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Torres C., R. A.
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Torres C., R. A.
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- PublicationOpen AccessUnusual seismic signals associated with the activity at Galeras volcano, Colombia, from July 1992 to September 1994(1996-03)
; ; ; ;Torres C., R. A.; INGEOMINAS, Observatorio Vulcanológico y Sismológico, Unidad Operativa Pasto, Colombia ;Gómez M., D. M.; INGEOMINAS, Observatorio Vulcanológico y Sismológico, Unidad Operativa Pasto, Colombia ;Narvàez M., L.; INGEOMINAS, Observatorio Vulcanológico y Sismológico, Unidad Operativa Pasto, Colombia; ; After the emplacement of a lava dome at Galeras volcano in 1991, seven eruptions occurred from July 16, 1992, to September 23, 1994, six of which were preceded by quasi-monochromatic, long-duration seismic events with slowly decaying coda named «tornillos» (screws). The dominant frequencies of these unusual seismic signals are related to source characteristics and show temporal changes, diminishing and then tending to stabilize before an eruption. At the same time, the accumulated number and the duration of these signals increase several days prior to the eruption. The increase in the duration of the tornillo events and the decline of the dominant frequencies both suggest an increasing impedance contrast between the surrounding solid material and the fluid. These characteristics may be associated with an increase in the free gas phase in the magma produced by saturation of volatiles due to cooling, crystallization and partial solidification of the column of magma plugging the conduits. The solidified magma can contribute to sealing the conduits and preventing free gas escape, with consequent generation of overpressure. An eruption is initiated when the overpressure exceeds the resistance strength of the solid material.306 484 - PublicationOpen AccessTornillo seismic events at Galeras volcano, Colombia: a summary and new information from broadband three-component measurements(1999-06)
; ; ; ; ; ;Gómez M., D. M.; INGEOMINAS - Observatorio Vulcanológico y Sismológico Pasto, Colombia ;Torres C., R. A.; INGEOMINAS - Observatorio Vulcanológico y Sismológico Pasto, Colombia ;Seidl, D.; Seismologisches Zentralobservatorium Gräfenberg, Germany ;Hellweg, M.; Institut für Geophysik, Universität Stuttgart, Germany ;Rademacher, H.; Geo Enterprises Orinda, CA, U.S.A.; ; ; ; Long-duration events have been recorded at several active, andesitic volcanoes. Their main characteristics are a single, sharp frequency peak, and an exceptionally long coda. Because their seismograms resemble a screw, these signals are called "tornillos" in Colombia. These events have been recorded during different stages of volcanic activity at various volcanoes worldwide. Tornillos have occurred for example, as a short-term precursor to eruptions at Galeras volcano, Colombia (1992-1993); and at Asama volcano, Japan, (1983). At Tokachi volcano, Japan, they were recorded after an eruption (1989). The Tornillo's dominant frequency appears to be related to the time of occurrence during an eruption cycle. It is independent of epicentral distance, azimuth, travel time, and lapse time, indicating that it is a source characteristic. Damping coefficients for the tornillo's coda range between 0.002 and 0.02. In contrast, damping coefficients for normal long-period events lies between 0.010 and 0.025 and for volcano-tectonic events between 0.010 and 0.040. In March 1996, the Galeras seismic network, which consists of short period, single-component seismometers, was augmented with a broadband, three-component station. This station, installed 1.5 km south of Galeras active cone, recorded a series of six tornillos. Narrowband and broadband tornillo records have similar characteristics in the time and frequency domains.233 274 - PublicationOpen AccessThe anatomy of a tornillo: puzzles from three-component measurements at Galeras volcano (Colombia)(1999-06)
; ; ; ; ; ;Seidl, D.; Bundesanstalt für Geowissenschaften und Rohstoffe, Seismologisches Zentralobservatorium Gräfenberg, Erlangen, Germany ;Hellweg, M.; Institut für Geophysik, Universität Stuttgart, Germany ;Rademacher, H.; Geo Enterprises Orinda, CA, U.S.A ;Gomez M., D. M.; INGEOMINAS - Observatorio Vulcanológico y Sismológico, Pasto, Colombia ;Torres C., R. A.; INGEOMINAS - Observatorio Vulcanológico y Sismológico, Pasto, Colombia; ; ; ; Many of the recent ash eruptions of Galeras volcano (Colombia) have been preceded by tornillos. These unusual seismic events of unknown origin have screw-like profiles on seismograms and can last up to several minutes. In March, 1996, a broadband, three-component seismometer station was installed 1.6 km south of Cono, Galeras' active vent. With high quality recordings acquired by this system, and methods which work well with narrow band signals, we investigate the frequency content and polarization properties of two tornillos. They have slighty different predominant frequencies, 3.0420 Hz and 3.0595 Hz which remain highly stable for the entire tornillo duration. The polarization analysis shows that the polarization for the two events also differs slightly but remains constant. Most of their energy is linearly polarized and lies in the horizontal plane. If these waves are taken to be SH waves, the remaining energy is confined to the beginning of the tornillo during its initiation or excitation. The remainder of the tornillo is characterized by exponential decay. These observations provide parameters for the classification of tornillos and constraints for source models.151 1003