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.

A Reversed Hierarchy of Surface Ruptures during Normal Faulting Earthquakes: the 6 April 2009, L'aquila (central Italy) Earthquake (mw 6.3)

BONINI, Lorenzo;
2013-01-01

Abstract

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.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11368/2836239
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