7.5 Quake Rocks New Caledonia: Here Is Everything You Need To Know

USGS ShakeMap of the event near New Caledonia, with the epicentre represented by a star.USGS

Just in case you needed a reminder that we live on a geologically active and – because we live on it – troublesome planet, a magnitude 7.5 earthquake struck the French-governed southwest Pacific island of New Caledonia and the nearby Loyalty Islands, located 1,210 kilometres (750 miles) east of Australia. Right now, they are being rocked by some pretty powerful aftershocks.

This is a developing story, and will be updated if need be. Here’s everything we know about the event so far, based on reporting from various geoscience agencies, including the United States Geological Survey (USGS).

What Happened?

An earthquake coming in at a magnitude 7.5 struck just east of New Caledonia at a pretty shallow depth of 10 kilometres (6.2 miles), at 15:18 on December 5th, local time. For most readers here (hello, America!), that would be in the middle of the night.

Due to the geography and bathymetry of the area, the National Weather Service issued a tsunami warning shortly afterwards for a thousands of kilometres of coastlines. Noting that “tsunami waves have been observed”, they forecasted hazardous waves for some coasts, including New Caledonia and Vanuatu. It was possible that waves reaching heights of 1 to 3 metres (roughly 3 to 10 feet) could occur along these coastlines, with a minor one hitting Fiji and a very small wave height occurring all over the Pacific, from Australia to Chile, from Japan to Mexico.

Now it appears the tsunami threat has largely passed. Although they recommend that those along shorelines should remain alert, it seems that the most that will be seen are minor fluctuations in sea level above and below normal tide for a few hours. So with that in mind, everyone can breathe a sigh of relief.

As the USGS data is indicating, plenty of aftershocks are occurring at the same site as the original earthquake. They’re pretty intense, with plenty of magnitude 4-5 events taking place; the most powerful so far was a magnitude 6.6 quake which took place a couple of hours after the original earthquake. It also seems that the mainshock was preceded by a foreshock coming in at a magnitude 6.8, although we only know it’s a foreshock in retrospect.

As I explain here, aftershocks like this as expected after a powerful quake. The fault that slipped is still slipping, which means it will release additional packages of energy over an undetermined amount of time. Faults that are within close distance to the original rupture may also slip somewhat too, producing additional aftershocks.

In the next few days, expect there to be hundreds of aftershocks, perhaps thousands, with most being of very small magnitudes and some being a little more powerful. There’s always a (very slim) chance that a more energetic earthquake than the original 7.5 event could occur in the near future too, which would then make that the mainshock, and all these the foreshocks.

Although the USGS hasn’t put any numbers on this, I’m guessing that the 7.5 event – pretty darn energetic – will turn out to be the mainshock, with everything in the coming weeks an aftershock. As a point of comparison, let’s look to Alaska: the tectonic settings are different, but the recent 2018 Anchorage earthquake, which came in at a magnitude 7.0, could have been trumped by a more energetic quake within a week. However, the USGS, on the day of the event, put those odds at 3 percent. That quickly dropped to less than 1 percent.

What’s The Damage?

Although shallow, with some major shaking at the site, this quake took place offshore – and with no tsunami, the destructive potential is pretty low. That’s a huge relief: the local population live in structures that are, in general, “highly vulnerable to earthquake shaking.”

Had this quake taken place closer to shore or onshore, we’d be telling a far grimmer story. Instead, the USGS estimates that there is both a low likelihood of economic damage, as well as infrastructural destruction and human fatalities.

The secondary threats (aside from the tsunami) also appear to be minimal if non-existent. Data suggests that little to no people or areas have been exposed to risk through landslides or liquefaction, a process wherein wet, unconsolidated soil moves like a fluid. The 270,000 people on New Caledonia and its Loyalty Islands are a-okay, with no injuries or damages yet reported, per BBC News.

What Caused This Earthquake?

This area is known for being pretty darn seismic. We are, after all, on the (sigh) Ring of Fire, which features 90 percent of the world’s earthquakes (as well as 75 percent of its volcanoes). Here, the Australia tectonic plate is moving toward the Pacific plate, whereupon it descends (subducts) beneath it and into the lower mantle.

This subduction zone involves plenty of friction, as well as the bending and fragmenting of the Australia plate, which leads to a plethora of earthquakes. In this case, it appears the quake originated on the part of the Australia plate that’s yet to subduct, just before the South New Hebrides Trench, where both plates meet. A fault slipped along what is known as the “outer rise”, where the Australian slab is bending and stretching, putting it in an extensional regime, like the top of an eraser you are bending in half.

When you have extension, you get normal faults; this means that a block of rock moves downwards with respect to another, as it follows gravity’s lead. As this took place within the plate, rather than the boundary between the two plates, it is known as a within-plate or “intraplate” earthquake.

These are common. As the USGS notes, this is the sixth quake above a magnitude 6.0 to occur within the last three months alone; the latest shaking is in fact part of a prolonged sequence of events, the first of which was the August 29th magnitude 7.1 event just to the east of the latest one. Plenty of these quakes have been thrust faults, which is when you have overall compression – not extension – and where a block of rock moves upwards, against gravity, relative to another segment.

Such sequences in the region are also not unheard of, with a similar one taking place in 2017. Magnitude 7.0 quakes are also to be expected from time to time, with 24 taking place in the region in the 20th Century alone. The most powerful was a magnitude 8.1 event back in September of 1920. This is what’s made possible by the inexorable subduction of the Australia plate beneath the gargantuan Pacific one.

Look at all those earthquakes! This region is, thanks to its tectonic interplay, one of the most seismically active parts of the planet.USGS

Baptiste Gombert, a seismologist at Oxford University, tweeted that the shallow nature of quakes like this one can produce significant tsunamis, so it’s perfectly understandable why a warning was sent out. Tsunamis can also be produced during megathrust earthquakes, when major fault slippage takes place on the boundary between these two tectonic plates itself. Some of those recent 7.0+ quakes in the past century or two were megathrust events, at least five of which produced tsunamis.

Megathrust earthquakes are of great concern to seismologists, as such tectonic machinations can also produce earthquakes above a magnitude 9.0. They can also generate truly devastating tsunamis, much like the one that swamped Japan’s eastern seaboard in March 2011.

Incidentally, I don’t remember the earlier part of the sequence near New Caledonia making many headlines (if any), despite similar magnitudes being registered. I suspect that this one has appeared all over the place because of its proximity to the recent shakings in Anchorage, Alaska…which neatly brings me on to the next section.

What Isn’t This Quake A Sign Of, Or Related To?

As you’ve probably guessed, this quake has no link to the one that shook Anchorage last week. They are nowhere near each other, and I can’t stress enough how the Ring of Fire isn’t an interconnected house of cards. If the faults were geographically very close – within a couple of rupture-length distances of each other – then there may be a chance that the first transferred stress to the other, but that’s certainly not the case here.

Remember, too, that the New Caledonia quake is part of an ongoing sequence that pre-dated the Anchorage one. Make no mistake: what you have here is a coincidence, and nothing more. So if you see rumours online that try to join the non-existent dots, please do not share them.

Similarly, if you see anyone making precise spatial and temporal predictions as to what’s going to happen next, ignore those too. What will definitely continue to happen are those aftershocks, with overall decreasing frequency and magnitude. Aside from that, it’s anyone’s guess, so don’t believe any soothsayers.

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