The week of 11-17 September saw just one significant earthquake of at least magnitude 6 (≥M6.0) recorded on the United States Geological Survey’s real time earthquake map.
The map, which includes tremors of all magnitudes in the US and its territories and those of ≥M4.0 elsewhere, included a total of 1492 tremors of which 34 were ≥M5.0.
As usual, the distribution of larger tremors is closely associated with the margins of the planet’s tectonic plates.
Most notably, perhaps, the cluster of earthquakes associated with the subsidence of the crater at Iceland’s Bárðarbunga volcano continues, with eight tremors of ≥M4.5 recorded this week, the largest of them with a magnitude of 5.3.
A second cluster of earthquakes, this time five of them, occurred in Afghanistan along the western margin of the Himalayas.
M6.7, Guam: The Week’s Biggest Earthquake
Initially recorded at M7.1 but later downgraded to M6.7, the largest tremor of the week occurred on 17 September around 45 km west of the island of Guam.
Despite its offshore location and its magnitude (which approached that generally considered a threshold for generating a potentially destructive wave) no tsunami occurred.
This is likely to be because the tremor was just not quite large enough – and also because of its depth at 136km. Deeper tremors are less likely to be tsunamigenic because their energy is more easily dissipated.
Tectonically speaking the tremor looks likely to to be a classic subduction zone earthquake. The westward movement of the old, cold, dense crust of the Pacific plate leads to its subduction beneath the Philippine Sea plate, and such a setting leads to frequent, sometimes very large, tremors.
The island chain of Guam lies along this subduction zone – which, incidentally, includes the deepest known point on the Earth’s surface where the Marianas Trench reaches almost 11,000 metres.
From the limited available information at the time of writing, it appears likely that the Guam earthquake of 17 September was the result of movement at or near the plate interface.
M4.7 Quake in Central Sweden
Small earthquakes can be fascinating. This week, one of noteworthy the features on the USGS map was a tremor of M4.7 in south-central Sweden, far from any tectonic plate margin.
Yeats draws attention to the existence of a series of recent (postglacial) faults which probably developed as a result of stresses in the crust following the last ice age.
Although these are largely to the north of this week’s tremor, it isn’t impossible that other faults resulting from the pressure of ice may exist further south and have gone unrecorded.
We may speculate, therefore, that the earthquake was the result either of movement along an old but possibly reactivated fault.
This quake may also have been caused by altering tectonic stresses following isostatic rebound (where the crust moves up once the weight of ice is removed).
US Earthquakes: Texas Gets in on the Act
So, Oklahoma….you can do earthquakes? Texas can do them too.
This week saw four small tremors across the state – two near Dallas, one near Charlotte and one near Snyder.
Although there’s no available evidence to link these directly to human activity, the USGS has linked the Oklahoma earthquake swarm to wastewater injection, and more recent work has suggested that this is also the cause of low-level seismic captivity in New Mexico and Colorado.
Bearing this in mind it seems possible that the Texas ‘quakes may have an anthropogenic source (although it’s also possible that they are natural).
Either way, Texas has a long way to go before it begins to match the scale of abnormal seismic activity being shown by its neighbour.
Varying Causes of Earthquakes
Larger earthquakes are generally the result of stresses at plane margins, as the Guam tremor shows. But intra-plate earthquake can also occur, either as result of other types of tectonic activity (as in Sweden) or as the result of human activity, which may be the case in Texas this week.