Macroseismic study of the Mw 7.5 21 January 2003 Colima, Mexico, across-trench earthquake

A field investigation of residential house damage in 83 localities of the Occidental part of Mexico allowed construction of a macroseismic map of the moment magnitude 7.5 2003 Colima earthquake. The 12-grade modified Mercalli (MM) scale and the 7-grade damage scale of Whitman (1988) were used for estimating the earthquake intensities. The macroseismic effects of this earthquake were unexpected for the Occidental part of Mexico. The earthquake occurred within the across-trench structure of the El Gordo Graben and was characterized by the down-dip across-trench directivity of its rupture, as reconstructed from seismic-waveform modeling. It was a reason for the severe damage to buildings in the localities situated at distances where usual along-trench rupturing subduction earthquakes of the same magnitude do not produce any significant damage. The macroseismic map of the 2003 earthquake shows that the narrow (30 kin wide) MM VII zone was elongated (up to 60 km) in the northeast direction toward the continental part of Colima state. As a result, the intensity in Colima city was 1.5 grades larger than would be expected for this magnitude according to the proposed attenuation of intensity with distance relationship for earthquakes from the Mexican subduction zone.