Array analysis of the seismic wavefield of long-period events and volcanic tremor at Arenal volcano, Costa Rica (original) (raw)
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Permanent tremor of Masaya Volcano, Nicaragua: Wave field analysis and source location
Journal of Geophysical Research, 1997
The Masaya Volcano, Nicaragua, is a basaltic caldera in a subduction zone. The permanent source of the volcanic tremor was located inside Santiago crater, at the lava lake's position and 400 m below the NE rim, and therefore corresponds to superficial magma activity. We used two tripartite arrays (90 m side), one semicircular array (r=120 m) in 1992, and two semicircular arrays (r=60 m) and a 2500 m long linear array radiating out from the source and on the flank of the crater in 1993. We used both a cross-spectrum method and a correlation method to determine the wave delay time between the reference station and the other stations of an array and to quantify the wave field. Using the delays therefore by intersecting the back azimuth wave directions from the arrays, we could pinpoint the source. Additionally, the correlation coefficients obtained as functions of frequency for the three components of motion confirm the inferred position of the source of tremor. The tremor's wave field is composed of comparable quantities of dispersed Rayleigh and Love surface waves, whose phase velocities lie in the ranges 730-1240 m/s at 2 Hz and 330-550 m/s at 6 Hz. The dispersive phase velocities were inverted to obtain crustal structures with a minimal number of layers. The resulting velocity models are similar for the northern and southern parts of the volcano. After geometrical spreading corrections, Q2Hz = 14 and Q3Hz =31 were determined along the northern linear array. The typical low velocities and low Q corresponding to the cone structure and are similar to those of other basaltic volcanoes like Puu Oo, Hawaii, and Klyuchevskoy, Kamchatka.
Journal of Volcanology and Geothermal Research, 2006
Typical records of volcanic tremor and explosion quakes at Arenal volcano are analyzed with a high-resolution time-frequency method. The main characteristics of these seismic signals are: (1) numerous regularly spaced spectral peaks including both odd and even overtones; (2) frequency gliding in the range [0.9-2] Hz of the fundamental peak; (3) frequency jumps with either positive or negative increments; (4) tremor episodes with two simultaneous systems of spectral peaks affected by independent frequency gliding; (5) progressive transitions between spasmodic tremor and harmonic tremor; (6) lack of clear and systematic relationship between the occurrence of explosions and tremor. Some examples of alternation between two states of oscillation characterized by different fundamental frequencies are also observed. Some tremor and explosion codas are characterized by acoustic and seismic waves with identical spectral content and frequency gliding, which suggests a common excitation process. We propose a source model for the tremor at Arenal in which intermittent gas flow through fractures produces repetitive pressure pulses. The repeating period of the pulses is stabilized by a feedback mechanism associated with standing or traveling waves in the magmatic conduit. The pressure pulses generate acoustic waves in the atmosphere and act as excitation of the interface waves in the conduit. When the repeating period of the pulses is stable enough, they produce regularly spaced spectral peaks by the Dirac comb effect and hence harmonic tremor. When the period stability is lost, because of failures in the feedback mechanism, the tremor becomes spasmodic. The proposed source model of tremor is similar to the sound emission process of a clarinet. Fractures in the solid or viscous layer capping the lava pool in the crater act as the clarinet reed, and the conduit filled with low velocity bubbly magma is equivalent to the pipe of the musical instrument. The frequency gliding is related to variations of the pressure in the conduit, which modify the gas fraction, the wave velocity and, possibly, the length of the resonator. Moreover, several observations suggest that two seismic sources, associated with two magmatic conduits, are active in Arenal volcano. They could explain in particular the apparent independence of tremor and explosions and the episodes of tremor displaying two simultaneous systems of spectral peaks.
Complex behavior and source model of the tremor at Arenal volcano, Costa Rica
Journal of Volcanology and Geothermal Research, 2006
Typical records of volcanic tremor and explosion quakes at Arenal volcano are analyzed with a high-resolution time-frequency method. The main characteristics of these seismic signals are: (1) numerous regularly spaced spectral peaks including both odd and even overtones; (2) frequency gliding in the range [0.9-2] Hz of the fundamental peak; (3) frequency jumps with either positive or negative increments; (4) tremor episodes with two simultaneous systems of spectral peaks affected by independent frequency gliding; (5) progressive transitions between spasmodic tremor and harmonic tremor; (6) lack of clear and systematic relationship between the occurrence of explosions and tremor. Some examples of alternation between two states of oscillation characterized by different fundamental frequencies are also observed. Some tremor and explosion codas are characterized by acoustic and seismic waves with identical spectral content and frequency gliding, which suggests a common excitation process. We propose a source model for the tremor at Arenal in which intermittent gas flow through fractures produces repetitive pressure pulses. The repeating period of the pulses is stabilized by a feedback mechanism associated with standing or traveling waves in the magmatic conduit. The pressure pulses generate acoustic waves in the atmosphere and act as excitation of the interface waves in the conduit. When the repeating period of the pulses is stable enough, they produce regularly spaced spectral peaks by the Dirac comb effect and hence harmonic tremor. When the period stability is lost, because of failures in the feedback mechanism, the tremor becomes spasmodic. The proposed source model of tremor is similar to the sound emission process of a clarinet. Fractures in the solid or viscous layer capping the lava pool in the crater act as the clarinet reed, and the conduit filled with low velocity bubbly magma is equivalent to the pipe of the musical instrument. The frequency gliding is related to variations of the pressure in the conduit, which modify the gas fraction, the wave velocity and, possibly, the length of the resonator. Moreover, several observations suggest that two seismic sources, associated with two magmatic conduits, are active in Arenal volcano. They could explain in particular the apparent independence of tremor and explosions and the episodes of tremor displaying two simultaneous systems of spectral peaks.
Geophysical Research Letters, 1997
Broadband seismic data recorded 2.3 km from the active vent of Arenal Volcano, Costa Rica, provide new constraints on tremor source processes. Arenal's tremor contains as many as seven harmonics, whose frequencies vary temporally by up to 75 percent, from initial values of 1.9 Hz for the first peak immediately following explosive eruptions to 3.2-3.5 Hz several minutes later. We infer that gas bubble concentration is variable within the conduit and also changes as a function of time, thereby changing the acoustic velocity. We infer that the source is a shallow, 200-660 m-long, vertically oriented 1-D resonator with matched boundary conditions, radiating seismic energy from a displacement antinode. Polarization analyses show that particle motion azimuths abruptly rotate, which may be explained by a decrease of the incidence angle. We suggest that energy is radiated predominantly from a displacement antinode that is changing position with time. Tremor consists mainly of transverse waves that travel at speeds of about 800 m/s. P waves in the magma conduit will couple very efficiently into S waves in the surrounding medium when there is virtually no impedance contrast between the two media for these two types of waves. The tremor at Arenal is similar to tremor at nine other volcanoes.
Bulletin of Volcanology, 2003
Following an initial phreatic eruption on 21 December 1994, activity at Popocatepetl has been dominated by fumarolic emissions interspersed with more energetic emissions of ashes and gases. A phase of repetitive dome-building and dome-destroying episodes began in March 1996 and is still ongoing at present. We describe the long-period (LP) seismicity accompanying eruptive activity at Popocatepetl from December 1994 through May 2000, using data from a three-component broadband seismometer located 5 km from the summit crater. The broadband records display a variety of signals, with periods ranging in the band 0.04-90 s. Long-period events and tremor with typical dominant periods in the range 0.3-2.0 s are the most characteristic signals observed at Popocatepetl. These signals appear to reflect volumetric sources driven by pressure fluctuations associated with the unsteady transport of gases beneath the crater. Very-long-period (VLP) signals are also observed in association with LP events and tremor. The VLP signals which accompany LP events display Ricker-like wavelets with periods near 36 s, whereas VLP signals associated with tremor waveforms typically show sustained oscillations at periods ranging up to 90 s. The spectra and particle motion patterns remain similar from event to event for the majority of LP and tremor signals analyzed during the time span of this study, suggesting a repeated, non-destructive activation of a common source. Hypocenters determined by phase pick analyses of selected LP events recorded by the seven-station, permanent Popocatepetl short-period network suggest that the majority of these events are confined to a source region in the top 1.5 km below the crater floor. The repetitive occurrences of VLP signals with closely matched waveform characteristics are consistent with a non-destructive reactivation of at least two sources. One source appears to coincide with the main source region of LP seismicity, whereas the second is a deeper source whose activity appears to be intimately linked with episodes of monochromatic tremor.
Journal of Volcanology and Geothermal Research, 2012
ABSTRACT Moment tensor inversions of volcanic tremor are synthetically tested and subsequently applied to a dataset recorded on Arenal volcano in 2005. We selected 15 harmonic tremor events showing an emergent but very clear onset with a fundamental frequency range of 0.8-1.5 Hz. These events constitute an excellent opportunity to broaden our knowledge of tremor generation models. The inversions were performed for a common source location, the position of which was retrieved through the evaluation of the joint probability density function of the misfit values obtained by the moment tensor inversion of all the events and all predefined source locations (grid search). Events are located beneath the summit crater, in a shallow position. The inversion procedure was tested through the use of synthetic tremor, generated using full wavefield numerical simulations. The maximum amplitude decomposition method is used to analyse the solutions of the synthetic tests. The results highlight the stability of the moment tensor solution when the whole length of the signal is used in the inversion procedure. Hence the whole length of the tremor is utilized to retrieve the source mechanism generating the 15 tremor events. A sliding window is utilized to investigate the time variability of our solution. A crack dipping 20° to the North-Northeast is reconcilable with all 15 tremor events. This mechanism is found to be constant for the entire length of each tremor and for different events.