This is the week when, if you believe the Internet, the alignment of the planets is due to cause mega-earthquakes all around the Pacific.
(I won’t bother to link to the articles; they aren’t hard to find though probably not worth the effort.)
At Decoded Science, we are comfortable enough in our seismological knowledge to say that it won’t happen — and we haven’t lost any sleep over it.
The number of earthquakes varies from week to week but that variability is natural and normal. This week the United States Geological Survey’s real time earthquake map, which records tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed nothing out of the ordinary. Its total of 1625 tremors included 3 ≥M6.0 (all in the western Pacific); 28 ≥M5.0; 116 ≥M4.0; and 245 ≥M2.5.
The Week’s Largest Earthquake: M6.9, Solomon Islands
The area along the South Solomon Trench has been rumbling for a while, and this week does rather feel like Groundhog Day as we yet again come back to the Solomon Islands for the week’s largest earthquake.
This week’s major tremor weighed in at M6.9, just a little larger than last week’s and the biggest in the earthquakes series so far, although whether it’s actually the mainshock in the series, or whether it’s just a larger foreshock with the mainshock still to come, is something that we won’t know for certain until this particular episode of earthquake activity has quietened down.
Beyond that, there isn’t much to add to last week’s summary in which we noted that the tremor is almost certainly the result of subduction along the margin between the Pacific and Australian plates. The USGS earthquake archives suggest that the current episode of earthquake activity is at the higher end of what we might expect; but larger earthquakes can, and do, occur here.
M5.8 Earthquake, South Sandwich Islands
A long way away for the western Pacific, in the South Atlantic, there’s been a further rumbling of earthquake activity. In this extremely remote corner of the world, in the South Sandwich Islands, a short (in global terms) length of subduction zone marks the point where the South American plate descends beneath the Scotia microplate, trapped between the Antarctic and the South American continent.
In the barely-settled, hardly-explored and little-understood South Sandwich Islands, large subduction-related earthquakes aren’t uncommon but often pass without notice because of the remoteness and the lack of impact.
The USGS shows almost 150 tremors of at least M6.0 along the trench in the last century — a probable underestimate. The largest of these was an M8.1 in 1929, but as recently as 2013 the area was shaken by a tremor of M7.3.
US Earthquakes: Nevada
It’s hard to resist the temptation to look at the San Andreas and the possibility of California disappearing into the sea; but common sense and respect for magnitude leads us to King Spring Canyon, Nevada, where the extensional tectonics of the Basin and Range Province caused an M4.8 tremor which was felt in Las Vegas.
Quakes: The Big One
This week (22 May) was the anniversary of the largest earthquake on record. That was an M9.5 off Chile in 1960. In the first 15 years of this century we’ve seen two earthquakes of at least M9.0.
Because of the limits to the bearing strain of rock, M9.5 is about as big as it gets on a theoretically open-ended earthquake scale — so that even if every serious seismologist on the planet turns out to be wrong and the planets were to align and cause the Big One along the San Andreas, most of us will probably still be here to talk about it next week.