Report on Popocatepetl (Mexico) — October 1996
Bulletin of the Global Volcanism Network, vol. 21, no. 10 (October 1996)
Managing Editor: Richard Wunderman.
Popocatepetl (Mexico) Ash emitted on 28-29 October
Please cite this report as:
Global Volcanism Program, 1996. Report on Popocatepetl (Mexico) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 21:10. Smithsonian Institution.
19.023°N, 98.622°W; summit elev. 5393 m
All times are local (unless otherwise noted)
At 0905 on 28 October, the largest ash emission since 5 March (BGVN 21:03) was detected by a surveillance video camera. CENAPRED seismic and tilt monitoring network recorded the event as an emergent signal that produced high-amplitude records on the stations closest to the crater until 0913, when both amplitude and ash emission rate decreased. Heightened levels persisted until 0953, when the emission stopped, but high-frequency tremors continued for the most of the day.
The ash plume rose 2-3.5 km above the summit and was rapidly bent towards the W by the 30-45 km/hour winds. At 1630, NOAA/NESDIS reported that the ash cloud was between 5.3 and 6.3 km altitude. Minor ash falls were reported over towns W and NW of the volcano as far as 50 km away, and significant amounts of ash were observed on the volcano flanks and over the summit glacier, darkening the snow. Due to clear weather, visibility was >30 km and residents in towns on the W flank of the volcano witnessed the event; the explosion was also documented by Mexico City TV.
On the morning of 29 October the gas plume was drifting W at an altitude of 4.5-5.8 km. That day Jose Luis Macias, Claus Siebe, and Hugo Delgado of the Instituto de Geofisica of the University of Mexico (UNAM), Alejandro Mirano and Enrique Guevara of the National Center for Disaster Prevention (CENAPRED), and Mindy Brugman of the Columbia Mountains Institute of Ecology (CMI) observed the volcano using a helicopter provided by the Procuraduria General de la Republica. The helicopter flew at 6-6.5 km altitude to view the interior of the summit crater. The crater was still occupied by the lava body that first appeared in March 1996 (BGVN 21:04) and stopped growing by July. The concave lava body with a depressed central part observed during a reconnaissance flight on 25 October showed no substantial changes, but a new vent with an elongated trend NW-SE appeared on the central part of the lava dome. The dimensions of this craterlet were ~50 x 20 m (25% of the dome dimension) and 10-20 m depth.
Intense fumarolic activity was also detected within the SE part of the crater. Several other fumarolic sources were observed along the edge of the lava dome, at the crater wall and slope contact with the S side of the crater, and at the cracks on the lava dome. Along a N-S trending lineament a small amount of gases came from the SE flank at an approximate elevation of 5,000 m.
Although difficult to establish as a causal relationship, three A-type earthquakes at 0258 (M 2.2), 1728 (M 2.5) and 1811 (M 2.6) on 26 October may now be interpreted as precursors of this event. Similar A-type activity in the past was not necessarily followed by ash emissions. No other clear precursors were observed by the CENAPRED monitoring network. However, the explosive event of 28 October showed precursory evidence in terms of the SO2 flux and deformation of the volcanic edifice. The SO2 flux had averaged 11,000 metric tons/day (t/d) for the last 4.5 months, with a pattern of increasing flux starting on 30 September.
A peak in SO2 flux occurred on 21 October with >27,000 t/d recorded. By 25 October the SO2 flux decreased to 9,000 t/d, a 60% drop within three days. Two GPS stations located on the W flank of the volcano also started to show a continuous increase in the vertical (z) component since 5 October; this surface deformation pattern of uplift roughly followed parallel to the SO2 flux, having a peak on 21 October when the volcano suddenly rose at a rate of 40 mm/day. The vertical motion decayed in the same way as the SO2 flux pattern within the three days preceding the explosion. The correlation between the SO2 flux data and deformation data (derived from GPS) suggested repeated entrance of new magma into a large magma chamber beneath the volcano. The SO2 flux measurements are currently carried out under the sponsorship and collaboration of the Ministry of the Interior (CENAPRED) and Instituto de Geofisica (UNAM). Permanent GPS monitoring is carried out as a joint project UNAM-University of Miami/RSMA-MGG with funds from the NASA-DOSE program and support from CENAPRED.
Another ash-emission event occurred on 29 October at 2211. From the RSAM recordings, it was estimated that this new event released about one-fifth of the seismic energy of the previous one. The highest intensity phase in this case lasted ~7 minutes and then declined. Under clear moonlight conditions, local observers reported a similar short-lived plume rising ~2-3 km above the summit. Winds blew the ashcloud W, but no ashfall reports were received.
Popocatépetl at this stage efficiently releases the gases in a passive way and accommodates deformation due to intrusion of new magma. On 5 March 1996 the volcano started a new episode of activity with ash emissions comparable to those of December 1994 (BGVN 19:12). By the end of March, a lava dome was growing at the bottom of the crater. The dome continued growing at the moderate rate of a few cubic meters per second, as determined by aerial photogrammetry done by B. Cabrera from SCT, the Ministry of Communications and Transport, until July 1996, when the growth rate slowly declined. By September, the rate of growth could not be measured and ash emissions became smaller and less frequent.
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: Enrique Cabral-Cano and Hugo Delgado, Instituto de Geofisica, U.N.A.M. Circuito Cientifico; Servando De la Cruz-Reyna, Instituto de Geofisica, UNAM/CENAPRED; Roberto Quaas, Enrique Guevara, Bertha López, Alejandro Mirano, and Alicia Martinez, Centro Nacional de Prevencion de Desastres (CENAPRED); Marcos Galicia, Civil Protection, Amecameca EdoMex; NOAA/NESDIS Satellite Analysis Branch, USA.