Issue link: https://maltatoday.uberflip.com/i/1491702
maltatoday | SUNDAY •5 FEBRUARY 2023 8 INTERVIEW Raphael Vassallo rvassallo@mediatoday.com.mt Quake, rattle and roll… EVER since the current 'earth- quake swarm' first began to be felt, seismologists such as your- self have been saying that such occurrences are actually quite frequent around the Maltese is- lands. And yet – just to take my- self, as an example – I've lived on this island for 50 years: and I on- ly 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 regis- ters a series of jolt-like move- ments, because of small dis- placements 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 dis- placements, will only consist of tiny movements, which will usually not be felt at all. But oc- casionally – 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 is- lands, that's considered quite large. In fact, these are some of the largest earthquakes that we have measured, with epicen- tres 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 ac- tivity 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 pub- lic doesn't get to hear about them, because they tend to be small. As such, they're only interest- ing to us, as seismologists; be- cause they provide information about what's happening be- neath the Earth's surface, and give more information about the regional plate tectonic sit- uation. At the risk of a question that might be unanswerable: what causes the actual difference in magnitude, between earth- quakes 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: in- cluding 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 earth- quakes, also depends on their magnitude. A 7.4-magnitude earthquake, for example – which is very powerful – hap- pens very rarely. A magni- tude-6 happens more often; and the smaller the earth- quakes, the more frequent they are. In fact, in this particular epi- sode: since January 18, we have measured and located around 200 earthquakes… only a few of which were actually felt. But in reality, there have been prob- ably been many, many more, of much smaller magnitude… but we can't measure them, be- cause 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 detecting. But as I said earlier: each fault is different. If we talk about Sic- ily, for instance: people know of – and still talk about – the 1693 earthquake; and, more re- cently, the Messina earthquake of 1908. Those were both very large: greater than Magni- tude 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 mag- nitude are characteristic of the geological faults in that region. Every region has its own char- acteristics; 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 learn- ing about them – because in- strumentation is relatively re- cent; 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 per- spective of non-scientific peo- ple (such as myself) who were alarmed by the intensity of last Monday's quake: they certainly seem to be getting more pow- erful. 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 get- ting 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 It is not possible to predict when (or how powerful) the next earthquake might be. But Prof. PAULINE GALEA, of the University's Seismological Monitoring & Research Group, argues that the ongoing 'swarm' of seismological activity is nothing new for the Maltese islands