HR: 16:00h
AN: V14B-01    [Abstracts]
TI: One More Time: Tornillos From the Beginning
AU: * Hellweg, M
EM: peggy@seismo.berkeley.edu
AF: Berkeley Seismological Laboratory, U.C. Berkeley 215 McCone Hall # 4760, Berkeley, CA 94720-4760 United States
AU: House, L
EM: house@lanl.gov
AF: Los Alamos National Labortory Geophysics Group, EES-11, MS D-443, Los Alamos, NM 87545 United States
AU: Dreger, D
EM: dreger@seismo.berkeley.edu
AF: Berkeley Seismological Laboratory, U.C. Berkeley 215 McCone Hall # 4760, Berkeley, CA 94720-4760 United States
AB: In 1993, several ash eruptions at Galeras Volcano, Colombia were preceded by tornillos, distinctive seismic events with screw-like envelope profiles which can last up to several minutes. More than 90 of these events have been recorded since December 1999 with broadband seismometers installed as part of a cooperative project between the German and Colombian geological Surveys (BGR and INGEOMINAS). These tornillos are multichromatic with a varying number of spectral peaks between 1 and 40 Hz. Careful inspection of the three-component recordings shows that each tornillo waveform can be divided into three segments. The initial onset is very small compared to the amplitude of the long-lasting coda which gives tornillos their name. As the onset is usually impulsive and found on the vertical component, it is likely to be a P-wave. About 0.3 s later, a transition wavelet lasting between 2 and 3 s arrives on both horizontal components as well as the vertical component. This wavelet is generally complex, but rather than being broadband is made up of contributions at several discrete frequencies. It is followed by the coda. During the three year interval represented by this dataset, $t_{S-P}$ and the polarization for spectral peaks between 1.4 and 4 Hz change very little, suggesting that tornillos always occur in the same place. There are, however, both differences in the onset and the transition wavelet from one tornillo to the next and variations which appear to be time-dependent. We examine the beginnings of many tornillos in search of the process which excites them and investigate whether and how the first 3 s of the tornillo relate to the characteristics of the coda.
DE: 8414 Eruption mechanisms
DE: 8419 Eruption monitoring (7280)
DE: 8499 General or miscellaneous
DE: 7280 Volcano seismology (8419)
DE: 7299 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting