Infrasonic Stations Research Articles

Analysis of infrasonic and seismic events related to the 1998 Vulcanian eruption at Sakurajima

We present results from a detailed analysis of seismic and infrasonic data recorded over a four day period prior to the Vulcanian eruptive event at Sakurajima volcano on May 19, 1998. Nearly one hundred seismic and infrasonic events were recorded on at least one of the nine seismic–infrasonic stations located within 3 km of the crater. Four unique seismic event types are recognized based on the spectral features of seismograms, including weak seismic tremor characterized by a 5–6 Hz peak mode that later shifted to 4–5 Hz. Long-period events are characterized by a short-duration, wide spectral band signal with an emergent, high-frequency onset followed by a wave coda lasting 15–20 s and a fundamental mode of 4.2–4.4 Hz. Values of Q for long-period events range between 10 and 22 suggesting that a gas-rich fluid was involved. Explosive events are the third seismic type, characterized by a narrow spectral band signal with an impulsive high-frequency onset followed by a 20–30 second wave coda and a peak mode of 4.0–4.4 Hz. Volcano-tectonic earthquakes are the fourth seismic type. Prior to May 19, 1998, only the tremor and explosion seismic events are found to have an infrasonic component. Like seismic tremor, infrasonic tremor is typically observed as a weak background signal. Explosive infrasonic events were recorded 10–15 s after the explosive seismic events and with audible explosions prior to May 19. On May 19, high-frequency impulsive infrasonic events occurred sporadically and as swarms within hours of the eruption. These infrasonic events are observed to be coincident with swarms of long-period seismic events. Video coverage during the seismic–infrasonic experiment recorded intermittent releases of gases and ash during times when seismic and acoustic events were recorded. The sequence of seismic and infrasonic events is interpreted as representing a gas-rich fluid moving through a series of cracks and conduits beneath the active summit crater.

Source constraints of Tungurahua volcano explosion events

The most recent eruptive cycle of Tungurahua volcano began in May 2004, and reached its highest level of activity in July 2004. This activity cycle is the last one of a series of four cycles that followed the reawakening and major eruption of Tungurahua in 1999. Between June 30 and August 12, 2004, three temporary seismic and infrasonic stations were installed on the flanks of the volcano and recorded over 2,000 degassing events. The events are classified by waveform character and include: explosion events (the vast majority, spanning three orders of pressure amplitudes at 3.5 km from the vent, 0.1–180 Pa), jetting events, and sequences of repetitive infrasonic pulses, called chugging events. Travel-time analysis of seismic first arrivals and infrasonic waves indicates that explosions start with a seismic event at a shallow depth (<200 m), followed ∼1 s later by an out-flux of gas, ash and solid material through the vent. Cluster analysis of infrasonic signals from explosion events was used to isolate four groups of similar waveforms without apparent correlation to event size, location, or time. The clustering is thus associated with source mechanism and probably spatial distribution. Explosion clusters do not exhibit temporal dependence.

The Morávka meteorite fall: 1. Description of the events and determination of the fireball trajectory and orbit from video records

Abstract— The Morávka (Czech Republic) meteorite fall occurred on May 6, 2000, 11:52 UT, during the daytime. Six H5–6 ordinary chondrites with a total mass of 1.4 kg were recovered. The corresponding fireball was witnessed by thousands of people and also videotaped by 3 casual witnesses. Sonic booms were recorded by 16 seismic stations in the Czech Republic and Poland and by one infrasonic station in Germany. A total of 2.5% of the fireball eyewitnesses reported electrophonic sounds. Satellites in Earth orbit detected part of the fireball light curve.In this first paper from a series of 4 papers devoted to the Morávka meteorite fall, we describe the circumstances of the fall and determine the fireball trajectory and orbit from calibrated video records. Morávka becomes one of only 6 meteorites with a known orbit. The slope of the trajectory was 20.4° to the horizontal, the initial velocity was 22.5 km/s, and the terminal height of the fireball was 21 km. The semimajor axis of the orbit was 1.85 AU, the perihelion distance was 0.982 AU, and the inclination was 32.2°. The fireball reached an absolute visual magnitude of −20 at a height of 33 km.

The Morávka meteorite fall: 2. Interpretation of infrasonic and seismic data

Abstract— The sound production from the Morávka fireball has been examined in detail making use of infrasound and seismic data. A detailed analysis of the production and propagation of sonic waves during the atmospheric entry of the Morávka meteoroid demonstrates that the acoustic energy was produced both by the hypersonic flight of the meteoroid (producing a cylindrical blast wave) and by individual fragmentation events of the meteoroid, which acted as small explosions (producing quasispherical shock waves). The deviation of the ray normals for the fragmentation events was found to be as much as 30° beyond that expected from a purely cylindrical line source blast. The main fragmentation of the bolide was confined to heights above 30 km with a possible maximum in acoustic energy production near 38 km. Seismic stations recorded both the direct arrival of the airwaves (the strongest signal) as well as air‐coupled P‐waves and Rayleigh waves (earlier signals). In addition, deep underground stations detected the seismic signature of the fireball. The seismic data alone permit reconstruction of the fireball trajectory to a precision on the order of a few degrees. The velocity of the meteoroid is much less well‐determined by these seismic data. The more distant infrasonic station detected 3 distinct signals from the fireball, identified as a thermospheric return, a stratospheric return, and an unusual mode propagating through the stratosphere horizontally and then leaking to the receiver.

Array tracking of infrasonic sources at Stromboli volcano

Infrasonic arrays are a powerful tool for volcanic monitoring and hazard assessment. Explosions were recorded at Stromboli using a small aperture array of 4 infrasonic stations, allowing precise vent location. The acoustic signals were delayed‐and‐summed, revealing the existence of two main groups of infrasonic waves. The NE crater produces short (&lt;3 s) high amplitude (20–80 Pa) pressure waves while the SW crater produces small acoustic pressure (10–30 Pa) with long (5–15 s) coda. The two groups reflect different in explosive styles and similar spectral content, centered on 5–6 Hz. When stacked together, acoustic waveforms for each crater reveal the same pressure pulse, which indicates a common source process. We infer that the acoustic onset at both craters is generated by the burst of a large gas bubble while the acoustic coda is controlled by a sustained pressure release.

Monitoring of atmospheric nuclear explosions with infrasonic microphone arrays

A review is given of the various United States programs for the infrasonic monitoring of atmospheric nuclear explosions from their inception in 1946 to their termination in 1975. The US Atomic Energy Detection System (USAEDS) monitored all nuclear weapons tests that were conducted by the Soviet Union, France, China, and the US with arrays of sensitive microbarographs in a worldwide network of infrasonic stations. A discussion of the source mechanism for the creation and subsequent propagation around the globe of long wavelength infrasound from explosions (volcanic and nuclear) is given to show the efficacy of infrasonic monitoring for the detection of atmospheric nuclear weapons tests. The equipment that was used for infrasound detection, the design of the sensor arrays, and the data processing techniques that were used by USAEDS are all discussed.

Infrasonic precursors to a Vulcanian Eruption at Sakurajima Volcano, Japan

The May 1998 eruption sequence of Sakurajima Volcano was monitored by ten infrasonic stations, ten seismometers, and a video camera. During this seismo‐acoustic experiment, we recorded hundreds of infrasonic tremor and long‐period events associated with seismic signals, and observed a progression from relative quiescence to a Vulcanian eruption. The number of infrasonic events increased with escalating volcanic activity, and the dominant character of the infrasonic signals changed from impulsive to emergent. At 22:17 of May 19, Sakurajima released ash and gases to a height of 2 km above the vent, an event that was recorded continuously by one infrasonic and two seismic stations. We present the experimental setup as well as a procedure through which infrasonic signals may be incorporated into future eruption monitoring and forecasting algorithms for open‐vent volcanic systems. In addition, our recordings suggest that infrasonic signals are more representative of processes occurring within the volcanic interior than are seismic signals, which are strongly altered by diffraction and scattering in the volcanic edifice.

Naturally occurring sources of infrasound

There are growing numbers of potential applications for infrasonic observing systems. The evolving International Monitoring System for the nuclear test ban treaty (including 60 infrasonic stations) offers exceptional opportunities to make infrasonic data sets widely available and to explore their uses in geophysical studies. This is especially true because of the ability to perform synergistic studies with seismic and hydroacoustic systems in a processing environment where diverse data sets can be readily melded. Natural infrasound sources such as avalanches, earthquakes, geomagnetic activity, meteors, ocean waves, severe weather, turbulence, and volcanoes offer great resources for international cooperative research programs. Sources of low-frequency sound from civilization processes offer a set of parallel opportunities. This paper will review the broad range of investigations possible using both individual and multiple infrasonic observing systems, as well as infrasonic systems in combination with other remote sensing systems.

Burning Oil Fields as a Source of Infrasonic Waves

Burning oil fields in the Persian Gulf may be considered as an intense source of infrasonic waves. In this paper propagation of these waves to high latitudes is discussed. Methods for sensitive detection of such a distant source are presented. Infrasonic data from two Swedish infrasonic stations, recording at 2 Hz (Lulea (65.7N, 22.5E) and Lycksele (64.6N, 18.7E) during the periods: February 6-March 14, 1991 and June 7–13, 1991) are examined in order to demonstrate the reception of these long distance infrasonic waves.

Caldera Collapse in the Galápagos Islands, 1968

The summit caldera of Isla Fernandina, a large, uninhabited basaltic shield volcano, was further enlarged by 1 to 2 km(3) in June 1968. A small quake and large vapor cloud on 11 June were followed 4 hours later by a remarkable volcanic ash cloud and, after another hour, by a major explosion recorded at infrasonic stations throughout the hemisphere. Seismic activity increased to a peak on 19 June, when more than 200 events per day were recorded by a seismograph 140 km away. Several hundred quakes were in the magnitude range 4.0 to 5.4 m(b), but few such events were recorded after 23 June. Unusual lightning accompanied the major cloud, and, during the evening of 11 June, distant observers reported red glow and flashes from the area. Fine ash fell that night and much of the next day to distances at least 350 km from the volcano.

Infrasonic-Noise Power Level and Wind-Velocity Correlation Study

Measurements of infrasonic-noise power and analyses of wind-velocity data were made. Correlation was found between infrasonic-noise power levels at a particular location and wind-velocity measurements taken at a distance of approximately 11 miles away. This information is useful for studying infrasonic-signal data, improving the signal-to-noise ratio in physical measurements of infrasonic signals, and for the selection of sites for infrasonic stations.

Ionospheric Motions Caused by Rayleigh Waves

Rayleigh waves of considerable amplitude from the Alaskan earthquake of March 1964 passed across the United States of America. The waves radiated sound into the atmosphere. The sound pressures measured at infrasonic stations at Boulder and Washington were about 20 dyn/cm2, corresponding to vertical surface motions of 12 cm/sec. The sound waves had periods of the order of 25 sec, and were radiated almost vertically upward. Waves starting upward with an amplitude of 12 cm/sec will increase to 104 cm/sec at an altitude of about 160 km. At higher altitudes, shock waves are formed, and the oscillatory vertical motions are estimated to be of the order of hundreds of meters/second. Observations of the ionospheric motions were made by sending radio waves almost vertically upward at Boulder, and measuring the Doppler-shifted waves reflected from the ionosphere back down to the ground. Doppler shifts, caused by sound from the Rayleigh waves, of more than 3 cps occurred for radio waves at 4 Mc/sec. These correspond to vertical motions of the ionosphere of more than 100 m/sec.