ALBERT

Description: Over the past few years the assessment of the earthquake potential of large continental faults has increasingly relied on field investigations. State-of-the-art seismic hazard models are progressively complementing the information derived from earthquake catalogues with geological observations of active faulting. Using these observations, however, requires full understanding of the relationships between seismogenic slip at depth and surface deformation, such that the evidence indicating the presence of a large, potentially seismogenic fault can be singled out effectively and unambiguously. We used observations and models of the 6 April 2009, Mw 6.3, L'Aquila, normal faulting earthquake to explore the relationships between the activity of a large fault at seismogenic depth and its surface evidence. This very well-documented earthquake is representative of mid-size yet damaging earthquakes that are frequent around the Mediterranean Basin, and is somehow paradigmatic of the nature of the associated geologic evidence along with observational difficulties and ambiguities. Thanks to available high-resolution geologic, geodetic and seismological data aided by analogue modeling, we reconstructed the full geometry of the seismogenic source in relation with surface and sub-surface faults. We find that the earthquake was caused by seismogenic slip in the range 3–10 km depth, and that the slip distribution was strongly controlled by inherited discontinuities. We also contend that faulting was expressed at the surface by pseudo-primary breaks resulting from coseismic crustal bending and by sympathetic slip on secondary faults. Based on our results we propose a scheme for hierarchizing normal faults through which all surface occurrences related to faulting at depth can be interpreted in the frame of a single, mechanically coherent model. Appreciating such complexity is crucial to avoid severe over- or under-estimation of the local seismogenic potential.

Description: Over the past few years the assessment of the earthquake potential of large continental faults has increasingly relied on field investigations. State-of-the-art seismic hazard models are progressively complementing the information derived from earthquake catalogs with geological observations of active faulting. Using these observations, however, requires full understanding of the relationships between seismogenic slip at depth and surface deformation, such that the evidence indicating the presence of a large, potentially seismogenic fault can be singled out effectively and unambiguously. We used observations and models of the 6 April 2009, Mw 6.3, L'Aquila, normal faulting earthquake to explore the relationships between the activity of a large fault at seismogenic depth and its surface evidence. This very well-documented earthquake is representative of mid-size yet damaging earthquakes that are frequent around the Mediterranean basin, and was chosen as a paradigm of the nature of the associated geological evidence, along with observational difficulties and ambiguities. Thanks to the available high-resolution geologic, geodetic and seismological data aided by analog modeling, we reconstructed the full geometry of the seismogenic source in relation to surface and sub-surface faults. We maintain that the earthquake was caused by seismogenic slip in the range 3–10 km depth, and that the slip distribution was strongly controlled by inherited discontinuities. We also contend that faulting was expressed at the surface by pseudo-primary breaks resulting from coseismic crustal bending and by sympathetic slip on secondary faults. Based on our results we propose a scheme of normal fault hierarchization through which all surface occurrences related to faulting at various depths can be interpreted in the framework of a single, mechanically coherent model. We stress that appreciating such complexity is crucial to avoiding severe over- or under-estimation of the local seismogenic potential.

Description: Understanding the relationship between seismogenic slip at depth and surface deformation is fundamental in any seismic hazard analysis because the assessment of the earthquake potential of large continental faults relies largely on field investigations. The well-documented 6 April 2009, Mw 6.3, L'Aquila earthquake affords a unique opportunity to explore the relationships between the activity of the deep source and its surface evidence. We used available high-resolution geologic, geodetic and seismological data aided by analogue modeling to reconstruct the geometry of the seismogenic rupture in relation with surface and sub-surface faults. We contend that the earthquake was caused by a blind fault, controlled at depth by pre-existing discontinuities and expressed at the surface by pseudo-primary breaks resulting from coseismic crustal bending. Finally, we propose a scheme for hierarchizing normal faults that explains all surface occurrences related to blind faulting in the frame of a single, mechanically coherent, interpretative model. Failure to appreciate such complexity may result in severe over– or under-estimation of the local seismogenic potential.

Description: While scientists are paying increasing attention to the seismicity potentially induced by hydrocarbon exploitation, little is known about the reverse problem, i.e. the impact of active faulting and earthquakes on hydrocarbon reservoirs. The recent 2012 earthquakes in Emilia, Italy, raised concerns among the public for being possibly human-induced, but also shed light on the possible use of gas wells as a marker of the seismogenic potential of an active fold-and-thrust belt. Based on the analysis of over 400 borehole datasets from wells drilled along the Ferrara-Romagna Arc, a large oil and gas reserve in the southeastern Po Plain, we found that the 2012 earthquakes occurred within a cluster of sterile wells surrounded by productive ones. Since the geology of the productive and sterile areas is quite similar, we suggest that past earthquakes caused the loss of all natural gas from the potential reservoirs lying above their causative faults. Our findings have two important practical implications: (1) they may allow major seismogenic zones to be identified in areas of sparse seismicity, and (2) suggest that gas should be stored in exploited reservoirs rather than in sterile hydrocarbon traps or aquifers as this is likely to reduce the hazard of triggering significant earthquakes.

Description: While scientists are paying increasing attention to the seismicity potentially induced by hydrocarbon exploitation, so far, little is known about the reverse problem, i.e. the impact of active faulting and earthquakes on hydrocarbon reservoirs. The 20 and 29 May 2012 earthquakes in Emilia, northern Italy (Mw 6.1 and 6.0), raised concerns among the public for being possibly human-induced, but also shed light on the possible use of gas wells as a marker of the seismogenic potential of an active fold and thrust belt. We compared the location, depth and production history of 455 gas wells drilled along the Ferrara-Romagna arc, a large hydrocarbon reserve in the southeastern Po Plain (northern Italy), with the location of the inferred surface projection of the causative faults of the 2012 Emilia earthquakes and of two pre-instrumental damaging earthquakes. We found that these earthquake sources fall within a cluster of sterile wells, surrounded by productive wells at a few kilometres' distance. Since the geology of the productive and sterile areas is quite similar, we suggest that past earthquakes caused the loss of all natural gas from the potential reservoirs lying above their causative faults. To validate our hypothesis we performed two different statistical tests (binomial and Monte Carlo) on the relative distribution of productive and sterile wells, with respect to seismogenic faults. Our findings have important practical implications: (1) they may allow major seismogenic sources to be singled out within large active thrust systems; (2) they suggest that reservoirs hosted in smaller anticlines are more likely to be intact; and (3) they also suggest that in order to minimize the hazard of triggering significant earthquakes, all new gas storage facilities should use exploited reservoirs rather than sterile hydrocarbon traps or aquifers.