Giancarlo Neri

Seismic strain and seismogenic stress regimes in the crust of the southern Tyrrhenian region

Abstract An investigation has been performed to identify and characterize the seismic deformation zones active over the last decades in the region of Italy that has experienced the strongest seismicity during the last centuries. The study is based on the estimate of hypocenter locations, fault plane solutions, seismogenic stress and seismic strain tensor orientations carried out using the entire dataset of the national and local seismic networks, and the recently improved three-dimensional (3D) crustal velocity model of the study area. A series of simulation tests have been performed to evaluate the significance of the earthquake space distribution obtained and whether it was influenced by network geometry problems related to the sea and the lack of ocean bottom seismometers. In the sectors where hypocentral location was synthetically proved to be reliable, space distributions of earthquakes located with epicenter and focal depth errors less than 3 and 4 km, respectively, have been compared with local geology in order to identify seismogenic faults. The dataset of 32 fault plane solutions estimated with fault parameter errors less than 20° has been used to investigate space variations of seismogenic stress and seismic strain orientations over the study area. Stress was found to be uniform in the Messina Strait and southern Calabria where inversion of the available set of 11 fault plane solutions showed clear evidence of an extensional regime. The different orientations of the minimum compressive stress and strain found in this sector, together with the information available on local geology and tectonics, lead us to propose that the seismicity occurring over the last decades in the Messina Strait and southern Calabria was not in general produced at the main faults, but at minor faults activated by the main tectonic stress field acting in the area. To the west, in the sector including western Etna, the Nebrodi chain and the western Aeolian Islands, analysis of the available set of 16 fault plane solutions revealed a certain degree of stress heterogeneity with an apparent prevalence of north–south compression. This east to west change of stress–strain regimes is evaluated in the light of current hypotheses regarding the geodynamics of the study region.

GEODYNAMIC IMPLICATIONS OF EARTHQUAKE DATA IN THE SOUTHERN TYRRHENIAN SEA

Abstract Data from short-period seismic stations operating in Calabria, Sicily and the Aeolian Islands have been used to investigate earthquake properties of the southern Tyrrhenian lithosphere. The results have been analyzed, taking into account other geophysical and geological information available in the literature for the same region, with the main purpose of contributing to the definition of local geodynamic processes. Space distribution and focal mechanisms of earthquakes support the hypothesis of southeastward migration of the southern Tyrrhenian lithosphere, shared by several authors in their tectonic modelling of the area. We find that most seismicity appears to be related to two main faults crossing the Aeolian Island archipelago (Sisifo and Vulcano) and propose that these faults are important dislocation structures associated with the lithosphere migration process. We also give a detailed picture of the seismic behaviour of the above-mentioned fault systems by analyzing the earthquake depth distribution, focal mechanisms, seismic clustering and strain release. Some degree of activity was also detected at the eastern boundary of the southern Tyrrhenian area (Calabrian Arc). In particular, our results demonstrate that in this highly stressed zone of contact between different tectonic units (Tyrrhenian and Ionian basins) strain is released under seismic form up to depths of the order of 25 km in the crust and not just in the shallowest structures as proposed by other authors.

A Possible Seismic Gap within a Highly Seismogenic Belt Crossing Calabria and Eastern Sicily, Italy

A review of the seismic, geologic, and geodynamic information available for the Calabro-Sicilian region of southern Italy leads us to suggest a unifying view of the larger ( M ≥ 7) earthquakes that have occurred there in the last millennium coincidentally concentrated in the last 3.7 centuries. The seismicity coincides with a narrow curvilinear extensional belt that passes through western Calabria and eastern Sicily (wces belt) and which includes a nearly continuous north–south succession of primarily east-dipping normal faults. In our reconstruction of the seismotectonic process the faulting is activated by west-northwest–east-southeast extension induced by residual rollback of the Ionian subduction slab. Our analysis of the space-time distribution of strong earthquakes indicates a zone of conspicuous aseismicity ( M > 4.5 since 1700, M ≥ 7 since 1000) along the belt, corresponding to the 30-km-long Scaletta-Fiumefreddo segment of the Messina–Fiumefreddo fault in eastern Sicily. Moreover, because estimated recurrence times are on the order of a millennium for M 7 earthquakes along different parts of the wces belt, and because historical data for destructive earthquakes in the first millennium a.d. are not detailed enough to allow reliable identification of the source zones, we cannot definitely state whether or not there is a late-stage seismic gap for a large earthquake in eastern Sicily. By applying standard relationships, the potential for a magnitude 7 earthquake can be estimated for the Scaletta–Fiumefreddo 30-km-long normal fault segment. The level of seismic activity has been low in the possible gap area in the past two decades when the upgraded local network has detected tens of events above magnitude 2.5 with values up to 3.7. Hypocenter locations of these events seem to delineate the deep geometry of two faults reported in the surface geologic maps, one of which is the Scaletta–Fiumefreddo silent fault. Coulomb stress changes not larger than +0.6 bar produced on the silent fault by the most recent M 7 regional earthquake (1908), and the substantially nil Coulomb stress change on the same fault by M 7 earthquakes of 1693 and 1783, imply relatively small perturbation by previous earthquakes to the silent fault compared with the Coulomb stress perturbation of 2–2.5 bars estimated by other investigators on the 1908 earthquake source caused by major earthquakes of the previous centuries.

CRUSTAL STRUCTURE AND SEISMICITY OF SOUTHERN TYRRHENIAN BASIN

Abstract In this paper all the travel-time data from crustal earthquakes that occurred in the period 1978–1991 along the southern Tyrrhenian sea have been re-examined. In particular, 410 local earthquakes recorded at a minimum number of seven seismic stations and 52 shots fired on sea have been utilized to infer a tomographic image of the velocity model beneath a region located in the domain 37°41′–39°10′N and 14°10′–16°44′E. These earthquakes are selected from a database containing about 1000 local earthquakes. The results from the inversion of about 9000 travel-times confirm the strong lateral variations of the velocity model relative to compressional and shear waves. Below the volcanic center of the Aeolian Islands, in the upper crust, several low velocity bodies are found, in agreement with features inferred from seismicity and other geophysical data. The 3-D velocity images also show a thickening of the crust from the center of the Tyrrhenian sea towards the coasts of Sicily and Calabria. Finally hypocenter locations derived from inversion, which appear to be significantly less scattered than using 1-D velocity models, help to identify with some accuracy two notable fault systems of the southern Tyrrhenian basin.

Reply to comment by Andrea Argnani et al. on ''On the cause of the 1908 Messina tsunami, southern Italy''

Within minutes after the passage of the seismic waves, atsunami with maximum observed runup of almost 12 m hitthe coasts of Calabria and Sicily [Platania, 1909; Baratta,1910]. Soon after the catastrophic events, Omori [1909]concluded that: ‘‘One remarkable fact is that the tsunamiwasstrongestatthoseplaceswheretheearthquakeshockwasnot most violent, indicating the probable non-coincidencein position of the origin of the earthquake with that ofthe tsunami’’. By using computer simulations, Tinti andArmigliato [2003] reached a similar conclusion: ‘‘The firstconclusion we arrive at is that it is difficult to find a singlesource matching simultaneously both tsunami and levelingdata’’. Recently, we presented new results supporting thehypothesis that the tsunami was generated by a submarinelandslide triggered by the earthquake at c. 40 km from theepicentral area [Billi et al., 2008]. Argnani et al. [2009] nowproduce new marine data and numerical models to questionsome of our inferences. Here we reply to their criticisms.

Earthquake clustering on volcanic and tectonic structures

Abstract Temporal clustering of seismic shocks is investigated using the Generalized Poisson Process and assuming that grouped events are distributed according to Riemann statistics. The E-parameter (inversely proportional to the clustering level) has been calculated for both volcanic and tectonic earthquakes listed in the catalogues of the Aeolian Island local network (Southern Italy). E-values from other investigations have also been used in order to extend the analysis. Clustering is generally higher for volcanic earthquakes than for tectonic earthquakes. In volcanic regions, the tendency of shocks to group in time increases during eruptive phases with respect to quiescent periods. A similar relationship between earthquake clustering and the activity state of seismogenetic structures may be preliminarily hypothesized in tectonic environments. These findings reveal an interesting analogy between seismic clustering (1/E) and the coefficient b of the Gutenberg-Richter relationship which “controls” the earthquake distribution in the magnitude domain. A possible explanation of the behaviour of the E and b parameters is proposed, advancing some hypotheses on the role both of rock fracturing level and stress intensity, and on the combined effect of these factors on the statistical properties of earthquake sequences.

Seismotomographic detection of major structural discontinuity in northern Sicily

We present the results of tomographic inversion computed with the use of the LOTOS code for Sicily and surroundings, a region of great geodynamic interest located on the Nubia-Europe margin where previous analyses have progressively improved the knowledge of seismic velocity structure without, however, permitting fine detection of tectonic units and structural discontinuities. We used LOTOSs devices for inversion, grid rotation and adaptation to ray density for application to a dataset of 7105 local earthquakes of the period 1990-2012. Our tomographic model highlights a previously undocumented major discontinuity which is located approximately along the northern coast of Sicily and is characterized by a sudden transition from low velocity imbricate thrust sheets and accretionary wedge in mainland Sicily (to the south) to relatively high velocity Tyrrhenian continental crust (to the north). Combining this finding with available geological and geodynamic information, we conclude that this northern Sicily seismic velocity discontinuity, which approximately corresponds to a regional fault system known as Kumeta-Alcantara, may have played a major role in the Miocene to Middle Pliocene, when lithosphere tearing occurred between the Tyrrhenian sea and Sicily in response to trench retreat. The more recent geodynamic settings of northern Sicily and the southern Tyrrhenian can be unravelled from Quaternary geological observations, seismicity and GPS data, which indicate that (i) the northern Sicily discontinuity has ceased to be active in more recent times; and (ii) the reorganized slow convergence of Nubia with respect to Europe is currently accommodated ~100 km north of Sicily, along the east-trending seismogenic belt enclosing Ustica and the Aeolian Islands.

Present-day kinematics and deformation processes in the southern Tyrrhenian region: new insights on the northern Sicily extensional belt

We performed a new analysis of updated and accurate sets of seismic and GNSS data relative to the southern Tyrrhenian region. Detailed velocity field and crustal strain distribution coming from integration of episodic and continuous measurements at more than 160 geodetic sites (spanning the 1994-2015 period) have been evaluated together with the spatial distribution of recent seismicity and an updated catalogue of waveform inversion fault-plane solutions relative to the period 1976-2014. In agreement with previous investigations, we have found that the kinematics of the study area is quite homogeneous except for the north-eastern corner of Sicily which moves almost coherently with southern Calabria in response to the SE-ward rollback of the Ionian slab. The rest of the study region shows a NNW-trending velocity field in agreement with the direction of the Nubia-Eurasia convergence and it is mainly interested by a major compressive domain. NNW-oriented compression is particularly highlighted by seismic data along the E-W trending seismic belt located in the southern Tyrrhenian Sea. In the framework of such compressive regime, the E-W trending extensional domain of northern Sicily is also clearly depicted both by seismic and geodetic data. The cause of this extensional domain framed inside a mainly compressive one represents an open question in the recent scientific debate. Comparisons between our results and literature information on regional geology and crustal structure led us to investigate whether the extension could occur as local response to the thrusting dynamics of the southern Tyrrhenian belt, favoured by the presence of pre-existing weakness zones. We then propose a first attempt to evaluate such a possible causal relationship by means of Finite Element Method (FEM) and Coulomb Stress Change (CSC) modelling. In particular, we adopted a FEM approach to investigate the deformation pattern produced by thrust faulting of southern Tyrrhenian belt, along a 2D profile crossing both the compressive belt and the extensional one in northern Sicily. We also estimated the CSC due to the thrust faulting on normal receiving faults fairly reproducing pre-existing structures of northern Sicily. Modelling results indicate that the thrust faulting activity along the Southern Tyrrhenian compressive margin could be effective in promoting extensional processes in northern Sicily. We have so shown that the local response to thrust faulting activity may concur, even in combination with other processes, to generate the crustal stretching of northern Sicily.

Estimating Stability and Resolution of Waveform Inversion Focal Mechanisms

The main aim of this study is to describe several tools for testing the stability and resolution of waveform inversion focal mechanisms already successfully adopted for crustal earthquakes occurred in the Calabrian Arc region,