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Rademacher, H.
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- 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.233 273 - PublicationOpen AccessObservations of rapid-fire event tremor at Lascar volcano, Chile(1996-03)
; ; ; ; ; ;Asch, G.; GeoForschungsZentrum, GFZ-Potsdam, Germany ;Wylegalla, K.; GeoForschungsZentrum, GFZ-Potsdam, Germany ;Hellweg, M.; U.S. Geological Survey, Menlo Park, CA, U.S.A. ;Seidl, D.; Seismologisches Zentralobservatorium Graefenberg, SZGRF, Erlangen, Germany ;Rademacher, H.; 57 Overhill Rd., Orinda, CA, U.S.A.; ; ; ; During the Proyecto de Investigaciòn Sismològica de la Cordillera Occidental (PISCO '94) in the Atacama desert of Northern Chile, a continuously recording broadband seismic station was installed to the NW of the currently active volcano, Lascar. For the month of April, 1994, an additional network of three, short period, three-component stations was deployed around the volcano to help discriminate its seismic signals from other local seismicity. During the deployment, the volcanic activity at Lascar appeared to be limited mainly to the emission of steam and SO2. Tremor from Lascar is a random, «rapid-fire» series of events with a wide range of amplitudes and a quasi-fractal structure. The tremor is generated by an ensemble of independent elementary sources clustered in the volcanic edifice. In the short-term, the excitation of the sources fluctuates strongly, while the long-term power spectrum is very stationary.174 390 - 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 - PublicationOpen AccessA preliminary survey of the broadband seismic wavefield at Puu Oo, the active vent of Kilauea volcano, Hawaii(1996-03)
; ; ; ; ;Seidl, D.; Seismologisches Zentralobservatorium Graefenberg, SZGRF, Erlangen, Germany ;Hellweg, M.; U.S. Geological Survey, Menlo Park, CA, U.S.A. ;Okubo, P.; Hawaiian Volcano Observatory, Hawaiian Volcanoes National Park, HI, U.S.A. ;Rademacher, H.; 57 Overhill Rd., Orinda, CA, U.S.A.; ; ; The seismic wavefield near an active volcanic vent consists of superimposed signals in a wide range of frequency bands from sources inside and outside the volcano. To characterize the broadband wavefield near Puu Oo, we deployed a profile of three three-component broadband sensors in a 200 m long line about 1.5 km WSW of the active vent. During this period, Puu Oo maintained a constant, but very low level of activity. The digital data logger recorded the wavefield continuously in the frequency band between 0.01 and 40 Hz between June 25 and July 9, 1994. At the same time, local wind conditions along with air temperature and pressure were monitored by a portable digital weather station. On the basis of characteristic elements, such as waveform, spatial coherence between stations, particle motion and power spectra, the wavefield can be divided into three bands. The dominant signals in the frequency band between 0.01 and 0.1 Hz are not coherent among the stations. Their ground velocities correlate with the wind speed. The signals in the 0.1 to 0.5 Hz band are coherent across the profile and most probably represent a superposition of volcanic tremor and microseisms from the Pacific Ocean. Much of the energy above 0.5 Hz can be attributed to activity at the vent. Power spectra from recordings of the transverse components show complex peaks between 0.5 and 3 Hz which vary in amplitude due to site effects and distance. On the other hand, power spectra calculated from the radial components show a clearly periodic pattern of peaks at 1 Hz intervals for some time segments. A further remarkable feature of the power spectra is that they are highly stationary.144 346