There was just a major earthquake involving Mexico City. Can we add earthquakes to the list of Earth-related problems for humanity caused by extraterrestrials or are these always a natural phenomenon?

Earthquakes are almost entirely unnatural. They are rare events because of the slow movement of the tectonic plates, although there have been many fault-lines develop, sustaining tremendous inner pressures while representing areas of vulnerability to a sudden catastrophic release of energy that would disrupt the Earth's surface and cause havoc for human inhabitants of buildings in the area and so on. These vulnerabilities need not manifest as a sudden release of energy. They can be dissipated in other ways and the frequency, if left alone, would be much, much, less without extraterrestrial manipulation. They are the cause of 95% of earthquakes directly, whether causing great harm or not. This is done to have a mix of both, to maintain the illusion of a natural phenomenon and not arouse suspicion of a hidden hand.