HR: 17:45h
AN: T54A-08    [Abstracts]
TI: Fluid Overpressure and Earthquakes Triggering in the Natural Laboratory of the Northern Apennines: Integration of Field and Laboratory Data
AU: * De Paola, N
EM: nicola.de-paola@durham.ac.uk
AF: RRG, Earth Sciences Department, University of Durham (UK), South Road, Durham, DH1 3LE, United Kingdom
AU: Collettini, C
EM: colle@unipg.it
AF: GSG, Dipartimento di Scienze della Terra, Universita' di Perugia, P.zza Universita 1, Perugia, 06100, Italy
AU: Faulkner, D
EM: faulkner@liv.ac.uk
AF: Rock Deformation Lab, Earth and Ocean Sciences Department, University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, United Kingdom
AB: The integration of seismic reflection profiles with well-located earthquakes show that the mainshocks of the 1997-1998 Colfiorito seismic sequence (Central Italy) nucleated at a depth of ~6 km within the Triassic Evaporites (TE, anhydrites and dolostones), where CO2 at near lithostatic pressure has been encountered in two deep boreholes (4 km). In order to investigate the deformation processes operating at depth in the source region of the Colfiorito earthquakes we have characterized: 1) fault zone structure by studying exhumed outcrops of the TE: 2) rheology and permeability by performing triaxial loading tests on borehole samples of anhydrites at room temperature, 100 MPa confining pressure (Pc), and range of pore fluid pressures (Pf). Permeability and porosity development was continuously measured throughout the deformation experiments. The architecture of large fault zones within the TE is given by a distinct fault core, where most of the shear strain has been accommodated, surrounded by a geometrically complex and heterogeneous damage zone. Brittle deformation within the fault core is extremely localized along principal slip surfaces associated with dolomite rich cataclasite seams, running parallel to the fault zone. The damage zone is characterized by adjacent zones of heavily fractured rocks (dolostones) and foliated rocks displaying little fracturing (anhydrites). Static permeability measurements on anhydrite samples show increasing values of permeability for decreasing values of Pe, (k = 10E-20 - 10E-22 m2). During single cycle loading tests the permeability values immediately prior to failure are about three orders of magnitude higher than the initial values. The field data suggests that during the seismic cycle, the permeability of the dolostones, within the damage zone, is likely to be high and controlled by mesoscale fracture patterns. Conversely, the permeability of the anhydrites, due to the absence of mesoscale fracture patterns within Ca-sulphates layers, may be potentially as low as the values measured in the lab experiments (k = 10E-17 - 10E-22 m2). This suggests that fluid overpressure can be maintained in this lithology, within the damage zone, as far as the co-seismic period. Our observations and results can be applied to explain the seismicity of the Northern Apennines and other regions where fluids overpressures play a key role in triggering fault instability and earthquakes.
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting