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  • 1
    Publikationsdatum: 2017-04-04
    Beschreibung: Mt Etna lies on the footwall of a large normal fault system, which cuts the eastern coast of Sicily and crosses the volcano eastern flank. These faults are responsible for both large magnitude historical earthquakes and smaller damaging seismic events, closer to the volcano. We investigate here the two-way mechanical coupling between such normal faults and Mt Etna through elastic stress transfer. The comparison between eruptive sequences and historical seismicity reveals that the large earthquakes which struck the eastern Sicily occurred after long periods of activity along the Mt Etna rift zone. The larger the erupted lava volumes, the stronger the earthquake. The smaller earthquakes located on the eastern flank of the volcano occur during periods of rift zone eruptions.We point out that the seismicity rates are well correlated with the rate of erupted lava. By modelling elastic stress changes caused by earthquakes and eruptions in a 3-D elastic half-space, we investigate their interaction. Earthquake dislocations create a vertical stress gradient along fissures oriented perpendicular to the minimum compressive stress and compress shallow reservoirs beneath the volcano. This may perturb the magmatic overpressures in the Etna plumbing system and influence the transport and storage of the magma as well as the style of the eruptions. Conversely, the large rift zone eruptions increase up to several tenths MPa the Coulomb stress along the eastern Sicily normal fault system and may promote earthquakes. We show that the seismic activity of the normal faults that cut the eastern flank of the volcano is likely to be controlled by Coulomb stress perturbations caused by the voiding of shallow reservoirs during flank eruptions.
    Beschreibung: Published
    Beschreibung: 697-718
    Beschreibung: reserved
    Schlagwort(e): Coulomb stress modelling ; earthquakes–volcanoes interaction ; historical eruptions ; Mt Etna ; stress transfer ; 04. Solid Earth::04.06. Seismology::04.06.08. Volcano seismology
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
    Standort Signatur Erwartet Verfügbarkeit
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  • 2
    Publikationsdatum: 2017-04-04
    Beschreibung: The spatial distribution of 414 earthquakes (1.0 ≤ ML ≤ 4.6), recorded from 1994 to 2002 in Southeastern Sicily (Italy), has been analyzed; it generally coincides with mapped Plio-Quaternary faults, including the NNW-SSE offshore fault system which is the most important tectonic structure of the area. For the best located events, we computed 70 focal mechanisms by combining P-wave polarities with S-wave polarizations. A predominance of strike-slip and normal faults was observed. Focal mechanisms were then inverted for stress tensor parameters using the algorithm of Gephart and Forsyth. The results highlighted a region governed mainly by a compressional stress regime. Moreover, anisotropy analysis of shear-waves showed a polarization of fast S-waves prevalently aligned in the NNW-SSE to NW-SE direction over the whole area. A finer analysis of stress tensor evidenced three regions characterized by slightly differing orientation of the greatest principal stress axis, 1. The eastern sector displays a nearly horizontal 1 trending NW-SE; the central sector is affected by a low dip NNW-SSE 1; whereas in the western sector a 1 NNW-SSE oriented, with a higher dip angle, was detected. Finally, the comparison of the spatial distribution of seismicity occurring during 1994-2002, with locations of previous instrumental earthquakes and larger (M ≥ 5.0) historical events showed that the seismic patterns are persistent.
    Beschreibung: Published
    Beschreibung: 1359–1374
    Beschreibung: JCR Journal
    Beschreibung: reserved
    Schlagwort(e): Stress direction ; focal mechanisms ; Shear-Wave Anisotropy ; 04. Solid Earth::04.06. Seismology::04.06.99. General or miscellaneous
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
    Standort Signatur Erwartet Verfügbarkeit
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  • 3
    Publikationsdatum: 2017-04-04
    Beschreibung: An increase of seismicity occurred at Mount St. Helens between May and July 1998, with more than 900 events (Md 2.2) recorded by the Pacific Northwest Seismograph Network. This article describes an attempt to estimate the temporal and spatial variation in seismic attenuation using 200 microearthquakes that occurred before (January 1995–April 1998), during, and after (August–December 1998) the period of higher seismicity and recorded on three different one-component seismic stations. Epicentral distances of the studied events ranged between 0.5 and 15 km. We performed the analysis distinguishing the deep (depth 5.5 km) from shallow (depth 5.5 km) events. We used the frequency decay method to estimate the quality factor Q and station site correction S for P waves in the frequency bands 2–7 and 18–30 Hz. The results show that the attenuation varies from site to site systematically and decreases with depth. Lower Qp values are obtained for focal depths less than 5.5 km. Moreover, the spatial variations of Qp show minimum values ( 30) in the crater area that are interpreted as due to a low-density mass distribution under the crater. We find that Qp was 30% higher before the period of high seismicity than after. This change may be attributed to an increase of pressure in the magma chamber producing new cracks and/or reopening of pre-existing cracks, which are the most viable mechanisms for increasing attenuation.
    Beschreibung: Published
    Beschreibung: 1441–1455
    Beschreibung: 3.6. Fisica del vulcanismo
    Beschreibung: JCR Journal
    Beschreibung: reserved
    Schlagwort(e): seismicity ; attenuation ; 04. Solid Earth::04.06. Seismology::04.06.03. Earthquake source and dynamics
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
    Standort Signatur Erwartet Verfügbarkeit
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  • 4
    Publikationsdatum: 2017-04-04
    Beschreibung: The unrest of Stromboli volcano leading to the February 27–April 2, 2007 eruptive period and to the March 15 paroxysm is constrained by combining broadband seismic data and 1-Hz GPS (High Rate GPS – hereinafter HRGPS) measurements. During the pre-eruptive stage, the simultaneous examination of seismic and HRGPS data, together with weather parameters, suggests the possible influence of external perturbations on the magmatic system, which evolved toward a critical state after January 2007. Some days after the onset of the eruption, a sudden change of the seismic and eruptive behaviour was recognized, while ground deformation began to show a deflation. The March 15 paroxysm was preceded, about two days before, by a peak of the HRGPS spectral power densities (a small inflation) and by the occurrence of a few VT earthquakes located at depths down to 3.5 km b.s.l.. These findings constrain, for the first time at Stromboli volcano, the deep origin of a fast rising magma batch, rich in gas, that led to a strong explosive event.
    Beschreibung: Published
    Beschreibung: L22309
    Beschreibung: 3.6. Fisica del vulcanismo
    Beschreibung: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive
    Beschreibung: JCR Journal
    Beschreibung: reserved
    Schlagwort(e): magmatic system ; 04. Solid Earth::04.08. Volcanology::04.08.06. Volcano monitoring
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
    Standort Signatur Erwartet Verfügbarkeit
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  • 5
    Publikationsdatum: 2017-04-04
    Beschreibung: The adaptive mesh double-difference tomography algorithm (tomoADD) was applied to absolute and differential P, S and S–P data to determine three dimensional VP, VS and VP/VS variations and event locations in southeastern Sicily (Italy). The obtained velocity images highlight vertical and lateral heterogeneities that can be associated with different geological units and main tectonic features. In particular, the sharp velocity contrasts are consistent with previously recognized active faults, allowing us to better determine their shapes and geometries at depth. Moreover, a striking correspondence between areas ruptured by earthquakes and velocity anomalies is observed. In fact, seismicity is mostly confined in the high velocity volumes and/or along the high–low velocity boundaries at mid-crustal depths, whereas it tends to avoid regions with lower than average VP and VS values and higher VP/VS ratios.
    Beschreibung: Published
    Beschreibung: 74-85
    Beschreibung: 3.1. Fisica dei terremoti
    Beschreibung: JCR Journal
    Beschreibung: reserved
    Schlagwort(e): Continental crust ; Tomography ; 04. Solid Earth::04.06. Seismology::04.06.07. Tomography and anisotropy
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
    Standort Signatur Erwartet Verfügbarkeit
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  • 6
    Publikationsdatum: 2017-04-04
    Beschreibung: Tectonic stress in the Pacific Northwest Washington is dominated by a N-S major compressive axis s1 and a minor compressive axis s3 which varies from E-W to near vertical. Minor variations in this pattern occur in different parts of the region. In this study we used ca. 550 earthquakes in the central Washington Cascade Mountains to study in detail the uniformity of the stress tensor in this volcanic arc. Earthquakes from the Pacific Northwest Seismograph Network (PNSN) catalogue were divided into several subsets based on epicentral and depth groupings and stress-tensor inversions using the Gephart and Forsyth technique were computed for each group. As in previous similar studies the maximum compressive stress axis (s1) is nearly horizontal and trending ca. N-S and NNE-SSW in all but one subset. Shallower events directly under Mount Rainier have a near vertical s1. For other subsets the minimum compressive stress axis (s3) deviates from vertical to horizontal for different groups of events. In particular, events in the depth range of 10-14 km in the Western Rainier Seismic Zone (WRSZ) have near vertical s3 direction while other depth ranges in this area show a near horizontal, E-W s3 orientation. We hypothesize that the change in orientation of 3 for the 10-14 km depth range in the WRSZ is probably due to the influence of the nearby Mount Rainier magmatic system.
    Beschreibung: Published
    Beschreibung: 811-821
    Beschreibung: 3.2. Tettonica attiva
    Beschreibung: JCR Journal
    Beschreibung: reserved
    Schlagwort(e): focal mechanism ; stress tensor ; 04. Solid Earth::04.06. Seismology::04.06.03. Earthquake source and dynamics
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
    Standort Signatur Erwartet Verfügbarkeit
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  • 7
    Publikationsdatum: 2017-04-04
    Beschreibung: Four hundred forty-seven well-recorded earthquakes at Mount St. Helens during the late 1990s are relocated with high precision using a combination of a cross-correlation technique and then a procedure to obtain a one-dimensional velocity structure solution together with station corrections. The resulting high-resolution pattern of the spatialtemporal evolution of the earthquake activity, along with focal mechanisms of events provide additional evidence for the presence of a magma reservoir in the depth range 5.5– 10 km, connected to the surface by a thin vertical conduit. Moreover, the distribution of the events below 5.5 km yielded a much clearer view of the structures that generate the seismicity at that depth. Some of the deeper events occur along a sequence of faults under the influence of a radial stress field; however, a large number of the events occur on a NNE-SSW striking steeply dipping fault with slip consistent with magma being periodically injected into a truncated dike on the northwest side of this fault.
    Beschreibung: Published
    Beschreibung: 2264
    Beschreibung: 3.1. Fisica dei terremoti
    Beschreibung: JCR Journal
    Beschreibung: reserved
    Schlagwort(e): magma ; hypocenter ; 04. Solid Earth::04.06. Seismology::04.06.03. Earthquake source and dynamics
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
    Standort Signatur Erwartet Verfügbarkeit
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