I’m a day late with the earthquake digest this week, and it means I can offer you some breaking news — one of the largest earthquakes we’ve seen in the mainland US for a while, in Montana.
This event apart, the rest of the week of 30 June-6 July was reasonably run of the mill — which is not to belittle the impacts of larger earthquakes elsewhere, but merely to note that they aren’t necessarily unusual.
Aside from Montana, there wasn’t anything truly exceptional in the 1800-plus earthquakes recorded on the United States Geological Survey’s real time earthquake map, which shows (with caveats) tremors of all magnitudes in the US and its territories and those of at least M4.5 elsewhere. There were two tremors of at least M6.0, 20 of at least M5.0 and 102 of at least M4.0.
Normally, almost all of the larger events are associated with the margins of the Earth’s tectonic plates: this week there were a few scattered about in the continental interiors (the Montana earthquake being one of them). Africa and central Asia also saw intermediate-sized earthquakes, as did the North Sea. It’s maybe a little unusual to see so many in one week, but not exceptional.
The Week’s Biggest Earthquake: M6.5, Philippines
We’ve been in the Philippines before at this point in the weekly digest, and I venture to suggest it won’t be very long before we’re here again. The collision of the westwards-moving Philippine Sea plate with the eastern margin of Eurasia has given rise to a complicated jumble of plates and microplates and a consequent series of regular, large earthquakes.
This week’s earthquake seems relatively easy to explain. Although there’s no earthquake summary for it on the USGS site (which, in itself, implies that the event is not so unusual) we know the depth (6.5km), the location and the nature of the movement (lateral) which, along with a quick check at tectonic maps elsewhere, shows what probably happened.
This part of the Philippines lies between two subduction zones — the Philippines Trough (dipping westwards) and the Negros Trough (dipping eastwards). In the middle, a long series of active faults runs along the central Philippines, and it appears to be on one of these faults (either the Philippine Fault or one associated with it) that the earthquake occurred.
M4.8 Earthquake, North Sea
The North Sea is a rough and inhospitable place at the best of times, but it’s not usually associated with earthquakes. This week, for a change, the good folk of Shetland and western Norway might have got a little excited by the earth moving. Not on a large scale, but large enough. The USGS map recorded a tremor of M4.8 between these two pieces of land.
It’s unusual, but it isn’t unprecedented, and the clues to its origin can be found in the geology, rather than in any earthquake summary (the USGS gives little or no information). The North Sea is a basin filling up with sediment, the weight of which eventually causes subsidence and faulting. And faults, as we know, are the sources of earthquakes.
Without access to detailed fault maps I can’t say exactly why this week’s earthquake occurred but it does seem to be associated with a major structural feature, the Viking Graben, in the north-central North Sea. With no information on the nature of the movement, I can’t say for certain — but I would expect that the cause was movement along a normal fault.
US Earthquakes: Montana
As I write, the Montana earthquake of 6 July is only hours old and daylight is barely breaking, so it’s likely that there’s as yet no assessment of any damage caused by the M5.8 event. The USGS summary of the event, however, has identified both the source and the cause (though not the exact fault on which the earthquake occurred): “The location and focal mechanism solution of this earthquake are consistent with right-lateral faulting in association with faults of the Lewis and Clark line, a prominent zone of strike-slip, dip slip and oblique slip faulting trending east-southeast from northern Idaho to east of Helena, Montana, southeast of this earthquake.”
The tremor is tectonic rather than volcanic, so not associated with the earthquake swarm at Yellowstone which I’ve discussed in recent weeks. It’s worth noting, too, that Montana is by no means unused to earthquakes, with nearly 100 of at least M3.0 recorded by the USGS between 2010-2015.
This week’s is large in its context, but it isn’t the largest on record. In 1959 there was a tremor of M7.2 in the state and, more recently, an earthquake of only slightly lesser magnitude (M5.6) occurred in 2005.
Last Thoughts: Non-Subduction Earthquakes
It’s rare that I write a digest which doesn’t discuss at least one earthquake associated directly with a subduction zone. This week, all the featured earthquakes were the result of shallow Earth processes, all within the top 20km of the crust.
Two of them, at least, (the Philippines and Montana) were the result of lateral movement: we don’t know about the third but the chances are that it resulted from extensional movement.
Subduction earthquakes tend to be the largest ones on the planet, but the Philippine and Montana earthquakes, at least, serve as a reminder that compressional forces aren’t the only ones capable of producing significant earth tremors.