The earthquake was centered about 75 miles southeast of Mexico City, not far from the city of Pueblo. At a magnitude 7.1 it's considered a 'major' earthquake. Depth is important as well. This one was 50 km deep below the earths crust. Surprisingly, this depth is considered shallow as the deepest quakes occur 500 miles down. These 'shallow' quakes often do the most damage at the surface.
The cause of the quake appears to be underground vulcanism. In that part of central Mexico, molten rock (magma) is very close to the surface of the earth. Over the eons this area has seen much volcanic activity. The area is elevated because of this. Today's quake was probably caused by magma moving just beneath the surface, causing the ground to move and shake. Today is also the anniversary of Mexico's most deadly quake, an 8.0 magnitude that struck on this day back in 1985.