High-Resolution Analysis of the 2023 Seismic Swarm Offshore Malta

In January-February 2023 a seismic sequence took place in the Central Mediterranean Sea,  ~90 km south of the island of Malta, and ~200 km from the coast of Sicily. The seismicity started in Mid January, in a region that experienced only sparse seismicity in the past. In the following days several M4+ events occurred. The largest earthquake, with moment magnitude Mw 5.3, took place a couple of weeks after the unrest onset, on January 30. The seismicity continued for several weeks, before fading down. Later seismicity was observed and still going on. This recent, unusual seismicity offers a unique opportunity to investigate seismogenic processes in this region with unprecedented detail. However, analyzing seismic sequences in offshore environments presents significant challenges due to the absence of optimal seismic monitoring conditions. These limitations compromise the effectiveness of conventional data analysis techniques, hindering the characterization of offshore seismic sequences. We tackled these limitations through the adoption of advanced, waveform-based seismic data analysis techniques that allow to investigate offshore seismic sequences, with the aim to provide insights into their origin. We combine full-waveform based detection and template matching methods to enhance the detection of events, advanced location techniques based on Distance Geometry Solvers (DGS), and probabilistic waveform-based methods for seismic source characterization. We combine the seismic source analysis for the 8 largest earthquakes in the sequence, with magnitude exceeding ML 4.5, with waveform-based and statistical analysis of the seismicity. About 500 events are identified. Their locations map a narrow lineament extending ~NW-SE. Full moment tensors for the largest events identify normal faulting mechanisms with a similar orientation, and shallow centroids of ~5 km depth. This result, combined with a waveform similarity analysis, suggests a predominant mechanism for the entire sequence. Using different seismicity indicators we classify the 2023 sequence as a seismic swarm. Indeed, the largest events in the sequence occur weeks after the unrest onset. Compared to previous seismicity, the sequence was outstanding in terms of maximum magnitude, seismicity rate and moment rate. While normal faulting earthquakes are not unusual in the Central Mediterranean, they differ from the few focal mechanisms previously proposed for the swarm focal region, which has important implications, considering that normal faulting earthquakes at shallow depth pose a tsunami hazard in the region.