Tiziana Tuvè

Seismic hydraulic fracture migration originated by successive deep magma pulses: The 2011–2013 seismic series associated to the volcanic activity of El Hierro Island

In this manuscript we present a new interpretation of the seismic series that accompanied eruptive activity off the coast of El Hierro, Canary Islands, during 2011–2013. We estimated temporal variations of the Gutenberg-Richter b value throughout the period of analysis, and performed high-precision relocations of the preeruptive and syneruptive seismicity using a realistic 3-D velocity model. Our results suggest that eruptive activity and the accompanying seismicity were caused by repeated injections of magma from the mantle into the lower crust. These magma pulses occurred within a small and well-defined volume resulting in the emplacement of fresh magma along the crust-mantle boundary underneath El Hierro. We analyzed the distribution of earthquake hypocenters in time and space in order to assess seismic diffusivity in the lower crust. Our results suggest that very high earthquake rates underneath El Hierro represent the response of a stable lower crust to stress perturbations with pulsatory character, linked to the injection of magma from the mantle. Magma input from depth caused large stress perturbations to propagate into the lower crust generating energetic seismic swarms. The absence of any preferential alignment in the spatial pattern of seismicity reinforces our hypothesis that stress perturbation and related seismicity, had diffusive character. We conclude that the temporal and spatial evolution of seismicity was neither tracking the path of magma migration nor it defines the boundaries of magma storage volumes such as a midcrustal sill. Our conceptual model considers pulsatory magma injection from the upper mantle and its propagation along the Moho. We suggest, within this framework, that the spatial and temporal distributions of earthquake hypocenters reflect hydraulic fracturing processes associated with stress propagation due to magma movement.

Improving Seismic Surveillance at Mt. Etna Volcano by Probabilistic Earthquake Location in a 3D Model

The increasing accuracy of 3D velocity models developed recently for Mt. Etna has enabled their use today in routine earthquake locations. In this work, we tested the potential and performance of a global-search probabilistic earthquake lo- cation method (NonLinLoc) in a 3D velocity model, to improve earthquake locations for seismic surveillance. In addition, NonLinLoc hypocenter locations and those ob- tained by standard iterative-linear 3D locations, SimulPS-14, have also been com- pared. To this end, a dataset of 328 selected earthquakes, occurring during the 2002-2003 Etna flank eruption, and the recent highest resolution 3D velocity model, have been used. The results revealed that the differences in hypocentral coordinates between the two methods are typically of the same order or smaller than the spatial location uncertainty. To evaluate the consistency of results between the two 3D location algorithms, synthetic datasets with real source-receiver configuration are also considered. Furthermore, by using NonLinLoc we estimated the influence of the source-receiver geometry on the quality of hypocenter locations. If we vary the net- work geometry in a dense and well-distributed network like at Etna, reducing the number of stations (by 20% and 50%), it is significant that no large systematic hypo- central shifts of the relocated earthquakes are observed if they occur within the net- work. NonLinLoc is a fast and promising approach for automatic earthquake locations and surveillance purposes at Mt. Etna, because (1) it works well with a reduced num- ber of seismic pickings, which are usually available in the automatic locations; (2) it is not particularly sensitive to tolerable levels of random noise in arrival times; and (3) it produces full location uncertainty and resolution information with respect to standard iterative-linear 3D locations.

A reappraisal of seismic Q evaluated at Mt. Etna volcano. Receipt for the application to risk analysis

A new approach in dealing with seismic risk in the volcanic areas of Italy, by taking into account the possible occurrence of damaging pre- or syn-eruptive seismic events, is exciting the scientific interest and is actually the topic developed in several research projects funded by the European Community (e.g., UPStrat-MAFA, www.upstrat-mafa.ov.ingv.it/UPstrat/) and the Civil Defense Department of Italy. To achieve this goal, it is necessary to have a detailed knowledge of the local attenuation-distance relations. In the present paper, we make a survey of the estimates of the seismic quality factor of the medium reported in literature for the Etna area. In the framework of a similar paper published for the Campi Flegrei zone in Southern Italy, we first review the results on seismic attenuation already obtained for Etna and then apply a standard technique to separately measure intrinsic and scattering attenuation coefficients from passive seismic data recorded by the Etna seismological network. Indications are then given for the correct utilization of the attenuation parameters to obtain the best candidate quality factor Q to be used in this area for seismic risk purposes.

When probabilistic seismic hazard climbs volcanoes: the Mt Etna case, Italy. Part I: model components for sources parametrization

The volcanic region of Mt. Etna (Sicily, Italy) represents a perfect lab for testing innovative approaches to seismic hazard assessment. This is largely due to the long record of historical and recent observations of seismic and tectonic phenomena, the high quality of various geophysical monitoring and particularly the rapid geodynamics clearly demonstrate some seismotectonic processes. We present here the model components and the procedures adopted for defining seismic sources to be used in a new generation of probabilistic seismic hazard assessment (PSHA), the first results and maps of which are presented in a companion paper, Peruzza et al. (2017). The sources include, with increasing complexity, seismic zones, individual faults and gridded point sources that are obtained by integrating geological field data with long and short earthquake datasets (the historical macroseismic catalogue, which covers about 3 centuries, and a highquality instrumental location database for the last decades). The analysis of the frequency–magnitude distribution identifies two main fault systems within the volcanic complex featuring different seismic rates that are controlled essentially by volcano-tectonic processes. We discuss the variability of the mean occurrence times of major earthquakes along the main Etnean faults by using an historical approach and a purely geologic method. We derive a magnitude–size scaling relationship specifically for this volcanic area, which has been implemented into a recently developed software tool – FiSH (Pace et al., 2016) – that we use to calculate the characteristic magnitudes and the related mean recurrence times expected for each fault. Results suggest that for the Mt. Etna area, the traditional assumptions of uniform and Poissonian seismicity can be relaxed; a time-dependent fault-based modeling, joined with a 3-D imaging of volcano-tectonic sources depicted by the recent instrumental seismicity, can therefore be implemented in PSHA maps. They can be relevant for the retrofitting of the existing building stock and for driving risk reduction interventions. These analyses do not account for regionalM > 6 seismogenic sources which dominate the hazard over long return times (≥ 500 years).

From Seismic Input to Damage Scenario: An Example for the Pilot Area of Mt. Etna Volcano (Italy) in the KnowRISK Project

In this paper we present a multidisciplinary approach aimed at assessing seismic risk regarding non-structural damage. The study has been carried out in the framework of the European KnowRISK Project and focuses on the pilot area of Mt. Etna volcano (Italy). Both instrumental data and as well as macroseismic observations provide unique opportunities for testing innovative and classical approaches for assessing seismic risk. Starting from the seismic hazard analysis, we first identify a test site (Zafferana) affected by non-structural damage. We produce seismic scenarios based on macroseismic and ground-motion data and finally obtain the relevant risk map using the Italian census data to classify buildings into vulnerability classes and a model to predict damage distribution.

Regionalization and dependence of coda Q on frequency and lapse time in the seismically active Peloritani region (northeastern Sicily, Italy)

The Peloritani region is one of the most seismically active regions in Italy and, consequently, the quantification of attenuation of the medium plays an important role for seismic risk evaluation. Moreover, it is necessary for the prediction of earth ground motion and future seismic source studies. An in depth analysis has been made here to understand the frequency and lapse time dependence of attenuation characteristics of the region by using the coda of local earthquakes. A regionalization is likewise performed in order to investigate the spatial variation of coda Q across the whole region. Finally, our results are jointly interpreted with those obtained from recently published 3D velocity tomographies for further insights.