V14B-01 16:00h
One More Time: Tornillos From the Beginning
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.
V14B-02 16:15h
Correlation of Seismic, Gravimetric And Geochemical Data During The 2002-2003 Mt. Etna Eruption
One of the most powerful and voluminous eruptions of Etna's recent history began on late October, 2002. During the first 16 days of eruption, the activity consisted of sustained ash emissions from a crater located at 2750 m elevation on the S flank of the volcano, accompained by lava outpouring from vents located on both the N and S sectors. Eruptive activity suddenly stopped at around 12 GMT on November 12, 2002. During the following 12 hours, the activity at the 2750-m vent evolved to energetic, pulsating Strombolian explosions and strong ash emissions. During this transition, contemporaneous seismic, gravimetric and geochemical measurements provide unprecedented observations of oscillatory phenomena characterising the eruptive dynamics at different time scale. The entire, 12-hour long transition period is associated to a saw-tooth variation of the energy of 1-2 Hz volcanic tremor, to which are superimposed large-amplitude, 24-minute-period oscillations. Both these changes are markedly anti-correlated with the signal recorded by a gravimetric station located about 4 km from the active vent. Noticeably, such anticorrelation is not observed either before or after the 12-hour-long transition period. Location of tremor activity is retrieved through inversion of seismic amplitudes observed at a 6-element broad-band network, and shows a complicate migration of the source occurring at time scale of a few hours. The dynamics of the transition activity is furtherly parameterised through data from a Fourier Transform Infrared spectrometer (FTIR) that recorded the intensity of infrared radiation and the SO2/HCl ratios of emitted gas. Intensity depicts a pulsating behaviour occurring at cycles of about 28 minutes, which are significantly correlated with tremor cycles, lagging, by $\sim$5 minutes. Moreover, intensity and SO2/HCl ratios are almost perfectly anti-correlated throughout the whole transition period. These data offer unprecedented insights into the complex mechanisms governing the eruptive dynamics at Etna Volcano.
V14B-03 16:30h
Source Process of Volcanic Tremor Inferred From Waveform Inversion
Guagua Pichincha located 12 Km west of Quito, Ecuador, is an active volcano in the Pichincha volcanic complex. Beginning on August 7 1998, Guagua Pichincha activity was characterized by phreatic eruptions, dome growth and collapse, and pyroclastic flows, accompanying volcano-tectonic earthquakes, tremor, and long-period events. In January 2003, after a two-year-long quiescent period, we observed tremor showing harmonic oscillations with a dominant spectral peak at around 2 Hz, which occurred periodically showing well-constrained tremor episodes with active periods of 11 minutes and repose periods of 30 minutes. This type of tremor sequence continued for a few months, and has intermittently occurred until the present. We performed waveform inversions of the tremor to quantify the source process. Since it was recorded only by three seismic stations (three-component seismometers at two stations and a vertical-component seismometer at one station) which are not enough for an accurate description of a moment tensor for the source of tremor, we performed waveform inversions assuming the moment tensors for both horizontal and vertical cracks, and a vertical pipe, which are possible geometries for the source of tremor. We obtained a vertical crack with the strike direction of N80°E in the northern rim of a dome at a depth nearly 200 m below the caldera floor as the best-fit solution for our inversions. The tremor may be interpreted as the resonance of a vertical crack-like conduit excited during repetitive releases of hot gases which are originating from the contact of hydrothermal water with magma inside the dome.
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V14B-04 16:45h
Unusual Volcanic Tremor Observations in Fogo Island, Cape Verde
Volcanic tremor is a ground motion characterized by well-defined frequencies, and has traditionally been explained by the movement of fluids, namely magma, in conduits or cracks (Chouet, 1996). Thus tremor has the potential to reveal key aspects of volcanic structure and dynamics. Two types of previously unreported seismic signals have been observed in Fogo volcano: a) tide-modulated seismic noise and volcanic tremor, and b) high-frequency low-attenuation harmonic tremor. Amplitude modulation of seismic noise can be detected by simple eye-inspection of raw data in some stations of the VIGIL Network, Fogo Volcano. A more detailed analysis shows that certain frequency bands which we interpret as volcanic tremor, mainly in the range 2.0-3.0Hz, are preferentially modulated. The main frequency of modulation is 1.93 c.p.d., which corresponds to M2, the semi-diurnal lunar harmonic. Air pressure and temperature, which are continuously monitored in Fogo Island, have been analyzed and cannot explain the observed periodicity. Thus we conclude that seismic noise and tremor amplitudes are controlled by tides (Custodio et al., 2003). A relation between the tidal modulation and hydrothermal systems activity is suspected and under investigation. High-frequency (HF) tremor (5-20 Hz) has been recorded simultaneously in several stations in Fogo Island and even in different islands of the Cape Verde archipelago (up to distances of 120 km). In volcanic environments high-frequency motions are normally recorded in a small area close to the source, due to the strong attenuation of seismic waves. Non-volcanic origins for HF tremor were examined: cultural noise, whale vocalizations, ship noise, electronic/processing artifacts and path and/or site effects were all considered and dismissed. Emergent arrivals and strong site effects render source location a difficult task, but the analysis of wave polarizations and amplitude distributions seems to point to an offshore source. Two alternative mechanisms are presently being considered: a) propagation in the ocean sound channel of T-waves generated by resonance in a shallow conduit/chamber, and b) existence of a deep strong source, such as a large fluid-filled crack, capable of producing tremor with a complex pattern that propagates to large distances.
V14B-05 17:00h
Precise Hypocenter Location of High-Frequency-Onset Earthquakes, Tomography, and Changing Stress Conditions Beneath Soufriere Hills Volcano, Montserrat
The andesite stratovolcano of Soufriere Hills (SHV), Montserrat, has been intermittently erupting now for 9 years. Previous work (Aspinall et al., 1998) has examined hypocenter locations for events 1995 - 1997 to map possible magma migration pathways and domains sensitive to stress redistributions caused by magmatic processes. This indicated a NE-SW alignment of epicenters under the NE flank, and nests occurring under the volcano, and 3 km W of SHV. Recent instrumentation at SHV with hardware by the CALIPSO Project (Voight et al., 2003; Mattioli et al., 2003) have made it desirable to re-examine the seismic database to more precisely constrain the magma storage and transport system, and stress changes which have occurred during eruption activity. Recent developments have allowed more precise hypocenter relocations using relative positioning techniques. Studies using such methods (e.g., Prejean et al., 2002) have demonstrated improvements in location precision, with distinct lineaments emerging where previous techniques had merely identified earthquake clusters. Therefore we have examined seismic data at SHV utilizing the Waldhauser and Ellsworth (2000) precise relocation technique. The aim of the study is to provide clarification of distinct structures that may have influenced magmatic processes, and which might influence the modeling of crustal strains as deduced by CALIPSO data. Velocity structure is examined, and fault plane solutions are also explored to detect any stress changes that have occurred in concert with pulsatory magmatic pressure episodes. Examination of the data for both spatial and temporal patterns provides constraints on magma storage and transport dynamics at SHV.
V14B-06 17:15h
Source mechanisms of strong vulcanian explosions at Soufri\`{e}re Hills Volcano, Montserrat, B.W.I., from moment tensor inversions of very-long-period data
Seismic signals associated with explosions at Soufri\`{e}re Hills Volcano (SHV) as recorded by broadband seismic stations, were analyzed to better understand eruption dynamics and physical eruptive processes. We discuss recent data from July 2003. Several explosions in July followed conduit decompression associated with the 12-13 July 2003 lava dome collapse. The collapse/eruption in July and explosion events were preceded by long-period and hybrid seismicity. Waveforms of the explosions from an array of broadband seismometers also indicated VLP (8-30s period) signals. We explore the source mechanisms and locations of source centroids of the explosive events by using the linear inversion method of Ohminato et al. (1998) with a 3D mesh. Preliminary results suggest that the VLP waveforms are broadly consistent with a quasi-horizontal crack (sill) source, with residuals minimal at a depth of several km. Further analysis yields evidence of shallower initiation at depth of a few hundred m, also involving a flat subhorizontal source, followed by propagation of source to deeper level. We suspect that our data may be consistent with a "flat crack" that represents initiation and descent down-conduit of the fragmentation surface for the explosion. Although the seismic network is far from having ideal coverage for a robust estimate of source parameters, the data provide an interesting snapshot of explosion dynamics.
V14B-07 17:30h
Seismicity of Mt. Veniaminof, Alaska: Implications for an active magmatic system.
A preliminary one-dimensional velocity model for Mt. Veniaminof, Alaska, was derived by a trial-and-error approach based on a group of 63 volcano-tectonic (VT) earthquakes with magnitudes ranging between 0 and 2.8 and mostly occupying a volume of 140 x 70 x 40 km. We tested 2,164 different velocity models and found that a modified version of the regional velocity model used in routine locations yielded the smallest RMS values of travel times, on average. The epicenters of earthquakes did not vary significantly among all the models tested and we infer that these locations are reasonably well constrained. The focal depths of earthquakes have been constrained to within one kilometer, or less, with the use of Wadati-Riznichenko diagrams. A systematic decrease with depth of the Vp/Vs ratio is apparent in the Mt. Veniaminof area but lateral variations in velocity cannot be resolved with the available data. We found that the hypocenters are located between 0.2 and 50.3 km below sea level and in the upper 20 km they cluster in two regions that diverge from the active vent and enclose a relatively aseismic volume. We found that VT earthquakes are uncommon close to the active vent and within the caldera but persistently occurr off to the sides of the SE-NW alignment of quaternary cones in the Mt. Veniaminof area. This suggests that the active magma system beneath the volcano consists of a dike or system of dikes whose pressure variations cause stresses in the crust and movement along pre-existing faults. From January 2004 until the time of this writing (August 2004) Mt. Veniaminof has had enhanced shallow local seismicity characterized by low-frequency earthquakes and tremor episodes near the vent, each associated with ash-laden puffs that reach a few kilometers above the active cone.
V14B-08 17:45h
Blasts, Roars, and Chugs at Tungurahua Volcano
Tungurahua volcano began a significant strombolian style eruptive cycle in May, 2004, which peaked in July. Between June 30 and August 12, 2004, we installed temporary seismic and infrasonic instruments on the southwest, northwest and northeast flanks of the volcano. Three PASSCAL type portable stations, recording at 125 samples/s included two mid-band Guralp 40T's and one broadband Guralp 3T instruments. Microphones were 1 Hz Larson-Davis. We recorded hundreds of explosions with high fidelity on both the infrasonic and seismic instrumentation. The signals can be classified as: -Explosion events: impulsive short duration blasts on infrasound and spindle-shape long-duration seismic signals. Amplitudes of the blast signals span a four order range from 0.1 to 180 Pa (July 21, 03h32 GMT, the largest signal recorded on the NW station). -Roaring events, composed of complex seismic and infrasound signals with large frequency bandwidths at stations close to the vent. -Chugging events, composed of sawtooth shaped infrasound signals in the infrasound records which follow small explosion signals. These events correspond to tremor episodes observed on seismic records. -Man-made explosions recorded by one station and which can be used for calibration purposes. Cluster analysis and waveform similitude will be used to identify variation in vent physics over time and space.