Results of a reconnaissance microearthquake survey of Bucaramanga, Colombia

Abstract

Six University of Wisconsin portable, continuously‐recording seismographs were operated for 3½ days in late 1976 in the Eastern Cordillera of Colombia in a 200‐km‐diameter array around Bucaramanga, where there are also three permanent stations of the Instituto Geofísico de Los Andes Colombianos. Twenty‐seven microearthquakes were recorded. Most can be well located. Only one event, located along the eastern boundary of the Eastern Cordillera, is at crustal depths. At least two events, beneath the western half of the Eastern Cordillera, are 110‐115 km in depth. Twenty earthquakes are associated with the Bucaramanga nest (6.83°N, 73.12°W), at a depth of 158 ± 4 km. For these well‐recorded events, relative arrival times between stations vary by up to ± 0.3 sec, and the epicentral area is 4‐5 km in diameter. The complex character of these seismograms, compared with those from 110‐115 km depth events outside the nest, suggests either complexity in the rupture process or, more likely, multi‐pathing which originates near the source region. For the upper mantle and crust above the nest, the average values for Q are about 450 for shear waves and probably over 1,000 for compressional waves, suggesting that the rays travel through lithospheric material for their entire path. If the two events at intermediate depths outside the nest and the nest lie at the same horizon within the same slab, they define a slab that strikes N‐S and dips to the east at 33°.