Abstract The Pacific coastal basin of Colombia covers an area of approximately 56,000 km 2 between the Pacific Ocean and the Western Cordillera. Another basin of deposition—the Pacific shelf basin—lies offshore and west of the coastal basin and comprises an even larger area. These basins are the least-explored, potentially petroliferous areas in Colombia. Only 6 wells (2 offshore, 4 inland) have been drilled; 3 are in the northern Atrato area of the basin. A partial reason for lack of exploration is the relative inaccessibility of the area, one of the most humid and unhealthy tropical rainforest zones in the western hemisphere. Unsuccessful results may also be traced to the lack of a concentrated effort at deciphering the regional geology. Surface geology does not everywhere accurately reflect the subsurface picture; in many places it masks the true underground structure. The Pacific coastal basin is currently regarded as one of promising potential for a country that badly needs to find new oil to meet ever-rising demands. Seismic investigations and photogeologic work permitted the mapping of a body of igneous and metamorphic rocks that crop out between the Docampadó and Baudó Rivers in the central or San Juan area of the basin (San Juan paleohigh). The petroleum potential of the area also appears clear, considering that sediments in excess of 20,000 ft (6,100 m) have accumulated in the basin, and that possible source rocks of both Tertiary and Late Cretaceous age occur within the sedimentary area. Furthermore, numerous oil and gas seeps have been reported throughout, and previously drilled wells, though not commercial, had encouraging oil and gas shows.

Abstract The macroseismic intensity of the 18 October 1992 Murindo-Atrato earthquake that affected the northwestern states of Colombia (Chocó and Antioquia) is reassessed using the newly developed INQUA Environmental Seismic Intensity Scale (ESI 2007) which is based on the evaluation of earthquake environmental effects. To generate the ESI 2007 isoseismal map of northwestern Colombia, a geographical information system was used. Unifying the available information on the seismological and active tectonics framework including historical seismicity, hypocentral depths, foreshocks, aftershocks, focal mechanism, macroseismic data under the same GIS and the map of Quaternary faults allowed us to reinterpret the geological and environmental effects of the 1992 earthquakes sequence. A total of 24 sites from the areas of Quibdó, Bojayá, Rio Sucio, Murindo, Vigía del Fuerte and Turbo were evaluated. A systematic comparison among evaluated intensities (Modified Mercalli and ESI scale) revealed differences from one to two degrees. According to the ESI 2007 scale, the epicentral intensity I o is XI. This represents one degree higher than the epicentral intensity obtained using MM and Medveded Sponhauer Karnik (MSK) intensity scales, probably due to the lack of suitable observations on building damage in this poorly populated and developed region. This information is also useful in order to shed some light on the persistent question of the exact location and dimension of the main rupture zone associated with the earthquake. The isoseismal map derived from the integration of the whole set of environmental effects with other macroseismic data strongly suggests that the causative tectonic structure is the Murindo fault. However, the rupture length derived from the distribution of ground effects is greater than the Murindo fault length, implying that other nearby fault segments were activated during the 1992 event. The new isoseismal map resulting from this work is relevant for the assessment of future seismic risk in the northwestern region of Colombia. Overall, the application of the ESI 2007 scale to the 18 October 1992 earthquake, and to similar strong events in the region, can be useful for disaster management and planning, estimation of damage, and post-earthquake recovery efforts.