Smithsonian Institution Global Volcanism Network Bulletin v. 19, no. 11, November 1994 Popocatepetl (Mexico) Small eruption on 21 December ends decades- long slumber Popocatepetl central Mexico (19.02N, 98.62W) All times are local (= GMT - 6 hours) A new episode of explosive activity began at Popocatepetl volcano on 21 December (figure 1). The eruption followed increases in seismicity, SO2 flux, and fumarolic activity during the last 13 months (Bulletin v. 18, no. 11; v. 19, nos. 1-4, 6, 8, 10). Although in the last year seismicity rose and fell several times, during late-October there was a sudden, prominent (roughly 1.6- to 10-fold) increase in daily earthquakes compared to previous months. Measurements of the volcano's total SO2 flux were consistently large (some airborne measurements averaged >1,000 tons/day). During October-November 1993 a cluster of steam vents in the summit crater produced clouds that reached 6,000 m elevation, several-hundred meters above the 5,465 m summit. These clouds sometimes stretched for 50 km. Eruptive activity. Near midnight on 22 December, Servando de la Cruz sent the following report. "The fumarolic activity that has been developing during the last two years or so culminated on early 21 December 1994, when a series of volcanic earthquakes, probably associated with phreatic explosions, marked the beginning of a new stage of eruptive activity. The seismic events, detected at 0131, 0132, 0138, 0140, and 0148, were very impulsive, high-frequency, short-duration signals, and were followed by a major, lower-frequency event at 0153. The events were recorded by 4 telemetric stations, within 11 km of the volcano, operated jointly by CENAPRED (National Disaster Prevention Center) and the Institutes of Geophysics and Engineering of the National University of Mexico (UNAM). As the day cleared an ash plume was observed for the first time in decades emerging from the volcano's crater. The ash emission was moderate and produced an almost horizontal plume causing a light ashfall over the city of Puebla, about 45 km E of the volcano's summit. A helicopter flight at 1030 showed that most of the ash issued from near the lower NE rim of the inclined crater. A radial fissure on the NE flank of the cone displayed some steam-producing vents, though the cloudy conditions made this interpretation equivocal. Old cracks in the glacier appeared to have extended a significant amount towards the W. A second flight at 1430 the same day revealed a substantial increase in ash production (about 3-4 times the amount observed in the morning). The light-gray ash appeared to be emitted episodically, with "puffs" every few minutes. "The seismicity consisted of mostly low-amplitude B-type earthquakes and concurrent high-frequency A-type events. Though this seismicity remained lower than during the night of 21 December, during the next day the seismicity again increased. At this stage and after several consultations between the scientific group and the Civil Protection authorities, an evacuation of the 19 most vulnerable towns and villages on the E sector of the volcano was started around 2100 of December 21, and about 31,000 persons were moved during the night to shelters in safer areas. Since then the situation has remained fairly stable, though long-duration, low-amplitude tremors appeared in the night of 21-22 December, and continue." Claus Siebe reported that climbers at Popocatepetl reached the summit, which lies along the W margin of the gaping summit crater's rim, both on the day before the eruption, and hours after the 21 December eruption started. On the day before the eruption visiting climbers could see the crater lake and sparse fumaroles. They reportedly heard no hissing sounds and they smelled less odor from sulfur-bearing gases than in previous months. Curiously, the six volcanic earthquakes that took place between 0130 and 0200 on 21 December were not felt, and the presumably associated phreatic summit explosions were not heard by any of about 25 mountain climbers at Tlamacas, 4 km N of the summit (figure 1). The climbers, who said they started ascending the mountain around 0400 on 21 December, did not notice anything unusual until they neared the crater rim. Just prior to reaching the rim, a few minutes before 0800, climbers were stunned by what they thought was the sound of jet engines. At the crater rim they saw new bombs as large as 40 cm that had been thrown out of the 250-m-deep crater and had burrowed deep impact-pits in the snow. According to Siebe: "Most climbers who reached the summit that morning thought that the activity was normal, because they had never visited Popocatepetl before." At the summit, the climbers said they could not see the crater floor even though a strong wind was blowing. They descended the mountain without incident. Siebe was at Tlamacas at 0900 on 21 December during clear weather. He observed a continuous ash plume rising 100-500 m above the crater with pulses at 1-5 minute intervals. The plume was carried at least 60 km E. Enough silt- and sand-sized material reached Puebla to produce a thin coating on cars. The ejecta appeared to be non-juvenile, and contained pyrite, sulfur, and Ca-sulfate. A report from Steve McNutt indicated that the volcano began to quiet down on the afternoon of 25 December. During the night of 27-28 December a M 2 earthquake took place; the largest prior event in the recent past was M 2.9. On 27 December tremor was barely perceptible and a few small low-frequency events took place. During the 24-hour period ending about midday on 28 December there were ~30 low-frequency events. Tremor roughly doubled between 23 and 24 December, but then during 25-28 December it dropped and became barely detectible. No specific seismic data were available for dates after that, though seismicity did increase again and an explosion was heard roughly 10 km from the summit at about 1300 on 31 December. Investigators planned to install about four new seismic stations to improve spatial and azimuthal coverage, and to add one station close in. By 27 December all but three of the previously evacuated towns had been reoccupied; those towns not reoccupied were subject to lahar hazard. A glaciologist made an initial helicopter inspection of the glacier looking especially for signs of abnormal melting. No report was available at the time of this publication, but plans to monitor the glacier included both a daily inspection flight and a video camera aimed at it from 5 km away. The last of the three previously evacuated towns was reoccupied by 28 December. News reports. A 21 December AP story said Popocatepetl, "spewed a column of roiling black ash Wednesday, dusting villages and farmland but causing no injuries" and that "television footage from traffic helicopters showed a dense column of ash belching from the summit." As of 23 December, an AP report noted that the Puebla state government said 75,000 people would be evacuated from the countryside around the volcano. Other news reports put the number of evacuees at ~50,000. One of the evacuated towns, Santiago Xalitzintla, is located ~13 km NE of the summit. The town sits along the road over "Paso de Cortez," the pass between Popocatepetl and the adjacent Quaternary stratovolcano to the N, Iztaccihuatl (figure 1). A 26 December UPI report noted that "Jorge Martinez Soto, a researcher at the University of Puebla, said the amount of smoke and ash being emitted ... diminished by about 75 percent since last week ...." Plume imagery and transport modeling. Although the 21 December eruption plume may appear on satellite imagery, to our knowledge no investigator has yet announced having found it. There is an AVHRR (channel 1) image of a Popocatepetl plume on 22 December at 0818 (1418 GMT). That image shows a SE-directed plume tens of kilometers long. There are also three other AVHRR images for plumes on 26, 27, and 28 December. All four images are available from Melissa Seymour, LSU Earth Scan Lab. We learned of these images at press time and although we have not had time to see them and tabulate plume orientations, the imaged plumes reportedly trailed southward. The Synoptic Analysis Branch (SAB) of NOAA/NESDIS first reported Popocatepetl activity at 1530 (2130 GMT) on 26 December for an eruption that took place at around 1300. A SIGMET (Significant Meteorological Event) notice was posted from Mexico City announcing that a new eruption had taken place and that the plume from this eruption reached an altitude of about 6.7 km (22,000 feet). SAB later described the shape of the plume associated with this eruption based on GOES-7 and -8 data (table 1 and figure 2). A report later on 26 December indicated that the volcano had continued to erupt, creating a visible plume that at 1745 extended to 50 km E. At 0745 the next day (27 December), a GOES-8 visible satellite image of the plume suggested a gently curving, funnel-shaped mass tracking NE (figure 2). Based on the lack of infrared signatures and on their visible signatures, all the plumes reported in table 1 and figure 2 were thought to be of low density. A modeling program called "VAFTAD" was used to forecast the transport and dispersion of the plume from the 26 December eruption (see references and description of VAFTAD in the report for Rinjani, Bulletin v. 19, no. 6). VAFTAD produced a series of visual ash-cloud forecasts such as those on figure 3, which showed the plume initially covering both quadrants in the E half of the volcano and then traveling NE along about the same path taken by actual plumes seen in the GOES imagery (table 1 and figure 2). The models forecasted that after about 24 hours the plume would travel NE over the Gulf of Mexico. VAFTAD uses wind and pressure data updated twice daily on grids with spacings of 91 km in the USA and 1 degree over the rest of the globe. The model assumes the eruption delivers a mass load to the atmosphere. The mass load is not scaled to the actual mass of the eruption, but rather the load is assumed to be 1 gram (composed of spherical particles with a density of 2.5 x 10^6 grams/m^-3 in a size range of 0.3-30 fm in diameter). VAFTAD computes transport and dispersion assuming particles are carried by advection both horizontally and vertically, diffuse with a bivariate normal distribution, and fall according to Stoke's law with a slip correction. Calculated ash concentrations have been correlated with satellite imagery for defining the visual ash cloud forecasts. One noteworthy aspect of the Popocatepetl plumes is the relatively large height of the summit crater (elevation ~5,215 m). Even small, low-energy eruptions from this high-altitude vent can eject material to 6 km (~20,000 feet) elevation. So in essence, these ash cloud forecasts serve best for hazards planning purposes. A key use, in fact, is to warn airline pilots of the airspace most likely to contain volcanic ash particles. Besides the other hazards discussed in Boudal and Robin (1989), a large eruption from Popocatepetl could affect air travel in routes over parts of NE Mexico and much of the Gulf of Mexico. Eruptive history. In the Holocene Popocatepetl has produced both effusive and pyroclastic activity. The latter has ranged from mild steam-and-ash emissions to Plinian eruptions accompanied by pyroclastic flows and surges. Vigorous Holocene explosive activity took place in three periods (in years before present, ybp): a) 10,000 to 8,000, b) 5,000 to 3,800, and c) 1,200 to present (Boudal and Robin, 1989). An effusive period from 3,800 to 1,200 ybp ended with a vigorous explosive eruption that both enlarged the summit crater and generated St. Vincent-type pyroclastic flows. Another large explosive eruption, about 1,000 ybp, produced pyroclastic flows that descended the N flank. Historical eruptions depicted on Aztec codices date back to 1345 AD. About 30 eruptions have been reported since then, although documentation is poor. Most historical eruptions were apparently mild-to-moderate Vulcanian steam and ash emissions. Lava flows restricted to the summit area may also have occurred in historical time, but cannot be attributed to specific eruptions. Larger explosive eruptions, possibly Plinian in character, were recorded in 1519 and possibly 1663. The last significant activity took place from 1920 to 1922. Then, intermittent explosive eruptions produced 6.6-km-tall columns and a small lava plug extruded onto the floor of the summit crater. Ash clouds were also reported in 1923-24, 1933, 1942-43, and 1947. Reference: Boudal, C., and Robin, C., 1989, Volcan Popocatepetl: Recent eruptive history, and potential hazards and risks in future eruptions: IAVCEI Proceedings in Volcanology 1, J.H. Latter (Ed.): Volcanic Hazards, Springer-Verlag Berlin, Heidelberg, p. 110-128. Information Contacts: Servando de la Cruz-Reyna, Instituto de Geofisica, UNAM, Ciudad Universitaria, 04510 Mexico DF, Mexico (Email: email@example.com); Claus Siebe, Instituto de Geofisica, UNAM, Coyoacan, 04510, Mexico DF, Mexico (Email: firstname.lastname@example.org); Steve McNutt, Alaska Volcano Observatory, Univ. Alaska Fairbanks, Geophysical Inst., Fairbanks, AK, 99775-0800 USA (Email: email@example.com); Melissa Seymour, LSU Earth Scan Lab, Coastal Studies Institute, 412 Howe-Russell Geoscience Complex, Baton Rouge, LA 70803-7257 USA (Email: firstname.lastname@example.org); Nick Heffter, National Oceanic and Atmospheric Administration (NOAA), Air Resources Laboratory, SSMC3, Room 3151, 1315 East West Hwy., Silver Spring, MD 20910 USA (Email: email@example.com); Jim Lynch, Synoptic Analysis Branch, NOAA/NESDIS, Room 401, 5200 Auth Road, Camp Springs, MD 20746 USA. Figure 1. Base map of Popocatepetl and vicinity (elevations taken from the 1986 Mexico City 1:250,000 topographic sheet). Figure 2. Popocatepetl ash plume at 0745 (1345 GMT) on 27 December (black) and 0815 (1415 GMT) on 28 December (stipple) as seen on satellite imagery. The N edge of the longer plume just reached the Gulf Coast near Tampico. Courtesy of Nick Heffter. Table 1. Visible (GOES-7 and -8) satellite images reported for Popocatepetl. The time of initial eruption for all these plumes was around 1300 (1900 GMT) on 26 December. The third and fifth plumes listed are shown graphically on figure 2. Courtesy of SAB. Date Local Time Plume Length; Estimated Height in km (GMT) Greatest Width (in 1,000's of feet)* 26 Dec 1300 (1900) 50 km; not reported 6.7 km (22,000 ft) 26 Dec 1745 (2345) 50 km E; not reported 6.7 km (22,000 ft) 27 Dec 0745 (1345) 250 km NE; ~75 km 7.6 km (25,000 ft) 27 Dec 1400 (2000) 85 km; not reported 7.0 km (23,000 ft)** 28 Dec 0815 (1415) 160 km; 40 km 6.1 km (20,000 ft)*** * Height of plumes were generally based on SIGMETs from Mexico City. ** Based on upper air data from Mexico City at 0600 (1200 GMT). SIGMET ALFA 2 indicated ash cloud 17,000-20,000 ft at 1500 GMT. *** Previous SIGMETS and weather balloon (radiosonde) data from Mexico City Figure 3. Examples of forecasts of the Popocatepetl plume after a large eruption. Both of these forecasts were for an initial erupted plume height of 7.6 km (25,000 feet) and an eruption duration of 24 hours. They both portray the elevation range from 6-10 km (20,000-35,000 feet). Forecasts were based on an eruption beginning at 1300 (1900 GMT) on 26 December. The map on the left shows the forecast plume 12 hours after the eruption began, the map on the right, 24 hours after the eruption began. Courtesy of Nick Heffter.