Landslides caused by the M 7.6 Tecomán, Mexico earthquake of January 21, 2003

The Tecomán, Mexico earthquake (also called the “Colima earthquake”) of January 21, 2003 (M 7.6) triggered several hundreds of landslides in the coastal cordilleras of Colima State, near the earthquake source, and several thousands in the volcanic highlands north and northwest of Colima City. These landslides, mostly shallow and disrupted failures, caused minor damage to roads, to a railroad, and to irrigation systems. In one area, extensive, post-earthquake rock-fall activity indicates a possible long-term instability that could threaten dwellings and other infrastructure located nearby. In the coastal cordilleras, most of the landslides were generated by failures of artificially cut slopes, especially along roads. The rocks of the coastal cordilleras are generally well indurated, and landslides occurred only where the rocks were made locally susceptible by weathering, the presence of prominent discontinuities with unfavorable orientations, or intense fracturing or shearing.In contrast to the coastal cordilleras, the volcanic rocks to the north were more susceptible to the occurrence of seismically triggered landslides. The greatest number and concentrations of landslides occurred there, and the landslides were larger than those in the coastal cordilleras, even though this volcanic terrain was farther from the earthquake source. Here, stretches of river bluffs several hundred meters long had been stripped of vegetation and surficial material by coalescing landslides, and several days after the main shock, thousands of small rock falls were still occurring each day, indicating an ongoing hazard. The high susceptibility of volcanic materials to earthquake-generated landslides conforms to findings in other recent earthquakes.