May 15, 1997

Here is an update of Popocatepetl volcano's report for the last two
weeks to be posted on the Listserv:

There have been several explosions at Popocatepetl of different magnitudes.
After the initiation of the last episode of activity of the volcano in late
April the volcano has had several explosions but the largest in magnitude
have occurred on May 11, 13 and 14.

The event of May 11 has been one of the largest in duration and amplitude
since the activity was renewed in December 1994. It occurred at night at
8:39 PM (local time) and ashes were dispersed to the NE towards the city of
Puebla and as far as the coast of the Gulf of Mexico (there are reports of
ash fall at Boca del R=EDo, Veracruz, 260 km towards the east). The ashes
that fell at Cholula (33 km from the vent) were up to 3-4 mm in maximum
size with average of 1-2 mm. In the vicinity of the volcano, several
incandescent blocks produced bush fires but no damage was reported from the
nearest towns to the volcano except ash fall.

A less strong event occurred during the night of May 13 (10:31 PM, local
time). This event was seen in the video- survelliance camera installed 11
km from the vent. Due to darkness, no details on the images were observed
but the glare of the incandescent materials expelled during the explosion
and trajectories of the blocks were confirmed. Those blocks also produced
fires in the nearby forests. The event was also recorded by the doppler
radar system installed by the USGS at CENAPRED (National Center for
Disaster Prevention) and the movement of the ash plume was monitored in
real time. The ashes were dispersed towards the NE and ash falls were
reported mainly in the nearby towns but not farther than 35 km from the
vent.

The last explosion to date occurred on May 14 at 9:50 AM (local time) and
was similar in magnitude to the one of May 13. Due to overcast conditions
it was not observed on the monitor but it was recorded by the radar as it
dispersed also to the NE. A COSPEC crew was flying at that time around the
volcano in order to measure the SO2 flux and ashes were observed moving
through the clouds reaching an altitude of 45,000 ft (<14,000 m).

The measurements of SO2 flux show a very interesting pattern of decrease in
the values before the explosions and increase of the flux after the events.
At least the last 4 larger events have been preceded by this pattern,
suggesting a process of obstruction of the conduits and opening of them
through the explosions. The reason for this sealing of the system is
perhaps the presence of viscous lava at the conduits. During the last three
months a lava dome was growing at the crater of the volcano and was blown
out during the events of April. More lava should be still obstructing the
conduits and thus, the volcano is trying to open again the plumbing system
to resume the open vent degassing that occurred during 1996. Several
hypothesis are now under consideration about the behavior of the volcano
but more analyses of the data is needed and is underway.

The permanent real-time deformation network consisting of tiltmeters at the
same sites of seismic stations do not show significant changes. Sameways
the continous GPS surface deformation monitoring system shows no
significant change on the southwest flank until the last available
processed data on May 9.

During all these events the inhabitants in the vicinity have felt the
quakes associated to them and also have heard the explosions. Civil defense
officials are under alert and monitoring is being performed at the highest
level.

Personal of UNAM and CENAPRED currently involved in the monitoring of the
volcano include: Carlos Valdez, Servando de la Cruz, Roberto Quaas, Enrique
Cabral, Alicia Martinez, Berta Lopez, Alejandro Mirano, Lucio Cardenas
, and Hugo Delgado.


Yours,

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
Fax:    (525) 550-2486
Internet: hugo@tonatiuh.igeofcu.unam.mx