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

Fault plane orientations of microearthquakes at Mt. Etna from theinversion of P-wave rise times

A crucial point in the analysis of tectonic earthquakes occurring in a volcanic area is the inference of the orientation of the structures along which the ruptures occur. These structures represent zones of weakness which could favor the migration of melt toward the surface and the assessment of their geometry is a fundamental step toward efficient evaluation of volcanic risk. We analyzed a high-quality dataset of 171 lowmagnitude, tectonic earthquakes that occurred at Mt. Etna during the 2002–2003 eruption. We applied a recently developed technique aimed at inferring the source parameters (source size, dip and strike fault) and the intrinsic quality factor Qp of P waves from the inversion of rise times. The technique is based on numerically calibrated relationships among the rise time of first P waves and the source parameters for a circular crack rupturing at a constant velocity. For the most of the events the directivity source effect did not allow us to constrain the fault plane orientation. For a subset of 45 events with well constrained focal mechanisms we were able to constrain the “true” fault plane orientation. The level of resolution of the fault planes was assessed through a non linear analysis based on the random deviates technique. The significance of the retrieved fault plane solutions and the fit of the assumed source model to data were assessed through a χ-square test. Most of the retrieved fault plane solutions agree with the geometrical trend of known surface faults. The inferred source parameters and Qp are in agreement with the results of previous studie

Fault plane orientation of microearthquakes at Mt. etna from the inversion of P-wave rise times

crucial point in the analysis of tectonic earthquakes occurring in a volcanic area is the inference of the orientation of the structures along which the ruptures occur. These structures represent zones of weakness which could favor the migration of melt toward the surface and the assessment of their geometry is a fundamental step toward efficient evaluation of volcanic risk. We analyzed a high-quality dataset of 171 low-magnitude, tectonic earthquakes occurred at Mt. Etna during the 2002-2003 eruption. We applied a recently developed technique aimed at inferring the source parameters (source size, dip and strike fault) and the intrinsic quality factor Qp of P waves from the inversion of rise times. The technique is based on numerically calibrated relationships among the rise time of first P waves and the source parameters for a circular crack rupturing at a constant velocity. For the most of the events the directivity source effect did not allow us to constrain the fault plane orientation. For a subset of 45 events with well constrained focal mechanisms we were able to constrain the “true” fault plane orientation. The level of resolution of the fault planes was assessed through a non linear analysis based on the random deviates technique. The significance of the retrieved fault plane solutions and the fit of the assumed source model to data was assessed through a χ-square test. Most of the retrieved fault plane solutions agree with the geometrical trend of known surface faults. The inferred source parameters and Qp are in agreement with the results of previous studies

La sequenza sismica nel versante nord-occidentale dell'Etna del 19-27 Dicembre 2009 : evidenze di ricarica magmatica profonda?

E’ stata analizzata la sequenza sismica che ha interessato il versante nord-occidentale dell’Etna nel periodo 19-27 dicembre 2009 (Fig. 1). Essa è stata caratterizzata da oltre 400 scosse di magnitudo compresa tra 1.0 e 4.8, localizzate ad una profondità tra 20 e 30 km, con un rilevante rilascio energetico, come si osserva dalla distribuzione temporale del numero delle scosse e dell’energia ad esse associata nel tempo (Fig. 2). È interessante notare come l’energia rilasciata durante la sequenza risulti essere quasi il triplo dell’energia del periodo sineruttivo 2008, pur essendo pressoché uguale il numero di scosse registrate. In questo settore dell’area etnea, caratterizzato da sismicità profonda, poco frequente e di modesta energia, la modalità di rilascio sismico della sequenza in oggetto costituisce un elemento di novità. Infatti, più del 50% delle scosse si sono verificate nel corso delle prime 24 ore, come tipicamente osservato nel corso di sciami vulcanici sineruttivi. E’ importante evidenziare che nell’area etnea eventi sismici con profondità focali comprese tra i 10 e i 30 km vengono considerati dei veri e propri “markers” di attività vulcanica (e.g. Puglisi et al., 2001), in quanto si verificano abbastanza regolarmente durante i periodi intra-eruttivi e possono essere messi in relazione con i meccanismi di ricarica magmatica (e.g. Bonaccorso, 2001). Essi sono principalmente localizzati nei settori occidentale e meridionale del vulcano lungo strutture orientate NO-SE e NNO-SSE e, occasionalmente, lungo strutture orientate NE-SO (Patanè et al., 2004). Pertanto è ragionevole ipotizzare che il fenomeno oggetto del presente studio possa essere ricondotto ad una fase di ricarica profonda del sistema magmatico etneo

Instrumental seismic catalogue of Mt. Etna earthquakes (Sicily, Italy): ten years (2000-2010) of instrumental recordings

Instrumental seismic catalogues are an essential tool for the zonation of the territory and the production of seismic hazard maps. They are also a valuable instrument for detailed seismological studies regarding active volcanoes and, above all, for interpreting the magma dynamics and the evolution of eruptive phenomena. In this paper, we show the first instrumental earthquake catalogue of Mt. Etna, for the period 2000-2010, with the purpose of producing a homogeneous dataset of 10 years of seismological observations. During this period, 16,845 earthquakes have been recorded by the seismic network run by the Istituto Nazionale di Geofisica and Vulcanologia, Osservatorio Etneo, in Catania. A total of 6,330 events, corresponding to approximately 40% of all earthquakes recorded, were located by using a one-dimensional VP velocity model. The magnitude completeness of the catalogue is equal to about 1.5 for the whole period, except for some short periods in 2001 and 2002-2003 and at the end of 2009. The reliability of the data collected is supported by the good values of the main hypocentral parameters through the time. The spatial distribution of seismicity allowed the highlighting of several seismogenetic areas characterized by different seismic rates and focal depths. This seismic catalogue represents a fundamental tool for several research aiming to a better understanding of the behavior of an active volcano such as Mt. Etna

Seismological constraints for the dyke emplacement of the July-August 2001 lateral eruption at Mt. Etna volcano, Italy

In this paper we report seismological evidence regarding the emplacement of the dike that fed the July 18 - August 9, 2001 lateral eruption at Mt. Etna volcano. The shallow intrusion and the opening of the eruptive fracture system, which mostly occurred during July 12, and July 18, were accompanied by one of the most intense seismic swarms of the last 20 years. A total of 2694 earthquakes (1 £ Md £ 3.9) were recorded from the beginning of the swarm (July 12) to the end of the eruption (August 9). Seismicity shows the upward migration of the dike from the basement to the relatively thin volcanic pile. A clear hypocentral migration was observed, well constraining the upwards propagation of a near-vertical dike, oriented roughly N-S, and located a few kilometers south of the summit region. Earthquake distribution and orientation of the P-axes from focal mechanisms indicate that the swarm was caused by the local stress source related to the dike intrusion

TOMO-ETNA experiment at Etna volcano: Activities on land

In the present paper we describe the on-land field operations integrated in the TOMO-ETNA experiment carried out in June-November 2014 at Mt. Etna volcano and surrounding areas. This terrestrial campaign consists in the deployment of 90 short-period portable three-component seismic stations, 17 broadband seismometers and the coordination with 133 permanent seismic station belonging to Italy’s Istituto Nazionale di Geofisica e Vulcanologia (INGV). This temporary seismic network recorded active and passive seismic sources. Active seismic sources were generated by an array of air-guns mounted in the Spanish oceanographic vessel “Sarmiento de Gamboa” with a power capacity of up to 5200 cubic inches. In total more than 26,000 shots were fired and more than 450 local and regional earthquakes were recorded. We describe the whole technical procedure followed to guarantee the success of this complex seismic experiment. We started with the description of the location of the potential safety places to deploy the portable network and the products derived from this search (a large document including full characterization of the sites, owners and indication of how to arrive to them). A full technical description of the seismometers and seismic sources is presented. We show how the portable seismic network was deployed, maintained and recovered in different stages. The large international collaboration of this experiment is reflected in the participation of more than 75 researchers, technicians and students from different institutions and countries in the on-land activities. The main objectives of the experiment were achieved with great success.PublishedS04272SR. VULCANI - Servizi e ricerca per la SocietàJCR Journalope

Seismic site characterization of the Kastelli (Kissamos) Basin in northwest Crete (Greece): Assessments using ambient noise recordings

Crete is actively seismic and site response studies are needed for estimating local site conditions subjected to seismic activity. In order to collect basic data, we performed ambient noise recordings to estimate the site response of the surface and near subsurface structure of the small-scale Kastelli Basin in northwest Crete. The spatial horizontal to vertical spectral ratios (HVSR) resonance pattern of the investigated sites in the centre of the Basin consists of either one or two peaks divided into low to high frequency range in different sites as follows: (a) in some sites only one amplified peak at low frequencies (0.6–1.2 Hz), (b) in other sites only one amplified peak at medium frequencies (2.9–8.5 Hz) and (c) in yet other sites two amplified peaks in the low to high frequency range (0.6–15.5 Hz). The investigated sites are amplified in the frequency range 0.6–15.5 Hz, while the amplitude reaches to a factor of 4 in the spectral ratios. The one HVSR amplified peak at low frequencies is related to locally soft or thick Quaternary deposits. Microtremors were measured in the coastal northwest part of the Basin in a well—lithified Cretaceous limestone site characterized by fractures and faults striking predominantly in a sector NNE to NNW. Sites of one amplified peak at medium frequencies are extended from coastal northwest to southwest delineating a structure striking to NNW. The two amplified peaks are attributed to shallow subsurface heterogeneities/irregularities, locally induced by fault zones and to the overlying Quaternary deposits. Spatial HVSR variations in the frequency and HVSR shape delineate four structures striking NNE, NNW and in a sector NW to WNW, crosscutting the dense populated Basin suggesting that microtremors could be a valuable tool for providing a first approximation of fault zone delineation at least for the Kastelli-Kissamos Basin. The Basin is classified into the X soil category of the Greek Seismic Code 2000.This work was implemented through the project entitled “Interdisciplinary Multi-Scale Research of Earth-quake Physics and Seismotectonics at the Front of the Hellenic Arc (IMPACT-ARC)” in the framework of action “ARCHIMEDES III—Support of Research Teams at TEI of Crete” (MIS380353) of the Operational Program “Education and Lifelong Learning” and is co-financed by the European Union (European Social Fund) and Greek national fund

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 ?1. 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

Stress tensor computations at Mount St. Helens (1995-1998)

Fault plane solutions of 459 events occurring between 1995 and 1998 at Mount St. Helens (State of Washington, Northwest U.S.A.) were considered in order to infer the state of stress beneath the volcano. These events occurred in two distinct depth zones. The shallower zone is between 2 and 5.5 km, with shocks clustering in a tight cylindrical distribution about 1 km in radius directly beneath the crater. The deeper events are spread over a larger volume from 5.5 to 10 km depth and surround an aseismic zone below and slightly west of the lava dome. Faulting is characterized by a mixture of strike-slip, reverse and normal faults with maximum compression axes which do not cluster around a single direction. In the deep zone, between 5.5 and 10 km, P axes define a wheel-spoke pattern pointing radially away from the center of the aseismic zone. The 459 fault plane solutions were inverted for stress tensor parameters using the algorithm of Gephart and Forsyth. The inversion of the whole data set revealed that faulting was not produced by a uniform stress distribution. The subdivision of the zone into smaller volumes significantly reduced misfit and confidence areas of the solutions, whereas temporal subdivision of the sample did not lead to significant improvements in terms of stress uniformity. We suggest that the inhomogeneous stress field is consistent with a varying pressure source originating from the inferred crustal magma chamber and a thin conduit extending above it