It’s difficult to predict how powerful earthquakes are going to be | Pauline Galea

Ever since the current ‘earthquake swarm’ first began to be felt, seismologists such as yourself have been saying that such occurrences are actually quite frequent around the Maltese islands. And yet – just to take myself, as an example – I’ve lived on this island for 50 years: and I only ever remember maybe two or three (very minor) tremors, in all that time. So what do you mean, exactly, that this sort of seismic activity is ‘common’? 

Let me start with this: for us seismologists, an earthquake doesn’t have to be ‘felt’ to be important or interesting. In the past years, we have recorded a number of these swarms – that is, episodes of activity, when a certain [geological] fault registers a series of jolt-like movements, because of small displacements in the rock. These phases can sometimes last for weeks, or even months. And when that happens, we say that the fault is ‘active’. 

Most of these jolts, or displacements, will only consist of tiny movements, which will usually not be felt at all. But occasionally – as in this case – the activity produces one or more larger earthquakes: measuring a magnitude of more than 5 [on the Richter Scale]. In this region, around the Maltese islands, that’s considered quite large. In fact, these are some of the largest earthquakes that we have measured, with epicentres below the sea-bed around our islands, since our seismic network has been in place. And these are the earthquakes that people feel.  

So when we say that this kind of seismic activity is ‘common’, it’s because we have observed similar episodes of seismic activity in various places around the Maltese islands, over the years. For example, in  2020, we recorded a small swarm around 20km south of Malta – which is a lot closer than the one we are experiencing today. There was another, around Easter of 2011, to the east of Malta; and another again to the West of Gozo. But very often, the public doesn’t get to hear about them, because they tend to be small. 

As such, they’re only interesting to us, as seismologists; because they provide information about what’s happening beneath the Earth’s surface, and give  more information about the regional plate tectonic situation.

At the risk of a question that might be unanswerable: what causes the actual difference in magnitude, between earthquakes along the same fault? Take the area we’re looking at now, around 120km to the south of Malta. If, as you say, it’s been active all along: why does it produce only minor tremors most of the time… but then, once in a while, a more powerful one that literally ‘shakes buildings’? 

Every fault is different. And the intensity of seismic activity depends on a lot of factors: including the size of the fault; the forces that are placing stress upon it – i.e., the tectonic movements that are ‘pushing’ (or in some cases, ‘pulling’, or ‘stretching’) the fault; and also, how long it’s been since the last major earthquake. 

One thing to bear in mind is that the frequency of earthquakes, also depends on their magnitude. A 7.4-magnitude earthquake, for example – which is very powerful – happens very rarely. A magnitude-6 happens more often; and the smaller the earthquakes, the more frequent they are.  

In fact, in this particular episode: since 18 January, we have measured and located around 200 earthquakes… only a few of which were actually felt. But in reality, there have been probably been many, many more, of much smaller magnitude… but we can’t measure them, because their epicentre is too far away from us.  

The smallest earthquake we measured was around 2.2. But definitely, there will be others, of magnitude 1 or smaller, that we are not detected.  

But as I said earlier: each fault is different. If we talk about Sicily, for instance: people know of – and still talk about – the 1693 earthquake; and, more recently, the Messina earthquake of 1908. Those were both very large: greater than Magnitude 7. And in fact, they both caused extensive destruction in Sicily; and were felt - and even caused damage - here in Malta. 

But earthquakes of that magnitude are characteristic of the geological faults in that region. Every region has its own characteristics; and what we call ‘earthquake potential’. Sicily is clearly capable of producing such powerful earthquakes, from time to time. But other regions are not.  

As for the other faults around the Maltese islands: although we are still studying, and learning about them – because instrumentation is relatively recent; and seismic activity has been going on for millions of years – they appear not to be capable of producing large earthquakes. Or at least: not as far as we know.  

Nonetheless, from the perspective of non-scientific people (such as myself) who were alarmed by the intensity of last Monday’s quake: they certainly seem to be getting more powerful. Is there any truth to this? And if so, can we expect even bigger quakes, in future? 

That was, in fact, the pattern of how the ongoing swarm has progressed. But while some of the later earthquakes were larger than others, it doesn’t mean that they will ‘keep getting more powerful in future’. 

This is, in fact, the difference between a ‘swarm’ – like we are experiencing now – and what we call a ‘mainshock’. Usually, when there is a large earthquake of Magnitude 7, or higher: it will be followed by a number of ‘aftershocks’. So you have the ‘mainshock’ – and sometimes, another big one afterwards – but in general, the aftershocks that follow will decrease in magnitude exponentially, over time.  

A swarm, on the other hand, doesn’t follow the same pattern. There is no way of knowing, with certainty, how the magnitudes are going to progress. It could be a series of small ones, followed by a big one, then more small ones; or there could be a few big ones in sequence… the pattern varies, from swarm to swarm. 

So it’s difficult to predict, not just how powerful the earthquakes are going to be; but also, how long the swarm will last. Yesterday, for instance [Thursday] we measured eight earthquakes, all at the same general location. And today, there have already been four or five smaller ones. So hopefully, the swarm is beginning to subside. 

But while it is certain that it will, in fact, die out eventually: it is not possible to predict exactly when that will happen. The truth is that we are still learning; and there is still a lot we have to observe.  

It was only in 2014, for instance, that the Seismic Monitoring & Research Group starting building up its seismic network. Today, we have eight stations, installed in Malta, Gozo and Comino, to help us locate these earthquakes more accurately. Before 2014, however, we only had one station, at Wied Dalam. 

So the more instrumentation we have - and the more we network with other stations in Sicily, Italy, and elsewhere - the more accurately we can assess what’s happening.

All the earthquakes we’ve been talking about so far were cases where the epicentre was actually quite far way. Yet when you look at the geological structure of the Maltese islands, there seems to be evidence of major seismic activity in the past. Malta itself is divided into two by a natural fault-line; and there are even ‘sinkholes’ here and there (such as Maqluba in Qrendi). Does this mean that Malta was once itself the epicentre of earthquakes?  

Well, this is one of our main concerns, really: whether earthquakes can happen ‘onshore’. In reality, we haven’t ever measured any seismic activity on shore with certainty, although some events also happened very close to the coastline . But it is, nonetheless, still a possibility. 

The fault-lines you mention, for example, are parts of an entire system of geological faults within the central Mediterranean: which, apart from being at the forefront of the collision between the African and European continental plates – which took place millions of years ago; but the scars are still visible, so to speak – is also subject to other, ongoing geological effects: such as ’extension’. The Earth’s crust, between Sicily and Tunisia, is actually being ‘stretched’… and therefore, it ‘cracks’.  

The faults in Malta and Gozo are all part of this network of ‘cracks’; and potentially, they could be activated, as well. But so far, we have measured nothing to indicate that this might happen.

Meanwhile, there has been a lot of online speculation that what is now happening 120 km to the south, is comparable to the underwater volcanic activity around nearby Sicily. In the late 19th century, an underwater volcanic eruption briefly caused the appearance of an entire island, in the Sicily-Tunisia channel (‘Graham’s Island’: which promptly sank beneath the waves, while European powers were still arguing over who owned it). Is there any link between the ongoing swarm, and the possibility of an underwater volcanic eruption (with all the added concerns about ‘tsunamis’, etc.)? 

That would be more true of the area further to the west of the Maltese islands: which is certainly volcanic. But generally speaking: whatever takes place on the surface, is an indication of what happens underneath. The energy that produces phenomena such as earthquakes, and volcanoes, all comes from deep within the earth. And beneath the crust, there is the mantle: which is very hot [anywhere between 1,000 and 3,700 degrees Celsius], and is more ‘fluid’ than the crust. 

There have, in fact, been studies centered around the area of Pantelleria – even by researchers from our own department – about what’s happening to the Mantle, in that region. Both Pantelleria and Linosa are themselves volcanic islands; so there is certainly enough volcanic activity, in the region, to form islands: even temporary ones, as happened in the case of Graham’s Island. 

In the particular area we are talking about today, however we cannot conclude that what’s happening beneath the crust, is comparable to the underwater volcanic activity to the West. Personally, I’d rather not speculate, until we have more research – and above all, long-term research – and analyse more carefully the large amount of data that we are collecting.

One other question that inevitable arises, when discussing earthquakes, is whether Malta is actually ‘prepared’ to handle such an eventuality. For example: in other earthquake-prone regions (Los Angeles springs to mind) buildings are specifically designed to withstand earthquakes above a certain magnitude. In Malta, this doesn’t appear to be the case. On the basis of your own research: how prepared would you say we really are, to cope with a major earthquake? 

I’m not the person to talk about buildings, really: because that falls within the remit of structural engineers. But what I can say is that part of our job, as seismologists, is to analyse what we call the “seismic hazard’: i.e., a quantitative measure of what kind of ‘ground-shaking’ – or to give it a more technical definition: the ground acceleration caused by earthquake-shaking at a particular site, and how it would impact buildings – that we can expect over the next 50-or-so years.  

Obviously, this information is of great interest to structural engineers; and we obtain it from studying the history of earthquakes around us; analysing the type of geological faults that exist in our region; and so on and so forth. 

The collation of all this data gives us a value: for example, that the possibility of a ground-shaking acceleration of ‘0.1g’, or higher – in other words, 10% of the Earth’s gravitational pull [which is enough to possibly cause damage to buildings] – is around 10%, over the next 50 years.  

That’s just an example, though. The actual values for Malta are being evaluated and discussed, and will be incorporated into future updates of our building codes.  

On another level, however, we have also carried out studies into the geological structure of the islands themselves. There are different types of rock beneath the surface: for instance, in the Western part of the islands – including Gozo – there is a sub-surface layer of clay, which doesn’t exist in the Eastern part. So we have conducted studies in recent years to see how this will be affected, in the case of a large earthquake. 

All this information is then communicated to structural engineers; and used in the design of new buildings. So there are buildings that are properly designed, to withstand the sort of earthquake shaking expected; but unfortunately, there may be a number of others, which are not.