1U.S. Geological Survey.
2Philippine Institute of Volcanology and Seismology.
During the 1991 Pinatubo volcanic crisis, seismicity was monitored in part by a Seismic Spectral Amplitude Measurement (SSAM) system. This personal-computer-based system continuously monitors seismic amplitudes within 16 user-defined frequency bands. The SSAM system is particularly useful for monitoring the spectral character of seismic signals over time. In this paper, SSAM data are displayed in time-frequency plots where the spectral amplitude is represented by an 11-step gray scale that is produced by a computer program called SSAM_VU. The program allows for a number of user-defined parameters for producing the spectrograms. We review the various effects that each parameter can have on the appearance of the spectrogram and its interpretation.
The 1991 eruption of Mount Pinatubo produced a wide variety of seismicity including swarms of volcano-tectonic earthquakes, long-period events, volcanic tremor, and explosive eruptions. Swarms of volcano-tectonic earthquakes and periods of continuous volcano-tectonic seismicity exhibited strong spectral peaks between 2.5 and 4.5 hertz and occurred during the emplacement of a lava dome on June 5-7, prior to the onset of explosive activity (June 8-12), and following the cataclysmic eruption on June 15. Long-period events and volcanic tremor produced a strong signal between 0.5 and 2.5 hertz. Energy in this frequency range increased dramatically between June 1 and the cataclysmic eruption on June 15. Explosive eruptions generated signals with spectral peaks between 0.5 and 1.5 hertz. SSAM data from the cataclysmic eruption on June 15 shows a shift in the spectral peak from 0.5 to 1.5 hertz to between 1.5 and 3.5 hertz that is coincident with the onset of magnitude 4+ earthquakes. Although SSAM did not play a major role in forecasting eruptive activity at Pinatubo, analysis presented here, as well as experience at other volcanoes, indicates it is a valuable tool for quickly analyzing seismicity at restless volcanoes.
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The cataclysmic eruption of Mount Pinatubo on June 15, 1991, was preceded by at least 10 weeks of unrest characterized by increasing seismic activity, high SO2 emissions, emplacement of a lava dome, and numerous smaller explosive eruptions (Punongbayan and others, this volume). In response to the initial unrest, a seven-station radiotelemetered seismic network was deployed around the volcano during late April and early May (Lockhart and others, this volume). A data acquisition and analysis system at Clark Air Base (Murray and others, this volume) received the data and provided rapid calculation of hypocenters, Real-time Seismic Amplitude Measurements (RSAM) (Endo and Murray, 1991), and Seismic Spectral Amplitude Measurements (SSAM) (Rogers, 1989).
Unlike earthquake location techniques and RSAM, SSAM is a relatively new technique in real-time volcano monitoring. At the time of this writing, eruptive cycles at only three volcanoes have been monitored by use of the SSAM system, and only data from the 1989-90 eruption of Redoubt Volcano, Alaska, have been analyzed in detail (Stephens and others, 1994). SSAM continuously monitors seismic signals and calculates 1-min average signal strengths in each of 16 specified frequency bands for each seismometer. It is ideal for monitoring changes in spectral character through time, particularly when a certain type of event with a given characteristic frequency dominates. During the Redoubt eruptions, SSAM was particularly valuable for detecting and tracking small swarms of long-period events and low-level volcanic tremor that preceded many of the tephra-producing eruptions (Stephens and others, 1994).
Our goal in this paper is to examine the precursory and eruption seismicity at Mount Pinatubo between May 13 and June 18, 1991, using SSAM measurements, and to evaluate the SSAM system as a monitoring tool. First we review the seismic instrumentation at Pinatubo and the technical aspects of the SSAM system. This is followed by a discussion of methods and techniques for analyzing SSAM data. We then develop a chronology based on SSAM, helicorder records, and pertinent observations reported by others, which is followed by a brief discussion of the implications of SSAM observations for the source processes of seismicity during various time periods prior to and during the June 15 eruption. We conclude with an evaluation of the SSAM system as it was used at Pinatubo and recommendations for future SSAM deployments. The chronology presented here is only intended to provide context to the SSAM data and is intentionally kept brief. All times and dates are referenced to Philippine time (to convert to G.m.t., add 8 h). Detailed chronologies of the 1991 eruption are presented by Punongbayan and others, (this volume), Hoblitt, Wolfe, and others, (this volume), Harlow and others, (this volume), Wolfe, (1992), Wolfe and Hoblitt (this volume).
Following the initial precursory explosions in April 1991, a network of seven telemetered high-gain, short-period seismometers was installed around Mount Pinatubo. The installation was completed on May 13. The stations were from 1 to 19 km from the summit of the volcano, and except for one three-component 2-Hz station (PPO), all were single-component vertical 1 Hz seismometers (fig. 1). Lockhart and others (this volume) describe the installation, operation, and technical specifications of the network. Data from this network were telemetered to Clark Air Base and recorded by a computer system described by Murray and others (this volume). The network remained intact until June 11, when eruptive activity destroyed station UBOZ and vandalism at a telemetry repeater caused the loss of signals from PPO and BURZ. PIEZ, GRNZ, and BUGZ were damaged by pyroclastic flows during eruptions on June 15. Only station CABZ survived the eruption, although recording of its signal was disrupted on June 15.
Figure 1. Locations of telemetered seismic stations (black triangles) in the Pinatubo network between May 13 and June 18, 1991.
The SSAM system acquires data on a PC/AT-compatible computer in conjunction with the program MDETECT (Tottingham and others, 1989) and a digital signal processor (DSP) board (Rogers, 1989). More recent versions of the SSAM system run in conjunction with the program XDETECT (Rogers, oral commun. 1992). Sixteen analog seismic signals are digitized at 100 samples per second. After 512 samples for each channel have been collected, the samples are transferred to the DSP board and a Fast-Fourier-Transform (FFT) is computed for each channel. As a result of limitations in the speed of the computer system used at Pinatubo, the amplitudes of individual spectral lines were approximated by use of a method described by Gledhill (1985). The average spectral amplitude is determined for a band by averaging the values for all of the spectral lines that fall within the band. Every minute the average FFT value for all 16 bands is calculated and stored on computer disk for each station. A comprehensive description of the SSAM system is given by Stephens and others (1994).
At Pinatubo, the first 10 SSAM bands were defined from 0.5 to 10.5 Hz in increments of 1 Hz. The remaining six bands each covered a broader spectrum in order to determine if SSAM could be used for specific applications such as replacing RSAM or detecting lahars. Data from these wider bands were not analyzed in this study.
As the SSAM system is a fairly recent development, its response characteristics are not completely understood. At the time of this writing we know little about the type and amount of distortion that is introduced if the seismic signal is electronically clipped. In theory, the square waves produced by electronic clipping should only introduce frequencies higher than those ordinarily of interest in volcanic environments, though the telemetry system may introduce some distortion (Chris Stephens, oral commun., 1993). Much of the data from stations close to the volcano (PIEZ, BUGZ, GRNZ) is continuously electronically clipped starting about 1230 on June 14. Data from CABZ began to clip continuously about 1345 on June 15.
A variety of techniques have been developed to display and analyze SSAM data. These include bar graphs of amplitude plotted against frequency for individual 1-min samples (see fig. 3 of Murray and others, this volume), time series of individual frequency bands (see Hoblitt, Wolfe, and others, this volume), contoured two-dimensional or pseudo-three-dimensional time-frequency-amplitude plots (Marso and Murray, 1991), time-frequency plots that use a gray scale in which the density of dots is proportional to the spectral amplitude (Stephens and others, 1994), and time-frequency-amplitude plots where the spectral amplitude is displayed as a contoured surface (see fig. 4 of Murray and others, this volume).
The primary method of display in this paper is a time-frequency-amplitude plot in which the spectral amplitude corresponds to an 11-step gray scale. The different steps are constructed by varying the spacing between black lines. The lowest amplitude is represented by white (infinite space between lines), the highest amplitude by solid black (no space between lines). These spectrograms are produced by the computer program SSAM_VU, which runs on a PC/AT-compatible computer. Figures 2-12 are examples of spectrograms produced by SSAM_VU. When displayed on a computer monitor, a color scale is used instead of a gray scale. The gray scale interval is user defined and can range between 1 and 99 amplitude counts, which allows even very strong seismic signals to be analyzed. The program provides an option to display the spectrum from 0 to 5 Hz or 0 to 10 Hz and also allows a user-defined time scale that can range from 1 to 144 h (see fig. 9 for an example of a spectrogram with an expanded time scale). To overcome limitations in the resolution of printers and video displays, data are plotted for individual cells, the time represented by each cell can vary from 1 to 60 min. The program allows the user to specify whether the value represented by the cell is the highest data value or an average of the values within the time-period of the cell and allows the user to select a greater cell duration if desired. Generally, in this paper, the highest data values are shown.
A number of physical and computational factors can greatly affect the appearance of spectrograms produced from SSAM data when SSAM_VU is used. The physical factors include the source process of the seismicity, the effects of attenuation between the source and receiver, and the response characteristics of the instrumentation. The computational factors include whether the high or average values are plotted for a given cell, the duration of the cell, and the amplitude interval used for defining the gray scale.
To illustrate the effect of the computational factors, figure 2 shows a number of spectrograms from the swarm of volcano-tectonic earthquakes on June 7 demonstrating the effect of different display parameters in SSAM_VU. Figures 2A and 2B show spectrograms for stations PIEZ and UBOZ with high values for each 3-min cell and an amplitude interval of 2. When high values are selected the spectrum is more heavily influenced by the frequency characteristics of the largest individual event in a given cell. Note that there is less high-frequency energy at PIEZ than at UBOZ. This is likely the result of attenuation of the higher frequencies caused by the greater distance from the volcano (10 km for PIEZ and 1 km for UBOZ). At Redoubt Volcano, significant spatial variations also were observed in swarms of long-period events at different stations in the network (Stephens and others, 1994). Choosing average values instead of highest values favors the overall character of the seismicity because the effects of distinct events are averaged over the duration of the cell. Figures 2C and 2D show the same swarm at PIEZ and UBOZ for averaged values in 3-min cells. Figure 2E shows highest values at station PIEZ plotted in 10-min cells. The longer cell puts greater emphasis on the spectral character of larger individual events because the cell represents the highest value in a longer time. Figure 2F shows high values at PIEZ in 3-min cells at an amplitude interval of 4, twice that of figures 2A-2E. Increasing the amplitude interval shows that the seismicity between 1500 and 1600 has only about half the energy as that between 1630 and 1730. An important consideration in viewing SSAM records when the amplitude scale is increased is that only the strongest portion of the spectrum is preserved. Consequently, if a very strong swarm of volcano-tectonic earthquakes with a strong spectral peak between 3 and 4 Hz and weak volcanic tremor peaked at 1.5 Hz are occurring simultaneously, the peak at 1.5 Hz may not rise above the lowest graduation.
Figure 2. Spectrograms illustrating the effects of the various input parameters for SSAM_VU. Spectrograms are from stations PIEZ and UBOZ for the 9-h interval from 1100 to 2000 on June 7 during a swarm of volcano-tectonic earthquakes associated with dome emplacement. A and B show data from stations PIEZ and UBOZ with 3-min cells and high values. C and D show averaged values in 3-min cells. E shows highest values in 10-min cells at PIEZ, and F shows highest values in 3-min cells at a doubled amplitude interval. See text for further discussion.
The seismic activity that preceded the cataclysmic eruption on June 15 is a complex sequence comprising volcano-tectonic earthquakes, long-period events, hybrid events, and dome-building and explosive eruptions (Harlow and others, this volume; White, this volume). In characterizing this seismicity, we have relied primarily on the SSAM and helicorder records. Earthquake hypocenters and waveforms are described by Harlow and others (this volume), White (this volume), and Mori, White, and others (this volume).
In describing the various waveforms and event types at Pinatubo, we use terminology and event classifications similar to that developed by Chouet and others, (1994), Lahr and others (1994), Power and others (1994), and Harlow and others (this volume). This classification is based on the present understanding of the physical processes associated with the seismic source. It identifies volcano-tectonic (VT) earthquakes as representing a purely elastic source and long-period (LP) events, which represent a more complex process involving gas and liquid phases (Chouet, 1992). We do not identify events as hybrids in this paper, as SSAM and helicorder records do not provide adequate spectral information for individual events. Volcanic tremor is generally defined as a continuous signal generated within the volcano (Koyanagi and others, 1987). Volcanic tremor frequently occurs in close association with long-period events (Chouet and others, 1994). In this paper we use volcanic tremor (or just tremor) to refer to continuous signals with frequencies between 0.5 and 3.5 Hz in which we cannot identify individual events on helicorder records. The term "sustained long-period seismicity" as developed by Stephens and others (1994) describes periods when both volcanic tremor and long-period events were occurring simultaneously. We use the term "continuous volcano-tectonic seismicity" to describe periods when VT earthquakes are occurring so rapidly the individual events became indistinguishable. Those events associated with the forcible ejection of steam and ash from the volcano are referred to as explosive eruptions.
Between April 4 and June 3, the activity at the volcano was characterized by vigorous fumarolic activity, high gas output, and VT earthquakes (Punongbayan and others, this volume). Seismicity during this interval consisted primarily of VT earthquakes and indistinct periods of volcanic tremor that occurred in association with a few LP events. Most of the earthquakes during this period were located on the northwest flank of the volcano between 2 and 7 km depth. Harlow and others (this volume), have described this as the metastable phase, because the seismic activity was elevated but relatively stable. SSAM records from this period are fairly quiet because the individual events were too infrequent to generate a sustained signal.
Between roughly June 1 and June 3, locatable VT earthquakes began to occur beneath the summit of Mount Pinatubo (Harlow and others, this volume). At 1939 on June 3 a small explosion occurred which produced minor amounts of ash (Punongbayan and others, this volume). This event was followed by 30 to 40 mins. of volcanic tremor (fig. 3). Figure 3 also shows a variety of microseismic and cultural noise. The occurrence of volcanic tremor and the rate of VT earthquakes increased over the next several days and by early on June 6 had reached the point where their spectral character was recorded well on the SSAM system. Data from station PIEZ for June 6 and 7 show that most of the energy associated with these earthquakes had a fairly broad spectrum (between 0.5 to 9.5 Hz), and peaked between 2.5 and 4.5 Hz (fig. 4). Figure 2 shows this period of seismicity in greater detail. These spectral characteristics are generally attributed to VT earthquakes (Lahr and others, 1994). Near the end of June 6 and for the first 3 h on June 7, the rate of VT earthquakes fluctuated several times, reaching a rate of about 1 per min by 1100. By 1600 they were occurring so frequently they formed a continuous signal on the helicorder records that we call continuous volcano-tectonic seismicity. Throughout this period of continuous volcano-tectonic seismicity the spectrogram shows a fairly strong signal that is present at all frequencies and peaks between 2.5 and 4.5 Hz. This seismicity culminated in a small explosion at 1700, which was followed by roughly 8 h of increased activity that slowly returned to background levels (figs. 4 and 5).
Early on the morning of June 8 (0800) a lava dome was seen on the northwest flank of the volcano (Hoblitt, Wolfe, and others, this volume). Tilt measurements suggest that extrusion of lava began about 1700 on June 7 (Ewert and others, this volume), coinciding with the height of the continuous volcano-tectonic seismicity.
Figure 3. SSAM record from station PIEZ on June 3 plotted with high values in 3-min cells at an amplitude interval of 2. Strong pulses noted by the letter H at roughly 0900 and 1100 reflect helicopter landings near the station (note increased signal in higher bands). Activity between pulses reflects geophysicists' movements while servicing the station (battery change for radio transmitter). Diffuse signal between roughly 1350 and 1430 with a slight peak in the 3.5- to 4.5-Hz band (noted by letter R) results from a strong rainstorm. Arrow at 1928 notes onset of explosion which was followed by approximately 45 min of volcanic tremor. Additional spikes during the day result from large volcano-tectonic earthquakes, short bursts of tremor and entrained long-period events, and a second small explosion at 2258 (noted by second arrow).
Figure 4. SSAM record from station PIEZ on June 6 and 7 plotted with high values in 7-min cells at an amplitude interval of 2. Spectrogram shows signature of short swarms of volcano-tectonic earthquakes on June 6. These intensified to form a continuous volcano-tectonic signal on June 7 that preceded the extrusion of magma. A small explosion at 1700 (noted by arrow) coincided with the onset of magma extrusion.
Figure 5. SSAM record from station PIEZ on June 8 and 9 plotted with high values in 7-min cells. Strong swarms of volcano-tectonic earthquakes form the broad spectral signals noted by the solid triangles. A strong period of tremor between approximately 1100 and 1400 on June 9 forms the prominent signal in the 0.5- to 1.5-Hz band noted by the letter T.
From June 8 to June 12, the seismicity was characterized by discrete swarms of VT earthquakes and a gradual increase in tremor throughout the phase. Small amounts of ash were continuously emitted from the margins of the lava dome, and visual observations suggested that the lava dome continued to grow until it was destroyed on June 12 (Hoblitt, Wolfe, and others, this volume).
Following the emplacement of the lava dome on June 7, seismic activity returned to near pre-June levels for much of June 8. Several short swarms of VT earthquakes occurred on June 9, the strongest of which was an hour-long swarm at roughly 0800, and this rivaled the strength of any previously recorded based on SSAM records. Starting at approximately 1100 a very strong 3-h period of volcanic tremor began with a dominant low-frequency signal in the 0.5- to 1.5-Hz band (fig. 5). Following this energetic low-frequency tremor the volcano entered a period of cyclic swarms of VT earthquakes that continued until the first plinian eruption at 0851 on June 12. On June 9 these swarms occurred at intervals of approximately 2 to 3 h; each episode lasting about an hour. These swarms form strong vertical stripes on SSAM spectrograms (figs. 5 and 6), but lack some of the higher frequency energy (>4.5 Hz) associated with the swarms observed prior to the dome emplacement on June 7. A less intense but more protracted swarm occurred between approximately 2145 on June 9 and 0300 on June 10. SSAM measurements indicate that this swarm contained a higher percentage of energy below 2.5 Hz, which is typical of volcanic tremor. The intensity and frequency of these swarms increased on June 10, while the duration of individual swarms declined (fig. 6).
The protracted earthquake swarm starting at 2145 on June 9 initiated a gradual increase in LP seismicity that continued until the plinian eruption on June 12 (fig. 6). A more distinct increase in the level of long-period seismicity occurred at about 2205 on June 11, coincident with the destruction of seismic station UBOZ by a small pyroclastic flow (Lockhart and others, this volume). A small explosive eruption at 0330 was followed by roughly 2 h of low-level volcanic tremor. Then a 4-h period of relative seismic quiescence immediately preceded the first plinian eruption at 0851 on June 12 (fig. 6).
Figure 6. SSAM record from station PIEZ on June 10, 11, and first 10 h of June 12. Plot reflects high values in 8-min cells at an amplitude interval of 2. Swarms of volcano-tectonic earthquakes are noted by solid triangles. A small explosion at 0330 on June 12 and the first plinian eruption at 0851 on June 12 are noted by arrows.
Following the initial plinian eruption on June 12, the general level of seismicity increased dramatically. The June 12 eruption initiated a series of explosive eruptions that increased in rate of occurrence until the cataclysmic eruption on June 15. The onset times of many of these explosions correlate closely with atmospheric pressure waves observed on a microbarograph at Clark Air Base. A comparison of pressure waves and the seismic record indicates that large explosive eruptions occurred at 2252 on June 12, 0841 on June 13, 1309, 1410, 1516, 1853, 2218, 2320, on June 14, 0115, 0257, 0555, 0810, 1027, 1117, 1158, 1222, 1252, 1315 on June 15 (Hoblitt, Wolfe, and others, this volume). Between each of these eruptions, large VT earthquakes, swarms of LP events, and volcanic tremor occurred. Seismicity during this interval is very complex, and the record is often difficult to interpret, as a variety of event types were occurring simultaneously.
Because of the increase in seismic intensity, the amplitude scale on spectrograms produced by SSAM_VU needed to be increased by a factor of 6 to stay on scale (figs. 7 and 8). In viewing spectrograms from this period, recall that only the strongest portion of the spectrum is preserved when the amplitude scale is increased. Spectrograms from PIEZ for June 12 through 15 (figs. 7 and 8) show that the spectrum was dominated by two prominent peaks. The first is between 2.5 and 4.5 Hz and correlates with swarms of VT earthquakes on the helicorder records and intervals of continuous volcanic-tectonic seismicity. The second peak is in the 0.5- and 1.5-Hz band and correlates with episodes of volcanic tremor, swarms of LP events, and intervals of sustained long-period seismicity.
Two strong swarms of VT earthquakes followed the 0851 eruption on June 12 (fig. 7). Following the 2252 eruption on June 12, the volcano-tectonic bands began a gradual increase that persisted at station PIEZ until it was destroyed by a pyroclastic flow at 1409 on June 15. The gradual increase was punctuated by several more intense swarms on June 13 and 14 (figs. 7 and 8).
Swarms of LP events and tremor followed a similar pattern from June 12 to 15 (figs. 7 and 8). A strong burst of tremor coincided with a strong swarm of VT earthquakes between roughly 1530 and 1700 on June 12 (fig. 7). LP seismicity also began a gradual increase following the 2252 eruption, which continued to intensify until PIEZ was destroyed on June 15.
A very unusual seismic signal began at about 1830 on June 12 as a continuous string of VT earthquakes with a strongly peaked signal in the 3.5- to 4.5-Hz band at station PIEZ. The signal lasted a little over 1.25 h, and three times the peak in seismic energy shifted to progressively lower frequency bands for short periods. During approximately the last 20 min of the signal, the spectral peak shifted from the 3.5- to 4.5-Hz band to the 2.5- to 3.5-Hz band, and then to the 1.5- to 2.5-Hz band (fig. 9). The signal was localized at the volcano, as it did not record well at station CABZ, and there was no known associated eruptive activity. Throughout the analysis of the Pinatubo seismicity we have referred to the this event as "the groan."
To gain further insight into the seismicity between June 12 and 18 we turn to station CABZ, which, at a distance of 19 km was the only station to survive the eruption on June 15 (figs. 10, 11, and 12). Fortunately, CABZ was also the station with the lowest gain (Lockhart and others, this volume); therefore, it was more suited to record the rapidly escalating seismicity. The signal from station CABZ did not experience significant electronic clipping until June 15.
The individual explosive eruptions generally form signals having energy from 0.5 to 10.5 Hz on SSAM spectrograms (figs. 6, 7, 10, and 12), with the strongest spectral peak in the 0.5- to 1.5-Hz band (fig. 10). The SSAM record suggests that relative seismic quiescence preceded the explosive eruptions at 0851 and 2252 on June 12 and at 0115 on June 15. In contrast, increases in LP seismicity preceded explosive eruptions at 0841 on June 13, 1309, 1516, 1853, 2218, on June 14, and 0257 on June 15. Following the 0257 eruption on June 15, the level of background seismicity remained elevated, and it is difficult to establish a clear relation between seismicity and later individual explosive eruptions.
Figure 7. SSAM record for June 12 and 13 plotted with high values in 7-min cells and an amplitude interval of 12. The amplitude interval is increased by a factor of 6 from spectrograms shown in the previous figures. Strong swarms of volcano-tectonic earthquakes form the signals in the 2.5- to 3.5- and 3.5- to 4.5-Hz bands (noted by the solid triangles). Increased occurrences of volcanic tremor generate the strong signals in the 0.5- to 1.5-Hz band. Note the gradual escalation in both volcano-tectonic and long-period seismicity throughout June 12 and 13. Explosive eruptions at 0851 and 2252 on June 12 and at 0841 on June 13 are marked with arrows. ³The groan² is noted with the letter G.
Figure 8. SSAM record for station PIEZ on June 14 and 15 plotted with high values in 7-min cells and an amplitude interval of 12. This plot shows the continued escalation of both volcano-tectonic earthquakes and long-period seismicity on June 14 and 15. Station PIEZ was destroyed by a pyroclastic flow at 1409 on June 15. Arrows correspond to times of known explosions. See text for further discussion.
Figure 9. SSAM record from station PIEZ between 1800 and 2000 on June 12 plotted with data in 1-min cells at an amplitude interval of 12. This spectrogram shows the strong period of continuous volcano-tectonic seismicity that began at roughly 1830, which we have called ³the groan.² The peak in energy shifted to lower frequency bands several times during the signal. During the last 20 min the signal shifted in a stepwise manner to lower frequency bands. See text for further discussion.
Figure 10. SSAM record from station CABZ for June 13 through 15. Spectrogram shows highest values in 10-min cells at an amplitude interval of 4. Arrows note the times of known explosive eruptions.
Figure 11. SSAM record for station CABZ for June 15 plotted with high values in 3-min cells at an amplitude interval of 20. This spectrogram shows the eruptions early on June 15 and the onset of the cataclysmic eruption at roughly 1342 (as noted by last arrow). The first 3 h of the cataclysmic eruption are characterized by a strong signal in the 0.5- to 1.5-Hz band. The onset of strong energy at roughly 1630 approximately corresponds to the onset of large volcano-tectonic earthquakes recorded on distant stations as the caldera formed. See text for further discussion.
Figure 12. SSAM record for station CABZ for June 13 through 18 plotted with high values in 21-min cells at an amplitude interval of 8. This spectrogram provides a synoptic view of the temporal changes in seismicity before and after the cataclysmic eruption on June 15. The precursory explosions on June 13-15 are noted by arrows. The strong signal crossing all bands corresponds to the cataclysmic eruption. The strong peak between 1.5 and 3.5 Hz on June 16-18 reflects the vigorous volcano-tectonic earthquake activity resulting from the stress adjustments in response to the removal of magma.
At approximately 1342 on June 15, the volcano began to erupt continuously, eventually forming a 2.5-km-wide caldera where the previous summit had been. Volume estimates of erupted material range between 3.7 and 5.3 km3 of dense magma. (W.E. Scott and others, this volume). The intense seismicity associated with the eruption lasted over 9 h, and recording of data from CABZ was interrupted when various components of the acquisition system were temporarily removed during an evacuation (0730 to 1012 June 15) and when power to the acquisition system was lost (2014 to 2047 and 2102 to 2343 June 15) (figs. 8, 10, 11, 12). A sequence of large (Mb 4.3 to 5.7) earthquakes began at 0739 on June 15. Most of these events occurred between 1616 on June 15 and 0032 on June 16, when their approximate rate was 1 event every 15 min. Forty-eight events were located by the worldwide seismographic network on June 15 and 16 (U.S. Geological Survey, 1991). Additional aspects of the seismicity as observed at distant stations are discussed by Kanamori and Mori (1992), and Zürn and Widmer (this volume).
The SSAM record for June 15 shows that the energy at the onset of the cataclysmic eruption was concentrated in the lowest frequency band and later abruptly shifted to higher bands. The eruption shows as the dominant wide vertical black stripe in figure 10. By increasing the amplitude scale by a factor of five (fig. 11) we see that the eruption began with a spectral peak in the 0.5- to 1.5-Hz band. At approximately 1630 the energy in the lowest band declined and a new dominant peak developed between 1.5 and 3.5 Hz. This change roughly coincides with the onset of large VT earthquakes, as detected on the worldwide network.
The spectral peak between 1.5 and 3.5 Hz at station CABZ dominated throughout much of the remainder of June (fig. 12). helicorder records from June 16 to 18 show large VT earthquakes which occur at a rate of roughly 1 per minute, as well as strong volcanic tremor.
The SSAM records from Pinatubo provide numerous examples of a wide variety of signals common at active volcanoes. In this section we review the character of the SSAM record and discuss the physical implications for the various processes active at the volcano.
The Pinatubo eruption provided the first opportunity for an SSAM system to record strong swarms of VT earthquakes and what we have called continuous volcano-tectonic seismicity. At stations PIEZ, UBOZ, and CABZ, VT seismicity had a broad spectrum generally ranging from 1.5 to 9.5 Hz with a well-defined peak generally between 1.5 and 4.5 Hz. Using SSAM makes it easy to distinguish between episodes of continuous VT seismicity and episodes of LP seismicity. The SSAM record is dominated by VT seismicity during the June 5-7 episode of dome emplacement, during the cataclysmic buildup, and after about 1630 on June 15, during the cataclysmic eruption. All of these episodes take place when we would expect changes in magmatic pressure to exert excessive stress on the brittle rock surrounding the Pinatubo magmatic system. The VT earthquakes that preceded the extrusion of the lava dome represent the forceful passage of magma to the surface. This interpretation is supported by the observed shoaling of hypocenters (Harlow and others, this volume) and ground deformation (Ewert and others, this volume). The VT earthquakes during June 12 to 15 reflect the destruction, induced by the magma's destabilization, of the brittle rock above the Pinatubo magma body. Most located earthquakes during this period occur at shallow depth (Harlow and others, this volume). The numerous VT earthquakes initiated by the June 15 eruption are related to the adjustment of stresses resulting from the evacuation of material from the Pinatubo magma chamber (Mori, White, and others, this volume).
LP seismicity has been attributed to the dynamics of pressurized fluids associated with the magma (Chouet and Shaw, 1991; Chouet, 1992; and Chouet and others, 1994). Long-period seismicity became increasingly prevalent on the SSAM record between June 3 and 15. Tremor and LP events generally formed a strong signal in the 0.5- to 1.5-Hz band (figs. 5, 7, and 11). Episodes of LP seismicity may reflect time periods when pressurized fluids could accumulate. The increased occurrence of LP seismicity is somewhat expected following the emplacement of the lava dome. The magma associated with the lava dome is thought to have formed a seal on the magmatic system behind which volatiles could accumulate (White and others, this volume; Hoblitt, Wolfe, and others, this volume). This interpretation is supported by observations of decreased gas flux from the volcano beginning several days prior to the extrusion of magma (Daag, Tubianosa, and others, this volume). Between June 12 and 15, increased LP seismicity generally preceded explosive eruptions (fig. 10). This seismicity likely represents the pressurization of fluids that were trapped behind temporary barriers that were destroyed in the ensuing eruption.
The SSAM signals from individual explosive eruptions peak strongly in the 0.5- to 1.5-Hz band (figs. 7, 10 and 11), which is much like the LP seismicity preceding the events. This peak dominates throughout the duration of the eruption signals. SSAM measurements of explosive eruptions at Redoubt Volcano, Alaska, also show that LP seismicity and eruptions share similar spectral peaks. Stephens and others (1994) suggest that the similarity in spectra is an indication that the source mechanism of the precursory LP seismicity continued to be active throughout each eruption. They suggest that the stronger signals observed during eruptions may be produced by the increased flow of magmatic fluids through cracks as obstructions at the vent are removed. This physical interpretation agrees with the observations of explosive eruptions at Pinatubo between June 12 and 15.
The signal between 1830 and 1940 on June 12, which we have called "the groan", is dominated by energy concentrated in the 3.5- to 4.5-Hz band. We feel this signal represents the movement of magma and associated volatiles into a new system of cracks and passageways. The continuous VT seismicity represents the brittle failure of competent rock as new cracks were forced open. The occasional shifts to lower frequencies may reflect the movement of fluids into the new cracks and passages.
The cataclysmic eruption on June 15 began with 3 h of seismicity with a strong spectral peak in the 0.5- to 1.5-Hz band (fig. 11). The SSAM data on June 15 must be viewed with some caution, as the data were electronically clipped. This LP seismicity likely represents increased flow of fluids through cracks resulting from the removal of obstructions at the vent. The onset of strong energy in the VT bands (1.5- to 3.5-Hz) at roughly 1630 likely represents the onset of large stress adjustments in response to the removal of magma from beneath Mount Pinatubo. The onset of large earthquakes (Mb4.3+) suggests this is approximately the time that the summit of Mount Pinatubo was destroyed and the caldera began to form. The coincident decline in LP events would result from the destruction of the cracks and passageways through which the fluids were transported.
The strong VT peak between 1.5 and 3.5 Hz continued beyond June 16 (fig. 12). The continued occurrence of VT earthquakes represents stress adjustments in response to the removal of magma from beneath Mount Pinatubo. That this signal continued in the bands from 1.5 to 3.5 Hz suggests that the clipped signals on June 15 did not drastically affect the data in these bands.
During the 1989-90 eruption of Redoubt Volcano, SSAM measurements proved valuable both for detecting small swarms of LP events that preceded many of the tephra-producing eruptions and in reconstructing the details of the seismic record during intense periods of seismic activity. At Pinatubo, SSAM played only a minor role in formulating eruption forecasts. The strength of the various seismic signals was much greater than at Redoubt; consequently, it was much easier to recognize these signals on standard helicorder records. Additionally, at the time of the Pinatubo crisis, analysis software for SSAM data on PC/AT-compatible computers was not as flexible as that which now exists. These advances in software design should make the incorporation of SSAM data in real-time interpretation much easier in the future. In studying the Pinatubo seismicity, SSAM has again proven to be a valuable tool for characterizing changes in the seismic spectrum through time.
In deploying SSAM systems at future active volcanoes, bands should be chosen that provide the greatest resolution in those areas of the spectra where critical activity is most likely to occur. Sustained seismic activity during the Pinatubo eruption covered a broad range of frequencies and resulted from periods of sustained LP seismicity as well as the continuous occurrence of VT earthquakes. At Redoubt Volcano, critical activity was associated with swarms of LP events and volcanic tremor concentrated between 0.9 and 1.9 Hz. To provide greater resolution in this range, the SSAM bands were adjusted during the Redoubt eruptions. At Pinatubo, the first nine bands provided adequate coverage from 0.5 to 10.5 Hz but lacked the resolution that the Redoubt bands provided. In future deployments of SSAM systems, bands should initially provide coverage between 0.1 and 12.0 Hz and have as much resolution as possible between 0.1 and 8 Hz, because most events commonly associated with active volcanism fall within this range of frequencies (Lahr and others, 1994). It is important to dedicate a few bands above this range, as many signals such as storm noise frequently have significant energy at higher frequencies, and SSAM data help with their identification. Suggested band definitions based on experience with SSAM systems from both Pinatubo and Redoubt are contained in table 1. Established bands can be modified to monitor more closely a given type of seismicity once the seismic style and eruptive character of a given volcano are established. Band redefinition should be accomplished by widening some bands and narrowing others instead of deleting bands; it is important to keep the entire spectrum monitored, as seismicity may change unexpectedly. As is the case in any monitoring situation, care must be taken in redefining SSAM bands so that the continuity of baseline measurements is not disrupted at a critical time.
Table 1. Suggested spectral band definitions as based on experience with SSAM systems from Mount Pinatubo and Redoubt Volcano.
Band |
Frequency limits (Hz) |
|---|---|
|
1 |
0.1-0.5 |
|
2 |
0.5-1.0 |
|
3 |
1.0-1.5 |
|
4 |
1.5-2.0 |
|
5 |
2.0-2.5 |
|
6 |
2.5-3.0 |
|
7 |
3.0-3.5 |
|
8 |
3.5-4.0 |
|
9 |
4.0-4.5 |
|
10 |
4.5-5.0 |
|
11 |
5.0-5.5 |
|
12 |
5.5-6.0 |
|
13 |
6.0-7.0 |
|
14 |
7.0-8.0 |
|
15 |
8.0-10.0 |
|
16 |
10.0-12.0 |
We thank the many individuals involved in the Pinatubo response who assisted with data acquisition and reduction. This paper benefited greatly from discussions with Andy Lockhart, Randy White, Dave Harlow, Rick Hoblitt, Chris Stephens, Bernard Chouet, and John Ewert. Randy White, E.G. Ramos, and Robert Page provided formal reviews of the manuscript.
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