The correct answer is - The coast of Mexico is on the edge of two tectonic plates, which results in frequent earthquakes.
Mexico is located on a geologically very active place. From both the eastern and western side, there are borders between tectonic plates, which contributes to the creation of lots of earthquakes, as well as volcanic activity. Because Mexico is not very wide in east-west manner, when an earthquake strikes on the shores, or further mainland, it is felt from one shore to the other, and Mexico City is in the middle of it all, thus it is very often affected by earthquakes.