Systematic Triggering of Large Earthquakes by Karst Water Recharge: Statistical Evidence in Northeastern Italy

Abstract

It is known that surface water accumulation by natural or anthropic causes like precipitation and reservoir impoundment can trigger earthquakes. The phenomenon is amplified and sped up in karst areas, where fracture systems can store large quantities of water and facilitate its percolation to seismogenic depths, increasing both elastic stress and pore pressure on pre-existent faults. The present work explored the possibility that this mechanism had systematically triggered major earthquakes in northeastern Italy, where seismicity is concentrated along the pre-Alpine thrust belt, characterized by the alignment of a series of karst massifs. The time occurrence of damaging and destructive earthquakes (moment magnitude Mw between 4.8 and 6.4) since 1901 was compared with the evolution of the Palmer Drought Severity Index (PDSI), an index of soil moisture that summarizes precipitation and, through temperature, water evaporation. Statistical analysis based on the bivariate Ripley’s K-function shows a significant time correlation between earthquakes and the peaks of PDSI since 1934, with the two strongest earthquakes (1936 Alpago-Cansiglio earthquake and 1976 Friuli earthquake, Mw 6.1 and 6.4, respectively) placed by the two PDSI maxima. The analysis was extended back in time to the last millennium, showing a time correlation between the occurrence of destructive earthquakes (Mw ≥ 6.2) and the peaks of ice extension in the European Alps, assumed as a proxy for groundwater accumulation in the study area. This evidence presented herein coupled with geological characteristics of the area and recent observations on large crustal deformations induced by heavy precipitation suggests that, if PDSI is a valid ground water indicator, karst water recharge may play a role in triggering major earthquakes in northeastern Italy, also relating their occurrence to the large scale climate changes affecting precipitation and evaporation.

Evidence of the climate control of strong seismicity in the Italian Apennines through groundwater recharge

“…The novelty emerged here is that the relationship is not episodic but systematic and persistent in the Apennines, at least in the last 800 years, involving the largest part of seismicity. This behavior resembles that observed for similar karst conditions in northeastern Italy (Bragato, 2021a) and relates the earthquake occurrence to multi-year climate fluctuations rather than to short-term meteorological effects. From the climatic point of view, the curves of scPDSI in Frontiers in Earth Science frontiersin.org faults as a consequence of the water load; the pore pressure diffusion thanks to the hydraulic continuity from the surface to the seismogenic layer.…”

“…Nonetheless, in many areas, it correlates well with the water storage changes estimated from GRACE (Gravity Recovery and Climate Experiment) satellites (Dai, 2011). It has been used successfully as an indicator of groundwater recharge in comparison with the occurrence of strong earthquakes in northeastern Italy (Bragato, 2021a) and Southern California (Bragato, 2021b). For the study of the time period between 1200 AD and 1900 AD, the author used the mainshocks with a moment magnitude of Mw ≥6.1 drawn from the seismic catalog CPTI15 as mentioned previously (green circles in Figure 1, a total of 28 earthquakes occurred before 1900) in comparison with the GA-length reconstructed by Holzhauser (1997).…”

“…Differently from the studies mentioned previously, the main interest here is on the possible systematic triggering of strong earthquakes over large karstic areas, a theme originally addressed for the seismicity of Greece Petropoulos, 1992, Liritzis andPetropoulos, 1994). In a previous statistical study conducted in the pre-Alpine karstic belt of northeastern Italy (Bragato, 2021a), it has been shown that the advent of multi-year wet periods (characterized by increased precipitation and reduced evaporation due to lower temperatures) is almost a necessary precondition for the occurrence of damaging and destructive earthquakes. The Apennines, with the exclusion of their southernmost part, are another zone of Italy where strong seismicity (moment magnitude up to 7.2) and carbonate rock outcrops coexist (Figure 1), suggesting a possible significant role of precipitation in earthquake triggering, like for the 1980 Irpinia Mw 6.8 earthquake (D'Agostino et al, 2018).…”

Contribution of Remote Sensing and GIS to the Inventory and Analysis of Factors influencing on the Development of Karst Features in the Middle Atlas, Morocco

“…It is known that surface water accumulation by natural or anthropic causes like precipitation and reservoir impoundment can trigger earthquakes. The phenomenon is amplified and sped up in karst areas, where fracture systems can store large quantities of water and facilitate its percolation to seismogenic depths, increasing both elastic stress and pore pressure on pre-existent faults [39]. The accumulation of water can trigger earthquakes, and the two main mechanisms are explained by poroelastic models: the addition to faults of further elastic stress caused by the groundwater load; the propagation through faults of pulses of fluid pore pressure that reduces their frictional strength.…”

Statistical Relationship Between the Decrease of Major Seismicity and Drought in Southern California Since 1900

“…The aim was to check if a coarse, large-scale groundwater indicator (which is also easily available and continuously updated) could be sufficient to evidence interesting characteristics of the groundwater/earthquake interaction. PDSI was used for a similar comparison in northeastern Italy (Bragato, 2021), where a systematic enhancement of seismicity following the peaks of PDSI was found. As any statistical investigation, that proposed in this study does not demonstrate the existence of a physical connection between groundwater recharge and seismicity.…”