It was an interesting week for earthquakes, with plenty of meaty earthquake steaks to get your teeth stuck into — so much so that we could probably have written a special US edition of the digest, never mind the earthquake that took place nearly at the top of the world.
Numerically, the earthquakes that appeared in the United States Geological Survey’s real time earthquake map (which includes tremors of all magnitudes in the US and its territories and those of at least magnitude 4 elsewhere) were nothing abnormal. The total of 1,639 events included one of M7.8, three of at least M6 (≥M6.0), 28 of ≥M5.0 and 109 ≥M4.0.
One of them, an M5.2 in the Norwegian Sea, was so far north at 86.1°N that it sits on the very edge of the USGS map and causes some problems for the digital cartographers (“Due to the high latitude of this event, the location map does not show the correct location. We are working on a solution for this problem”).
That wasn’t the only technical hitch — but more of that later.
The Week’s Biggest Earthquake: M7.8, Bonin Islands
This week’s main event was very big (M7.8); very deep (667km); and very remote (874 km south of Tokyo). The first of these factors gave it the potential to cause significant damage and the second two worked against it, with the result that, although the tremor was felt (weakly) across much of Japan, very few people were injured and there was little disruption.
Tectonically, the tremor occurred in the Bonin Islands (in the western Pacific), to the west of the Isuzu Trench, along which the Pacific plate subducts beneath the Philippine plate.
At first glance it looks as though it is a subduction zone earthquake but in fact its abnormal depth indicates otherwise.
The USGS summary solves the problem — the huge stresses generated by large-scale plate tectonics create stresses beyond the plate margins.
The earthquake, they say: “occurred as the result of oblique-normal faulting at a depth of over 660 km beneath the Pacific Ocean… This earthquake occurred in response to stresses generated by the slow distortion of the Pacific plate at depth, rather than occurring on the interface between the Pacific plate and the overriding Philippine Sea plate”.
M6.7 Quake in Alaska
Earlier this week, on 29 May (28 May at the epicentre) Alaska made a strong early pitch for the week’s biggest tremor. An earthquake with an initial magnitude of M7.0 (later revised downwards to M6.7) would have been the chart-topper in many, if not most, of Decoded’s weekly earthquake digests.
Like the Bonin Islands tremor, Alaska’s earthquake looks at first glance like a subduction earthquake but, on closer inspection, is probably not.
Again depth is the key and in this case it was relatively shallow (72 km) given the distance from the subduction zone. A look at fault maps provides additional clues.
The mainland of Alaska is being crumpled by the convergence of the Pacific and North American plates, both along the east-west trending Aleutian Trench (subduction) and the north-south trending Queen Charlotte Fault (strike-slip).
Between them, these movements have generated significant fault zones and it seems most likely that this week’s earthquake occurred along one of these — the Border Ranges Fault Zone.
US Earthquakes: Now You See Them, Now You Don’t
We were scratching our heads at goings-on in California this week. Alerted to reports of a large (M5.5) in California, we were met by a message that the event had been deleted.
This happened more than once. Who deleted these earthquake reports? Aliens? The FBI? Was this seismic meltdown foretold on the Internet, and if it was, why didn’t they want us to know?
As in so many cases, the answer is more prosaic (though no less interesting for that). The culprits are, in fact, the two earthquakes above. I’ll let the USGS explain:
“On Friday, a M6.7 earthquake occurred at a depth of approximately 60 km, 111 km off of Chirikof Island, Alaska. It was this earthquake that fooled the automatic processing of the Northern California Seismic System to issue the first false alert. Just 28 hours later, a M7.8 earthquake off of Japan with a depth of more than 660 km – the deepest earthquake of its size to have occurred during our history of recording – spawned two more phantom events in Northern California”.
In other words, the ‘phantom’ quakes in California were just echoes of the Japan and Alaska tremors.
Earthquakes: How Little We Know
This week’s earthquakes were fascinating, because (as is often the case) they weren’t what they appeared — or in the case of the ‘phantom quakes’ of California, just weren’t. A first, not unreasonable, reaction may turn out to be inaccurate in the light of interpretation of the data.
I hope you learned something this week. Here at Decoded we certainly did — and that’s always a good thing.
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