Is it holiday lethargy? The week of 7-13 July was yet another quiet week for earthquakes. The United States Geological Survey’s real time earthquake map included a total pretty much as we might expect (almost 1700 earthquakes) but there wasn’t a lot of energy expended in terms of large tremors.
The map shows (broadly speaking) all tremors in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere. This week, it includes just two of ≥M6.0 — one in Ecuador and the other in Tonga. There were 25 of at least M5.0; 127 of at least M4.0; and 238 of at least M2.5.
As usual, the distribution of the larger earthquakes (those ≥M4.5) strongly reflected the margins of the earth’s tectonic plates, both those where the plates converge and those where they diverge. Two outlying earthquakes, in central Asia and East Africa, occurred in tectonic settings where extension is occurring — with the potential for plate breakup millions of years in the future.
The Week’s Largest Earthquake: M6.3, Ecuador
The Pacific margin of both South and Central America is marked by subduction — of the Cocos plate beneath the North American and Caribbean plates in the north, and the Nazca plate beneath South America in the south.
This week’s largest earthquake, an M6.3 150km from the Ecuadorian capital, Quito, occurred along the latter boundary — though close enough to the junction between the four plates to be complicated.
At first sight the earthquake looks likely to be a subduction earthquake, but the combination of depth (20km) and distance from the boundary (around 100km) implies otherwise.
A look at tectonic maps shows that the larger-scale relationships between the Nazca, Caribbean and South American plates has created alternative tensions and faulting. These maps suggest that the most likely cause is deformation caused by this coming together, probably along a strike-slip boundary running broadly parallel to the trench.
M5.2 Tremor, Western Australia
This week, the USHS map recorded an earthquake of M5.2 in the south of Western Australia. Geologically speaking, the Australian continent is referred to as a craton — an area which is tectonically stable and has been for many hundreds of years. We might, therefore, expect that such areas don’t experience any significant seismic activity at all.
This is not, in fact the case. Seismologist Robert Yeats lists five earthquakes of at least M6.0 in Australia since 1968. These tremors have typically been shallow focus and Yeats suggests that the majority of them occur in ancient mineralised zones — that is, those where, many millions of years ago, there was some kind of tectonic activity.
US Earthquakes: Nevada
Extensional tectonics dominates in the Western Rockies, where it produces a series of parallel mountain ranges and valleys known as the Basin and Range province. Typically, these extensional tectonics produce small or medium earthquakes along normal faults.
This week’s featured US earthquake is an M4.5 which occurred along one of these fault zones, the Walker Lane Seismic Belt, an area of historic faulting. The past year has seen four earthquakes of at least M4.0 in the immediate area, along with many smaller tremors.
Last Thoughts: Just a Little Bit Complicated
It’s generally true that we can separate earthquakes into those which occur at constructive, destructive and conservative margins. In such situations, as Ecuador shows, the actual cause of the earthquake can be a little more complicated than the dominant tectonics suggest.
Both the Nevada and Australian earthquakes show us that there can be different types of seismic belt away from the margins — and Australia, in particular, reminds us that there is no tectonic province on the planet so stable that it can’t produce an earthquake, even if earthquakes in these areas are generally much smaller than those felt elsewhere.