If there’s such a thing as a normal week in earthquake terms, the week of 8-14 June, 2017 was probably it (or certainly well within the expected range). The United States Geological Survey’s real time earthquake map includes a total of over 1800 events, which is a little bit more than it normally shows but (given the caveats attached to the map) nevertheless a significant underestimate of the real figure.
The map shows (broadly speaking) earthquakes of all magnitudes in the US and its territories and those of at least magnitude M4.5 elsewhere. Despite its limitations regarding the smaller earthquakes, it’s accurate for the larger-magnitude events. This week these included two of ≥M6.0; 28 of ≥M5.0; and 73 of ≥M4.5.
As usual, most of the larger-magnitude earthquakes were at or near the margins of the planet’s tectonic plates. There was one noteworthy exception (an M5.3 in the Hawaiian islands, right in the middle of the Pacific) which is discussed below.
The Week’s Biggest Earthquake: M6.9, Guatemala
At the time of writing, this week’s M6.9 earthquake in Guatemala is very recent (just a few hours old) and so there’s limited available information from the USGS. More may come up later, but at this stage there’s no detailed tectonic summary or any information on the type of movement.
The tremor occurred onshore, a short distance from the coast of Guatemala. Tectonically, this is an area where the coastline is marked by an offshore trench, the Middle America Trench, which is the axis along which the Cocos plate descends beneath Central America, close to the margin between the North American and Caribbean plates, and at a depth of 94km.
The proximity of three plates, rather than two, implies that the situation may be more complicated than the location relative to the Middle America Trench would suggest.
A look at more detailed tectonic maps shows that the tremor took place in a zone of parallel strike-slip faults associated with the margin between North America and the Caribbean plate, rather than as a result of the subduction of the Cocos Plate.
M6.3 Tremor, Greece
Earthquakes don’t recognise geopolitics: This week’s M6.3 in the eastern Mediterranean might technically have occurred in Greece, but it’s very much a product of Turkish (or rather, Anatolian) tectonics.
The Mediterranean as a whole, and the eastern part of the basin in particular, is complicated. The convergence of Africa and Eurasia over millions of years has led to the closure of an ancient ocean and the Mediterranean, with its many islands, is the last remnant of that ocean. South of Greece, Cyrus and Turkey much of that movement is accommodated by subduction zones, but much of it has to be taken up elsewhere.
Without detailed fault maps of the area, it’s difficult to be specific about the cause, but the location — some distance away from any major faults); the depth (10km); and the direction of movement (extensional) provide some clues. Robert Yeats notes that one of the main faults bounding Turkey, the North Anatolian Fault, splays into several branches in the Northern Aegean, and that these branches display different types of movement.
It seems most likely, therefore, that the earthquake results from movement associated with one of these diverging faults, rather than the northwards movement and subduction of the African continent. But I stand to be corrected on this.
US Earthquakes: Hawaii
I said above that there was an outlier in terms of the major earthquakes. In Hawaii, a shallow M5.3 earthquake almost 20km from the town of Volcano was clearly not the result of plate tectonics. The clue is in the name: Volcano. The earthquake occurred on the currently-active Kilauea volcano, one of several shield volcanoes which make up the island chain of Hawaii, all of them supplied by magma rising from deep within the mantle.
Most of the earthquakes recorded on the USGS map are the result of plate movement, but it never does any harm to remember that there are volcanic, as well as tectonic, earthquakes.
“Thousands of earthquakes occur every year in the State of Hawaii,” notes the USGS. “They are caused by eruptive processes within the active volcanoes or by deep structural adjustments due to the weight of the islands on Earth’s underlying crust.”
Volcanic earthquakes aren’t as large as those associated with major plate boundaries, so this was a significant event,. But it’s worth remembering that the magma plume beneath Hawaii is capable of generating much larger events the this.
Last Thoughts: A Crowded Surface
There was nothing too complicated this week, but both of the major earthquakes weren’t as straightforward as they might at first seem. Both resulted (I conclude) essentially from realignment of the crust as different forces come into play — the differing movements of the Cocos, Caribbean and North American plates in one case, the squeezing of the ocean crust of the Mediterranean in the second.