Report on Popocatepetl (Mexico) — November 1997
Bulletin of the Global Volcanism Network, vol. 22, no. 11 (November 1997)
Managing Editor: Richard Wunderman.
Popocatepetl (Mexico) Low activity through November; lava extrusion and explosion in December
Please cite this report as:
Global Volcanism Program, 1997. Report on Popocatepetl (Mexico) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 22:11. Smithsonian Institution.
19.023°N, 98.622°W; summit elev. 5393 m
All times are local (unless otherwise noted)
Low levels of eruptive and seismic activity characterized Popocatépetl through most of November. Typically, a few events occurred each day, including short episodes of low-amplitude harmonic tremor and gas- and-steam venting in plumes that drifted to the NE or SE. Tiltmeters showed little variation in November but indicated a slight increasing trend. Bad weather and poor visibility occurred frequently.
Table 9 lists type-A seismic events recorded during November. Two episodes of harmonic tremor were recorded on 1 November. A 2 November lightning strike disabled video monitoring until the 5th. Poor weather impeded observation on 11-12 November during a slight increase in activity. On 15 November, a slight increase in the number of events was accompanied by minor ash emission. Some ash was also emitted in conjunction with seismic events on 21 November.
|01 Nov 1997||0250||2.0||4.0||--|
|01 Nov 1997||0311||2.1||10.2||NE|
|01 Nov 1997||1849||2.8||6.1||SE|
|02 Nov 1997||1600||2.1||2.8||S|
|04 Nov 1997||0019||2.1||5.4||SE|
|04 Nov 1997||0036||2.2||5.5||--|
|05 Nov 1997||1538||1.9||6.0||NE|
|06 Nov 1997||0001||2.5||5.5||--|
|08 Nov 1997||1255||1.7||6.6||--|
|10 Nov 1997||1420||2.2||6.6||NE|
|22 Nov 1997||2204||2.3||2.1||SE|
|25 Nov 1997||0457||2.4||4.9||--|
|25 Nov 1997||0826||2.6||5.0||--|
|25 Nov 1997||0837||2.4||2.4||SE|
|26 Nov 1997||1517||2.9||3.6||N|
On 24 November, seismic and associated eruptive activity began to increase. Thirty-six low- to moderate- intensity seismic events were recorded, including significant exhalations at 0823, 0829, 0857, and 0953; during these events, ash plumes rose to 1 km and drifted NE. Low-amplitude harmonic tremor 3-5 minutes in duration occurred in the afternoon. On 25 November, 42 seismic events were recorded; some were accompanied by ash emissions and periods of tremor lasting 2-8 minutes. No significant deformation was observed. During a 25 November helicopter flight, increased gas and steam from fumaroles obstructed views of the crater and dome. By 27 November, activity was subsiding; 29 seismic events and tremor 2-3 minutes long were recorded. Levels of seismic activity continued to decline until the end of the month.
The last of several flow-detection monitoring stations (BGVN 22:10) was installed on 7 November; also, a temporary high-gain broad-band seismograph was installed at the Canario station to study the N flank in more detail. The Canario station's instrumentation included a triaxial short-period seismograph, a triaxial broad-band seismograph, a digital inclinometer, a flow detector, and a rain gage. To reinforce the seismic and geodetic monitoring system, a new station was installed on 28 November on the W flank just under Ventorrillo peak near the Nexpayantla ravine at 4,452 m elevation. The instrumentation includes a triaxial short- period seismograph and a biaxial tiltmeter.
Two important eruption episodes highlighted activity in December. Both eruptions involved extrusion of lava into the crater, creating a dome that sealed fumaroles.
Small- and moderate-intensity emissions of gas and steam characterized activity at the volcano for most of December. Small type-A tectonic events occurred regularly along with incidents of tremor. The first increase in activity began 5 December with 15 small gas-and-steam emissions, tremor of 90 minutes duration beginning at 1335, and two type-A seismic events in the late afternoon. At 0315 on 6 December activity increased considerably and continued throughout the day. A moderately large high-frequency tremor accompanied by continuous gas, steam, and ash exhalations lasted until 1700. Early in the morning of the 6th a faint glow at the fumarole was observed through the video monitor, indicating the presence of incandescent material in the crater interior. In the late afternoon the gas emission abruptly stopped, possibly due to obstructions of the vents. Observers speculated that these phenomena indicated the extrusion of new lava in the bottom of the crater. No significant changes of the other measured parameters could be observed.
The following day saw a considerable decrease in activity: only six moderate emissions including some short ash puffs. Continuing very low emissions over several days indicated the vents were partially closed. On the morning of 9 December personnel from CENAPRED and the Instituto de Geof¡sica, UNAM, made a helicopter overflight during which the presence of a large lava dome, spread across almost over the entire crater floor, was seen. This observation confirmed the assumption of a new lava extrusion on 5-6 December. No important changes on the volcano flanks or the glacier could be observed. SO2 measurements made the afternoon of 9 December gave preliminary values of 6,100 tons/day. The other parameters that are continuously monitored showed little variation. Popocatépetl returned to characteristic low levels of activity, although some earthquakes of M 2.2 occurred at a depth of ~4.7 km during 14-15 December. On 13 December strong winds and low temperatures caused damage to monitoring equipment, including the video transmission link.
After several weeks of very low activity an eruption clearly observed from neighboring towns started at 1930 on 24 December. The activity began with a 2-minute explosion followed by 15 smaller volcano-tectonic events and several moderate emissions. According to reports from the nearby towns of San Nicolas de los Ranchos and Amecameca, during the first event observed brightness around the summit was produced by the expulsion of incandescent materials, and an associated shock wave was felt. From Puebla grass fires were reported on the E flank of the volcano. Ashfalls were reported starting at 2045 in towns E of the volcano (Atlixco, Calpan, and San Nicolas de los Ranchos). The whole episode lasted a total of 30 minutes. All monitored parameters except for seismicity then returned to normal levels. This eruption was probably associated with the reopening of conduits inside the crater, obstructed since 6 December by lava extrusion. This obstruction was carefully monitored because pressurization of the system raised the possibility of explosive events. Following the eruption of 24 December activity returned to very low, stable levels for the remainder of the month. The volcanic- alert system remained on "yellow" (caution) through all of December.
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: Roberto Meli, Roberto Quaas Weppen, Alejandro Mirano, Bertha López Najera, Alicia Martinez Bringas, A. Montalvo, G. Fregoso, and F. Galicia, Centro Nacional de Prevencion de Desastres (CENAPRED); J.L. Macias, Instituto de Geofisica, UNAM, Circuito Cientifico.