THE INTERMEDIATE SEISMICITY ZONE OF THE WESTERN COLOMBIA

Western Colombia (4° - 6° N, 76° - 78° W) is characterized by a pronounced intermediate depth seismicity (50 °Ü h °Ü 120 km), typified by a periodicity of at least a M °Ý 5.0 quake every 5 years. The diffuse shallow seismicity (0 °Ü h °Ü 50 km), as documented by the SISRA and CERESIS catalogues, impedes the recognition of active crustal fault zones, rendering the assessment of seismic risks extremely difficult. Important historical events of this intermediate seismic activity are provided by the following examples: 7 - 30 - 1962, h °Ö 59 km, M = 6.7 and 11 - 23 - 1979, h °Ö 105 km, M = 6.3.

In this work we consider the extent of this intermediate depth seismicity by hypocentral profiles and local seismic tomography, as recorded by the National Seismic Network of Colombia (RSNC) during the period of 1993 - 2001. The focal mechanism of the 2 - 8 - 1995, h °Ö 93 km, M = 6.8 earthquake of Pereira City was especially helpful to deduce the regional state of stress. The widely distributed seismicity points to the existence of an asperity, as defined by Cloos (1992) and Scholtz & Small (1997), and reveals a change in slab angle of the subducting Nazca plate.

We propose that this asperity represents the northeastern continuation of the Malpelo Ridge and originated in the Neogene, as the slowly expanding Galápagos Ridge intercepted with East Pacific Rise. The convergence with the Caribbean, North America and South America plates resulted in a strong constriction of Nazca plate, giving rise to the radial array of the aseismic Cocos, Malpelo and Carnegie ridges. As the disintegration of the Nazca plate reached its climax in the Late Neogene large oblique transform faults formed, as exemplified by the Panama Fault Zone, which separates the Cocos from the Coiba Ridge. Subduction of the Malpelo Ridge beneath Northwestern South America produced a flat slab segment between 6° - 7,5° N and a change in slab angle to the steeply dipping slab segment between 4° - 6° N. These changes in slab angle are similar to those observed at the collision of the Nazca and Juan Fern¨¢ndez Ridges at latitudes 12° - 16° S and 28° - 33° S.