There was no shortage of earthquakes in the week of 5-11 March 2014.
The United States Geological Survey’s real time earthquake map, which records all tremors in the US and its territories, and those that were of at least magnitude 4 (≥M4.0) elsewhere, showed 1,728 seismic events in total — but the tremors weren’t particularly large.
Just one tremor registered in excess of M6.0 and there were 14 greater than, or equal to, M5.0.
All of this week’s quakes were along boundaries of the Earth’s tectonic plates, five of them along the Andean margin and four in the western Pacific.
The Week’s Biggest Earthquake: M6.2, Colombia
The week’s biggest earthquake was an M6.2 which occurred around 270km north east of the country’s capital, Bogota. The overall tectonic context of northern South America is complicated, with interactions between four plates — the South American, Caribbean, Cocos and Nazca plates — taking place at different levels within the crust.
The epicentre of the earthquake (the point on the Earth’s surface above the focus, which is the place where the shaking actually occurred) was in the region of the Boconos fault, a shallow fault zone resulting from the juxtaposition of the South American and Caribbean plates where movement is lateral.
The focal depth, however, suggests an alternative source. The earthquake occurred at a depth of around 150km, implying that it is more likely to have been caused either by movement along the (shallowly-dipping) plane along which the Nazca plate subducts beneath South America, or as a result of internal deformation within either the Nazca or South American plate.
This mechanism as a result of the most recent earthquake is supported by the USGS commentary on seismicity in the region, which notes that “This subduction results in relatively high rates of seismicity … intermediate-focus earthquakes occur within the subducted Cocos plate to depths of nearly 300 km.”
M5.2 Earthquake in Iran
Crustal deformation is also the probable cause of an earthquake which occurred in southern Iran.
In the Arabian Sea the Arabian plate subducts beneath Eurasia but the nature of the boundary changes further to the north, where it becomes a convergent boundary with thrust faulting along the Zagros Mountains.
Both boundaries involve collision but subduction occurs where oceanic crust, which is denser, collides with more buoyant continental crust and mountain building occurs where continents collide.
This week’s earthquake, which occurred along the thrust zone at a depth of just 10km, is most likely to be associated with uplift along the thrust belt.
US Earthquakes: Not California (Well, Not This Week)
The US was as quiet as everywhere else this week, which gives us an opportunity to cast an eye over the revised earthquake forecasts for California. Earthquake forecasting is a bit of a mug’s game but science is constantly improving and updating models on the basis of, among other things, improved mapping of existing faults and understanding of past earthquakes.
So what’s the headline news? In a nutshell, fewer smaller earthquakes but an increased likelihood of a larger one. “Compared to the previous assessment issued in 2008, UCERF2, the estimated rate of earthquakes around magnitude 6.7, the size of the destructive 1994 Northridge earthquake, has gone down by about 30 percent. The expected frequency of such events statewide has dropped from an average of one per 4.8 years to about one per 6.3 years,” says the USGS.
“However, in the new study, the estimate for the likelihood that California will experience a magnitude 8 or larger earthquake in the next 30 years has increased from about 4.7% for UCERF2 to about 7.0% for UCERF3.”
Don’t forget that these are probabilities not predictions. We can be pretty sure that California will experience the Big One at some point, but these forecasts suggest it’s (slightly) more likely to be sooner rather than later.
Movement on All Fronts
This week may have been fairly quiet but our featured areas show the variation of tectonic settings — two types of compression plus lateral movement. Fortunately there was nothing out there of any size or which caused any serious damage. A good week in the seismic world.