Robin Andrews, Contributor
If you’re in Greece, you likely already know this, but for everyone else: a rather sizeable magnitude 5.1 earthquake just rocked the nation, particularly Athens, which news reports tell me knocked out phone networks in areas and caused plenty of people to run around in a bit of a panic.
You know the deal by now: here’s what happened, what triggered it, what to expect next, and what misinformation you might spot that you get throw into the sea, where it belongs. I’ll update this post if new, relevant, verified information comes in.
So, what happened?
The quake struck north of Athens – specifically, three kilometres (1.9 miles) north-northwest of Magoula, or roughly 22 kilometres (13.7 miles) north of the capital. Coming it an a magnitude 5.3, according to the United States Geological Survey (USGS)’s report on the temblor. The Athens Institute of geodynamics, per the Los Angeles Times, pipped the quake as a magnitude 5.1. It occurred at a depth of 10 kilometres (6.2 miles).
Small differences in magnitude early on are common: They are calculated based on the amplitude (the maximum height of depths of a wibbly wave) of the seismic waves emanating from the quake’s hypocentre, its point of origin below ground. Depending on how seismometers, each calibrated slightly differently in each location, receive these waves, minor discrepancies in the quake’s magnitude can crop up. Over time, as these differences flatten out as those seismic waves are studied in greater detail.
As ever, magnitude is probably not what you think it is. Crudely speaking, it relates to the amount of energy that a rupturing fault (or faults) unleash during the duration of their rupture. A single-digit increase on the moment magnitude scale – say, from a 3 to a 4, or a 5 to a 6 – means a 32-fold increase in the amount of energy that earthquake released over its duration.
As a very crude rule of thumb, the more powerful an earthquake is and the closer it is to a city, the higher the potential for damage and destruction. As this evergreen post explains, though, it isn’t all about magnitude: an earthquake’s power means nothing if it happens far from civilisation and doesn’t trigger any dangerous secondary events, like landslides or tsunamis. Depending on depth of the quake, the type of fault (or faults) that slip and the type of sediment it’s shaking, two temblors of equivalent magnitude can cause very different degrees of shaking at the surface.
When someone speaks about the intensity of the quake, that refers to the shaking at the surface. Generally speaking, this is measured by how fast the ground manages to accelerate during the earthquake, as well as how this shaking is perceived at the surface.
A magnitude 5.1 quake close to a major metropolitan city certainly sounds dicey, and news reports suggest the ground shook for a rather mortifying 15 seconds. Per the USGS, at the epicentre (the point above the quake’s hypocentre), the shaking intensity came in at a VII on the modified Mercalli scale, which goes up to X (and, potentially, higher than that, but that’s vanishingly rare.) This means the shaking was “very strong” near the source. In Athens, intensity mostly came in as IV, V, or VIs: “light” to “moderate” to “strong”, but still enough of a surprise to cause people to stream out of their buildings.
— EMSC (@LastQuake) July 19, 2019
Any reports of damages or deaths?
Plenty of reports coming in of some structural damage, power outages, people trapped in elevators and minor injuries – but, so far, no reports of any major injuries or deaths. At that magnitude and intensity near several metropolitan areas, it’s not surprising that damage has occurred: the modified Mercalli scale this time around says the damage will be “moderate” near the epicentre, and anywhere from “none” to “light” in the peripheries, including Athens.
The US has an automated system that, based on magnitude, intensity and other factors, estimates the potential number of deaths and the potential economic damage that the quake may cause. In this case, there is a 50% chance that there will be 0 to 1 deaths, and a 41% chance of there being no more than 10. So far, it looks like there are zero, fortunately. In terms of the bill, it looks like there is a 37% chance that it’ll be no more than $1 million, a 35% chance it’ll be no more than $10 million, and a 21% chance it’ll be no more than $100 million.
Was that the mainshock? Will there be aftershocks?
At the moment, the magnitude 5.3/5.1 event looks like the mainshock, although we’ll have to wait a little to make sure this is the case. Earthquakes don’t happen in isolation, but in clusters, as faults tend to keep jutting forwards after letting loose over time, freed up a little after that initial big push. The mainshock is the most powerful quake in that cluster, which means a mainshock can only be applied in retrospect. Anything weaker that happened before in the same cluster is known as a foreshock, and anything weaker afterwards is known as an aftershock.
There have already been a few aftershocks rocking the region, with one coming in at a magnitude 4.2 not too far from the mainshock. Although the USGS normally posts an aftershock forecast after an event, they don’t always do this; although I’m not 100% sure on this, I don’t think they make these mathematically determined forecasts for quakes outside the US unless they are particularly devastating or powerful.
I’ll update this if an aftershock forecast comes in, but in general, the mostly likely pattern is this: in the first few days of the mainshock, there will be plenty of aftershocks at varying magnitudes, with some being close to the mainshock’s own. Generally speaking, as time goes by, the number of powerful aftershocks, and the frequency of aftershocks, will go down. Saying that, there is a non-zero chance that an aftershock more powerful than the mainshock will crop up, particularly in the hours and days nearer to the mainshock. If that’s the case, then that’ll become the new mainshock, and the magnitude 5.3/5.1 event will become a powerful foreshock.
What caused this quake?
The beach ball-like object on the USGS’ site indicates that the fault that slipped is known as a normal fault. That means that one block of earth, obeying gravity, slipped downwards with respect to another. These are common in areas of extension, where the region is stretching itself apart.
As detailed by the USGS’ report, earthquakes in this part of the world – the Mediterranean – aren’t as uncommon as you may think. As it has been for 50 million years or so, the African tectonic plate is moving into the Eurasian plate, something that once ended up closing up a sea known as the Tethys, whose contemporary remnants can be found in the form of the Mediterranean Sea.
The plate boundary here is pretty complicated: it’s produced a earthquake-producing subduction zone, where one plate sinks under another, in southern Greece; you also have the North Anatolian Fault Zone in western Turkey, itself home to both devastating and super sneaky earthquakes; you also have another subduction zone in Italy’s Calabria region.
A magnitude 5.1 (ML by NOA) to 5.3 (GFZ) #earthquake in Greece. 20km NW of Athens, on an ~E-W normal fault. Shallow epicenter (~13km). Strongly felt in Athens, possibly damaging.https://t.co/I3B66RPZZF pic.twitter.com/FWF4fc6vd4
— Robin Lacassin (@RLacassin) July 19, 2019
The Hellenic subduction zone may involve two plates squishing into (and over-and-under) each other, which causes compression in parts of this region. But wait, you may wonder – I thought you said this quake was caused by extension? Well, as the downgoing tectonic plate keeps being dragged downwards, the part of the plate behind it stretches out, leading to extension and plenty of normal faulting. In this case the region of extension is, among other places, in Greece.
Will this trigger more earthquakes on other faults nearby?
That depends. As Caltech seismologist Lucy Jones has regularly pointed out, an earthquake can only trigger another earthquake on a fault is the distance of those peripheral faults is no further than three to four times the length of the fault that ruptured. So, sure, it’s possible, but it’s very unlikely you’re about to see a cascade of earthquakes rocking all parts of Greece.
— Derek Gatopoulos (@dgatopoulos) July 19, 2019
I’ve seen news reports of a lot of earthquakes at the moment. Is this related?
Not in the slightest. From those recent, powerful quakes in Southern California to that other potent quake near western Australia and even those rather intense temblors over in Indonesia, these are all happening in isolation. They aren’t connected to each other in any manner; they are so far apart that they cannot be even remotely related, geologically speaking.
In fact, the reason so many quakes are making the news lately is that those Californian shakes understandably made plenty of headlines at the time, so now the media is, in general, on the lookout for any other quakes of significant magnitude to report on. The public are more interested in earthquakes after a major, if non-lethal, one in somewhere like California, so there will be more reporting in general on them, wherever they happen.
This will likely diminish in time – that is, until the next major earthquake that causes widespread devastation in some way or another. The same uptick in reporting will happen for any quakes that had the potential to cause devastation, too, even if the reality is fortunately quite different – although in this case, I suspect near-miss quakes like these will be more likely to cause a spike in news articles if they happen in the US, because of an inherent bias in how such events are reported.
So this isn’t related to the Ring of Fire, the ‘big one’, the end of the world, or anything like that?
No – and if anyone tells you that it is, and that they just know when the next major quake is about to occur, and how powerful it will be, call them out on their bullshit and throw their ideas into the sea.