THE JANUARY 25TH, 1999 EARTHQUAKE IN THE COFFEE GROWING REGION OF COLOMBIA: TECTONIC AND SEISMOLOGICAL ASPECTS

2 SUMMARY The northwestern corner of South America is a very complex tectonic environment where three major tectonic plates - Nazca, South-America and Caribbean - are presently converging. Nazca plate forms a subduction zone underneath the steady South American plate; and the Caribbean plate also compresses the south American Plate towards the south-east. The stress field generated by the interaction of these three plates induces a rather high seismic activity along several geological faulting systems. The seismic history of the region reports several damaging earthquakes in the past during the 20 th Century. The main shock of the January 25 th , 1999 earthquake occurred at 1:19 PM local time, it had a magnitude mb of 5.9 and a depth of 10 km. The main aftershock occurred 4 hours and 20 minutes after the main shock and had a magnitude mb of 5.4. The aftershock sequence is contained within a volume of 7 km long, 6 km wide and as deep as 25 km. This paper presents the tectonic situation of Colombia and some details of the complex faulting in the region where the earthquake occurred. Also, some of the seismological observations and conclusions shall be presented. Analysis of the aftershock sequence as well as observations in the field suggest that the event maybe be assigned to the large Romeral faulting system along a segment known as Silvia Pijao, 15 km away from the city of Armenia, capital of the Quindio Province. The focal mechanisms show that the main shock and the biggest aftershock may be related to a strike slip fault trending NE with a nearly vertical plane and left lateral movement. Peak horizontal accelerations from the main shock which were recorded at ground level in Armenia. were as high as 58% g (acceleration of gravity). Accelerations recorded across the country show a rapid decrease with distance; 50 Km away from the epicentre, peak accelerations were as low as 5%g.