Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 4, April 1995

Popocatepetl (Mexico)   Located seismic events and summit crater
      observations

Popocatepetl
central Mexico
19.02N, 98.62W; summit elev. 5,465 m

A new episode of explosive activity began at Popocatepetl volcano
on 21 December 1994. The eruption followed increases in seismicity,
SO2 flux, and fumarolic activity seen in the 13 months proceeding
it (Bulletin v. 18, no. 11; v. 19, nos. 1-4, 6, 8, 10, 12). More
recent observations have included ash plumes, SO2 flux, seismicity,
and temperature and chemistry of springs around the volcano
(Bulletin v. 20, nos. 1-3). The following report was submitted by
Carlos Valdes-Gonzalez, Guillermo Gonzalez-Pomposo, and A.
Arciniega-Ceballos (UNAM).
"We report on Popocatepetl seismic activity during the interval 21
December 1994 to 2 May 1995. Activity was monitored using 7 seismic
stations located around to the volcano above 2,600 m elevation
(figure 8). These stations are part of the Popocatepetl Seismic
Network. Beginning 21 December, the volcano changed dramatically in
its seismic and fumarolic activity. Several explosions emitted ash
that fell on Puebla City, an area located about 50 km away
[Bulletin v. 19, no. 11; v. 20, nos. 1-3]. About 22 hours after
this activity, seismic tremor was observed for the first time at
several stations.
"In the 21 December-2 May interval we located 75 seismic events in
the vicinity of the volcano (figure 8). We used arrival times from
digital records from at least three stations and located the events
using Hypocenter software. The average standard location errors in
the horizontal and vertical directions do not exceed 1 km with a
standard deviation of 0.14 km (figure 9). Earthquake magnitudes
(calculated using a coda length magnitude for tectonic events in
Mexico) ranged between 1.4 and 3.4 (as represented by different
sized dots on figure 9). The E-W cross section of the hypocenters
(figure 9) shows a concentration of seismic events in a circle of
3.0 km diameter and in a conduit that connects to the overlying
crater. These results crudely suggest a magma chamber located below
sea level and connected to the volcano crater. A N-S cross section
suggests the same findings.
"During the first four days (21-24 December) seismic tremor was
continuous and of high amplitude. During the following 20 days (25
December-13 January) tremor was also continuous, but the amplitude
diminished five-fold compared to the first four days. After that,
in the next 45 days (14 January-28 February), tremor turned
sporadic with durations of about 10 minutes and with amplitudes
comparable to those in the first four days. During the last 60
days, tremor became more sporadic with smaller durations, but it
still had amplitudes similar to, and in some cases exceeding, those
of the first four days.
"On 12 March an expedition lead by Enrique Chaves-Popuard reached
the volcano's summit. The meteorological conditions allowed the
team to videotape the interior of the crater. The following
observations were made: a) the crater lake disappeared, b) three
new craters appeared at the foot of the main crater's E wall, c)
most of the fumarolic emissions came from these new craters, d) the
number of small fumarolic vents has increased in the older inner
crater, and e) several fumarolic vents were observed in the S and
E walls of the main crater."
Information Contacts: Carlos Valdes-Gonzalez, Guillermo
Gonzalez-Pomposo, and A. Arciniega-Ceballos, Departamento de
Sismologia y Volcanologia, Instituto de Geofisica, UNAM, Ciudad
Universitaria 04510 D.F., Mexico (Email:
ssn@ollin.igeofcu.unam.mx).

Figure 8. Stations of the Popocatepetl Seismic Network (triangles)
and epicenters for located events detected 21 December to 2 May
1995 (dots). Courtesy of Instituto de Geofisica, UNAM.

Figure 9. An E-W cross section of the hypocenters beneath
Popocatepetl for the interval 21 December 1994 to 2 May 1995.
Earthquake magnitudes are shown by dot sizes; the size of error
bars are discussed in the text. Courtesy of Instituto de Geofisica,
UNAM.