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maltatoday | SUNDAY •5 FEBRUARY 2023 9 INTERVIEW higher: it will be followed by a number of 'aftershocks'. So you have the 'mainshock' – and sometimes, another big one af- terwards – but in general, the aftershocks that follow will de- crease in magnitude exponen- tially, over time. A swarm, on the other hand, doesn't follow the same pat- tern. There is no way of know- ing, with certainty, how the magnitudes are going to pro- gress. 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 earth- quakes are going to be; but also, how long the swarm will last. Yesterday, for instance [Thurs- day] we measured eight earth- quakes, 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 eventual- ly: 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 in- stance, that the Seismic Mon- itoring & 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, how- ever, 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 actu- ally 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. Mal- ta itself is divided into two by a natural fault-line; and there are even 'sinkholes' here and there (such as Maqluba in Qren- di). 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 'on- shore'. In reality, we haven't ever measured any seismic ac- tivity on shore with certainty, although some events also hap- pened very close to the coast- line . But it is, nonetheless, still a possibility. The fault-lines you mention, for example, are parts of an en- tire system of geological faults within the central Mediterra- nean: which, apart from being at the forefront of the colli- sion 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, ongo- ing 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 noth- ing 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 activ- ity around nearby Sicily. In the late 19th century, an underwa- ter volcanic eruption briefly caused the appearance of an entire island, in the Sicily-Tuni- sia 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 under- water 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 phe- nomena such as earthquakes, and volcanoes, all comes from deep within the earth. And be- neath the Crust, there is the Mantle: which is very hot [any- where between 1,000 and 3,700 degrees Celsius], and is more 'fluid' than the Crust. There have, in fact, been stud- ies centred around the area of Pantelleria – even by research- ers 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 vol- canic 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 under- water 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 inevi- table arises, when discussing earthquakes, is whether Malta is actually 'prepared' to handle such an eventuality. For exam- ple: in other earthquake-prone regions (Los Angeles springs to mind) buildings are specifically designed to withstand earth- quakes above a certain mag- nitude. 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: be- cause that falls within the re- mit 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 quanti- tative measure of what kind of 'ground-shaking' – or to give it a more technical definition: the ground acceleration caused by earthquake-shaking at a par- ticular 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 structur- al engineers; and we obtain it from studying the history of earthquakes around us; analys- ing the type of geological faults that exist in our region; and so on and so forth. The collation of all this da- ta gives us a value: for exam- ple, that the possibility of a ground-shaking acceleration of '0.1g', or higher – in other words, 10% of the Earth's grav- itational 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 incor- porated 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 be- neath the surface: for instance, in the Western part of the is- lands – including Gozo – there is a sub-surface layer of clay, which doesn't exist in the East- ern 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 de- sign of new buildings. So there are buildings that are properly designed, to withstand the sort of earthquake shaking expect- ed; but unfortunately, there may be a number of others, which are not.