Tectonic evidence for the ongoing Africa-Eurasia convergence in central Mediterranean foreland areas: A journey among long-lived shear zones, large earthquakes, and elusive fault motions.Daniela Di Bucci, Pierfrancesco Burrato, Paola Vannoli, Gianluca Valensise
We investigate the role of the Africa-Eurasia convergence in the recent tectonic evolution of the central Mediterranean. To this end we focused on two sectors of the Adriatic-Hyblean foreland of the Apennine-Maghrebian chain as they allow tectonic evidence for relative plate motions to be analyzed aside from the masking effect of other more local tectonic phenomena (e.g., subduction, chain building, etc.). We present a thorough review of data and interpretations on two major shear zones cutting these foreland sectors: the E-W Molise-Gondola in central Adriatic and the N-S Vizzini-Scicli in southern Sicily. The selected foreland areas exhibit remarkable similarities, including an unexpectedly high level of seismicity and the presence of the investigated shear zones since the Mesozoic. We analyze the tectonic framework, active tectonics, and seismicity of each of the foreland areas, highlighting the evolution of the tectonic understanding. In both areas, we find that current strains at midcrustal levels seem to respond to the same far-field force oriented NNW-SSE to NW-SE, similar to the orientation of the Africa-Eurasia convergence. We conclude that this convergence plays a primary role in the seismotectonics of the central Mediterranean and is partly accommodated by the reactivation of large Mesozoic shear zones.
Present-day stress in the surroundings of 2009 L’Aquila seismic sequence (Italy)
AUTORI: M.T. Mariucci, P. Montone, S. Pierdominici
Geophysical Journal International, in press
Seismotectonic setting of the area. (a) 170 TDMT T-axis orientations of the 2009 L’Aquila sequence (http://www.eas.slu.edu/Earthquake Center/MECH.IT/); CMT focal mechanisms of the three mainshocks (A, B, C; Pondrelli et al. 2010); σ3 orientation from the 1992, 1994 and 1996 seismic sequences (Boncio et al. 2004); the Shmin orientation inferred from breakout analysis in the two deep wells Campotosto (C1) and Varoni (V1) and the major active normal and thrust faults. In the inset: the rose plot shows the orientation of the 186 coseismic ruptures identified at surface (Emergeo Working Group 2010). (b) Azimuth frequency histogram of TDMT T-axis orientations; the red lines represent the T-axis orientations from CMT data (Pondrelli et al. 2010). (c) The chart shows the TDMT T-axis azimuth distribution versus depth together with the Shmin orientation of the boreholes (yellow stars) and the σ3 (red circle). (d) The chart shows TDMT T-axis versus their magnitude.
The axial zone of the Apenninic belt in central Italy is a tectonically active region affected by post-orogenic Quaternary extension. The present-day stress field is characterized by a minimum horizontal stress (Shmin) ∼NE–SW oriented, derived mainly from earthquake focal mechanisms and secondarily from borehole breakouts and fault data. The paper describes the computation of the Shmin orientation along two deep boreholes located in the vicinity of the area hit by the 2009 April 6, Mw 6.3 L’Aquila earthquake. The analysed wells show breakout zones at a depth range between 1.4 and 4.6 km, giving precious information on a depth interval usually not investigated by any other data. The results show an Shmin N81±22° and N74±10° oriented for Varoni 1 and Campotosto 1 wells, respectively. The comparison among the breakouts, the 2009 seismic sequence, the past seismicity and the Quaternary faults indicates a small rotation of Shmin orientation from ∼NE, in the southern, to ∼ENE in the northern sector of the study area, where the wells are located. These differences are linked both to the natural variations of data and to the orientation of the main tectonic structures varying from NW–SE in the Abruzzi region to ∼N–S moving toward the Umbro-Marchean Apennines. The identification of constant Shmin orientations with depth derived from all the examined active stress data, confirms the breakouts as reliable stress indicators also for aseismic areas.
link pdf Mar_et_al2010-GJI
Trace element behaviour during interaction between basalt and crustal rocks at 0.5-0.8 GPa: an experimental approach
Silvio Mollo, Valeria Misiti, Piergiorgio Scarlato
We experimentally investigate the trace elements behavior during the interaction between a partially molten crustal rock (MK72, meta-anorthosite) and a mantle-related magma (PF1, calc-alkaline basalt) at the Moho depth. Results show that a hybrid melt is formed at the basalt-crust contact, where plagioclase crystallizes (Figure a). This contact layer is enriched in trace elements which are incompatible with plagioclase crystals (Figure b). Under these conditions, the trace element diffusion coefficients are one order of magnitude larger than those expected. Moreover, the HFSE diffusivity in the hybrid melt is surprisingly higher than the REE one. Such a feature is related to the plagioclase crystallization that changes the trace elements liquid-liquid partitioning (i.e. diffusivity) over a transient equilibrium that will persist as long as the crystal growth proceeds.
Evidence for surface rupture associated with the Mw 6.3 L'Aquila earthquake sequence of April 2009 (central Italy)EMERGEO Working Group
An earthquake of Mw = 6.3 struck L'Aquila town (central Italy)
on 6 April 2009 rupturing an ca.18-km-long SW-dipping normal
fault. The aftershock area extended for a length of more than
35 km and included major aftershocks on 7 and 9 April and
thousands of minor events. Surface faulting occurred along the
SW-dipping Paganica fault with a continuous extent of
ca.2.5 km. Ruptures consist of open cracks and vertical dislocations
or warps (0.1m maximum throw) with an orientation of
N130°–140°. Small triggered slip and shaking effects also took
place along nearby synthetic and antithetic normal faults. The
observed limited extent and small surface displacement of the
Paganica ruptures with respect to the height of the fault scarps
and vertical throws of palaeo-earthquakes along faults in the
area put the faulting associated with the L'Aquila earthquake in
perspective with respect to the maximum expected magnitude
and the regional seismic hazard.
Clues to the identification of a seismogenic source from environmental effects. The case of the 1905 Calabria (southern Italy) earthquake.A. Tertulliani and L. Cucci
The September 8, 1905 Calabria (southern Italy) earthquake belongs to a peculiar family of highly destructive (I0=XI) seismic events, occurred at the dawning of the instrumental seismology, for which the location, geometry and size of the causative source are still substantially unconstrained. During the century elapsed since the earthquake, previous Authors identified three different epicenters that are more than 50 km apart and proposed magnitudes ranging from M≤6.2 to M=7.9. Even larger uncertainties were found when the geometry of the earthquake source was estimated. In this study, we constrain the magnitude, location and kinematics of the 1905 earthquake through the analysis of the remarkable environmental effects produced by the event (117 reviewed observations at 73 different localities throughout Calabria). The data used in our analysis include ground effects (landslides, rock falls and lateral spreads) and hydrological changes (streamflow variations, liquefaction, rise of water temperature and turbidity). To better define the magnitude of the event we use a number of empirical relations between seismic source parameters and distribution of ground effects and hydrological changes. In order to provide constraints to the location of the event and to the geometry of the source, we reproduce the coseismic static strain associated with different possible 1905 causative faults and compare its pattern to the documented streamflow changes. From the analysis of the seismically-induced environmental changes we find that: 1) the 1905 earthquake had a minimum magnitude M=6.7; 2) the event occurred in an offshore area west of the epicenters proposed by the historical seismic Catalogs; 3) it most likely occurred along a 100°N oriented normal fault with a left-lateral component, consistently with the seismotectonic setting of the area.
L’AQUILA (CENTRAL ITALY) EARTHQUAKES: THE PREDECESSORS OF THE APRIL 6, 2009 EVENT
Andrea Tertulliani, Antonio Rossi, Luigi Cucci, Maurizio Vecchi
On April 6th 2009 a Mw=6.3, Imax=9-10 earthquake struck the Abruzzi region of Central Italy (). We find similarities in some peculiar characteristics of the 2009 sequence and of some historical events occurred in the area, in particular in 1461 and 1762. We observe strong analogies in the distribution of the intensities and in the areas of the strongest effects produced by these shocks, as well as in the temporal evolution.
The 2009 quake severely devastated the town of L’Aquila (~73,000 inhabitants) and tens of villages located along the Middle Aterno Valley (hereinafter MAV, see Figure 1). One month after the event, its social impact was not yet definitive but certainly burdensome: 308 casualties, 47% homes damaged in the epicentral area, 20% heavily damaged (with an even worse situation in downtown L’Aquila), and about 40,000 people left homeless.
Although the magnitude of the event was not among the largest to have occurred in the Apennines, the Abruzzi earthquake can be considered as one of the most disastrous of the last century. The effects of the shaking were recorded not only in the epicentral zone but also in distant areas (e.g. as far as Rome - ~90 km from the epicenter – where it produced light damage), and heavily affected the cultural and socio-economic fabric of a wide region characterized by high seismic risk.