Faults may rupture across several segments of plate boundaries, generating earthquakes of M 9 class. Along the Pacific coast of Mexico, a rupture of 450 km on 28 March 1787 generated an Mw 8.6 earthquake. Intensity values (I) of VII were reported at the Atlantic coast, and the maximum was XI in Oaxaca. Here, we estimate the number of casualties if an M 9 rupture were to occur at present. We verified that our software tool, QLARM, with its database, estimates intensities and fatalities approximately correctly for historic earthquakes along the Pacific coast. Our test set consists of 10 earthquakes (Mw 7.3–8.6) that caused between 0 and ∼10,000 fatalities between 1787 and 2003. Requirements for a satisfactory match are that the maximum I agrees within 0.5 units, the extent of the I=VII area agrees approximately, and the fatality count differs by not more than a factor of 2 or by 200 fatalities, whichever is larger. Results for Mexico, without the special case of Mexico City, suggest that an M 9 earthquake would cause approximately 27,000 fatalities and 120,000 injuries. Applying amplification factors to the ground motion in 11 of 15 districts, we estimate that, in an M 9 along the Pacific coast, 100,000 people may perish and 480,000 may be injured in Mexico City. The number of people in settlements with fewer than 20,000 inhabitants exposed to large I exceed the exposed population in larger cities by factors of 3–10. The estimated mortality rate in rural areas is several times higher than in major population centers.