Abstract
Long-term geochemical monitoring performed in the seismic area of the Umbria-Marche region of Italy (i.e. Central Apennines) has allowed us to create a model of the circulation of fluids and interpret the temporal chemical and isotopic variations of both the thermal springs as well as the gas vents. Coincident with the last seismic crisis, which struck the region in 1997-1998, an enhanced CO2 degassing on a regional scale caused a pH-drop in all the thermal waters as a consequence of CO2 dissolution. Furthermore, much higher 3He/4He isotope ratios pointed to a slight mantle-derived contribution. Radon activity increased to well above the ±2 sinterval of the earlier seismic period, after which it abruptly decreased to very low levels a few days before the occurrence of the single deep-located shock (March 26, 1998, 51 km deep). The anomalous CO2 discharge was closely related to the extensional movement of the normal faults responsible for the Mw 5.7, 6.0 and 5.6 main shocks that characterized the earlier seismic phase. In contrast, a clear compressive sign is recognizable in the transient disappearance of the deep-originating components related to the Mw 5.3, 51 km-deep event that occurred on March 26, 1998. Anomalies were detected concomitantly with the seismicity, although they also occurred after the seismic crisis had terminated. We argue that the observed geochemical anomalies were driven by rock permeability changes induced by crustal deformations, and we describe how, in the absence of any release of elastic energy, the detection of anomalies reveals that a seismogenic process is developing. Indeed, comprehensive, long-term geochemical monitoring can provide new tools allowing us to better understand the development of seismogenesis.
Highlights
It is well-known that earthquakes can provoke modifications in the natural setting of a seismicAntonio Caracausi, Francesco Italiano, Giovanni Martinelli, Antonio Paonita and Andrea Rizzo et al, 2002a,b) have suggested that the best way to approach the broad-ranging problem of fluid geochemistry and seismogenesis was to collect as many locally significant geochemical data as possible and to evaluate it within an interpretative geochemical model
The chemical composition of the sampled thermal waters was established to be a mixture of a carbonate component, equilibrated with calcite and dolomite, and a selenitic component circulating in the Evaporitic Triassic basement (Chiodini et al, 1982, 1999; Quattrocchi et al, 2000)
The long-term monitoring program, which started in July 1997, allowed us to interpret the geochemical information in such a way as to model the circulation pattern of the fluids released in the area
Summary
It is well-known that earthquakes can provoke modifications in the natural setting of a seismic. The long-term geochemical monitoring of any seismic area enables us to define the background and detect any anomalies, which, besides the knowledge of fluid circulation and interaction, allow us to define the behaviour of these anomalies as a seismogenic process develops. This paper contains new data mainly collected at a single site, namely the thermal spring of Bagni di Triponzo, where the emitted fluids have undergone the widest geochemical modifications of all the monitored sites (e.g., the temperature of the thermal water, 29.6 ± 0.2°C constant over time and with no seasonal effect, dropped by 2°C two weeks before and > 20°C two days before the occurrence of the March 26, 1998 shock). Long-term geochemical monitoring: case study of the Umbria Region (Central Italy) rocks, highlighting the geochemical phenomena related to extension versus those related to the compressive behaviour of the faults involved
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