Okay, I’m not going to make any jokes about political earthquakes. I’ve done that before and I daresay I’ll do it again. This week we’ll stick to solid Earth.
The United States Geological Survey’s real time earthquake map wasn’t over-busy this week, wth just one tremor in excess of magnitude 6. The map, which includes tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, recorded a total of just under 1500 tremors, of which 17 were ≥M5.0.
Most of the larger tremors were exactly where we’d expect them to be, around the margins of the planet’s major tectonic plates. There’s one outstanding feature on the map, though — an earthquake of M5.0 in Oklahoma. More of that (of course) later.
The Week’s Biggest Earthquake: M6.4, Chile
Last week, we looked at an M5.9 earthquake in southern Chile, the second largest of the week. This week we’re back again. Same area, same tectonic setting (the Peru-Chile Trench), slightly larger earthquake — in this case an M6.4.
These quakes were, however, different. Although both are the product of the overall collision between the Nazca and South American plates (the former subducting beneath the latter) they have very different initiating mechanisms. Last week’s M5.9 was offshore; this week’s was onshore. Last week’s was caused by compressional forces; this week’s was by extensional forces.
The collision of the two plates has led to uplift of the Andes — and that’s the difference. Major mountain chains like the Andes are anything but tectonically simple. The USGS map shows the plate boundary but doesn’t show the detail of the fault zones — and without that, it’s hard to identify the exact origin of the tremor. The most likely cause, however, is crustal deformation related to the uplift of the Andes.
M5.8 Tremor, Indonesia
Like the Peru-Chile Trench, the Java Trench is a classic example of a subduction zone, with (in this case) the Indo-Australian plate subducting beneath the Sunda microplate.
The available data suggest that this week’s M5.8, which occurred close to the plate boundary, is actually fairly straightforward, and a classic example of a subduction zone earthquake.
The relative similarity of location (about 15km from the trench, in the overriding plate) and depth (33km) suggest the it was not caused by movement directly at the plate interface but near it. The direction of movement is compressional.
Taken together, these suggest that the earthquake was caused by movement near the plate boundary in the overriding plate.
US Earthquakes: Oklahoma
We were in Oklahoma last week, too, when I observed that: “It is worth noting when a larger-than-normal tremor shows up. I’ve always taken this to be one which reaches M4.5 and that’s what we saw this week. I should perhaps increase my threshold for what’s noteworthy, because the number of earthquakes breaching that M4.5 threshold does seem to be increasing”.
This week Oklahoma treated us to an M5.0 near the town of Cushing. Since the beginning of the year, north-central Oklahoma has seen tremors of M4.7; M5.1; M5.8; M4.5; and now M5.0. The M5.8 in September was the largest on record in the state, surpassing that of 2011.
It’s clear that not only is the incidence of earthquakes in this area increasing — the magnitude is increasing, too.
Last Thoughts: Deja Vu All over Again
I worry, sometimes, that this digest might be a bit repetitive — the same tectonic settings, the same areas, the same driving mechanisms and only the occasional different event. But actually, this week’s Oklahoma earthquake is an illustration of the fact that it’s worth keeping track.
Earthquakes in Chile and Sumatra (and Alaska and Japan and California and…and…and) follow the same pattern. What makes an earthquake noteworthy in these cases is unusual magnitude. Oklahoma is different. In Oklahoma we’re seeing a trend of increasing seismic activity.
Something tells me we’ll back in Oklahoma again before too long.