Fluid geochemistry as a tool for the discrimination of fault systems and fault interaction areas and their stress-strain pathfinders parameters: case histories from Italy
by
Luca Pizzino
INGV-Rome
Coauthors: Quattrocchi F.(*), Guerra I.(**), Moretti A.(***), Cinti D.(*), Galli G.(*), Voltattorni N.(*). (*) INGV-Rome, (**) Università della Calabria, (***)Università dell'Aquila.

The modern concept of deterministic Seismic Hazard Assessment (SHA) involves the individuation and parameterisation of seismogenic sources, and their spatial and temporal interactions. This task could be accomplished by means of different and independent observations and techniques: seismicity, patterns of recent and historical seismicity, structural and geo-morphological data, geophysical prospecting, analysis of deformation by geodetic and SAR data, photo interpretation of aerial and satellite images and, more recently, by fluid geochemistry methods, as exploited in recent EC projects (Geochemical Seismic Zonation, Corinth Rift Laboratory, Corseis).

Fifteen years of activity performed by the Fluid Geochemistry Laboratory of the Istituto Nazionale di Geofisica e Vulcanologia of Rome in different Italian and European seismic and volcanic areas pointed out the potentiality and the soundness of the fluid geochemistry methods in discriminating fault systems. Fractures and faults provide permeable pathways for fluids at a variety of scales, from great depth in the crust to flow through fractured aquifers, geothermal fields and hydrocarbon reservoirs.In some cases fault systems were defined by different authors as geochemical active fault zone (GAFZ), tip-line area and fault trace, tentatively linking their geochemical “activity” to the definition of “active” fault from a structural or a seismological point of view. Presently, any definitive answer, that may link unequivocally and quantitatively the presence of peculiar geochemical patterns to an “active” seismogenic segment, mostly if the SHA authorities require information on an impending earthquake, has been stated.

Nevertheless, the most important insight gathered applying the geochemical techniques to the SHA studies is the evidence that some geochemical tectonic pathfinders (e.g. Radon, He, CO2, CH4, 3He/4He, temperature, trace metals, etc.) are able to map in details the areas where different fault systems interact. These areas, defined by different authors as geochemical interaction fault zone (GIFZ) or breakdown regions, show an enhanced permeability with respect to the surrounding sectors. In the fault interaction areas, stress concentrations cause active fracturing and continual re-opening of fluid-flow conduits, permitting long-lived hydrothermal flow despite a tendency to clogging of fractures due to mineral precipitation. Moreover, in these areas, discrete and continuous monitoring of shallow and ground waters as well as soil gases in the occurrence of relevant earthquakes demonstrated a very close relationship between the observed variations in both physical-chemical features and composition of the fluids and the ongoing seismic activity, confirming that fault interaction areas are “sensible” to the changes of the acting stress-strain field.

Based on these concepts, we present a review of some inferred GAFZ and GIFZ case histories from Italy, with a particular emphasis to the Calabria and Sicilia regions.

Date received: September 10, 2004

Copyright © 2004 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caon-40.