A reappraisal of shear wave splitting parameters from Italian active volcanic areas through a semiautomatic algorithm

Bianco, Francesca; Zaccarelli, Lucia

doi:10.1007/s10950-008-9125-z

Cited by 32 publications

(17 citation statements)

References 35 publications

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“…Note that measurement 2, which is the nearest the array site of Piano Pernicana, shows similar spectral properties to the array area: i.e., the HVSR peak at about 1 Hz and the 160° polarization (see Figures 6, 8, and 11), although at a smaller level of HVSR amplitude (a peak amplitude of 4). It is also interesting that our finding of a predominant north‐south polarization (excluding the sites near the NE rift) observed through tremor data is consistent with the fast polarization direction found by Bianco and Zaccarelli [2009]. Bianco and Zaccarelli used shear wave splitting analysis on local earthquakes recorded on the eastern flank of Mount Etna by local seismic stations (see Figure 12) considering different time periods including the 1989 and 2001 eruption.…”

Section: Resultssupporting

confidence: 83%

Evidences for strong directional resonances in intensely deformed zones of the Pernicana fault, Mount Etna, Italy

et al. 2009

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[1] In this paper we investigate ground motion properties in the western part of the Pernicana fault. This is the major fault of Mount Etna and drives the dynamic evolution of the area. In a previous work, Rigano et al. (2008) showed that a significant horizontal polarization characterizes ground motion in fault zones of Mount Etna, both during earthquakes and ambient vibrations. We have performed denser microtremor measurements in the NE rift segment and in intensely deformed zones of the Pernicana fault at Piano Pernicana. This study includes mapping of azimuth-dependent horizontalto-vertical spectral ratios along and across the fault, frequency-wave number techniques applied to array data to investigate the nature of ambient vibrations, and polarization analysis through the conventional covariance matrix method. Our results indicate that microtremors are likely composed of volcanic tremor. Spectral ratios show strong directional resonances of horizontal components around 1 Hz when measurements enter the most damaged part of the fault zone. Their polarization directions show an abrupt change, by 20°to 40°, at close measurements between the northern and southern part of the fault zone. Recordings of local earthquakes at one site in the fault zone confirm the occurrence of polarization with the same angle found using volcanic tremor. We have also found that the directional effect is not time-dependent, at least at a seasonal scale. This observation and the similar behavior of volcanic tremors and earthquake-induced ground motions suggest that horizontal polarization is the effect of local fault properties. However, the 1-Hz resonant frequency cannot be reproduced using the 1-D vertically varying model inferred from the array data analysis, suggesting a role of lateral variations of the fault zone. Although the actual cause of polarization is unknown, a role of stress-induced anisotropy and microfracture orientation in the near-surface lavas of the Pernicana fault can be hypothesized consistently with the sharp rotation of the polarization angle within the damaged fault zone.

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Section: Resultssupporting

confidence: 83%

Evidences for strong directional resonances in intensely deformed zones of the Pernicana fault, Mount Etna, Italy

et al. 2009

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“…The polarization of earthquakes (white squares) is also drawn. Diagrams in white circles show the results of velocity anisotropy found by Bianco et al [1996] and Bianco and Zaccarelli [2007] using earthquakes recorded at the stations in the map (gray triangles).…”

Section: Discussionmentioning

confidence: 94%

Evidence for ground motion polarization on fault zones of Mount Etna volcano

et al. 2008

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[1] During local and regional earthquakes, an evident amplification of horizontal ground motion is observed at two seismological stations near the Tremestieri fault, on the southeastern flank of Mount Etna volcano. Rotated component spectral ratios show a narrow spectral peak around 4 Hz along a N40°E direction. A conventional polarization analysis using the eigenvectors of the covariance matrix confirms the very stable directional effect enhancing the approximately NE-SW elongation of the horizontal ground motion in the fault zone. The effect is evident during the entire seismogram and independent of source back azimuth as well as distance and depth of earthquakes. The same polarization is observed in ambient noise as well. This consistency allowed us to use microtremors for checking ground motion polarization along and across the Tremestieri fault zone with a high spatial resolution. The result is a stable polarization of horizontal motion in the entire area that can be observed in a broad frequency band. To check whether this ground motion property is recurrent and to understand a possible relationship with fault strike, faulting style, or orientation of fractures, ambient noise was recorded on other mapped faults of the Mount Etna area, the Moscarello, Acicatena, and Pernicana faults. The latter, in particular, is characterized by different strike and faulting style. A systematic tendency of ambient noise to be polarized is found in all of the faults. A picture emerges where normal faults of the eastern flank show an E-W to NE-SW polarization that changes on the Pernicana fault, which develops approximately E-W and is characterized by a prevailing NW-SE to N-S polarization. Directions of polarization were never parallel to the fault strike. Moreover, polarization persists too far away from the fault trace, excluding an effect limited to a narrow low-velocity zone hosted between harder wall rocks. Both these observations rule out an interpretation in terms of faulttrapped waves. The cause of observed polarizations will be the subject of future studies. However, the consistency with recent results of velocity anisotropy in a part of the investigated area suggests a possible role of attenuation anisotropy on horizontal amplitude variations versus azimuth.

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“…S1 and Supplementary Table S1). Although this result may seem surprising when considering the dramatic changes in f within the caldera, small changes in the magnitude of anisotropy can create large variations in f because f and dt are not coupled linearly, particularly when the mechanism of anisotropy varies 9,12 . At the times of the 2008 changes, therefore, subtle variations in dt may be masked by scatter in the measurements 11 .…”

Section: Sws Delay Timesmentioning

confidence: 98%

“…There is mounting evidence, however, that the dominant mechanism for SWS can switch between a static condition, such as aligned fractures in fault zones, and a dynamic process, such as maximum horizontal compressive stress causing aligned microcracks to dilate 9,10 . In areas where there are strong changes in maximum compressive stress direction and magnitude on observable timescales, such as at active volcanoes, SWS analysis has proven a valuable tool when combined with ground deformation or other seismological observations for interpretation of volcanic processes such as magma migration [8][9][10][11][12][13] . Such changes are always detected in hindsight, however, and seismologists struggle to employ monitoring of seismic anisotropy in real-or near-real-time to assist in eruption forecasting.…”

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confidence: 99%

Seismic detection of increased degassing before Kīlauea's 2008 summit explosion

Johnson

Poland

2013

Nat Commun

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The 2008 explosion that started a new eruption at the summit of Kı %lauea Volcano, Hawaìi, was not preceded by a dramatic increase in earthquakes nor inflation, but was associated with increases in SO 2 emissions and seismic tremor. Here we perform shear wave splitting analysis on local earthquakes spanning the onset of the eruption. Shear wave splitting measures seismic anisotropy and is traditionally used to infer changes in crustal stress over time. We show that shear wave splitting may also vary due to changes in volcanic degassing. The orientation of fast shear waves at Kı %lauea is usually controlled by structure, but in 2008 showed changes with increased SO 2 emissions preceding the start of the summit eruption. This interpretation for changing anisotropy is supported by corresponding decreases in V p /V s ratio. Our result demonstrates a novel method for detecting changes in gas flux using seismic observations and provides a new tool for monitoring under-instrumented volcanoes.

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A reappraisal of shear wave splitting parameters from Italian active volcanic areas through a semiautomatic algorithm

Cited by 32 publications

References 35 publications

Evidences for strong directional resonances in intensely deformed zones of the Pernicana fault, Mount Etna, Italy

Evidences for strong directional resonances in intensely deformed zones of the Pernicana fault, Mount Etna, Italy

Evidence for ground motion polarization on fault zones of Mount Etna volcano

Seismic detection of increased degassing before Kīlauea's 2008 summit explosion

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