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Ortiz, R.
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- PublicationRestrictedSeismovolcanic signals at Deception Island volcano,Antarctica: Wave field analysis and source modeling(2000-06-10)
; ; ; ; ; ; ; ;Ibanez, J.; University of Granada, Spain ;Del Pezzo, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Almendros, J.; University of Granada, Spain ;La Rocca, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Alguacil, G.; University of Granada, Spain ;Ortiz, R.; CSIC, Madrid ;Garcia, A.; CSIC, Madrid; ; ; ; ; ; Island (Antarctica), recorded during three Antarctic summers (1994- 1995, 1995-1996 and 1996-1997), are analyzed using a dense small-aperture (500 m) seismic array. The visual and spectral classification of the seismic events shows the existence of long-period and hybrid isolated seismic events, and of low-frequency, quasi-monochromatic and spasmodic continuous tremors, All spectra have the highest amplitudes in the frequency band between 1 and 4 Hz, while hybrids and spasmodic tremors have also significant amplitudes in the high-frequency band (4-10 Hz). The array analysis indicates that almost all the well-correlated low-frequency signals share similar array parameters (slowness and back azimuth) and have the same source area, close to the array site. The polarization analysis shows that phases at high-frequency are mostly composed of P waves, and those phases dominated by low frequencies can be interpreted as surface waves. No clear shear waves are evidenced. From the energy evaluation, we have found that the reduced displacement values for surface and body waves are confined in a narrow interval. Volcano-tectonic seismicity is located close to the array, at a depth shallower than 1 km. The wave-field properties of the seismovolcanic signals allow us to assume a unique source model, a shallow resonating fluid-filled crack system at a depth of some hundreds of meters. All of the seismic activity is interpreted as the response of a reasonably stable stationary geothermal process. The differences observed in the back azimuth between low and high frequencies are a near-field effect. A few episodes of the degassification process in an open conduit were observed and modeled with a simple organ pipe.249 28 - PublicationOpen AccessObservations of volcanic earthquakes and tremor at Deception Island - Antarctica(1999-06)
; ; ; ; ; ; ; ; ;Alguacil, G.; Instituto Andaluz de Geofisica, Universidad de Granada, Spain ;Almendros, J. C.; Instituto Andaluz de Geofisica, Universidad de Granada, Spain ;Del Pezzo, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Garcia, A.; Museo Nacional de Ciencias Naturales, CSIC, Madrid, Spain ;Ibañez, J. M.; Instituto Andaluz de Geofisica, Universidad de Granada, Spain ;La Rocca, M.; Dipartimento di Fisica, Università di Salerno, Baronissi (SA), Italy ;Morales, J.; Instituto Andaluz de Geofisica, Universidad de Granada, Spain ;Ortiz, R.; Museo Nacional de Ciencias Naturales, CSIC, Madrid, Spain; ; ; ; ; ; ; Deception Island - South Shetlands, Antarctica is site of active volcanism. Since 1988 field surveys have been carried out with the aim of seismic monitoring, and in 1994 a seismic array was set up near the site of the Spanish summer base in order to better constrain the source location and spectral properties of the seismic events related to the volcanic activity. The array was maintained during the Antarctic summer of 1995 and the last field survey was carried out in 1996. Data show the existence of three different groups (or families) of seismic events: 1) long period events, with a quasi-monochromatic spectral content (1-3 Hz peak frequency) and a duration of more than 50 s, often occurring in small swarms lasting from several minutes to some day; 2) volcanic tremor, with a spectral shape similar to the long period events but with a duration of several minutes (2-10); 3) hybrid events, with a waveform characterised by the presence of a high frequency initial phase, followed by a low frequency phase with characteristics similar to those of the long period events. The high frequency phase of the hybrid events was analysed using polarisation techniques, showing the presence of P waves. This phase is presumably located at short epicentral distances and shallow source depth. All the analysed seismic events show back-azimuths between 120 and 330 degrees from north (corresponding to zones of volcanic activity) showing no seismic activity in the middle of the caldera. Particle motion, Fourier spectral and spectrogram analysis show that the low frequency part of the three groups of the seismic signals have similar patterns. Moreover careful observations show that the high frequency phase which characterises the hybrid events is present in the long period and in the tremor events, even with lower signal to noise ratios. This evidence suggests that long period events are events in which the high frequency part is simply difficult to observe, due to a very shallow source and/or hypocentral distance higher than that of hybrids, while the tremor is composed of rapidly occurring hybrid events. We propose a possible interpretation for the three groups of seismic events. These may be generated by multiple pressure-steps due to the rapid phase change from liquid to vapour in a shallow aquifer which comes in contact with hot materials. The pressure change can put a crack in resonance or excite the generation of multiple surface waves modes in the shallow layered structure.188 343 - PublicationOpen AccessTwin digital short period seismic Array Experiment at Stromboli Volcano(1998)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;Del Pezzo, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;La Rocca, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Petrosino, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Grozea, B.; Università di Salerno ;Maritato, L.; Università di Salerno ;Saccorotti, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Simini, M.; Università di Salerno ;Ibanez, J.; Instituto Andaluz de Geofisica ;Alguacil, G.; Instituto Andaluz de Geofisica ;Carmona, E.; Instituto Andaluz de Geofisica ;Abril, M.; Instituto Andaluz de Geofisica ;Almendros, J.; Instituto Andaluz de Geofisica ;Ortiz, R.; Museo Naccional de Ciencias Naturales de Madrid ;Garcia, A.; Museo Naccional de Ciencias Naturales de Madrid ;Pingue, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Esposito, T.; Osservatorio Vesuviano; ; ; ; ; ; ; ; ; ; ; ; ; ; ; Two small arrays composed by short period (1 Hz) digital seismic stations, with an aperture of approximately 400 meters, were set up at Stromboli volcano (one at semaforo Labronzo, the other at Ginostra- Timpone del Fuoco) with the purpose of the spatial location of the high frequency source of the explosion quakes. About 75 explosion-quakes were recorded at both arrays, and constitute the available data base. We have planned to apply the zero-lag cross-correlation technique to the whole data set in order to obtain back-azimuth and apparent slowness of the coherent seismic phases. A preliminary analysis for both arrays show that the predominant back-azimuth for the first phase is oriented in the direction of , but not strictly coincident to, the crater area. Moreover some back-scattered arrivals are quite evident in the seismogram.184 365 - PublicationOpen AccessNew inferences from spectral seismic energy measurement of a link between regional seismicity and volcanic activity at Mt. Etna, Italy(2009-04-19)
; ; ; ; ;Ortiz, R.; CSIC, Volcanology, Madrid, Spain ;Falsaperla, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Marrero, J.M.; Departamento de Geologia. Universidad de La Laguna, Tenerife, Spain ;Messina, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; ; ; The existence of a relationship between regional seismicity and changes in volcanic activity has been the subject of several studies in the last years. Generally, activity in basaltic volcanoes such as Villarica (Chile) and Tungurahua (Ecuador) shows very little changes after the occurrence of regional earthquakes. In a few cases volcanic activity has changed before the occurrence of regional earthquakes, such as observed at Teide, Tenerife, in 2004 and 2005 (Tárraga et al., 2006). In this paper we explore the possible link between regional seismicity and changes in volcanic activity at Mt. Etna in 2006 and 2007. On 24 November, 2006 at 4:37:40 GMT an earthquake of magnitude 4.7 stroke the eastern coast of Sicily. The epicenter was localized 50 km SE of the south coast of the island, and at about 160 km from the summit craters of Mt. Etna. The SSEM (Spectral Seismic Energy Measurement) of the seismic signal at stations at 1 km and 6 km from the craters highlights that four hours before this earthquake the energy associated with volcanic tremor increased, reached a maximum, and finally became steady when the earthquake occurred. Conversely, neither before nor after the earthquake, the SSEM of stations located between 80 km and 120 km from the epicentre and outside the volcano edifice showed changes. On 5 September, 2007 at 21:24:13 GMT an earthquake of magnitude 3.2 and 7.9 km depth stroke the Lipari Island, at the north of Sicily. About 38 hours before the earthquake occurrence, there was an episode of lava fountain lasting 20 hours at Etna volcano. The SSEM of the seismic signal recorded during the lava fountain at a station located at 6 km from the craters highlights changes heralding this earthquake ten hours before its occurrence using the FFM method (e.g., Voight, 1988; Ortiz et al., 2003). A change in volcanic activity – with the onset of ash emission and Strombolian explosions – was observed a couple of hours before the occurrence of the regional earthquakes. It can be interpreted as the magmatic response to a change of the distribution of tectonic stress in the edifice before the earthquake. In the light of this hypothesis, we surmise that the magmatic system behaved similar to a dilatometer and promise news lines to forecasting the volcano activity.119 50 - PublicationOpen AccessA search for the volcanomagnetic signal at Deception volcano (South Shetland I., Antarctica)(1997-03)
; ; ; ; ; ; ;Garcìa, A.; Departamento de Volcanología, Museo Nacional de Ciencias Naturales, CSIC, Madrid, Spain ;Blanco, I.; Departamento de Volcanología, Museo Nacional de Ciencias Naturales, CSIC, Madrid, Spain ;Torta, J. M.; Departamento de Volcanología, Museo Nacional de Ciencias Naturales, CSIC, Madrid, Spain ;Astiz, M. M.; E.T.S. Arquitectura, Universidad Politécnica de Madrid, Spain ;Ibáñez, J. M.; Instituto Andaluz de Geofísica, Granada, Spain ;Ortiz, R.; Departamento de Volcanología, Museo Nacional de Ciencias Naturales, CSIC, Madrid, Spain; ; ; ; ; After the increase in seismic activity detected during the 1991-1992 summer survey at Deception Island, the continuous measurement of total magnetic intensity was included among the different techniques used to monitor this active volcano. The Polish geomagnetic observatory Arctowski, located on King George Island, served as a reference station, and changes in the differences between the daily mean values at both stations were interpreted as indicators of volcanomagnetic effects at Deception. A magnetic station in continuous recording mode was also installed during the 1993-1994 and 1994-1995 surveys. During the latter, a second magnetometer was deployed on Deception Island, and a third one in the vicinity of the Spanish Antarctic Station on Livingston Island (at a distance of 35 km) and was used as a reference station. The results from the first survey suggest that a small magma injection, responsible for the seismic re-activation, could produce a volcanomagnetic effect, detected as a slight change in the difference between Deception and Arctowski. On the other hand, a long term variation starting at that moment seems to indicate a thermomagnetic effect. However the short register period of only two stations do not allow the sources to be modelled. The future deployment of a magnetic array during the austral summer surveys, throughout the volcano, and of a permanent geomagnetic observatory at Livingston I. is aimed at further observations of magnetic transients of volcanic origin at Deception Island.234 203 - PublicationRestrictedArray tracking of the volcanic tremor source at Deception Island Antarctica(1997)
; ; ; ; ; ;Almendros, J.; University of Granada, Spain ;Ibanez, J.; University of Granada, Spain ;Alguacil, G.; University of Granada, Spain ;Del Pezzo, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Ortiz, R.; CSIC Madrid; ; ; ; We have found experimental evidence which shows that the volcanic tremor recorded at Deception Island (South Shetland Islands, Antarctica) is a superposition in time of overlapping hybrid events.181 25 - PublicationRestrictedIntermediate-focus earthquakes under South Shetland Islands (Antarctica)(1997)
; ; ; ; ; ; ;Ibanez, J.; University of Granada, Spain ;Morales, J.; University of Granada, Spain ;Alguacil, G.; University of Granada, Spain ;Almendros, J.; University of Granada ;Ortiz, R.; CSIC Madrid ;Del Pezzo, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia; ; ; ; ; Intermediate-focus seismicity (50192 22