Much of the world is gearing up for the holiday season. There are frantic presses of shoppers in the traditional last-minute panic, everywhere you look. And as if to counteract this flurry of activity, the world has been strangely quiet in seismic terms this week, with just over 1350 tremors recorded on the United States Geological Survey’s real time earthquake map.
The map, which shows earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 elsewhere, included just three events of at least magnitude 6 (≥M6.0), 18 of ≥M5.0 and 80 of ≥M4.0. That’s relatively low, though not abnormally so — and we expect variation in numbers from week to week.
The Week’s Largest Earthquake: M6.6, Mexico
The largest earthquake of the week came in at a respectable M6.6 and had its epicentre (the point on the earth’s surface immediately above the actual rupture) in southern Mexico.
Central America is tectonically interesting, with four plates (the Caribbean, Cocos, North American and South American plates) all in play. The earthquake occurred on the North American plate, close to its junction with the first two.
With several plates involved there are many potentially complicating factors to consider; but in fact, the evidence suggests that the tremor was pretty straightforward and resulted from the subduction of the Cocos plate beneath Central America along the Middle America Trench.
The clue is in the depth. The earthquake occurred at a depth just shy of 100km. Maps of the descending plate suggest that this is consistent with the location of the actual plate margin at this point and that therefore it is associated with movement at or near the plate interface.
M4.7 Earthquake, Democratic Republic of the Congo
It wasn’t one of the biggest earthquakes of the week, but it was one of the less usual. Earthquakes in continental interiors are smaller and less frequent than those at continental margins — but they are nevertheless an indication of potential large-scale tectonic activity, even if that activity may be far, far in the future.
Africa is an old continent, but its major topographic feature, the East African Rift, is relatively young and evolving. Extending for over 4000km, the rift system is an area of extension. There are no plate margins as we normally understand them, but the two sides of the rift are moving apart (at varying rates). Millions of years on, Africa may well be split into two or more continents.
The extension leads to downward slippage of the land in the centre of the rift. The result is earthquakes. They tend not to be large but they are an interesting indicator of possible future continental break-up. Since the beginning of 2015 there have been at least 30 earthquakes of at least M4.0 along the length of the risk. That may not be spectacular but it’s indicative of significant ongoing activity.
US Earthquakes: Nevada
The western part of the North American continent is also an area of extension, though topographically this shows up as a series of broadly parallel mountain and valley features (basin and range). This week’s largest earthquake in the contiguous United States, an M4.2 near Reno, was along one of these fault zones. Again, interesting, but not unusual. It’s worth noting, however, that although extensional earthquakes are typically smaller than subduction ones, Nevada’s largest earthquake, in 1932, was an unusual M7.2.
Last Thoughts: Extensional Tectonics
Extensional tectonics don’t always represent continental breakup and the African Rift is an extreme example. Many areas of the world, often in stable continental areas, experience minor earthquakes resulting from normal faulting associated with extension — and this is often the explanation for earthquakes occurring at a distance from plate margins.