ALBERT

Description: BREVIA

Description: Current emission inventories require an additional "unknown" source to balance the global atmospheric budgets of ethane (C2H6). Here, we provide evidence that a substantial part of the missing source can be attributed to natural gas seepage from petroliferous, geothermal, and volcanic areas. Such geologic sources also inject propane (C3H8) into the atmosphere. The analysis of a large data set of methane (CH4), ethane, and propane concentrations in surface gas emissions of 238 sites from different geographic and geologic areas, coupled with published estimates of geomethane emissions, suggests that Earth's degassing accounts for at least 17% and 10% of total ethane and propane emissions, respectively.

Description: Published

Description: 478

Description: 3.8. Geofisica per l'ambiente

Description: JCR Journal

Description: reserved

Description: Episodes of nonvolcanic tremor and accompanying slow slip recently have been observed in the subduction zones of Japan and Cascadia. In Cascadia, such episodes typically last a few weeks, and differ from “normal” earthquakes in their source location and momentduration scaling. The three most recent episodes in the Puget Sound/Southern Vancouver Island portion of the Cascadia subduction zone have been exceptionally well recorded. In each episode, we see clear pulsing of tremor activity with periods of 12.4 and 24-25 hours, the same as the principal lunar and lunisolar tides. This indicates that the small stresses associated with the solid-earth and ocean tides influence the genesis of tremor much more effectively than they do “normal” earthquakes. Because the lithostatic stresses are 105 times larger than those associated with the tides, we argue that tremor occurs on very weak faults.

Description: Published

Description: 186 -189

Description: 3.1. Fisica dei terremoti

Description: JCR Journal

Description: reserved

Description: It is increasingly apparent that faults are typically not discrete planes but zones of deformed rock with a complex internal structure and three-dimensional geometry. In the last decade this has led to renewed interest in the consequences of this complexity for modelling the impact of fault zones on fluid flow and mechanical behaviour of the Earth’s crust. A number of processes operate during the development of fault zones, both internally and in the surrounding host rock, which may encourage or inhibit continuing fault zone growth. The complexity of the evolution of a faulted system requires changes in the rheological properties of both the fault zone and the surrounding host rock volume, both of which impact on how the fault zone evolves with increasing displacement. Models of the permeability structure of fault zones emphasize the presence of two types of fault rock components: fractured conduits parallel to the fault and granular core zone barriers to flow. New data presented in this paper on porosity–permeability relationships of fault rocks during laboratory deformation tests support recently advancing concepts which have extended these models to show that poro-mechanical approaches (e.g., critical state soil mechanics, fracture dilatancy) may be applied to predict the fluid flow behaviour of complex fault zones during the active life of the fault. Predicting the three-dimensional heterogeneity of fault zone internal structure is important in the hydrocarbon industry for evaluating the retention capacity of faults in exploration contexts and the hydraulic behaviour in production contexts. Across-fault reservoir juxtaposition or non-juxtaposition, a key property in predicting retention or across-fault leakage, is strongly controlled by the three-dimensional complexity of the fault zone. Although algorithms such as shale gouge ratio greatly help predict capillary threshold pressures, quantification of the statistical variation in fault zone composition will allow estimations of uncertainty in fault retention capacity and hence prospect reserve estimations. Permeability structure in the fault zone is an important issue because bulk fluid flow rates through or along a fault zone are dependent on permeability variations, anisotropy and tortuosity of flow paths. A possible way forward is to compare numerical flow models using statistical variations of permeability in a complex fault zone in a given sandstone/shale context with field-scale estimates of fault zone permeability. Fault zone internal structure is equally important in understanding the seismogenic behaviour of faults. Both geometric and compositional complexities can control the nucleation, propagation and arrest of earthquakes. The presence and complex distribution of different fault zone materials of contrasting velocity-weakening and velocity-strengthening properties is an important factor in controlling earthquake nucleation and whether a fault slips seismogenically or creeps steadily, as illustrated by recent studies of the San Andreas Fault. A synthesis of laboratory experiments presented in this paper shows that fault zone materials which become stronger with increasing slip rate, typically then get weaker as slip rate continues to increase to seismogenic slip rates. Thus the probability that a nucleating rupture can propagate sufficiently to generate a large earthquake depends upon its success in propagating fast enough through these materials in order to give them the required velocity kick. This propagation success is hence controlled by the relative and absolute size distributions of velocity-weakening and velocity- strengthening rocks within the fault zone. Statistical characterisation of the distribution of such contrasting properties within complex fault zones may allow for better predictive models of rupture propagation in the future and provide an additional approach to earthquake size forecasting and early warnings.

Description: Published

Description: 5-33

Description: 2.3. TTC - Laboratori di chimica e fisica delle rocce

Description: N/A or not JCR

Description: reserved

Description: The influx of fluids into fault zones can trigger two main types of weakening process that operate over different timescales and facilitate fault movement and earthquake nucleation. Short- and long-term weakening mechanisms along faults require a continuous fluid supply near the base of the brittle crust, a condition satisfied in the extended/extending area of the Northern Apennines of Italy. Here carbon mass balance calculations, coupling aquifer geochemistry to isotopic and hydrological data, define the presence of a large flux (c. 12 160 t/day) of deep-seated CO2 centred in the extended sector of the area. In the currently active extending area, CO2 fluid overpressures at 85% of the lithostatic load have been documented in two deep (4–5 km) boreholes. In the long-term, field studies on an exhumed regional low-angle normal fault show that, during the entire fault history, fluids reacted with fine-grained cataclasites in the fault core to produce aggregates of weak, phyllosilicate-rich fault rocks that deform by fluid assisted frictional–viscous creep at sub-Byerlee friction values (m , 0.3). In the short term, fluids can be stored in structural traps, such as beneath mature faults, and stratigraphical traps such as Triassic evaporites. Both examples preserve evidence for multiple episodes of hydrofracturing induced by short-term cycles of fluid pressure build-up and release. Geochemical data on the regional-scale CO2 degassing process can therefore be related to field observations on fluid rock interactions to provide new insights into the deformation processes responsible for active seismicity in the Northern Apennines

Description: Published

Description: 175-194

Description: 4.5. Degassamento naturale

Description: N/A or not JCR

Description: reserved