Search

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.

Nuove interpretazioni sulla sismotettonica dell’Abruzzo derivanti dallo studio del terremoto di Mw 6,3 del 6 aprile 2009 de L’Aquila Il distruttivo terremoto de L’Aquila del 6 Aprile 2009 (Mw 6,3) ha fornito importanti lezioni agli studiosi dei terremoti, alcune delle quali anche sorprendenti. Nonostante che dal punto di vista del rilascio energetico questo terremoto non sia stato tra i più forti, se confrontato con altri eventi recenti a scala mondiale e con altri forti eventi della storia sismica italiana, esso ha permesso la raccolta di una mole notevole di dati che hanno portato ad una revisione approfondita della geometria e in alcuni casi dello stile che caratterizza la fagliazione cosismica della regione abruzzese. Una delle osservazioni principali emerse a seguito degli studi condotti è che il terremoto è stato associato ad una fagliazione della superficie terrestre molto limitata, lanciando così un campanello d’allarme per quelli che sono i classici strumenti geologici tradizionalmente e più diffusamente utilizzati per l’identificazione delle faglie attive e sismogenetiche. L’Abruzzo è una regione che dal punto di vista della geologia dei terremoti mostra alcuni paradossi, il primo fra tutti è rappresentato dal fatto che nonostante esista una documentazione storica molto lunga sull’attività sismica, e che per i forti terremoti essa sia molto attendibile, il numero dei terremoti incluso nei cataloghi appare di molto inferiore al numero di eventi che ci si sarebbe attesi di registrare considerando il numero e la lunghezza delle faglie attive mappate, potenziali sorgenti di terremoti di M, Semestrale di studi e ricerche di geografia, N. 1 (2012): L'Aquila e il cratere sismico a cura di Cristiano Pesaresi

Fault propagation depends on the applied stress and on the mechanical characteristic of the host rocks. In an isotropic system, a fault develops with substantial constant rate and dip. Natural systems, differently, are far from being uniform and anisotropies affects the development of faults. Rocks variability, fluids, and pre-existing faults are examples of anisotropies that may accelerate, stop or decelerate the propagation of the faults from depth up to the surface. Active tectonic studies on extensional systems relay on mapping of their surface traces. In many active areas, however, the extension is a young process and the related faults may be still confined at depth or poorly expressed at the surface. A commonly used tool in earthquake geology is to infer the seismic potential of a fault from its surface displacement and from empirical relationships relating magnitude with fault length. This approach cannot be applied when active faults are blind. During the blind phase, the fault propagation is hard to be detected and also the role of pre-existing faults on the upward propagation of extensional faults cannot be observed. To understand these topics, we simulated a blind extensional fault interacting with differently oriented mechanical discontinuities in analogue models. This tool can effectively describe blind faults evolution, their surface effects, and their interactions with pre-existing discontinuities. We simulated seven different settings where pre-existing discontinuities are placed above the upper tip of a 45° dipping master fault. We quantified the strain distribution within the models and we reconstructed the evolution of the fault-related folds at significant steps. The results indicate that during the evolution of a growing extensional fault, pre-existing discontinuities affect the ability of new faults to reach the surface, the development of secondary brittle structures related to fault-propagation folding, and the shape of the related basins together with the migration through time of the folds axes. All these elements are differently affected depending on the geometrical relationships between the main fault and the inherited discontinuities. These results can provide clues for interpreting surface evidence while analyzing areas where active faults are blind or hidden.

How do we detect active faults in a rocky terrain that has been folded and faulted more than once in past tectonic regimes? And how can the hierarchy level of an active fault be assessed in a region cut by a number of faults at all scales? Does a fault that can be proved to be active under the current tectonic regime automatically qualify as the surface expression of a large seismogenic fault at depth? An increasing number of earthquakes worldwide has shown complex if not ambiguous relationships between their causative source at depth and tectonic features seen and mapped at the surface prior to the event. Understanding such relationships is fundamental in any seismic hazard analysis because the assessment of the earthquake potential of large continental faults still relies mostly on field investigations. Failure to appreciate the proper hierarchy of a set of active faults may lead to underestimate - but more frequently to severely overestimate - the true earthquake potential, especially in regions that are mostly prone to moderate-size earthquakes such as the Euro-Mediterranean region. The 6 April 2009, Mw 6.3, L'Aquila, central Apennines earthquake is currently the best documented normal faulting event worldwide. As such it affords a unique opportunity to explore the relationships between the activity of the source at seismogenic depth and its surface evidence. We used available high-resolution geologic, geodetic and seismological, co-seismic and post-seismic data aided by analogue modeling to reconstruct the geometry of the seismogenic rupture in relation with surface and sub-surface faults. We argue that the earthquake was caused by seismogenic slip at 3-10 km depth, strongly controlled by inherited discontinuities and expressed at the surface by pseudo-primary breaks resulting from coseismic crustal bending and by sympathetic slip on secondary faults. We used observations and models of the L'Aquila earthquake complemented by analogs from a number of other seismogenic areas to derive a scheme for hierarchizing all normal faults affecting the rock volume encompassed by the seismogenic source. Our model is based on the geometrical and kinematic properties of all faults and is aimed at explaining all surface occurrences related to faulting at depth in the frame of a single, mechanically coherent interpretative scheme. We remark that the combination of the age of existing faults with the geometry of the seismogenic source and with the vigorous bulk uplift of the entire region creates the conditions for a "reversed hierarchy", such that the most evident faults may be the least relevant to seismic hazard assessment and viceversa. Similar conditions are encountered in most Euro-Mediterranean countries and in many other seismic regions worldwide, calling for a reconsideration of data and methods used to derive active faulting data to be used in seismic hazard analyses.