Flash floods are triggered by high-intensity and short-duration rainfalls and represent a major hazard for small river basins1,2. Given the heavy rainfall in a short period of time and the following rapid concentration of the runoff, flash floods populate the upper tail of the flood frequency distribution in small- to medium-size catchments (103 − 104 km2), posing at high-risk large communities and infrastructures. The flash floods risk is exacerbated by the increasing in frequency of extreme meteorological events. This increase is nowadays more and more imputed by the scientific community to the consequences of the climate changes; for instance, according to climate model projections, the annual damage caused by flooding in the United Kingdom is expected to increase by more than one fifth over the next century if the COP26 and Net Zero promises are not collectively met3.
The Mediterranean region is particularly exposed to the consequences of the climate change, and despite the foreseen overall drying, extreme precipitation events are expected to increase4. Even the vulnerability to floods is expected to increase at regional scales given the population growth that is experiencing the Mediterranean basin.
Despite the damage potential of such phenomena, the capability to monitor them can be limited by the quantity and type of instruments normally adopted for monitoring. In fact, the existing hydrometric and raingauge monitoring networks are not everywhere dense as needed and generally prone to low recording frequency and lack of information beyond stage height5. Furthermore, there are still remote mountain areas where precipitation and hydrometric information are not available at all. This can be a relevant limitation for understanding the hydro-meteorological processes that control flash floods. In this concern, there is the urgence to make the monitoring system more efficient and reliable, even by the opportunistic integration of different sensors.
Italy is especially exposed to the hydrological risk. A consistent number of flood events occurred there in the recent years, such as the one occurred in Liguria on October 20216, or in the Island of Ischia on October 20097 and on November 2022, or the recent flood that hit the Marche region on September 20228, which is the target of this work. A detailed list of floods occurred in Italy and all over the word can be found at FloodList (https://floodlist.com/).
The flash flood that hit the Marche region (Central Italy) between the 15 and the 16 September 2022 (Fig. 1) cumulated a rainfall peak of 437 mm in less than 12 hours (Cantiano rain gauge) that corresponded to a return period exceeding 500 years9. The rainfall triggered 1687 landslides in an area of 550 km2 affected by the peak rainfall intensity10 and led to 12 fatalities and severe damages to transport, infrastructures and buildings, especially in the Misa basin (Fig. 1). Such exceptionally severe event occurred at the end of a climatic anomaly of prolonged drought and warm conditions over Europe and the Mediterranean region. In particular, on the 15 September 2022, a deep trough over Scandinavia, a secondary lower one over the Iberian Peninsula and a high pressure on North Africa produced convective systems on the Tyrrhenian side. The slow movement of such baric structures determined the stationarity of its western flow in which these convective structures were developing. As a consequence, they continued to affect the same area for several hours, resulting in very high accumulations. In the early afternoon of the 15 September 2022, storm cells developed on the Apennines side in the mountain part of Marche region, generating intense, localized, and stationary phenomena. In the final part of that day, the system progressively moved towards the coast, gradually weakening in its intensity. At the same time, another thunderstorm system developed in the southern part of the region persisting for a more limited period, with intense phenomena but with considerably lower accumulations. In order to highlight the strength of the 15 September 2022 hydro-meteorological event, which leads us to define it as an extreme event, we recall that the total rainfall accumulation of this event (437 mm) was more the half of the total accumulation value for the Marche region over an entire year (i.e., the mean annual precipitation over the 1981–2010 period is 803,6 mm, https://www.reterurale.it/flex/cm/pages/ServeBLOB.php/L/IT/IDPagina/16319).
Although Italy has an advanced multi-parametric monitoring system (i.e., raingauges, disdrometers, hydrometers), and the national civil protection agency has at its disposal even ground-based weather radars and satellite-borne active and passive sensors, the 15 September 2022 event highlights that dealing with extreme floods calls for a paradigm change monitoring. Hydrometric network, in particular, seems not fully adequate to monitor the evolution of flash floods, given: (i) the low temporal resolution of the measurement, (ii) the exposure to damage by water discharge, coarse sediment and large wood transport, (iii) the lack of information beyond stage height, and (iv) malfunctioning due to erosion/deposition processes5,11−12. For instance, most of the hydrometers in Italy sample water level once in 30 min, which seems insufficient to characterize rapidly evolving floods featuring short-duration pulses.
In this retrospective study, we leverage seismic data from a national monitoring network to characterize the hydrometeorological event that between the 15 and the 16 September 2022 hit the Marche region.
Different structural and environmental factors are responsible for the temporal and spatial variation of seismic noise13. Non-tectonic seismic sources such as landslides, debris flows, dam collapses, floods, and avalanches generate seismic signals that are considered as “exotic” sources of noise when the objective is to perform a classical seismological analysis. Building upon a pioneering experiment carried out in the early 90s14, a growing number of studies dealing with the monitoring of river networks during monsoons15, typhoons 16, and controlled floods17 are nowadays relying on seismometers placed near torrents. Separating the contribution of various “exotic” seismic sources – including precipitation, bedload transport, and flow turbulence – would allow to characterize different processes with a single sensor at very high temporal resolution.
Fluvial seismology has already shown that important characteristics about river flow processes, such as bedload transport and turbulence, are encoded in ground vibrations18. Worth to mention, during the last twenty years, the Italian seismological community made great efforts in establishing dense seismic networks at national scale, standardizing formats for data transmission and archiving, and creating open data repositories for sharing real-time and archived data streams (e.g., the Observatories and Research Facilities for European Seismology European Integrated Data Archive, https://www.orfeus-eu.org/data/eida/). Such infrastructure and methodological developments open new scientific avenues, including the upscaling of fluvial seismology from single rivers to watersheds, which would be precious during large-scale flash floods.
In the following, we show as during the main precipitation event of the Marche hydrological crisis (i.e., between 12:00 and 20:00 UTC of the 15 September 2022) the seismic power registered by the seismic stations of the national seismic network (i.e., we used data from the network IV, INGV Seismological Data Centre, 2006), coherently increases and shows maxima in the frequency band 30–50 Hz. The spatio-temporal evolution of seismic noise recorded by stations distributed over central Italy highlights a remarkable temporal coincidence of seismic signals, which in turn also agrees with peak of rainfall from rainguages. Worth to note, our results highlight that anomalous high amplitudes on seismic signals culminate about 6 hours before the major flooding, with high spatial coherence of seismic amplitude at different stations during the rainfall crisis (i.e., at the 17:50 UTC of the 15 September 2022). Our results show higher seismic amplitudes in coincidence with the principal rivers of the area, where indeed most damaged villages are located and indicate eventually that the analysis of the seismic noise recorded by dense, regional seismic networks can provide powerful information on the spatio-temporal evolution of the flood that can efficiently complement hydrometric data.