Back to normal for the digest this week, with a collection of earthquakes which fall more or less where we’d expect them to. Not that that makes them dull, of course. Far from it. There’s always something…
The United States Geological Survey’s real time earthquake map recorded a total of almost 2000 tremors this week — perhaps the one unusual thing, as this number is slightly larger than normal. This is probably a methodological variation as the map doesn’t show all earthquakes: it shows those of at least magnitude 4 (≥M4.0) worldwide and some, though not all, smaller tremors in the US and its territories.
The numbers of larger earthquakes are pretty much in line with the typical weekly pattern — 2 of at least M6.0; 21 of at least M5.0; and 92 of at least M4.0. And, apart from one outlier of M4.8 in the Ukraine, the larger tremors were pretty much where we’d expect them to be — that is, associated with the margins of the earth’s tectonic plates.
The Week’s Biggest Earthquake: M6.3, Bonin Islands
Like last week’s largest earthquake, this week’s top magnitude event was in the western Pacific along the margin between the westward-moving Pacific plate and the Philippine Sea plate, beneath which it subducts.
Could the two be linked? The answer is that they almost certainly are not. While earthquakes can trigger aftershocks, these are almost always relatively close to the mainshock: the distance between these two events is around 1000 km. And while seismologists believe that that large earthquakes can transfer stress onto nearby faults, the distances here are significant.
The USGS observes that: “There is evidence to suggest that earthquakes in one area can trigger seismic activity within a few hundred miles, including aftershocks clustered near the main shock. There is also evidence that some major earthquakes manage to trigger seismicity over much greater distances (thousands of miles), but these triggered quakes are small and very short lived”.
In addition, this week’s earthquake, at 510km, was much deeper than last week’s; the two may both be subduction earthquakes in a similar tectonic setting, but it’s most unlikely that the first is the cause of the second.
M6.1 Earthquake, Argentina
The largest earthquake of the week was caused by the collision of two oceanic plates: the second largest was the result of the coming together of the oceanic crust of the Pacific plate and the continental crust of South America along the Peru-Chile trench. The stresses of this collision have uplifted the mighty Andean range.
The earthquake occurred inland, close to the border between Argentina and Bolivia. It occurred at a depth of 254km and around 300km from the trench. Together these facts, along with the tectonic setting and the moment tensor diagram produced b the USGS (moment tensors are one of last week’s favourite things) suggest that the earthquake is a result of fracturing at or near the interface between the two plates — which is pretty much what we might expect.
US Earthquakes: California
California shakes a lot, so this week’s M5.1 in the coastal ranges is nothing new (and, just for reference, the 1906 San Francisco earthquake released over 11,000 times as much energy). The Coastal Ranges are part of the San Andreas Fault Zone and, as we might expect, the motion of this earthquake was lateral, with the Pacific and North American plates sliding past one another.
Thought relatively large, the earthquake didn’t make much of a ripple. The USGS impact summary suggests little or no damage and that only around 200,000 people felt the earthquake — and even then, they didn’t feel much.
Last Thoughts: Normal Service Resumed
Two subduction earthquakes in subduction settings. A strike-slip earthquake in the San Andreas fault zone. I often talk about interesting anomalies but, in actual fact, most earthquakes do follow (broadly) the generalisations that are taught in schools.
There are no surprises in this week’s digest. Another week, there will be.