Explosive Event at Popocatepetl Volcano, Mexico

On October 28, 1996 at 8:59 AM, an explosive event took place at
Popocatepetl Volcano (19.02 N; 98.62 W; 40 km SE from Mexico City) through
the main vent. It consisted of an emission of ashes directed towards the
west by the explosion itself and the column rose to 9,000 meters above sea
level. The prevailing winds distributed ashes towards the west. Due to
clear weather, the visibility was more than 30 km and the populations in
the towns and villages on the western flank of the volcano witnessed the
event and the explosion was documented by Mexico City TV.

On October 29 at 8:00 AM, Jose Luis Macias, Claus Siebe and Hugo Delgado of
the Instituto de Geof=EDsica 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) directly observed the volcano directly using a helicopter provided by
the Procuraduria General de la Republica.

The helicopter flew at 6,000-6,500 m.a.s.l. (18-19,000 feet above sea
level) around the crater of the volcano to view the inside the summit
crater. The crater is still occupied by the lava body which appeared in
March, 1996 and stopped growing by July, 1996. Comparing the views of today
with those taken last Friday (October 25) during an earlier helicopter
reconnaissance flight, the concave lava body which showed a sinking of the
central part, looked today about the same but a new vent hole (about 50 m x
20 m) appeared on the central part of the lava dome with an elongated trend
NW-SE. Intense fumarolic activity was seen today at the SE part inside the
crater. Several other fumarolic sources were along the edge of the lava
dome at the crater wall and slope contact with the s of the crater.
Additional fumaroles were seen at the cracks on the lava dome. Along a N-S
trending lineament a lesser amount of gases came out from the volcano's SE
flank at an approximate altitude of 5,000 masl. This morning the gas plume
was drifting towards the west at an altitude between 4,500-5,800 masl
(15,000' and 19,100').

This explosive event showed clear precursory evidence in terms of the SO2
flux and deformation of the volcanic edifice. The SO2 flux has averaged
11,000 t/d for the last 4.5 months, with a pattern of increasing flux
starting on September 30. A peak in SO2 flux ocurred on October 21 with
more than 27,000 t/d. By the weekend (October 25) the SO2 flux dropped down
to 9,000 t/d --- a 60 % drop within 3 days. Two GPS stations located on the
western flank of the volcano also started to show a continuous increase in
the vertical (z) component since October 5, this surface deformation
pattern of uplift roughly followed parallel to the SO2 flux having a peak
on October 21 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
3 days preceding the explosion.

Results suggest the correlation between the SO2 flux data and deformation
data (derived from GPS) indicates repeated entrance of new magma to a large
magma chamber beneath the volcano. Popocatepetl presently seems to be
efficiently in release the gases in a passive way and accomodate the
deformation due to the new magma intrusion.

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 NASA-DOSE program
and support from CENAPRED.

Information contacts: Enrique Cabral-Cano
(ecabral@tonatiuh.igeofcu.unam.mx) and Hugo Delgado

Dr. Hugo Delgado Granados Instituto de Geofisica, U.N.A.M. Circuito Cientifico, C.U. 04510, Mexico D.F. Phone: (525) 622-4145; 622-4119; 622-4124 Fax: (525) 550-2486 Internet: hugo@tonatiuh.igeofcu.unam.mx