Report on Popocatepetl (Mexico) — July 1998
Bulletin of the Global Volcanism Network, vol. 23, no. 7 (July 1998)
Managing Editor: Richard Wunderman.
Popocatepetl (Mexico) Ongoing exhalations; mid-August earthquake and 4-5 km ash plume
Please cite this report as:
Global Volcanism Program, 1998. Report on Popocatepetl (Mexico). In: Wunderman, R (ed.), Bulletin of the Global Volcanism Network, 23:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199807-341090.
19.023°N, 98.622°W; summit elev. 5393 m
All times are local (unless otherwise noted)
During July 1998 Popocatépetl issued sporadic minor-to-moderate exhalations, steam plumes, and occasional minor ash. The daily log for July posted on the Internet by CENAPRED (see below) shows that activity was very similar to that reported for June (BGVN 23:06). That report also discussed the discovery of a defective calibration cell in a spectrometer used to measure SO2 at the volcano. Fortunately, the required corrections needed only to be made for low SO2 flux conditions. Although some of these uncorrected low fluxes may have been cited in previous Bulletin reports, the volcano has typically produced high SO2 fluxes.
Summary of daily activity during July. These observations and hazards were posted on the CENAPRED web site. Some of the original text has been edited.
1 July: Activity remained stable with a tendency to decrease. The number of exhalations decreased slightly; they were of low-to-moderate intensity and sometimes accompanied by steam and gases. In the morning a small plume blew SW. The recommended minimum approach distance was 4 km from the crater and the hazard status remained yellow.
2 July: Activity remained stable. The number of exhalations decreased; all of low to moderate intensity, some were accompanied by steam and gases.
3 July: Bad weather and clouds limited visibility.
4-5 July: Short exhalations of low-to-moderate intensity; some accompanied by steam and gases.
6 July: Except some isolated low intensity exhalations, the activity remained stable and at low levels. In the morning a small steam-and-gas plume blew SW.
7 July: Seismicity remained low and only small isolated exhalations were recorded. Small steam-and-gas plume.
8 July: Generally low activity. Seismic signals indicated that only a few, moderate exhalations took place. In the morning a gas and steam plume rose ~700 m above the crater and then dispersed .
9-11 July: Intense cloudiness and bad weather.
12 July: Low activity. Small and isolated exhalations, some accompanied by light steam-and-gas puffs. A small steam-and-gas plume was observed all day, blown W.
13 July: Activity increased slightly in the morning at 0931. A moderate exhalation produced a small but persistent ash emission directed to the W. Emissions and tremor continued until 0950.
14 July: Stable, low activity. Small gas plume blew W.
15 July: A few moderate exhalations were recorded seismically and were accompanied by gas-and-steam emissions. Some signals indicated rockfalls. In the morning only a small gas plume blew W .
16 July: Low seismicity and some fumarolic activity prevailed. Bad weather obstructed visibility.
17 July: Although cloudiness obscured the volcano most of the day, in the morning a steam-and-gas plume blew W.
18-21 July: In the morning on each day a dense steam-and-gas plume blew W.
22 July: Seismic signals revealed isolated, short exhalations; some contained ash. The latter was seen in the morning at 0743. The ash produced was light; it rose up to ~1 km above the summit and rapidly dispersed SW.
23-28 July: Seismic signals indicated isolated, short exhalations; some were accompanied by gas and steam. In the morning a dense steam plume blew W.
29-31 July: Activity remained stable and, in general, at low levels. Bad weather obstructed visibility.
Activity during 13-14 August. Servando De la Cruz-Reyna provided this description of a mid-August earthquake followed the next day by an eruptive outburst. At 1447 on 13 August a M 3.9 volcano-tectonic earthquake took place at a depth of ~12 km beneath the central part of the volcano (~6.5 km below sea level). During the earthquake, several tiltmeter stations recorded a step-like down-going displacement. Two smaller earthquakes occurred at various depths beneath the edifice that day. That night some low-amplitude, harmonic tremor signals were detected between 2200 and 2400; afterwards seismicity declined.
At 1850 on 14 August a moderate ash emission lasting about 15 minutes produced a column that rose to about 4-5 km over the summit. A low velocity wind (10 km/hour) distributed very light ash falls on some towns in the NW sector of the volcano about two hours later. On August 16, at 2149, a similar event produced a 2-3 km high column over the summit. The pattern of light ashfall repeated. Afterwards, Popocatepetl volcano returned to the previous low-level of activity prevailing since May 1998.
Technology versus rumors. Besides a rapidly growing web site and a broad network of seismic and tilt stations, CENAPRED has also adopted other innovative approaches. For example, a near real-time image of the N summit area is transmitted via microwave linkage and can be viewed on the CENAPRED web site at two resolutions. An infrared camera discloses thermal signatures of erupting plumes.
Still, despite these advances in monitoring technology and communication, during mid-1998 members of the lay public became increasingly concerned about rumors of doomsday scenarios involving Popocatépetl, some of which were broadcast via the media. In response, in June 1998 Roberto Meli, the Director General of CENAPRED, posted an informative note on their web site. He addressed the rumors and explained that there was an absence of scientific evidence for substantive changes in the volcano's behavior in the near future.
Geologic Background. Volcán Popocatépetl, whose name is the Aztec word for smoking mountain, rises 70 km SE of Mexico City to form North America's 2nd-highest volcano. The glacier-clad stratovolcano contains a steep-walled, 400 x 600 m wide crater. The generally symmetrical volcano is modified by the sharp-peaked Ventorrillo on the NW, a remnant of an earlier volcano. At least three previous major cones were destroyed by gravitational failure during the Pleistocene, producing massive debris-avalanche deposits covering broad areas to the south. The modern volcano was constructed south of the late-Pleistocene to Holocene El Fraile cone. Three major Plinian eruptions, the most recent of which took place about 800 CE, have occurred since the mid-Holocene, accompanied by pyroclastic flows and voluminous lahars that swept basins below the volcano. Frequent historical eruptions, first recorded in Aztec codices, have occurred since Pre-Columbian time.
Information Contacts: Servando De la Cruz-Reyna1,2, Roberto Quaas1,2 Carlos Valdés G.2, and Alicia Martinez Bringas1; 1Centro Nacional de Prevencion de Desastres (CENAPRED) Delfin Madrigal 665, Col. Pedregal de Santo Domingo,Coyoacan, 04360, México D.F. (URL: https://www.gob.mx/cenapred/); 2Instituto de Geofisica, UNAM, Coyoacán 04510, México D.F., México.