2,184 research outputs found
A slip tendency analysis to test mechanical and structural control on aftershock rupture planes
No full textLarge portions of intraplate regions are characterised by relatively uniform stress fields with moderate to large main shock fault-ruptures nucleating on planes successfully predicted by 2D frictional fault reactivation theory. Here we use a slip tendency analysis, based on the notion that slip on a fault is controlled by the ratio of shear stress to normal stress acting on the plane of weakness, to test whether aftershock sequences are also governed by fault reactivation theory within the regional stress field. We observe that aftershocks for two well-documented seismic sequences occurring in extensional and compressional environments, the 1997 M(w)=6.0 Colfiorito sequence (Central Italy) and the 1999 M(w)=7.5 Chi-Chi sequence (Taiwan), respectively, nucleate on planes favourably oriented for frictional fault reactivation. In particular, 89% of 329 and 81% of 121 events for the Colfiorito and Chi-Chi sequences respectively, are the result of fault reactivation processes on geological structures that represent well oriented planes within the regional stress field. This suggests that stress rotations induced by the main shock for these two intracontinental sequences are unlikely. In addition, the percentage of well oriented aftershock rupture planes reaches 100% for Colfiorito and 86% for Chi-Chi if we consider a magnitude threshold above M(w)=3.7 and M(w)=5.0, respectively. We interpret this as the fact that stress heterogeneities if present are generally localised and can influence only small structures capable of generating small magnitude aftershocks. (c) 2007 Elsevier B.V. All rights reserved
Evaporites bearing faults as a tool to understand the deformation processes into the seismogenic zone of the Northern Apennines [Zone di faglia in rocce evaporitiche ed interpretazione dei processi defoimativi nella zona sismogenetica dell'Appennino Settentrionale]
No full textIn the Northern Apennines the integration of commercial seismic reflection profiles with the location of the mainshocks of the recent seismic sequences (the 1997 Mw = 6.0 Colfiorito and the 1998 Mw = 5.1 Gualdo Tadino) constrains the nucleation of the major events at ∼6 km of depth within the Triassic Evaporites, TE, formation. In order to investigate the deformation processes responsible for earthquake nucleation we have studied ancient and exhumed Evaporites bearing normal faults cropping out in Tuscany. Within the TE formation, that is a 2.5 km thick sequence, composed of decimetric-to-decameter scale interbeds of foliated gypsumanhydrites and brecciated dolostones, we have studied fault zone architecture and deformation processes of both small (100 m) displacement faults. Small and large scale faults display a relatively well structured internal architecture with a sharp reduction in grain-size between the damage zone (coarse grained protocatclasite) and the fault core (fine/very fine grained cataclasite). Extreme localization of slip to discrete and very thin (Y and B shear up to 10-100 mm) sliding surfaces is observed within the fine-grained dolomitic bearing cataclasite layers within the fault core. Fluid assisted processes have been inferred by fault fracture mesh development with crack and seal textures within dolomite-dominated damage fault zone. The deformation processes observed in the field are consistent with elastic friction behaviour, recorded by random fabric fault rocks developed within localised zones of high shear strain. Coseismic slip along very narrow slip zones ( 700 °C); it is currently under investigation whether such temperature increase which produces thermal decomposition of dolomite and phase transition in anhydrite rocks is able to cause dramatic coseismic slip weakening within the fault zone
Data for: The seismic signature of heavy oil on carbonate reservoir through laboratory experiments and AVA modelling
No full textSamples measurement
Physical-transport properties variations within carbonate-bearing fault zones: insights from the Monte Maggio Fault (Central Italy)
No full textPhysical properties of fault zones vary with time and space
and in particular permeability variations are strictly related to
fault zone processes. Results from previous laboratory studies,
conducted on sedimentary fault rocks outcropping in Central
Italy, show that permeability ranges from 10-16 m2 to 10-20 m2,
(Agosta et al., 2007) in response to the intrinsic petrophysical
properties of the material, state of stress and fault rock
microstructures.
Here we investigate the physical properties of carbonate
samples collected along the Monte Maggio normal Fault
(MMF) that is a regional structure (length ~10 km and
displacement ~500 m) located within the active system of the
Apennines. In particular we have studied an exceptionally
exposed outcrop of the fault within the Calcare Massiccio
formation that has been “exhumed” by new roadworks (Fig.
1A). Large cores (100 mm in diameter and up to 20 cm long)
drilled perpendicular to the fault plane (Fig. 1B,C) have been
used to: 1) characterize the damage zone adjacent to the fault
plane and 2) to obtain smaller cores, 38 mm in diameter both
parallel and perpendicular to the fault plane, for rock
deformation experiments.
The MMF shows two types of damage zone (Fig. 1C,D): 1)
a cemented and indurated cataclasite (CC), that extends up the
20 cm from the fault plane and 2) a porous cataclasite (PC),
that is located adjacent the CC.
We performed laboratory measurements of Vp, Vs, and
permeability at effective confining pressures up to 100 MPa in
order to simulate crustal conditions, at the HP-HT Laboratory
of experimental Volcanology and Geophysics (INGV, Rome1).
From ambient pressure to 100 MPa, P-wave velocity ranges
from 4,9 km/s to 5,9 km/s for PC samples, whereas it is
constant at 5,9 km/s for CC samples. Vs show the same
behaviour resulting in a constant Vp/Vs ratio of 1,5 and 1,6 for
PC and CC respectively. Permeability of CC samples is about
10-19 m2 and it is pressure independent; in contrast, it is higher
and pressure dependent for PC samples starting from 10-17 m2
at ambient pressure to 10-18 m2 at 100 MPa of confining
pressure.
Permeability variations are intimately related to fracture
density as well as P-wave velocity. To test the applicability of
laboratory data to in-situ condition we have compared laboratory and deep borehole data by assuming 25 MPa = 1 kmof lithostatic load. We selected four boreholes drilled in central
Italy that have encountered the Calcare Massiccio (CM)
formation: Pieve Santo Stefano 1, Tavullia 1, Villa Degna 1,
and Daniel 1. Borehole velocities show significant values in the
range of 5,7 – 6,9 km/s and this variation is not depth
dependent. The most frequent in-situ values of Vp ~6.3 km/s is
~5% higher than the average velocity registered for both PC
and CC samples at 100 MPa in the laboratory. A detailed
microstructural analysis conducted by using image analysis on
large cores of the MMF damage zone (Fig. 1D) will help in
clarifying the relationship between crack density and geometry and elastic wave velocities and permeability
Fault zone architecture and deformation processes within evaporitic rocks in the upper crust
No full textRecently, in the Northern Apennines, geophysical data have identified the Triassic Evaporites (TE, anhydrites and dolomites) as the source region of the major extensional earthquakes of the area (M-6). In order to characterize fault zone architecture and deformation processes within the TE, we have studied exhumed evaporite-bearing normal faults within the upper crust. The structure of large displacement (>100 m) normal faults is given by 1) a zoned fault core with a wider portion of fault-parallel foliated Ca-sulphates (ductile deformation), overprinted by an inner fault core (IFC) of localized brittle deformation, and 2) wide (dolostones) to absent (Ca-sulphates) damage zones of fault fracture patterns. Fault rock assemblage within the IFC is characterized by fault breccia, gouge, and cataclasites of different grain size. Most of the deformation within the IFC is localized along thin and fault parallel principal slip surfaces (PSS) made of dolomite-rich fine-grained cataclasite. SEM analyses show an evolution from Ca- to St- to gypsum-rich mineralization, due to episodic fluid flow events channeled along the fault zones during different stages of fault exhumation. The development of the observed fault geometry can be explained by a mechanical fault evolution model where initial faulting occurs along broad and ductile shear zones within the anhydrites and causes fracturing within the dolostones. Progressive deformation within the fault core leads to the development of fault parallel dolomite-rich cataclastic layers. Their reactivation coupled with transient fluid overpressures can produce embrittlement and localization of brittle deformation within the IFC. Copyright 2008 by the American Geophysical Union
Modeling lateral facies heterogeneity of an upper Oligocene carbonate ramp (Salento, southern Italy)
No full textThe aim of this work is to reproduce a metre-scale facies heterogeneity 3D model of the Chattian Porto Badisco Calcarenite carbonate ramp outcropping in the Salento Peninsula (southern Italy). However, in shallow-water carbonate systems, capturing metre-scale facies heterogeneity in three-dimensional models remains controversial due to the possibility of facies coexistence and because their association can change through time and space. Within this context, the continuous and well-exposed Chattian Porto Badisco Calcarenite carbonate ramp allows detailed study of the distribution of lithofacies association and their architecture along the dip direction depositional profile. The lithofacies and the depositional model of the Porto Badisco Calcarenite are referred to those defined by Pomar et al. (2014). The Porto Badisco Calcarenite is a homoclinal carbonate ramp with a euphotic inner setting characterised by the extensive seagrass meadows, passing basinward into a large rotaliid packstone and coral mounds developed in mesophotic conditions. The deeper part of the oligophotic zone is characterised by rhodolithic floatstone to rudstone and large lepidocyclinid packstone. The distal part of the ramp is characterise by a fine calcarenite. The methodology used in this work combines classical field data collection (e.g., stratigraphic logs and field-facies mapping) and 3D stochastic modeling by using PetrelTM. All the data (top and base of stratigraphic logs, cross-section, key surfaces, lithofacies lateral extension etc.) were georeferenced and inserted into the software to build the digital outcrop model. The 3D facies model has been performed after several simulations through specific stochastic algorithms (SISim, TGSim), comparing the models reproduce by the two algorithms, matching the depositional geometries and the lithofacies association observed in the outcrop. The 3D modeling represents a useful tool to better understand the facies architecture and their complex heterogeneity. Moreover, a detailed 3D facies model provides an essential tool to characterise semi-quantitatively sedimentological features for subsurface reservoir studies
Laboratory experiments on the physical properties of the seismogenic Triassic evaporites of Umbria-Marche and their relation to crustal scale data
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