Measuring and Modeling Fluid Movements in Volcanoes: Insights from Continuous Broadband Seismic Monitoring at Galeras Volcano, Colombia

Margaret Hellweg, Leigh House (Los Alamos National Laboratory) and Douglas Dreger

Introduction

One important goal of volcano monitoring is to be able to reliably identify significant changes in a volcano's activity, in order to minimize the threat to the local population and infrastructure. Seismic signals are often the most immediate indicators of such changes (McNutt, 1996), and to interpret them, we must understand the processes which produce them. At volcanoes, we observe two basic types of seismic signals. Volcano-tectonic events are earthquakes, usually small ones, resulting from slip across fault planes. The second type of events are non-tectonic and are unique to volcanoes. They include volcanic tremor, long period (LP) events, and less commonly, tornillos. In seismic recordings, non-tectonic signals are often emergent, may continue for a long time, and have highly variable amplitudes. Very often their spectra contain one or a few distinct, sharp peaks. They are assumed to be associated with the movement of fluids in the volcanic system (for a review, see Konstantinou and Schlindwein, 2002).

Figure 15.1: Unfiltered three-component record of a tornillo recorded at the crater rim seismic station ANG. The coda of the E component bears the most resemblence to a screw. The very long period noise particularly apparent on the N component is most likely due to wind.

Tornillos

At Galeras Volcano, Colombia, `tornillos' occurred prior to explosive eruptions in 1992 and 1993 (Narváez et al, 1997). These distinctive seismic events have identifiable onsets and relatively long, gradually decaying event tails (codas) and their name comes from the resemblence of their shape on the seismic record to a screw (Spanish: tornillo. Figure 15.1). Their spectra have one or a few narrow peaks. That is, they are monochromatic or multi-chromatic (Figure 15.2). From December 1999 to December 2002, ninety tornillos occurred at Galeras Volcano, Colombia, and were recorded with broadband, three-component seismometers (Seidl et al, 2003). These tornillos have between 1 and 15 spectral peaks. To characterize the tornillos and learn what causes them, we are investigating the coda which gives them their name. The analysis of the coda follows the procedure described by Seidl and Hellweg (2003), where each spectral peak in each tornillo is treated separately. Based on the azimuth $Az_{n}$, inclination $In_{n}$ and rectilinearity $Re_{n}$ for each spectral peak, the seismograms are rotated from the Z-N-E coordinate system of the seismometer into a coordinate system of the wavefield, X1-X2-X3. From these records, the frequency of the peak's maximum, $f_{Pn}$, and its amplitude, $A_{Pn}$, are measured in the frequency domain, while the maximum velocity amplitude, $v_{n}$, damping factor, $Q_{n}$ and the signal energy, $E_{Pn}$, are measured in the time domain.

Figure 15.2: Linear-logarithmic spectrum of the E component for the tornillo shown in Figure 15.1. The spectrum during the tornillo is superimposed on the spectrum of the seismic noise before the tornillo (gray). Note the extremely narrow spectral peaks between 1 and 15 Hz.

Figure 15.3: Expanded seismograms for the beginning of the tornillo shown in Figure 15.1. These data have been bandpass filtered between 1 and 40 Hz to suppress long-period noise. Note the clear onset on the vertical component marked P and the onsets on the horizontal components 0.25 s later marked S. It takes a little over 2 s for the characteristic coda of the tornillo to develop.

Interpretation

For the characteristic coda of the tornillos, it is clear that the frequencies of the spectral peaks present, which may range from 1 Hz to 40 Hz, are related to the source, but we have not yet found a pattern allowing us to predict precisely which frequencies or families of frequencies will be present in any particular tornillo. For the spectral peaks below 5 Hz, the polarization both remains constant during an individual tornillo and varies little from one tornillo to the next. This suggests that tornillos are all generated within a limited volume of the volcanic edifice. The variation in the polarization at higher frequencies should allow us to constrain the size of this volume (Hellweg, 2003). With the high resolution data from broadband instruments, we can see that the onset of the tornillo is small but clearly impulsive (Figure 15.3). The initial P-pulse on the vertical component is followed 0.25 s later by S-waves on the horizontal components. A transition of about two seconds follows before the characteristic coda develops. As we determine details about the tornillo onset and this intermediate wavepacket, and their relationships to the characteristics of the tornillo coda, we shall be able to derive a more thorough picture relating the triggering mechanism to its effect on the coda.

Acknowledgements

This project is funded by U.C. Berkeley - Los Alamos National Laboratory collaborative Institute for Geophysics and Planetary Physics Project number 04-1407. Tornillo data have been acquired as part of a cooperative project between the Bundesanstalt für Geowissenschaften und Rohstoffe (Germany) and the Instituto de Investigación e Información Geocientífica Minero-Ambiental y Nuclear (Colombia) on Multiparameter Monitoring of Volcanoes.

References

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