Guagua Pichincha volcano, located in the Western Cordillera of the Ecuadorian Andes, is one of the most dangerous active volcanoes in the country. It is located 12 km to the west of Quito (population 1.5 million). Guagua Pichincha has shown a progressive reactivation since its onset in 1981 that consists rnainly in phreatic explosions, shallow seismicity and morphological changes in the interior
... [Show full abstract] of the caldera. This volcano has been permanently monitored by the Instituto Geoffsico since 1981.
Eleven years ago, from July to December 1988, a swarrn of 785 volcano-tectonic (VT) events occurred 8 km SE from the cardera. This activity migrated toward the caldera as weJI as the depth location did from 10 km to 4-5 km (Bonilla et al., 1992). Between 1989 and 1997, the average seismic activity was 130 LP and VT events per year, Since June 1998, a swarm of about 4000 events has been registered in the north of Quito (I6-km NE from the caldera, JO-20 km depth). It could be related with Guagua Pichincha present activity (Calahorrano et al., This volume).
On August 4,1998 a major subduction earthquake (Mw=7.1) (Segovia et al., This volume) hit the Ecuadorian coast near Bahia de Caraquez, 210 km west from the volcano. Three days later, on August 7, an abnormal phreatic activity began with a moderate explosion (Reduced displacement RD=0.63 cm"). For the past 8 months, until present, this activity has continued. More than 140 explosions have been recorded . October 1998 and February 1999 have been the months with the greatest number of explosion s (26 and 28 respectively), although the number of explosions has been moving up and down with time. It is remarkable that 8 of the 10 biggest explos ions have occurred since December 23, 1998.
