EL SISMO DEL 21/12/2001 EN LOS ANDES MERIDEÑOS

El dia 21 de diciembre de 2001 cerca de las cinco de la tarde un fuerte sismo sacudio gran parte de los Andes venezolanos. Este evento fue reportado por el Laboratorio de Geofisica (Universidad de Los Andes), con magnitud Mb = 5.7, epicentro con coordenadas 8.23o N – 70.97o O (al noreste de Aricagua, estado Merida) y profundidad focal de 6 Km. El mecanismo focal se determino utilizando 33 polaridades del primer arribo de la onda P, resultando la solucion focal: (a) rumbo = 228o, buzamiento = 75o y deslizamiento = 137o; (b) rumbo = 328o, buzamiento = 48o y deslizamiento = 17o. La distribucion de las replicas evidencia que el plano de falla es el plano (a) de la solucion focal, indicando un fallamiento rumbo deslizante dextral. La ubicacion de su epicentro, el tipo de fallamiento y la distribucion de sus replicas, permite asociar este sismo al sistema de fallas de Caparo, Venezuela. El dia 26 del mismo mes, aproximadamente a 20 km hacia el suroeste se produjo otra serie de eventos, tambien asociados a la falla de Caparo, y cuyo conjunto de polaridades es consistente con el mecanismo del sismo principal del dia 21. Aunque inicialmente no se reportaron danos graves, pocas semanas despues se conocio que este evento causo cierta destruccion en una poblacion meridena de dificil acceso: El Quino. Con la informacion macrosismica, obtenida a traves de diversas fuentes, se elaboro el mapa de isosistas, cuyas curvas muestran cierta tendencia a alargamiento NE-SO. La determinacion de los parametros del sismo a partir de las isosistas revela la factibilidad de aplicar, con buenos resultados, las ecuaciones de Shebalin a sismos historicos ocurridos en los Andes venezolanos. THE EARTHQUAKE OF 12/21/2001 IN THE MERIDA ANDES ABSTRACT On December 21, 2001, at nearly five o´clock in the afternoon, a strong earthquake shook a large extent of the Venezuelan Andes. This event was reported by the Laboratorio de Geofisica (Universidad de Los Andes) with magnitude Mb = 5,7; epicentral coordinates at 8.23o N - 70.96o W (northeast of Aricagua, Merida state) and 6 km depth. The focal mechanism was determined using 33 P-wave first motion polarities. The solution is given by: (a) strike = 228o, dip = 75o and slip = 137o; (b) strike = 328o, dip = 48o and slip = 137o. The distribution of aftershocks shows that (a) is the fault plane, indicating a right lateral strike slip faulting. Due to the location of its epicenter, its type of faulting and the distribution of its aftershocks, it is considered that this event broke the Caparo fault, Venezuela. Another set of events occurred on the 26th of the same month, at approximately 20 km to the southwest. They also were associated with the Caparo fault, and produced a set of polarities consistent with the focal mechanism of the main event of December the 21th. Initially, no serious damages were reported, but a few weeks later it was known that they occurred in the El Quino, a village of difficult access for its geographical location. The collected macroseismic information was enough to make an isosistic map, whose curves show some elongation with a NE-SW trend. Determination of earthquake parameters from this map point out the feasibility of the application, with good results, of the Shebalin seismic-field equations to historical earthquakes occurring in the Venezuelan Andes.