Earthquake scare in 2020 traced to seismic fault 23km south of Malta

An earthquake that shook southeastern Malta in September 2020 was caused by a geological 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 also 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 Sebastiano D’Amico from the University of Malta’s Department of Geosciences.

Pauline Galea, Matthew Agius, Daniela Farrugia and Sebastiano D’Amico

The researchers said proximity to the coastline of such a fault underlines the importance of understanding its characteristics and contribution to the seismic hazard assessment of the Maltese islands.

This is particularly relevant in view of the “ongoing rapid increase in the urban and population densities of the archipelago, 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 important contribution towards assessing the sources of seismic hazard to the islands,” the researchers said.

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

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 dynamics 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 Richter Scale and was strongly felt on the islands.

The first identified earthquake of the sequence occurred on 8 September 2020 which had a local magnitude of 2.3. This was followed in the next days by a number of similar events, most of them with magnitudes smaller than 2.0.

The major event was preceded by a magnitude 3.4 event on 29 September, which was also felt by residents. Then on the 30 September 2020, a magnitude 4.5 earthquake was registered 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 southeastern 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 also submitted on the website of the EuroMediterranean Seismological Centre.

Around 1,000 website hits were registered during the first few minutes following the largest event, right after 3am local time, with another peak of reporting during the following morning.

Close to the epicentre, the reports indicated strong shaking with 78% of respondents being woken from their sleep. Approximately half the respondents reported rattling doors and windows, and a minority reported falling small objects. Around 74% of respondents reported that some kind of sound (mostly a roaring sound) accompanied the shaking.

These reports allowed the researchers to assign an intensity of  between IV and V on the European Macroseismic Scale for this event.

An intensity IV is used for seismic events that are “felt indoors by many people but felt outdoors by very few during which a few people are awakened 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 objects are shifted, and doors and windows swing open or shut.”

Historical precedent

The study refers to a historical precedent for the 2020 seismic event, which occurred between  14 and 27 August 1886, when no less than 15 tremors were reported in local newspapers 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 nearby cities to flee outside and was accompanied by rumbling sounds. These events were reported not to have been felt in any nearby countries. According to the authors of this study “it is therefore likely that such historical earthquake sequences had similar origins and characteristics as the 2020 sequence”.