ABSTRACT

The Trans‐Mexican volcanic belt is an active continental volcanic arc related to subduction along the Middle America trench and characterized by several major arc‐parallel middle Miocene—Holocene lake basins formed by normal faults and related crustal seismicity. The Chapala graben, the largest of these basins in the western part of the Trans‐Mexican volcanic belt, is 115 km long and up to 30 km wide. Here, I document a 2 October 1847 earthquake with intensity magnitude MI of 5.7±0.4 that was locally devastating on the northern graben shoulder. It razed the villages of Poncitlán and Ocotlán in the state of Jalisco, where at least 58 persons perished. The macroseismic observations for this historical event and the elevated background seismicity indicate that the Chapala graben is active and poses a major ground‐shaking hazard to the nearby metropolitan areas of Ocotlán and Guadalajara. No historical earthquake had been previously documented from the Chapala graben, which was believed to be tectonically inactive. Furthermore, the 2 October 1847 event was not recognized as a crustal earthquake. In earthquake catalogs, the devastation at Ocotlán is aggregated with the damage caused on the same day, only two hours earlier, in the Colima region by a subduction‐zone earthquake that was devastating in Tecomán and Colima and caused minor damage in Mexico City, and which likely triggered dynamically the Chapala graben earthquake.

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