An earthquake swarm on the Malta graben, Central Mediterranean, September – November 2020

Abstract

The seafloor of the Sicily Channel is characterised by an extensional regime, governed by a network of normal and strike-slip fault systems. These faults generate a background level of seismicity that rarely exceeds magnitude 5.0. A number of these faults pass close to the Maltese islands. In particular the Malta graben lies less than 15 km to the south of Malta at its closest point, and the islands have been shaken a number of times by earthquakes originating on this, and other fault systems. In this study we describe the occurrence of a seismic sequence, that started in September 2020 and lasted for several weeks, the largest event having a local magnitude of 4.5 and being strongly felt throughout the archipelago. The sequence was located at a distance of around 23 km south of the eastern tip of Malta. We have used single station polarization analysis at seismic station WDD to estimate epicentral location for all events, down to magnitude 1.2. For the largest events, we have also used conventional network location, utilising phase picks at stations of the Malta Seismic Network and in Southern Sicily. We describe the time evolution of the sequence, felt effects, the public response and the implications for seismic hazard.