Issue link: https://maltatoday.uberflip.com/i/1506016
NEWS 16 maltatoday | SUNDAY • 20 AUGUST 2023 JAMES DEBONO AN earthquake that shook southeastern Malta in Septem- ber 2020 was caused by a geo- logical fault just 23km south of the island, a study reveals. The earthquake was preceded by a swarm of 100 tremors over a two-month span that were al- so traced to the same fault. The findings were published in Xjenza, an online science journal published by the Malta Chamber of Scientists. The study was co-authored by Pauline Galea, Matthew Agius, Daniela Farrugia and Sebas- tiano D'Amico from the Uni- versity of Malta's Department of Geosciences. The researchers said proxim- ity to the coastline of such a fault underlines the importance of understanding its character- istics and contribution to the seismic hazard assessment of the Maltese islands. This is particularly relevant in view of the "ongoing rap- id increase in the urban and population densities of the ar- chipelago, particularly in the southeastern region of Malta", the study concludes. "The earthquake swarm is important not only because it provides insight onto ongoing geological processes, but also because it represents an im- portant contribution towards assessing the sources of seismic hazard to the islands," the re- searchers said. The Malta graben which was the origin of these seismic events is less than 15km to the south of Malta at its closest point. In this case, the point of origin was traced to a distance of around 23km south of the eastern tip of Malta. The study establishes the dy- namics of a seismic sequence, that started in September 2020 and lasted for two months, continuing sporadically even to the present day. Most of the events were of a magnitude below 3.0, but the largest event reached a local magnitude of 4.5 on the Rich- ter Scale and was strongly felt on the islands. The first identified earth- quake of the sequence oc- curred on 8 September 2020 which had a local magnitude of 2.3. This was followed in the next days by a number of sim- ilar events, most of them with magnitudes smaller than 2.0. The major event was preced- ed by a magnitude 3.4 event on 29 September, which was also felt by residents. Then on the 30 September 2020, a magni- tude 4.5 earthquake was regis- tered at 3 am at 22.9km from the seismic station in southeast Malta. The earthquake produced "strong shaking on the Maltese islands and waking up most of the residents of the southeast- ern region". The magnitude 4.5 event was followed by more than 15 tremors on the same day. Strong shaking and rumbling sound The magnitude 4.5 event of 30 September was felt all over the archipelago, and especially in the eastern half of Malta. Close to 2,000 'felt reports' were submitted to the 'Did You Feel It?' form on the University of Malta's Seismic Monitoring & Research Group website. Another 282 reports were al- so submitted on the website of the EuroMediterranean Seis- mological Centre. Around 1,000 website hits were registered during the first few minutes following the larg- est event, right after 3am local time, with another peak of re- porting during the following morning. Close to the epicentre, the re- ports indicated strong shaking with 78% of respondents being woken from their sleep. Ap- proximately half the respond- ents reported rattling doors and windows, and a minority reported falling small objects. Around 74% of respondents re- ported that some kind of sound (mostly a roaring sound) ac- companied the shaking. These reports allowed the re- searchers to assign an intensi- ty of between IV and V on the European Macroseismic Scale for this event. An intensity IV is used for seismic events that are "felt in- doors by many people but felt outdoors by very few during which a few people are awak- ened and windows, doors and dishes rattle." An intensity V event applies to seismic events which are "felt indoors by most people and outdoors by a few during which many sleeping people awake and hanging objects swing considerably, small ob- jects are shifted, and doors and windows swing open or shut." Historical precedent The study refers to a histori- cal precedent for the 2020 seis- mic event, which occurred be- tween 14 and 27 August 1886, when no less than 15 tremors were reported in local news- papers to have been felt by the population, six of them on the same day, Sunday 15 August. Of these, the largest event, at 3:45am on Sunday, was large enough to cause most of the residents of Valletta and near- by cities to flee outside and was accompanied by rumbling sounds. These events were re- ported not to have been felt in any nearby countries. Ac- cording to the authors of this study "it is therefore likely that such historical earthquake se- quences had similar origins and characteristics as the 2020 sequence". Earthquake scare in 2020 traced to seismic fault 23km south of Malta Epicentre locations of the earthquake swarm from 08/09/2020 to 14/11/2020. Red circles are epicentres using single-station location at station WDD. The black stars indicate SeisComP locations Pauline Galea, Matthew Agius, Daniela Farrugia and Sebastiano D'Amico