Debora Presti

Probabilistic Location of Seismic Sequences in Heterogeneous Media

We have developed a method for the Bayesian location of seismic sequences in 3D heterogeneous velocity models. The method is based on a Bayesian algorithm for single earthquake location. Joint location of seismic sequences is performed by summing the probability density functions for individual earthquake locations. One of the main features of the method is the ability to accumulate travel times in the heterogeneous model, as a function of both the seismic stations and the grid points, simulating the seismogenic volume. The ability to compute travel times just once, separately from the location process, allows for fast 3D locations. This is a very attractive feature for real-time monitoring. Probabilistic location of seismic sequences, visualized in terms of contours of earthquake density, moment, and energy release, etc., can be obtained to give a much more seismotectonic insight than classical location algorithms. The continuous character of the output quantities is particularly indicated for seismic-network-testing purposes. It is in fact possible to compute the response of the location procedure, given the seismic network geometry, to various input earthquake distributions. Some applications of the method are also shown in this work, both to the location of synthetic earthquakes and of real sequences. Seismic sequences at Vesuvius and Campi Flegrei volcanic areas (southern Italy) have been located by the probability algorithm, allowing a better picture of the primary seismogenic structures. We also apply the method to assess the resolution of the operating monitoring networks at the two areas.

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.

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.

Seismogenic stress field estimation in the Calabrian Arc region (south Italy) from a Bayesian approach

A new high-quality waveform inversion focal mechanism database of the Calabrian Arc region has been compiled by integrating 292 mechanisms selected from literature and catalogs with 146 newly computed solutions. The new database has then been used for computation of posterior density distributions of stress tensor components by a Bayesian method never applied in south Italy before the present study. The application of this method to the enhanced database has allowed us to provide a detailed picture of seismotectonic stress regimes in this very complex area where lithospheric unit configuration and geodynamic engines are still strongly debated. Our results well constrain the extensional domain of Calabrian Arc and the compressional one of the southernmost Tyrrhenian Sea. In addition, previously undetected transcurrent regimes have been identified in the Ionian offshore. The new information released here will furnish useful tools and constraints for future geodynamic investigations.

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,