Hush; the Earth is sleeping. It certainly feels like it this week, with not a lot in the way of earthquake activity. But that’s the way it is: some weeks there are many earthquakes and some there are few.
This week is definitely in the latter category, with the United States Geological Survey’s real time earthquake map (which records tremors of all magnitudes in the US and its territories and those of at least magnitude 4 elsewhere) showing just 19 tremors larger than M5 and nothing in excess of M5.7.
As usual the earthquake activity was concentrated around the planet’s tectonic plate boundaries, particularly in the western Pacific (no change there) and, slightly less usual, a rumbling of seismic activity on the southern and western margins of the Caribbean plate.
The Week’s Biggest Earthquake: M5.7, off Panama
One of the tremors around the Caribbean margin crept in as the largest of the week at M5.7. Strictly speaking it isn’t part of the Caribbean plate, although it arguably lies in an area which is influenced by it, given its geographical location.
The boundaries of the plates are poorly defined in this location and are not marked on the US map (though they can be inferred from it). The earthquake’s epicentre lies south of the probable triple junction between the Cocos, Nazca and Caribbean plates with the South American plate brooding not too far (in tectonic terms) to the east.
Narrowing the context, we can see that the earthquake took place at the margin between the Nazca and Cocos plates (the Panama Fracture Zone), at a point where they are sliding past one another laterally in (broadly) the same direction but at different speeds. This lateral motion, which lacks a vertical component, along with the small magnitude of the tremor, explains why even a noteworthy undersea earthquake such as this didn’t generate a tsunami.
M5.3 Earthquake: Indian Ocean
In terms of intraplate settings, earthquakes don’t come much more isolated than this week’s M5.3 in the Indian Ocean. The closest land is 1300 km distant (the Cocos Islands) and it was 2400 west of Indonesia. Truly, this earthquake was in the middle of nowhere.
The pale line marked on the earthquake map is a clue to its origin. This is the Ninety East Ridge, according to Silva, et al: “an ∼5500 km long, north–south-oriented, submarine volcanic ridge in the eastern Indian Ocean that formed from magmatism associated with the deep-seated Kerguelen mantle plume.”
Although its remoteness means that science has not studied the ridge in detail, there’s evidence of rifting along its length and this implies that the most likely cause of this week’s earthquakes was normal faulting along the ridge.
US Earthquakes: Hawaii
This week we can leave the mainland US and its swarms of small earthquakes behind and head to the beaches of Hawaii, where a tremor of M4.3 struck on the south eastern slopes of the island.
This was a volcanic earthquake, triggered by magma movement within the (currently erupting) Kilauea, and the USGS notes that it’s most likely to have occurred along an existing fault.
It’s worth noting that though this is the largest earthquake on Hawaii for some time, the USGS map recorded 40 tremors on the island during the past week and 22 of at least M2.5 in the preceding 30 days.
Quakes: Not All About Subduction
Quiet weeks are often the most revealing. This week we we’ve looked at tremors along a fracture zone and an aseismic ridge as well as above an intraplate volcanic hot spot. In more active week these might have been overlooked in favour of larger, more classic (usually subduction zone) ‘quakes. But the Earth is endlessly fascinating and full of surprises.
Sometimes, for the earthquake geek as for everyone else, small can be beautiful.