The central part of the Trans-Mexican volcanic belt is being deformed by seismically active, west-east striking normal faults that cause pronounced scarps. Structural and seismic along-strike discontinuities allow the following segmentation: (1) The western segment, the area between Morelia and the Los Azufres volcanic center, is characterized by ≤25-km-long fault scarps that define the Cuitzeo graben, and the Santa Ana Maya horst which has a relief ≤850 m with respect to the Cuitzeo graben. The faults cut several Quaternary scoria cones as well as a alluvium. (2) The central neotectonic segment extends between Maravat¿o in the west and Altamirano volcano in the east. The most prominent structure of this segment is the 45-km-long Venta de Bravo fault with a relief ≤300 m. The February 22, 1979, mb=5.3 Maravat¿o earthquake occurred on this fault.

The late Quaternary vertical slip rate along he easternmost part of the Venta de Bravo fault is estimated at 2 mm/yr. The lateral extension of the segment is defined by the limits of the venta de Bravo fault and the eastern termination of the Cuitzeo graben. (3) The eastern segment is made up of the Acambay graben. The limit between the central and eastern segments is defined by an extensional jog between the Venta de Bravo and Pastores faults, which indicates a left-lateral strike-slip component. The most recent rupture of this segment occurred in the November 19, 1912, MS=6.9 Acambay earthquake that caused vertical displacements ≤50 cm along the faults flanking the graben. Stress determinations based on Quaternary scoria cone alignments, the focal mechanism of the 1979 earghquake, and the inversion of striation measurements indicate a stress field with SV>SENE>SNNW.

The fault striations as well as the focal mechanism indicate a minor but consistent left-lateral strike-slip component. The Observed stress and deformation can be explained by the superposition of two sources of stress. Isostatically compensated surface loads related to the high elevation of the Trans-Mexican volcanic belt probably cause horizontal deviatoric tension and the observed crestal normal faults which strike parallel to the axis of the volcanic belt.

The observed left-lateral strike-slip component, on the other hand, can be explained by compressional far-field stresses caused by loads at the boundary between the Cocos and North American plates; the orientation of the faults with respect to the direction of plate convergence is a prefered orientation for left-lateral slip along the observed faults. Alternatively, the observed left-lateral strike-slip component may accommodate trench-parallel motion of crustal blocks in southwestern Mexico. ¿ American Geophysical Union 1992