Smithsonian Institution
Global Volcanism Network Bulletin v. 19, no. 12, December 1994

Popocatepetl (Mexico)  Gas and visual observations following the
       small 21 December eruption

Popocatepetl
central Mexico (19.02N, 98.62W)
All times are local (= GMT - 6 hours)

Popocatepetl erupted for the first time in decades on 21 December
(see Bulletin v.19, no. 11; and pre-eruptive reports referenced
therein). In the wake of the eruption, Claus Siebe contributed the
following report.

"Visual observations since 21 January reveal that the emission of
ash has decreased in general and become more irregular. Longer time
intervals between single series of ash outbursts are observed.
Outbursts of ash still occur in single pulses, normally several
pulses in a row spaced at intervals of 2-5 minutes. During longer
periods of low ash-emissions fumarolic gases are still emitted,
preventing direct observation of the crater bottom. Despite this
restricted visibility, it is certain that the ash emissions
originate in the same area where previously a milky-green lake was
located. This lake occupied the depression of the small interior
crater that formed during the 1920-27 eruption, when a lava dome
was emplaced. During that eruption the lava dome was destroyed by
a Vulcanian explosion that created the depression. This interior
crater is not located in the center of the main crater but has an
eccentric position close to the E crater wall.

"In this respect, it is noteworthy that only one of the eight
springs monitored by me during the past months has shown a
conspicuous anomaly. This spring is located at Axocopan, near
Atlixco, 25 km SE of the volcano. Measurements of pH on 11 July, 18
August, and 17 November fluctuated between 5.8 and 6.1. The last
measurement taken prior to the 21 December eruption was on 10
December and yielded a pH of 2.5. This anomaly can be regarded as
another premonitory sign. The last measurement was taken 14 January
1995 and yielded a value of 5.5, which is almost back to normal.
Maybe, during the last weeks prior to the eruption, enough pressure
built up in the system to allow "magmatic juices" to be expelled
laterally and affect the pH of the Axocopan spring."

Following the 21 December eruption a new series of ultraviolet
absorption correlation spectrometry (COSPEC) measurements were made
by scientists of the University of Colima (I. Galindo, A. Gonzalez,
R. Ayala, and L. S. Ivlev). The measurements were requested by the
scientific committee organized through the auspices of the Centro
Nacional de Prevencion de Desastres (CENAPRED). The post-eruption
measurements began 48 hours after the eruption, and were also made
on 5 other days, with the latest reported being 14 January (figure
5 and table 1).

The COSPEC instrument was deployed from rented airplanes (flown by
R. Gonzalez and C. De la Cruz) in a variety of weather and
atmospheric conditions including air with fine suspended ash (23
December), high overcast clouds (27 December), and clear sky. The
plume top was situated at 4,300-5,740 m altitude and traverses
beneath the plume were made at altitudes between 3,000 and 4,545 m.
The plume was identified and two navigational bench marks were
placed using the aircraft's global positioning system (GPS) to
assure that the traverses were perpendicular to the plume's axis.
The speed and direction for both the plane and the wind were
computed at two points inside of the plume. In addition, at the end
of some sessions a parallel flight above the plume was used with
the GPS to resample the wind speed.

Figure 5 and table 1 suggest that the SO2-flux baseline for 1994
was ~1,000 tons/day, rising to ~4,000 tons/day during the eruption
on 24 December. The airborne observations also indicated that the
plume did not reach more than ~500 m above the summit, meaning that
the thermal output of the volcano was low. The plume mainly
consisted of water vapor with a minor ash component. Its color was
dark gray to light brown.

Two tiltmeters have now been installed within several kilometers of
the summit, and during their first 10 days of operation in
mid-January they remained stable (+-0.5 śradian). Reflectors for
survey lines have been replaced on the mountain's accessible N
side, and have been recently surveyed at 5-7 day intervals.
Reportedly, in mid-January the surveyed line-lengths also remained
stable. In addition to four or more seismometers on the mountain,
a video camera with a telephoto lens was aimed at the summit
crater, providing a view of the summit monitored from the CENAPRED
data center. John Ewert reported that in the first week of January
banded tremor prevailed, accompanied by nearly constant ash
emission. The seismicity has quieted since then. In mid-January
scientists saw several small earthquakes that were followed ~90
seconds later by distinct "puffs" seen coming from the summit on
the video monitor. This, and other behavior, were taken to suggest
that the magmatic system was open and under comparatively little
pressure. A terse summary is available on both activity as late as
the end of December, and recommendations for future monitoring
(McNutt, 1994).

Reference:  McNutt, Stephen R., 1994, Report to CENAPRED on
activity of Popocatepetl volcano and some recommendations for
improved monitoring and response: unpublished manuscript, 7 p.

Information Contacts:  Claus Siebe, Instituto de Geofisica, UNAM,
Coyoacan, 04510, Mexico DF, Mexico (Email:
csiebe@tonatiuh.igeofcu.unam.mx); I. Galindo, A. Gonzalez, R.
Ayala, and L. S. Ivlev, CUICT-Universidad de Colima, Mexico (Email:
ciencias@volcan.ucol.mx); John Ewert, USGS Cascades Volcano
Observatory, 5400 MacArthur Blvd., Vancouver, WA 98661 USA; Steve
McNutt, Alaska Volcano Observatory, University of Alaska Fairbanks
Geophysical Institute, PO Box 757320; Fairbanks, AK (Email:
steve@deus.gi.alaska.edu).

Figure 5. Graphical reproduction of Popocatepetl SO2 gas flux shown
with an arbitrary horizontal scale. ASU refers to data collected by
Stan Williams. Courtesy of Ignacio Galindo.

                   SO2 Measurements
Day        Average     Max    Min  Std. Dev.    n
5 Nov   1,261 +- 538  1,877    485    285      12
23 Dec  2,169 +- 435  2,488  1,617    244       8
24 Dec  3,961 +- 577  4,555  3,402    444       5
27 Dec  1,167 +- 263  1,513    987    140      13
29 Dec  1,237 +- 340  1,616    935    194      11
6 Jan     836 +- 169  1,054    716    123      12
14 Jan    533 +-  95    652    462    117       7

Table 1. Popocatepetl SO2 gas flux for 5 November 1994 through 14
January 1995. Courtesy of Ignacio Galindo.