1989–1995 Earthquake sequences in the Galeras volcano region, SW Colombia, and possible volcano–earthquake interactions

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The Galeras volcanic complex, located in the Nariño department, SW Colombia, includes the most historically active volcano in the country, Galeras, a 4276 m high stratovolcano located 9 km west of the city of San Juan de Pasto (400,000 inhabitants). The area is also affected by the continental faulting represented by the Buesaco, Aranda and Pasto faults belonging to one of the most seismically active structures in Colombia, the Romeral fault system. Several moderate to strong shallow earthquakes affected the city of San Juan de Pasto and its neighbouring region since the XVII century. The coexistence of an active volcanic complex and an active fault system complicates the study and interpretation of the different processes taking place in the region as well as the identification of any connection or interaction among them. The reactivation of the volcano in 1989 was characterized by three main magmatic events: (1) a series of Vulcanian eruptions during 4–9 May 1989, with semi-continuous ash emissions from a secondary crater; (2) the emplacement of an andesitic lava dome at the bottom of the main crater from September 1990 until December 1991; and (3) six Vulcanian eruptions from the main crater during 1992–1993, with destruction of most of the dome during the first one on 16 July 1992. During the same period, four earthquake sequences were located in a limited area N and NE of Galeras volcano on August–September 1989 (AUG1989), April–June 1993 (APR1993), November–December 1993 (NOV1993) and March–August 1995 (MAR1995). The last one included a Ml4.7 main shock on 4 March 1995 producing moderate to high damage in the epicentral region (MSK maximum intensity VIII), and in the city of San Juan de Pasto (VI–VII). The last damaging earthquake in the region was a MSK-intensity VIII–IX in 1947. A detailed analysis of the spatio-temporal characteristics of the four earthquake sequences allowed identifying their different origin and suggesting some interrelationship between the reactivated eruptive process and the contemporaneous seismic activity. The AUG1989 sequence presents a typical volcanic swarm-like pattern most probably related with the process of magma intrusion from depth at the beginning of the volcano's reactivation. The APR1993, the NOV1993 and the MAR1995 sequences show a clear tectonic origin with events occurring on rupture planes almost vertical that can be associated to the active faults in the area, The seismogenic process of these three sequences could have been activated or accelerated by the main eruptions during 1992–1993. These results suggest that constrains provided by improved relocations and the detailed analysis of the space–time characteristics of earthquake sequences in Galeras volcanic environment allow to establish the different generation mechanisms involved and to suggest feasible explanations on the possible interrelationships of the magmatic–volcanic processes and the seismicity observed.