Physical and Transport Property Variations Within Carbonate‐Bearing Fault Zones: Insights From the Monte Maggio Fault (Central Italy)

Abstract

The physical characterization of carbonate‐bearing normal faults is fundamental for resource development and seismic hazard. Here we report laboratory measurements of density, porosity, Vp, Vs, elastic moduli, and permeability for a range of effective confining pressures (0.1–100 MPa), conducted on samples representing different structural domains of a carbonate‐bearing fault. We find a reduction in porosity from the fault breccia (11.7% total and 6.2% connected) to the main fault plane (9% total and 3.5% connected), with both domains showing higher porosity compared to the protolith (6.8% total and 1.1% connected). With increasing confining pressure, P wave velocity evolves from 4.5 to 5.9 km/s in the fault breccia, is constant at 5.9 km/s approaching the fault plane and is low (4.9 km/s) in clay‐rich fault domains. We find that while the fault breccia shows pressure sensitive behavior (a reduction in permeability from 2 × 10−16 to 2 × 10−17 m2), the cemented cataclasite close to the fault plane is characterized by pressure‐independent behavior (permeability 4 × 10−17 m2). Our results indicate that the deformation processes occurring within the different fault structural domains influence the physical and transport properties of the fault zone. In situ Vp profiles match well the laboratory measurements demonstrating that laboratory data are valuable for implications at larger scale. Combining the experimental values of elastic moduli and frictional properties it results that at shallow crustal levels, M ≤ 1 earthquakes are less favored, in agreement with earthquake‐depth distribution during the L'Aquila 2009 seismic sequence that occurred on carbonates.

Assessment of earthquake locations in 3‐D deterministic velocity models: A case study from the Altotiberina Near Fault Observatory (Italy)

“…The average velocity of the whole Carbonatic Multilayer from borehole measurements is about 5.6 km/s ( Figure 2). Recent laboratory measurements of the Calcare Massiccio formation coming from the same area show pressure-independent velocities of 6.0-6.1 km/s [Trippetta et al, 2015].…”

“…Since 2010, this region hosts the Altotiberina Near Fault Observatory (TABOO) , a multidisciplinary research infrastructure that is providing a large amount of high-resolution geophysical data. We have developed a three-dimensional P and S wave velocity model (the AT seismic model) of the area by integrating a 3-D geological model, which is based on deep boreholes data and about 300 km of seismic reflection profiles [Mirabella et al, 2011, and reference therein], with sonic logs data and original laboratory measurements for different rock types [Trippetta et al, 2010[Trippetta et al, , 2013a[Trippetta et al, , 2015Smeraglia et al, 2014].…”

Rules for Flight Paths and Time of Flight for Flows in Porous Media with Heterogeneous Permeability and Porosity

“…In clastic sediments, it may occur along fractures and fault zones, which may either increase permeability due to dilational effects, or reduce permeability, for example, due to clay smear. For example, carbonate-bearing fault zones can be interpreted as damage zones with the permeability increasing 2 orders of magnitude relative to that of the undamaged protolith [48]. In many other cases, the nonuniform distribution of deviatoric stress could also lead to local permeability anisotropy.…”

Elastic Softening of Limestone Upon Decarbonation With Episodic CO2 Release

“…The development of soft (low aspect ratio, highly compliant) pore space such as cracks is a well‐known source of velocity and elastic moduli reduction (Ayling et al, ; e.g., Walsh, ) and enhancer of velocity sensitivity (Nur, ; Wyllie et al, ). Trippetta et al () note the decrease in velocity and increase in its sensitivity to pressure within a carbonate protolith that is cut by a fault as the protolith becomes brecciated, introducing low aspect ratio pores by way of cracking. In the decarbonated samples we observe a large drop in both V P (at least −1,200 m/s, average 25% decrease) and V S (at least −400 m/s, average 18% decrease), and large increases in Δ V P /Δ P C (average of 18 m/s/MPa) and Δ V S /Δ P C (average of 7 m/s/MPa)—common soft pore space indicators.…”