Structure and Holocene Rupture of the Morelia Fault, Trans‐Mexican Volcanic Belt, and Their Significance for Seismic‐Hazard Assessment

Abstract

Abstract The Trans‐Mexican volcanic belt is an active volcanic arc being deformed by an intra‐arc extensional fault network. The Morelia fault, with a length of 14 km and a maximum throw of 260 m, is a major normal fault in the central part of this volcanic arc and forms part of a 36‐km‐long fault array. Slickenlines measured on the primary fault surface indicate north–south extensional dip slip with a minor left‐lateral component. In a well‐exposed cross section in the hanging wall of the Morelia fault, some few meters from the master fault trace, two buried soil layers, containing ceramic shards and stone tools, yielded radiocarbon ages of A.D. 420–660 (prehispanic classic period, Chupicuaro culture) and A.D. 1290–1435 (postclassic period), respectively. The underlying bedrock (Atecuaro ignimbrite) is cut in a staircase pattern by three synthetic secondary normal faults that delimit two colluvial wedges. Three slip events, with displacements ranging between 46 and 185 cm (average 109 cm), can be inferred from this section. The latest of these events must have occurred after A.D. 1290–1435, during the postclassic period or possibly during early novohispanic times. The Morelia fault, which is capable of generating M w 6–7 earthquakes, poses a major seismic ground‐shaking hazard and locally a surface fault‐rupture hazard within the metropolitan Morelia region with a population of more than 900,000 habitants. The foci of a sequence of seven small 2007 earthquakes on the Morelia fault were located below the center of town, at a depth of 11 km, and the fault trace crosses urban residential areas. Moreover, the town repeatedly experienced severe ground shaking during historical earthquakes. The best documented is the 19 June 1858 earthquake, which caused Morelia extensive damage corresponding to a macroseismic intensity of VIII–IX on the modified Mercalli scale. Online Material: Radiocarbon analysis reports, local 1894–1904 and 1909–1911 earthquake catalogs.