High-Precision Relative Locations of Two Microearthquake Clusters in Southeastern Sicily, Italy

In November 1999 and in January 2000, two microearthquake swarms occurred in Southeastern Sicily (Italy). They were analytically located in the depth range 17-25 km, some kilometers northward from the buried front of a regional foredeep, below the active thrust zone of the Sicily mountain chain. Their hypocentral distribution showed two distinct clusters, and comparison of the waveforms revealed clearly that the two swarms formed two distinct families of multiplet events. This led us: i) to carry out a precise relocation relative to two chosen master events of the families, and ii) to better define the geometrical structure of the two clusters. The cross-spectral method was applied to obtain precise readings of the wave onsets. SH-wave onsets were used instead of P-waves, as they showed clearer onsets and a good signal-to-noise ratio. Residuals of the relative locations showed small values, no more than several meters on average. The vertical extent of the two relocated clusters was 500 m and 250 m, respectively, while the horizontal extent was 250 m. Hypocenters of the first cluster clearly delineate a NNW trending plane with almost vertical dip, matching one nodal plane of the focal mechanism obtained as a composite solution of all events of the cluster. Given the considerable gap angles, because of unfavorable network geometry with respect to the events, the stability of our results was tested carrying out a Montecarlo experiment. Varying the onset times randomly in the range of 5 ms, a dispersion of the locations less than 10 m in longitude, and less than 50 m both in latitude and depth was found. Similar results were obtained when comparing relocations carried out with different master events. Thus, the overall geometrical characteristics of the clusters were not affected seriously by random errors. Considering the geo-structural framework of the region, together with the location and time evolution of the two clusters, fluids of plutonic origin are suggested as the trigger mechanism

Current stress and strain-rate fields across the Dead Sea Fault System: Constraints from seismological data and GPS observations

The stress and strain-rate fields characterizing the Dead Sea Fault System are investigated by using seismological and geodetic observations. In order to assess spatial variations in the regional stress field, we compiled a multidisciplinary dataset of well-constrained horizontal indicators, by merging all available data reported in literature with the data obtained in this study through weighted stress inversions of focal plane solutions. Our findings indicate that the state of stress is characterized by the coexistence of a normal faulting stress regime with the primarily strike-slip one, according to the regional frame illustrated by previous geological and seismological observations. An updated velocity field computed from new observations and earlier published data depicts the general left-lateral motion of the Dead Sea Fault System well. In agreement with previous studies, we detected some differences in the slip-rate pattern between the northern and the southern sectors of the fault system. The geodetic strain-rate field highlights how much of the deformation is accommodated along the fault system itself in a narrow region. The comparison between the stress and the strain-rate directions reveals that both orientations are near-parallel, clearly indicating that present-day crustal stress and ground deformation patterns are chiefly driven by the same tectonic processes. (C) 2013 Elsevier B.V. All rights reserved

The lava flow invasion hazard map at Mount Etna and methods for its dynamic update

This special issue of Annals of Geophysics contains seventeen peer-reviewed papers that cover a wide variety of topics related to the V3-LAVA Project funded by the Italian Dipartimento della Protezione Civile in the framework of the 2007– 2009 Agreement with the Istituto Nazionale di Geofisica e Vulcanologia (INGV). The frequent eruptions of Mount Etna can produce lava flows that can cover distances long enough to invade vulnerable areas on the flanks of the volcano. These require improvements to our forecasting tools for the effective assessment of lava-flow hazards, to help the local authorities to make the necessary decisions during a volcanic eruption. The LAVA Project aims to develop, validate and unify methods for mapping the areas around Etna that are threatened by lava invasion within the next 50 years, and also within the immediate days after an eruption has begun. Both timescales of lava-hazard mapping call for estimations of the probabilities of vent openings – using geological evidence over the long-term, and monitoring data over the short-term

Ground deformation at Mt. Etna: a joint interpretation of GPS and InSAR data from 1993 to 2000

Combined GPS measurements and radar interferometry (InSAR) have been applied at Mt. Etna to study the ground deformation affecting the volcano both over the long- (1993-2000) and short-term (1997-1998 and 1998-2000). The aim was to better understand the dynamics of the volcano during the magma-recharging phase following the 1991-93 eruption. Since 1993, InSAR and GPS data indicate that Mt. Etna has undergone an inflation. A deep intrusion was detected by InSAR, on the western flank of the volcano, between March and May 1997. In the following months, this intrusion rose up leading to a seismic swarm occurring in January 1998 in the western sector. This now shallow intrusion is confirmed by GPS data. From 1998 to 2000, a general deflation affecting the upper part of the volcano was detected. Over the whole study period, a continuous eastward to south-eastward motion of the eastern sector of the volcano was also evidenced. The analytical inversions of GPS data inferred a plane dipping about 12°ESE, located beneath the eastern flank of the volcano at a depth of 1.4 km b.s.l.. The movement along this plane is able to reproduce the observed south-eastward motion of a sector bounded northward by the Pernicana fault, westward by the North-East Rift and the South Rift, and southward by the Mascalucia-Tremestieri-Trecastagni fault system. InSAR data have validated this model

Magma transport and storage at Mt. Etna (Italy): a review of geodetic and petrological data for the 2002-03, 2004 and 2006 eruptions

A detailed reconstruction of magma movements within the plumbing system of Mt. Etna volcano has been made by reviewing the eruptions occurring during the October 2002–December 2006 period. The availability of continuous GPS data allowed detecting at least ten different ground deformation stages, highlighting deflationary and inflationary episodes as well as the occurrence of a shallow dike intrusion. These data have been coupled with the available petrological datasets including major/trace elements and Sr-Nd-Pb isotope compositions for the volcanic rocks erupted in the 2002–2006 period. We identified two main magmatic reservoirs located at different depths along the plumbing system of the volcano. The former is located at a depth of ~ 7 km bsl and fed the 2001 and 2002–03 eruptions, while the latter, located from 3.5 to 5.5 km bsl, fed the 2004–05 and 2006 eruptions. Petrological characteristics of emitted products have been correlated with the inflation vs. deflation cycles related to the identified sources, providing evidence for changes through time of the evolutionary degree of the erupted magmas along with variations in their geochemical feature. Finally, we suggest that a modification of the deep plumbing system of the volcano might have occurred during the 2002–03 eruption, as a consequence of the major seaward motion of the eastern flank of the volcano

High-frequency spectral decay in P-wave acceleration spectra and source parameters of microearthquakes in southeastern Sicily, Italy

In widely used -2 source models the characteristics of high frequency radiation are described as being flat for frequencies between the source corner frequency and an upper limiting frequency fmax. Deviations from this behavior are described in a parameter which is understood as a general measure of the changes the signal undergoes on its way from the source to the receiver. In this study, we calculated  in Southeastern Sicily by using microearthquakes belonging to three different seismic sequences occurring in the area in 1990, 1999-2001, and 2002. The selected events form four different clusters whose seismic sources are located within a 2 km radius. Although the source-to-station paths are approximately the same inside a given cluster, the values of  change considerably at the same recording site from one event to another, also in the case of events having the same magnitude. We parameterized  in terms of event (E), and path (P and Diff) contributions. The term P represents the contribution on total  of both the whole source-to- station path and the near-surface geology, while Diff models the possible spatial variation in the parameter measured with respect to a reference source-station direction. Results show that the source contribution is not negligible and that there is a positive correlation with source size exists. Moreover, the hypothesis of a laterally homogeneous crustal structure within the area in question is not appropriate and significant variation in attenuating properties of the medium may occur in a very small distance range (also in the order of a few tens of meters). Our analysis suggests that the origin of the above mentioned variability is located near the recording site. Synthetic spectra are also computed in order to verify the actual significance of the parameterization employed and its capacity to separate the source and the path contribution to . We describe our spectra as a product of a Brune-type source spectrum and an exponential shaping term accounting for propagation effects. The seismic moments range between 3.8 ×1011 and 5.2 ×1013 N·m, the source radii range between 176 and 669 m, while the stress drop varies from 0.01 to 0.67 MPa

Seismological investigations in the Gioia Tauro Basin (southern Calabria, Italy)

This study provides new seismological information to characterize the seismically active area of the Gioia Tauro basin (southern Calabria, Italy). Seismic activity recorded by a temporary network from 1985 to 1994 was analyzed for focal mechanisms, stress tensor inversion, P-wave seismic attenuation and earthquake source parameters estimation. Fault plane solutions of selected events showed a variety of different mechanisms, even if a prevalence of normal dip-slip solutions with prevalent rupture orientations occurring along ca. NE-SW directions was observed. Stress tensor inversion analysis disclosed a region governed mainly by a NW-SE extensional stress regime with a nearly vertical σ1. These results are consistent with the structure movements affecting the studied area and with geodetic data. Furthermore, evaluation of P-waves seismic attenuation and earthquake source parameters of a subset of events highlighted a strong heterogeneity of the crust and the presence of fault segments and/or weakened zones where great stress accumulation or long-rupture propagation are hindered

Triggering mechanisms of static stress on Mount Etna volcano. An application of the boundary element method

In the last thirty years, numerous eruptions and associated deformation episodes have occurred at Mt. Etna volcano. Datasets recorded by continuous monitoring of these episodes provide a unique opportunity to study the relationships between volcanism, flank instability and faulting activity. We have investigated the stress triggering mechanism between magmatic reservoir inflation, intrusive episodes and flank dynamics. Using three-dimensional numerical Boundary Elements Models we simulated volcano-tectonic events and calculated Coulomb stress changes. Using this modeling approach, we analyzed four realistic scenarios that are representative of recent kinematics occurring at Mt. Etna. The main results obtained highlight how (1) the inflation of a deep spherical magma source transfers elastic stress to a sliding plane and faults (2) the opening of the NE Rift and S Rift (to a less efficient extent) favor movements of the instable sector and may encourage seismicity on the eastern flank faults, and (3) flank instability may trigger the uprising of magma. Defining the effects of the elastic stress transfer and relationships among the main forces acting on volcano, may help to forecast possible eruption scenarios during future episodes of unrest at Mount Etna and provide an important tool for decision makers during volcanic emergencies involving the highly populated areas of the volcano

How a complex basaltic volcanic system works: Constraints from integrating seismic, geodetic, and petrological data at Mount Etna volcano during the July-August 2014 eruption

Integrating geodetic, seismic, and petrological data for a recent eruptive episode at Mount Etna has enabled us to define the history of magma storage and transfer within the multilevel structure of the volcano, providing spatial and temporal constraints for magma movements before the eruption. Geodetic data related to the July-August 2014 activity provide evidence of a magma reservoir at similar to 4kmbelow sea level. This reservoir pressurized from late March 2014 and fed magmas that were then erupted from vents on the lower eastern flank of North-East Crater (NEC) and at New South-East Crater (NSEC) summit crater during the July eruptive activity. Magma drainage caused its depressurization since mid-July. Textural and microanalytical data obtained from plagioclase crystals indicate similar disequilibrium textures and compositions at the cores in lavas erupted at the base of NEC and NSEC, suggesting comparable deep histories of evolution and ascent. Conversely, the compositional differences observed at the crystal rims have been associated to distinct degassing styles during storage in a shallow magma reservoir. Seismic data have constrained depth for a shallow part of the plumbing system at 1-2kmabove sea level. Timescales of magma storage and transfer have also been calculated through diffusion modeling of zoning in olivine crystals of the two systems. Our data reveal a common deep history of magmas from the two systems, which is consistent with a recharging phase by more mafic magma between late March and early June 2014. Later, the magma continued its crystallization under distinct chemical and physical conditions at shallower levels

Seismic potential in the Italian Peninsula from integration and comparison of seismic and geodetic strain rates.

Seismological and geodetic data provide key information about the kinematics and active tectonics of plate margins. Focal solutions enable determining the directions in which the current tectonic stress acts when fault rupturing occurs; GPS measurements provide information on the crustal velocity field and on current interseismic strain rates. The comparison of the strain rates resulting from the two datasets provides further insight into how large an area is affected by aseismic deformation, which is a valuable indicator for seismic hazard mitigation and estimating the seismic potential. In this work, we investigate both seismic and geodetic strain rates and the combined field resulting from the joint inversion of the geodetic and seismic datasets, providing a picture of the overall deformation field and its variation during the last decades. In this way, we seek to give an overview of the seismic potential distribution across the Apennines and southern Italy, as a qualitative analysis of space-time variations in the released seismic strain rate, compared to the space-time distribution of the cumulated geodetic strain rate. The results show a variable distribution of the seismic efficiency over the peninsula. The Southern Apennines shows the greatest seismic potential, highlighting a significantly lower seismicity in the last two decades over an area affected by the highest total strain rates. The Messina Straits and eastern Sicily have a significant seismic potential, together with the Calabrian arc (from the Tindari-Letojanni and central Aeolian islands to the Mt. Pollino area), as a result of seismic gaps with respect to the combined strain rates in the investigated period. This long gap highlights the longer recurrence periods for the strongest earthquakes on this area. The central-northern Apennines and off-shore northern Sicily, show a lower seismic potential than central-southern Apennines, probably due to the more recent seismicity affecting these areas