This week produced just one large earthquake, along with a host of minor ones. 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 (≥M4.0) elsewhere, included just one tremor in excess of M7.0 and 20 of at least M5.0.
In total there were 1,845 tremors on the map and, apart from the slightly unusual fact that there were no earthquakes in the M6.0-M6.9 category, the spread in size and location was pretty much as we might normally expect.
All of the tremors of ≥M4.0 were located at or near the boundaries of the planet’s main tectonic plates.
The Week’s Biggest Earthquake: M7.1, Vanuatu
Some parts of the Earth are more prone to large earthquakes than others, and the complicated jumble of crustal slivers and microplates which marks the boundary between the Pacific and Australian plates between Tonga and the Philippines is one of these.
The direction of subduction changes along this part of the margin, and is further complicated by the existence of different types of plate boundary with all three types of plate movement (conservative, constructive and destructive) present.
This week, the area was once again the location of the largest tremor of the week, an M7.1 which occurred just north of Vanuatu, where the Australian plate subducts eastwards beneath the Pacific plate along the New Hebrides Trench.
The earthquake was large enough to merit a separate description from the USGS, which described the origin as “the result of oblique-reverse faulting at an intermediate depth, approximately 130 km beneath the Pacific Ocean and 100 km to the east of the New Hebrides Trench, within the lithosphere of the subducting Australia plate”.
The USGS report goes on: “Intermediate-depth earthquakes represent deformation within subducted slabs rather than at the shallow plate interfaces between subducting and overriding tectonic plates”.
Depth is significant. And although the earthquake was large enough, in theory at least, to create a tsunami, magnitude is just one factor in tsunamigenesis — and is the reason given in the Pacific Tsunami Warning Center’s event bulletin for the lack of any such tsunami event.
M5.9 Earthquake, Argentina
The Pacific margin of South America is another area known for its frequent, often high-magnitude, seismic activity.
The collision of the Pacific plate with the South American continent has generated some of the largest earthquakes on record and has raised the mighty Andes in the process.
This week, an earthquake of M5.9 occurred to the east of the Andean mountains, in Argentina. Although this looks at first sight to be associated with the subduction zone (which extends, at increasing depth, for many hundreds of kilometres beneath the continent) it merits a closer look.
Depth is significant again. Subduction earthquakes generally increase in depth with distance from the surface expression of the zone (the offshore trench). This week’s tremor occurred at a depth of just 5km, meaning that it is not a subduction earthquake but the result of shallow deformation, probably caused by thrust faulting caused by the uplift of the Andes.
US Earthquakes: California Shaking
California has been wobbling this week — well, some of it has. The area around the town of San Ramon, north of San Francisco, has been shaking like an autumn leaf along the Pleasanton fault, which makes up part of the San Andreas fault zone. There’s nothing unusual in this: California is yet another area of the world where seismic activity is to be expected.
And it’s worth noting that there’s further activity, in the shape of a second swarm, taking place further north in the mountains south of Clear Lake.
That’s not near San Francisco so it didn’t make the same news headlines.
Last Thoughts: How Deep is Your Focus?
The focus is that point at which an earthquake actually occurs (as opposed to the epicentre, which is the point on the Earth’s surface immediately above it, and is therefore the location seen on a map). And as I’ve said for two of this week’s earthquakes — depth matters. The depth gives us clues to the origin of the earthquake. More importantly, it influences damage. Energy from deep earthquakes, even larger ones, is more readily dissipated from a deep earthquake than from a shallow one. That means we have more to fear from shallow earthquakes, especially in subduction zones, than deeper tremors.