nature of the landscape and the mountainous terrain mean that such earthquakes can cause loss of life.
This week’s two earthquakes are the latest in a series which began in August with an M6.2 which killed almost 300 people. Although neither earthquake this week has caused any direct deaths, it’s possible that the earthquakes may have triggered a fatal avalanche that demolished a hotel.
The M5.7 earthquake was the second largest in the continuing series of aftershocks — and it may not be the last.
US Earthquakes: Oklahoma
Things seem to have gone a little quiet in the southern States in terms of earthquakes. The USGS map records just 16 earthquakes in Kansas and Oklahoma, the largest of them coming in at M3.1. We know that many of these earthquakes are associated with wastewater disposal as a byproduct of the oil extraction industry, and there’s extensive research ongoing alongside significant environmental concern.
I’ll keep an eye open for news reports on this subject. The earthquakes will keep coming, and hopefully they’ll keep being recorded on the USGS map.
Last Thoughts: Earthquakes and Hazard
Earthquakes don’t just cause damage directly: they produce a whole range of associated hazards. This week illustrated that. The potential for a major tsunami was certainly present in the Western Pacific, though fortunately not all the conditions were met — but a tsunami in this region could have killed many, many people, where the earthquake itself killed none.
Similarly, in Italy, it was an avalanche that claimed lives, not the earthquakes that preceded it — but the two can’t be regarded as separate. And when there’s no snow, earthquakes can trigger landslides and dam collapses, with associated damage and loss of life.
The Earth is a system. When something happens in one part of it, there are impacts elsewhere. That’s something it’s always wise to remember.