FIRE and MUD Contents

The West-Side Story: Observations of the 1991 Mount Pinatubo Eruptions from the West

By Julio P. Sabit,¹ Ronald C. Pigtain,¹ and Edwin G. de la Cruz¹

¹Philippine Institute of Volcanology and Seismology.

ABSTRACT

On April 2, 1991, during or just after phreatic explosions, vigorous steam vents opened on the upper north slope of the volcano. From April 2 to late May, steam jetted to heights of 300 to 800 meters and sometimes intensified to ash emissions that reached heights of 1,500 to 3,000 meters. Beginning in late May, steaming intensified and included increasing amounts of ash. By June 9, continuous ash emission and occasional small explosions produced ash clouds dense enough to flow slowly down the volcano's western slopes. A series of strong explosive eruptions began at 0851 on June 12, 1991, and, starting at 2218 on June 14, the eruptions slowly became more frequent and culminated in the violent climactic eruption of June 15.

Seismic activity was characterized by swarms of high-frequency volcanic earthquakes in April and May. Daily seismic counts averaged 74 high-frequency volcanic earthquakes in April and 90 high-frequency volcanic earthquakes in May. Early June brought a sudden increase in seismicity, including larger high-frequency volcanic earthquakes with distinct P-phases, low-frequency volcanic earthquakes, and harmonic tremor. On June 8, at a new, more distant seismic station in Poonbato, about 80 percent of the seismicity appeared as low-frequency volcanic earthquakes and low-frequency harmonic tremor. During the explosive eruptions (June 12-15), the Poonbato seismograms were marked by low-frequency volcanic earthquakes and low-frequency harmonic tremor.

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INTRODUCTION

Residents of Sitios Tarao and Yamut (fig. 1) felt earthquakes beginning on March 15, 1991, but did not notice any changes in the volume and color of the steaming from a preexisting thermal area on the northwest slope of the volcano. No rock falls or landslides were noted. Then, in the early morning of April 2, 1991, residents at these same sitios felt more earthquakes than they had experienced during the previous weeks. According to Erro and Palawig, members of the Lubos na Alyansa ng mga Katutubong Ayta ng Sambales (LAKAS) (Negrito People's Alliance of Zambales) (oral commun., 1991), the number of felt earthquakes increased significantly shortly before phreatic explosions began at about 1600 that afternoon.

Figure 1. The seismic stations (solid triangles) established at the western side of Mount Pinatubo in April 1991. Earthquake epicenters determined from this network during early April are shown as filled circles.

On April 4, 1991, Sister Emma Fondevilla of LAKAS reported these explosions to the Philippine Institute of Volcanology and Seismology (PHIVOLCS); she also reported felt earthquakes and a strong sulfur odor during and after the explosions. The series of explosions and the strong sulfur odor caused some residents of villages at the northwest, west, and southwest slopes of the volcano to evacuate voluntarily.

PHIVOLCS immediately dispatched a Quick Response Team to conduct an aerial survey and confirmed the reported explosions of Mount Pinatubo on the same day. The aerial survey showed a northeast-trending alignment of nine thermal vents vigorously emitting white to dirty white steam (fig. 2). Our initial interpretation of the activity was that explosions had occurred within the preexisting hydrothermal system.

Figure 2. Steaming vents of Mount Pinatubo. The two most vigorous vents are in two forks of the upper Maraunot River, shown in map view in figure 5. Photograph was taken on April 6, 1991.

To judge whether these events presaged more serious eruptions, we began monitoring operations on April 5, 1991. This paper presents the results of that monitoring, on the western side of Mount Pinatubo, before, during, and after the volcano erupted in June 1991.

SEISMIC NETWORK ESTABLISHED

Our first seismic station was established on April 5 at Sitio Yamut, Barangay Villar, Botolan, Zambales, about 7 km west-northwest of Mount Pinatubo (fig. 1, table 1). It was equipped with a portable seismograph (Kinemetrics Ranger, Model PS1A) that had a filter setting fixed at 2 Hz, gain at 48 dB and drum speed of 60 mm/min. A single-side-band radio transmitter was used to transmit reports to the PHIVOLCS central office in Quezon City.

Seismograms from this first station were analyzed immediately after the record was removed from the drum at about 0600 each day. Seismic observations were correlated with observations of steaming activity. Early seismograms showed numerous high-frequency volcanic earthquakes (HFVQ's), many of which were also felt (table 2), and steaming was strong.

Convinced that Mount Pinatubo was showing signs of unrest, and of the need to locate earthquakes, we established four additional seismic stations between April 8 and 15, at Barangays Maguisguis, Nacolcol, Moraza and Burgos (fig. 1). Each seismic station was equipped with a digital seismograph called an EDR (earthquake data recorder), previously donated by the Japan International Cooperation Agency (JICA) for use at Taal Volcano. Floppy disks with the data were retrieved and taken to Manila, where the data could be played back and reduced.

These were replaced by portacorder seismographs (Teledyne Geotech) during the first week of May (table 1). These portacorders had filter settings of 0.2 Hz for high pass and 12.5 Hz for low pass, gain at 48 dB, and drum speed of 60 mm/min.

Seismic data from these stations, plus visual observations, helped us to assess the daily activity of Mount Pinatubo, and our evaluation of these parameters was disseminated to the media, local officials, and the public through daily volcano updates (PHIVOLCS, 1991).

Table 1. Seismographs installed at the western side of Mount Pinatubo in 1991.

¹ Kinemetrics, Ranger seismograph (analog), installed in Yamut from April 5 to June 6; provided the seismic data used in preparing Pinatubo Volcano Updates (PHIVOLCS, 1991).

²EDR, Earthquake Data Recorder digital seismographs, donated to PHIVOLCS by the Japan International Cooperation Agency. Data gathered by the seismographs were used to locate earthquakes from April 8-30, 1991.

³Geotech, Teledyne Geotech Portacorder RV-320B (analog).

Table 2. Counts of high-frequency (HF) and low-frequency (LF) volcanic earthquakes as recorded at Yamut, and earthquakes felt on the western side of Pinatubo, April 5-June 6, 1991.

[Note that during this period, the greatest number of felt earthquakes occurred April 5-21, 1991. Int, Rossi-Forel intensity; MD, maximum deflection]

DETAILS OF THE PREERUPTIVE SEISMICITY

Mount Pinatubo's seismicity during April, May, and early June was characterized by swarms of HFVQ's. The daily count and amplitudes of earthquakes remained more or less constant from April 5 to May 26. However, on May 27, 1991, low-frequency volcanic earthquakes (LFVQ's) and tremor began and gradually increased in early June until they dominated the seismic data by June 8, 1991. Examples of HFVQ and LFVQ events are shown in figure 3.

Figure 3. A, High-frequency volcanic earthquakes as recorded at Yamut on April 15, 1991. B, Low-frequency volcanic earthquakes recorded at the Poonbato station on June 14, 1991. C, High-frequency volcanic earthquakes as recorded at station BUGZ on January 30, 1992. D, Low-frequency volcanic earthquakes recorded at station BUGZ on September 12, 1992. A and B were recorded on a smoked drum of a Kinemetrics Ranger seismograph; C and D were recorded by a Teledyne Geotech Portacorder.

HIGH-FREQUENCY VOLCANIC EARTHQUAKES

The first 24 h of seismic recording at the Yamut Station (1712 April 5 to 1730 April 6, 1991), showed 223 HFVQ's (table 2). Twelve of these quakes were recorded at maximum deflection. The rest had trace amplitudes that varied from 5 mm to 20 mm. These had dominant frequencies of 3 Hz to 5 Hz and S-P times (the difference in P-and S-wave arrival times) between 0.88 s and 1.36 s, suggesting that the source was about 4 to 8 km from the Yamut seismic station.

The seismograph malfunctioned, and no records were obtained from 1730 on April 6 to 1200 on April 8, 1991. When the seismograph began operating again on April 8, the Yamut seismic count had decreased from 148 (0000 to 1730, April 6) to 32 earthquakes (1200 April 8 to 0000 April 9) (table 2). Through the balance of April, 26 to 135 HFVQ's occurred each day (average 74), most of which were small and not felt. Around 20 to 25 percent of these earthquakes had trace amplitudes greater than 20 mm and only about 5 percent caused maximum deflection. Some of the larger HFVQ's were felt and heard as rumbling sounds. Measured S-P times were, at Yamut, 1.0-1.4 s; at Moraza, 1.5-1.6 s; at Nacolcol, 1.8-2.0 s; and at Burgos, 1.9-2.0 s. These S-P times suggested that the source was upslope from the Yamut Station (fig. 1).

Earthquake epicenters and hypocenters were determined graphically, using S-P times. Epicenters from April 8-30, 1991, clustered 5 to 7 km north-northwest of Mount Pinatubo's summit (fig. 1), at depths of 2 to 7 km beneath the surface.

In May, seismic activity increased slightly, to an average of 90 HFVQ's per day (fig. 4). The number of larger HFVQ's (>20 mm trace amplitude) gradually increased from 20 percent of the total count on May 17 to 45 percent by May 31. Felt earthquakes also increased. Residents of Moraza, Villar, and Belbel felt four to five earthquakes per day during this period.

Figure 4. Numbers of volcanic earthquakes per day (HF=high frequency; LF=low frequency) as recorded at the Yamut station, April 5-June 6, 1991.

In early June, the number and size of earthquakes continued to increase. About 40 to 45 percent of the recorded earthquakes had trace amplitudes >20 mm and had clear and distinct P-phases.

On June 6, 1991, all of our seismic stations on the western side were pulled out because of the rapidly deteriorating condition of Mount Pinatubo. A temporary seismic station was set up at Barangay Poonbato on the same day, 15 km northwest of Yamut and 22 km northwest of Mount Pinatubo's summit, to continue monitoring the volcano's seismicity. The first records from Poonbato show a few small LFVQ's and HFVQ's. (Note: Because Poonbato is far from Pinatubo, some shallow HFVQ's could have appeared as LFVQ's by the time their signals reached Poonbato, and others could have been missed entirely. Counts from Yamut and Poonbato are not directly comparable.) On June 7, the seismic count remained at a low level. By June 8, around 3 to 5 HFVQ's were recorded, and the number of LFVQ's and periods of tremor were increasing.

LOW-FREQUENCY VOLCANIC EARTHQUAKES AND TREMOR

LFVQ's and tremor were recorded by the Yamut seismic station from April 8-22, 1991 (table 2), during episodes of increased ash emission. The LFVQ's had maximum trace amplitudes of 25 mm, an average duration of 50 to 60 s, and dominant frequencies of 2 to 2.5 Hz. Discontinuous volcanic tremor had trace amplitudes of 0.5 to 1.5 mm, a dominant frequency of 1.0 Hz, and durations that ranged from 50 to 72 s. Curiously, from April 24 to May 26, no LFVQ's or tremor occurred.

On May 27, 1991, the Yamut seismograph recorded four small LFVQ's‹with trace amplitudes of 0.5 mm to 1.5 mm and durations from 30 s to 55 s. Beginning June 1, 1991, larger LFVQ's with trace amplitudes ranging from 1.0 mm to 8.0 mm were recorded. By June 3, seismic activity further increased, with five LFVQ's and harmonic tremor lasting about 25 min. On June 4, 5, and 6, the Yamut seismograph continued recording 3 to 4 LFVQ/d, until the Yamut seismograph had to be pulled out and transferred to Poonbato.

At Poonbato, the seismograph continued recording LFVQ's, but, predictably, because of the greater distance from the source, they were fewer and smaller than had been recorded at Yamut. By June 8, LFVQ's and low-frequency harmonic tremor (LFHT) gradually increased and comprised around 80 percent of the recorded seismic events.

The first large eruption began at 0851 on June 12, 1991, preceded by harmonic tremor from 0341 to 0530 of the same day and then several short bursts of volcanic tremor, each lasting about 60 s. The eruption itself was recorded on the seismograph as LFHT. Thereafter, the Poonbato seismograph recorded many LFVQ's and periods of LFHT.

VISUAL OBSERVATIONS

STEAMING ACTIVITY BEFORE APRIL 2, 1991

Before the April 2, 1991, explosions, steaming activity at Mount Pinatubo was weak and confined within a thermal area inside a 150-m-diameter depression, 1,180 m above sea level in the uppermost Maraunot River. The thermal area had small steaming vents, hot-water seeps, bubbling pools, and hot ground dotted with small quantities of sulfur sublimates (Delfin, 1983). According to the Aetas who frequented the area, steam emission was consistently weak (members of LAKAS, oral commun., 1991).

On August 3, 1990, residents of Yamut and Tarao reported a ground fracture and steam emission on the upper slopes of Mount Pinatubo. A PHIVOLCS Quick Response Team found that a large landslide had occurred on the steep northwest side of Mount Pinatubo but that the reported steaming activity corresponded to the preexisting thermal area (Ramos and Isada, 1990). The team concluded that the landslide and steaming were not related to any volcanic activity but, rather, that part of the former thermal area was exposed when the landslide occurred. No new steaming vent was noticed.

APRIL 2-MAY 28, 1991: EXPLOSIONS AND INTENSE FUMAROLIC ACTIVITY

At about 1600 on April 2, 1991, phreatic explosions occurred at and northeast of the thermal area on the upper northwest slope of Mount Pinatubo. The explosions reportedly lasted until nightfall. Several vigorous new steam vents developed in a northeast-trending alignment across the north slope of the volcano (fig. 5). Strong jets of dirty white steam rose from these vents.

Figure 5. Steaming vents (filled circles) at Mount Pinatubo. These vents formed during the April 2, 1991, explosions. The most vigorous vents are shown by the two larger filled circles in the upper Maraunot River. Contour interval 100 m.

During the rest of April and May, two of these vents were especially vigorous (fig. 2), both in the upper reaches of the Maraunot River. Steam routinely rose 300 to 800 m, and, sometimes, steaming intensified and small quantities of volcanic ash were ejected with the steam to heights of 1,500 to 3,000 m. Light ash fall occurred on the volcano's upper northwest, west, and southwest slopes, causing siltation of the Maraunot-Balin Baquero and the Marella-Santo Tomas River systems.

In May, the steaming activity gradually intensified, and ash emissions were occasionally observed. The steam columns changed from white to brown or gray during ash emissions and rose between 1,500 and 5,000 m above the vents. Beginning on May 16, ash emissions were observed frequently. One of the largest of these, on May 28, produced ash clouds that reached a height of 5,000 m and caused light ash fall in Yamut, Villar, Moraza and other areas within 15 km to the northwest, southwest, and west of Mount Pinatubo.

NEAR-ERUPTIVE STEAMING ACTIVITY (LATE MAY TO JUNE 1991)

Steaming progressively intensified and carried increasing amounts of ash after the May 28 ash emission. On June 1, intermittent ash puffs were common, and by June 3, intensified steam and ash emission caused intermittent light ash fall that hampered visual observation of the Pinatubo summit area. On June 5, ash fall reached areas 12 to 15 km west and southwest from the active vents. The activity continued to intensify, and by June 8, ash-laden steam clouds attained a height of 8,000 m. Ash fall from this event reached as far as Poonbato.

OTHER OBSERVATIONS

Steam plumes from the active vents had a strong sulfur odor that could be detected easily within 5 km distance from the volcano's summit. The sulfur-rich gas withered vegetation at and near the active vents.

Ash ejected during the April 2, 1991, explosions and during subsequent ash emissions caused siltation ofrivers that flow north and west from the volcano's summit. These rivers are the Maraunot Balin-Baquero River system, Marella-Santo Tomas River system, and the O'Donnell River. The silt destroyed aquatic life in the rivers. Water samples were highly acidic: a water sample collected on April 8 from the Maraunot River, 1.5 km below the preexisting thermal area, had a pH of 2.45, and samples from April 9 and 10 were equally acidic (Campita and Tansinsin, 1992; N.R. Campita written commun., 1993). In comparison, water collected from the Maraunot River in 1983 was neutral to basic with pH values ranging from 7.60 to 8.02 (Clemente, 1984).

JUNE 1991 ERUPTIONS

On June 9, 1991, at 1455 and again at 1504 and 1630, billowing ash clouds flowed down the northwest and west slopes of the volcano, down the Maraunot and Moraza Rivers. These density currents, which looked like pyroclastic flows from our vantage in Poonbato, started from ash columns reaching an altitude of 5,000 m and reached distances of around 4 to 5 km from the active vents. Although subsequent aerial observations found that these density currents had moved slowly and had left only a thin veneer of ash fall in the valleys (Hoblitt, Wolfe, and others, this volume), these currents nonetheless represented a significant intensification of activity.

Continuous emission of ash-laden steam columns followed, but column heights could not be estimated because the ash drifted and dispersed toward the west, nearly reaching our temporary station. At 1330 on June 10, a voluminous ash-laden steam column rose to a height of 4,000 m; other episodes of enhanced emission occurred on June 11 at 0515 and 0540, from which ash rose to 5,000 m and drifted to the southwest. At 1030 on June 11, steam emission gradually decreased, and by 1600, weak, white to dirty white steam clouds rose only 500 to 700 meters above the active vents.

Table 3. Ash emissions and eruptions observed from Poonbato, June 9-15, 1991.

[The eruption times and dates are as observed from Poonbato station through 0831 on June 15, after which the volcano was completely obscured. Estimates of column height through the afternoon of June 14 were based on vertical angles measured at Poonbato; estimates of column height during the evening of June 14 are from weather radar at Clark Air Base. n.d., not determined]

¹Discrete eruption, identified from combined data of seismograms, microbarograph records, weather radar, and visual observations (Hoblitt and others, this volume)

Felt earthquakes at 0341 on June 12 were followed by almost 2 h of harmonic tremor accompanied by continuing ash emission, the column of which rose to 4,000 m and drifted to the northwest. At 0851, an energetic eruption column rose to a height of about 20 km, and pyroclastic surge moved down the northwest, west, and southwest slopes of the volcano, nearly catching news reporters in Sitio Ugik (12 km northwest of the volcano). The eruption column formed an umbrella cloud that slowly extended over the whole western side of the volcano and caused moderate to heavy ash fall in the towns of Cabangan, San Felipe, San Narciso, San Antonio, San Marcelino, Castillejos, and Subic, all in the province of Zambales.

Sometime between 0900 and 1100, 2- to 5-cm-diameter pumice began to fall at Ugik and in Moraza and other barangays at the west and southwest foot of Pinatubo; ash began to fall in San Marcelino, Castillejos, San Antonio, and Cabangan about 0920. By 1100, the fall was nearly finished at Ugik. Another high eruption column observed at 2250 and 2305 was generated by an eruption that began at 2250. Tephra from this column drifted west and southwest from the volcano.

At 0841 on June 13, another large eruption occurred. The eruption cloud rose toward the sky and slowly expanded over the whole western and southern part of the province of Zambales. The eruption produced pyroclastic density currents that cascaded down most river valleys of the west flank, from the Maraunot valley on the north to the Marella valley on the south. Some of these ashy currents reached as far as Sitio Ugik. After the 0841 eruption, Mount Pinatubo's activity dropped back to continuous emission of ash-laden steam columns that rose to heights of 1,500 to 3,000 m.

At 0200 on June 14, steaming activity gradually decreased. From 0500 to 1300, steaming activity was weak, reaching only 300 to 400 meters above the vents. However, an increasing number of shallow earthquakes was recorded by the Poonbato seismograph from 2320 on June 13 to around 0700 on June 14. Harmonic tremor with an average double amplitude of 1 to 2 mm began at 0400. At 1309, Mount Pinatubo erupted anew, this time producing ash clouds that drifted to the southwest and dropped ash on the towns of Cabangan, San Felipe, San Antonio, San Narciso, San Marcelino, and Castillejos about an hour later. Eruptions producing large pyroclastic surges (R.P. Hoblitt, U.S.G.S., written commun., 1993) were observed at 1520, 1853, and 1921 (latter two might be from a single, continuous event). Starting at 2218, the eruptions became more frequent. Lightning in the mushroom-like eruption cloud helped us to see the drifting eruption cloud. Simultaneously, pyroclastic flows or pyroclastic surges rolled down the northwest, west, and southwest slopes of the volcano and exhibited beautiful fireworks displays like "dancing lights." These pyroclastic materials glowed and caused momentary brightness on the volcano's slopes. Another large explosion was seen at 2321.

At 0115 on June 15, a vigorous eruption occurred. This time, countless lightning flashes marked the rapidly spreading eruption cloud and covered the sky. At 0140, another mushroom-like eruption column was seen, and the whole volcano area was lit by lightning flashes in pyroclastic density currents that were flowing rapidly down the volcano slopes. Fireworks displays were observed in practically all drainages. Another eruption was observed at 0257. At 0517, we observed large pyroclastic density currents moving down the entire western and southwestern slopes of the volcano; flows in individual drainages could no longer be distinguished.

At 0556, 0611, 0809, and 0831 on June 15, 1991, powerful explosions and giant pyroclastic surges were observed. One large pyroclastic surge down the western slope of Mount Pinatubo, shown in figure 6, invaded Barangay Burgos, 15 km northwest of the volcano. At around 1100, heavy ash fall caused darkness and prevented further visual observation of the June 15 eruptions. Microbarograph records suggest that strong eruption ended about 2230 June 15 (Wolfe and Hoblitt, this volume); seismic records from Poonbato suggest further decline at about 0330 of June 16. The effects of the eruption were aggravated by the passing of typhoon Yunya (local name, Diding), which distributed volcanic ash in all directions and affected places not expecting it, like Metro Manila.

Figure 6. A giant pyroclastic surge cascading down the western side of Mount Pinatubo. Photograph taken at Barangay Burgos, Botolan, Zambales, during the early morning of June 15, 1991. (Photograph courtesy of Rene Arante, PHIVOLCS.)

SEISMICITY DURING AND AFTER THE JUNE 12-15 ERUPTIONS

From June 12 to 15, 1991, the seismograms obtained at Poonbato showed tremor and many volcanic earthquakes, mostly LFVQ's. Most of the harmonic tremor was recorded during eruptions. Only a few HFVQ's were recorded.

Before the 0841 and 1309 eruptions on June 13 and June 14, respectively, the Poonbato seismograph recorded increases in seismic activity. Seismicity increased from 0430 on June 13 until the 0841 eruption of that day (fig. 7). Likewise, seismicity increased from 2320 on June 13 to 0640 on June 14, before the 1309 eruption on June 14. The upsurge in seismicity was characterized by (1) a progressive increase in seismic count, (2) an increase in the duration and trace amplitude (size) of the earthquakes, and (3) increased amplitude and duration of harmonic tremor.

Figure 7. Increase in seismicity (to 1 to 2 earthquakes per minute) beginning about 0430 on June 13 and continuing until an eruption occurred at 0841 that day. The eruption itself was recorded as harmonic tremor.

Many felt earthquakes occurred during the June 12 to early June 15 eruptions. These registered at Poonbato at maximum deflection and were felt near the volcano at intensities I-IV (Rossi-Forel scale). Countless additional, and still stronger, earthquakes occurred on June 15-16, 1991, during and just after the climactic eruption, but the analog record obtained at Poonbato was not readable because the seismograms were saturated by large earthquakes and tremor generated by the eruption. Many of the earthquakes that occurred on June 15-16 were felt at intensities I-VII (Rossi-Forel scale) within a distance of 40 km from the volcano, even causing liquefaction at Sasmuan, Pampanga, about 35 km southeast of the volcano.

Seismic activity remained high after the June 12-15, 1991, eruptions. Earthquakes on June 16 remained uncountable. Beginning on June 17, 1991, the Poonbato seismograph recorded an average of 150 HFVQ's per day. Episodes of LFHT were also recorded, particularly during unusually energetic ash emission. On July 4, 1991, there was a significant increase in seismic activity (319 HFVQ's recorded at Poonbato). However, on July 5, the seismic count decreased to 120 HFVQ's and by July 13, this further dwindled to 76 HFVQ's (fig. 8).

Figure 8. Numbers of volcanic earthquakes per day (HF=high frequency; LF=low frequency) as recorded at Poonbato, June 18 to July 28, 1991. Tremor occurred on all days and was so strong in the last half of June that it obscured many discrete earthquakes; thus, the apparent increase in seismicity from middle to late June is probably an artifact of earthquake counting against strong background tremor.

No LFVQ's were reported by the Poonbato station from June 18 to July 4 because the seismograms were saturated by large-amplitude harmonic tremor and HFVQ's (fig. 8). The overprinting of relatively large HFVQ's and harmonic tremor prevented recognition of smaller LFVQ's on our analog record. By July 5, the size (trace amplitude) of HFVQ's and tremor decreased enough for us to recognize and report LFVQ's.

DISCUSSION

Phreatic explosions of April 2, 1991, occurred when ground water was heated by contact with magma or with hot gases rising from that magma. The hot ground water boiled, vaporized, and blasted through the country rocks. Rock alteration within the preexisting hydrothermal system could have prevented the escape of gases and allowed the buildup of high vapor pressure. Then, a sudden release of pressure may have caused the superheated fluid to expand explosively and produce phreatic explosions. Continued heat transfer to the hydrothermal system caused continued vigorous emission of white to dirty white steam from the newly formed thermal vents.

Stresses from inflating or rising magma caused active fracturing of the country rock and thus swarms of HFVQ's. The onset of LFVQ's on March 27 might indicate that magma had risen to shallow depths by that date.

Beginning June 1, 1991, seismic activity gradually intensified. Although the number of HFVQ's remained more or less constant, between 50 and 90/d, the trace amplitudes (size) of HFVQ's and the daily count of LFVQ's and harmonic tremor increased. Moreover, an increasing number of earthquakes with distinct P-phases was noticed. The fact that the number of HFVQ's recorded at Yamut during this period remained more or less constant, despite generally increasing overall seismicity (Endo and others, this volume; Harlow and others, this volume), might be due to increasing distance of the earthquake source from the Yamut station as the center of seismic activity migrated from the north-northwest cluster of hypocenters to the summit area of Pinatubo (Harlow and others, this volume).

CONCLUSIONS

Significant changes in seismicity and steaming activity at Mount Pinatubo were used to anticipate the onset of the June 1991 eruptions. Principal changes were:

1. An upsurge in seismic activity, including increases in the daily count and trace amplitude (size) of the earthquakes, a shift in the types of volcanic earthquakes, from high frequency to low frequency, and increased occurrence of harmonic tremor.
2. Increased steaming activity, including an increase in the volume and vigor (height) of steam emission, change in the color of steam from white to brown or gray with increasing ash content, and increasingly frequent episodes of enhanced ash emission. Even though the observations reported in this paper were relatively simple and used older equipment than observations described in other papers in this volume, they allowed us to keep abreast of the volcano's unstable condition and to issue appropriate warnings.

REFERENCES CITED

Campita, N., and Tansinsin, L., 1992, Water quality effects on Maraunot and other peripheral rivers by the recent volcanic activity of Mount Pinatubo: PHIVOLCS open file report, 24 p.

Clemente, V.C., 1984, A re-evaluation of Mount Pinatubo Geochemistry: Philippine National Oil Company open file memorandum report, 24 p.

Delfin, F.G., 1983, Geology of Mt. Pinatubo Geothermal Prospect: unpublished Philippine National Oil Company report, 45 p.

Endo, E.T., Murray, T.L., and Power, J.A., this volume, A comparison of preeruption real-time seismic amplitude measurements for eruptions at Mount St. Helens, Redoubt Volcano, Mount Spurr, and Mount Pinatubo.

Harlow, D.H., Power, J.A., Laguerta, E.P., Ambubuyog, G., White, R.A., and Hoblitt, R.P., this volume, Precursory seismicity and forecasting of the June 15, 1991, eruption of Mount Pinatubo.

Hoblitt, R.P., Wolfe, E.W., Scott, W.E., Couchman, M.R., Pallister, J.S., and Javier, D., this volume, The preclimactic eruptions of Mount Pinatubo, June 1991.

PHIVOLCS, 1991, Pinatubo Volcano Updates: PHIVOLCS open-file reports, April-December 1991, unpaginated.

Ramos, A., and Isada, M., 1990, Mount Pinatubo not erupting: PHIVOLCS Observer, July-September 1990, v. 6, no. 3, p. 6.

Wolfe, E.W. and Hoblitt, R.P., this volume, Overview of the eruptions.

FIRE and MUD Contents

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