Lecocq, Thomas

Microseism, the most continuous seismic signal on the Earth generated by the interaction between the hydrosphere, the atmosphere, and the solid Earth, is a useful tool for acquiring information about climate change. Indeed, several authors dealt with the relationship microseism-sea state and microseism-cyclonic activity, considering in particular tropical cyclones, hurricanes, typhoons, and recently Medicanes (small-scale tropical cyclones that occur in the Mediterranean Sea). In this study, we analyze, from a seismic point of view, several meteorological events that occurred in the Mediterranean Sea during the period November 2011 - February 2023. In particular, we consider 9 Medicanes and 4 more common storms. Despite the marked differences between them, each of these events caused heavy rainfall, strong wind gusts, violent storm surges with significant wave heights usually greater than 3 meters, and damage along the exposed coast. Occasionally, these events caused deaths and injuries. In this work, we analyzed the seismic signal recorded by 104 seismic stations, installed along the Italian, Maltese, Greek, and France coastal areas, and 15 seismic stations, installed in the Etnean area used only to perform array analysis. We deal with the relationships between the considered meteorological events and the features of microseism in terms of spectral content, space-time variation of the amplitude, and source locations tracked using two different methods (a grid search approach based on seismic amplitude decay and array techniques). By comparing the positions of the microseism sources, obtained from our analysis, with the areas of significant storm surges, retrieved from hindcast data, we observe that the microseism locations are in agreement with the actual locations of the storm surges for 10 out of 12 events analyzed (two Medicanes present very low intensity in terms of meteorological parameters and the microseism amplitude does not show significant variations during these two events). In addition, we also carried out two analyses that allowed us to obtain both the seismic signature of these events, by using a method that exploits the coherence of continuous seismic noise, and their strength from a seismic point of view, called Microseism Reduced Amplitude. By integrating the results obtained from these two methods, we can "seismically" distinguish Medicanes and common storms. Consequently, we demonstrate the possibility of creating a novel monitoring system for Mediterranean meteorological events by incorporating microseism information alongside with other techniques (e.g. wave buoy, wave gauge, and High-Frequency coastal radar) commonly used for studying and monitoring meteorological phenomena. In addition, since the seismometers were among the first geophysical instruments installed, it is possible to digitize old seismograms and examine historical data shedding new light on extreme weather events in a climate change scenario.

In this work, we analyze 12 meteorological events that occurred in the Mediterranean Sea during the period November 2011-November 2021 from a seismic point of view. In particular, we consider 8 Medicanes and 4 more common storms. Each of these events, in spite of the marked differences between them, caused heavy rainfall, strong wind gusts and violent storm surge with significant wave heights usually >3 m. We deal with the relationships between these meteorological events and the features of microseism (the most continuous and widespread seismic signal on Earth) in terms of spectral content, space-time variation of the amplitude and source locations tracked employing two different methods (amplitude decay-based grid search and array techniques). By comparing the positions of the microseism sources with the areas of significant storm surges, we observe that the microseism locations align with the actual locations of the storm surges for 10 out of 12 events analyzed (two Medicanes present very low intensity in terms of meteorological parameters and the microseism amplitude does not show significant variations during these two events). We also perform two analyses that allowed us to obtain both the seismic signature of these events, by using a method that exploits the coherence of continuous seismic noise, and their strength from a seismic point of view, called Microseism Reduced Amplitude. In addition, by integrating the results obtained from these two methods, we are able to "seismically" distinguish Medicanes and common storms. Consequently, we demonstrate the possibility of creating a novel monitoring system for Mediterranean meteorological events by incorporating microseism information alongside with other commonly employed techniques for studying meteorological phenomena. The integration of microseism with the data provided by routinely used techniques in sea state monitoring (e.g., wave buoy and HF radar) has the potential to offer valuable insights into the examination of historical extreme weather events within the context of climate change.

In this work, we study a Mediterranean cyclone, Helios, which took place during 9–11 February 2023 in the southeastern part of Sicily and Malta, by a multiparametric approach combining microseism results with sea state and meteorological data provided by wavemeter buoy, HF radar, hindcast maps and satellite SEVIRI images. The sub-tropical system Helios caused heavy rainfall, strong wind gusts and violent storm surges with significant wave heights greater than 5 m. We deal with the relationships between such a system and the features of microseism (the most continuous and ubiquitous seismic signal on Earth) in terms of spectral content, space–time variation of the amplitude and source locations tracked by means of two methods (amplitude-based grid search and array techniques). By comparing the location of the microseism sources and the area affected by significant storm surges derived from sea state data, we note that the microseism location results are in agreement with the real position of the storm surges. In addition, we are able to obtain the seismic signature of Helios using a method that exploits the coherence of continuous seismic noise. Hence, we show how an innovative monitoring system of the Mediterranean cyclones can be designed by integrating microseism information with other techniques routinely used to study meteorological phenomena.

Microseism is the most continuous and ubiquitous seismic signal on the Earth and is caused by the interaction between the atmosphere, the hydrosphere and the Solid Earth. In literature, there are several studies that deal with the relationship between microseism and cyclonic activity considering in particular hurricanes, tropical cyclones and typhoons. However, the relationships between microseism and the small-scale tropical cyclones that occur in the Mediterranean Sea, called Medicanes, have never been analysed. For this reason, we considered the Medicane Apollo, which developed in the Ionian Sea and impacted the eastern part of Sicily during the period 25th October to 5th November 2021 causing heavy rainfall (> 400 mm/48h), strong wind gusts (104 km/h) and violent sea waves (significant wave height > 3.5 m). Furthermore, the heavy rainfall induced by the presence of Apollo, caused damage to infrastructure and agriculture forcing the Sicilian regional government to declare a state of emergency for 32 municipalities (in the provinces of Catania, Messina, Siracusa and Ragusa) that were mostly affected by the Medicane Apollo.In this work, we analysed the microseism signal recorded by 78 seismic stations installed in South Italy, Malta and Greece coastline during the period under investigation. To obtain information about the significant wave heights, we consider the data obtained by hindcast maps and four wavemeters buoys. The spectral and amplitude analysis allowed us to obtain information about the space-time variations of the microseism amplitude and in addition, we were able both to differentiate the seismic stations that perceive Apollo (stations installed close to the Ionian Sea), the seismic stations that do not perceive the medicane (stations installed close to the Tyrrhenian sea) and the microseism bands influenced by the presence of the Medicane Apollo. Moreover, we tracked the position of the Apollo by using two different methods: i) grid search method based on the seismic amplitude decay using the 78 seismic stations first mentioned and ii) array technique by 15 seismic stations installed on Etna which may be considered an array thanks to their spatial distribution and geometry. We obtain a good match between the real positions of the Medicane Apollo derived from satellite images and the positions computed by the two analysis methods. This work shows that it is possible to extract information about the Mediterranean extreme meteo-marine events from microseism, a seismic signal that until not long ago was considered as noise, both for monitoring and research purposes.

Microseism is the continuous background seismic signal caused by the interaction between the atmosphere, the hydrosphere and the solid Earth. Several studies have dealt with the relationship between microseisms and the tropical cyclones, but none focused on the small-scale tropical cyclones that occur in the Mediterranean Sea, called Medicanes. In this work, we analysed the Medicane Apollo which impacted the eastern part of Sicily during the period 25 October-5 November 2021 causing heavy rainfall, strong wind gusts and violent sea waves. We investigated the microseism accompanying this extreme Mediterranean weather event, and its relationship with the sea state retrieved from hindcast maps and wave buoys. The spectral and amplitude analyses showed the space-time variation of the microseism amplitude. In addition, we tracked the position of Apollo during the time using two different methods: (i) a grid search method; (ii) an array analysis. We obtained a good match between the real position of Apollo and the location constraint by both methods. This work shows that it is possible to extract information on Medicanes from microseisms for both research and monitoring purposes.

This work analyzes temporal variations of seismic velocities at Mt. Etna from August 2018 to February 2019. During this time period, a strong flank eruption accompanied by intense seismicity and ground deformation took place along a fracture that opened on 24 December 2018 at the base of the New South-East summit crater. Furthermore, two moderate earthquakes—the 6 October 2018 ML 4.7 and 26 December 2018 ML 4.8—associated with the volcanic activity were recorded. In this study, we computed cross-correlation functions (CCFs) between windows of seismic ambient noise to identify seismic velocity variations within the volcano edifice. We calculated daily CCFs at 16 stations for 120 interstation pairs using the vertical component in the 1.0–1.5 Hz frequency band. We observe that dv/v starts to decrease rapidly from the beginning of October 2018 and reaches approximately −0.45% in the pre-eruption period. The spatio-temporal distribution of seismic velocities shows that the reduction of dv/v mostly occurs in the vicinity of the summit and close to the flank area and is interpreted to be affected by magmatic intrusion at 0–3 km depth. To infer the source mechanism of this eruption, we compared these observations with volcano-tectonic earthquakes, volcanic tremor, volcanic degassing, gravity, and ground deformation data. Our study suggests that a relationship between magma intrusion and associated crack opening is responsible for the decrease of dv/v.

During the COVID-19 pandemic, most countries put in place social interventions, restricting the mobility of citizens, to slow the spread of the epidemic. Italy, the first European country severely impacted by the COVID-19 outbreak, applied a sequence of progressive restrictions to reduce human mobility from the end of February to mid- March 2020. Here, we analysed the seismic signatures of these lockdown measures in densely populated eastern Sicily, characterized by the presence of a permanent seismic network used for earthquake and volcanic monitoring. We emphasize how the anthropogenic seismic noise decrease is visible even at stations located in remote areas (Etna and Aeolian Islands) and that the amount of this reduction (reaching 50 %–60 %), its temporal pattern and spectral content are strongly station-dependent. Concerning the latter, we showed that on average the frequencies above 10 Hz are the most influenced by the anthropogenic seismic noise.We found similarities between the temporal patterns of anthropogenic seismic noise and human mobility, as quantified by the mobilephone- derived data shared by Google, Facebook and Apple, as well as by ship traffic data. These results further confirm how seismic data, routinely acquired worldwide for seismic and volcanic surveillance, can be used to monitor human mobility too.

Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the coronavirus disease 2019 (COVID-19) pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. Although the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This quiet period provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of human activities.

On active volcanoes, ambient noise-based seismic interferometry can be a very useful monitoring tool as it allows to detect very slight variations in seismic velocity associated with magma transport towards the surface. In this work, we performed the autocorrelation of ambient seismic noise recorded at Mt. Etna volcano, by three stations located close to the active summit craters, during April 2013 - October 2014. Such an interval was chosen because of the number and variety of eruptions. In place of the classical cross-correlation, we implemented the phase cross-correlation of each component with itself, which does not require normalization of the signals. The detected seismic velocity variations were very consistent for all three stations throughout the study period, mainly ranging between 0.3 and -0.2%, and were time-related to both sequences of paroxysmal eruptions and more effusive activities. In particular, we observed seismic velocity decreases accompanying paroxysmal eruptions, suggesting an intense pressurization within the plumbing system, which created an area of extensional strain with crack openings. It is worth noting that classical cross-station approach failed to detect seismic velocity changes related to volcano activity. In addition, seismic velocity variations over time were integrated with ground deformation data recorded by GPS stations and volcanic tremor centroid locations. Finally, we showed that, although the investigated frequency band (1-2 Hz) contains most of the volcanic tremor energy, our results did not indicate a particular contamination of seismic velocity variation measurements by variations of tremor sources.

The aim of this paper is to propose the creation, in terms of European Macroseismic Scale (EMS-98), of the entire macroseismic fi eld of the 6 May 1976 Friuli earthquake. Only forty odd years have passed, and nothwithsatnding that there is a huge quantity of existing data, it was still disturbing to fi nd that much of the original data are missing and probably lost forever Efforts have therefore been made to fi nd additional and still unknown primary data. For the majority of the collected national data sets, a reevaluation was then possible. This study presents the comprehensive macroseismic data set for 14 European countries. It is, to our knowledge, one of the largest European data sets, consisting of 3423 intensity data points (IDPs). The earthquake was felt from Rome to the Baltic Sea, and from Belgium to Warsaw. The maximum intensity 10 EMS-98 was reached in eight localities in Friuli (Italy). Compared to previous studies, the Imax values have changed from country to country, in some cases being lowered due to methodological differences, but in the case of three among the most hit countries, Imax is now higher than in the previous studies, mainly due to the new data.