Jewish eschatology generally anticipated divine interventions at a point of terrible suffering of Israel. IOW God would rescue them when they needed him most. As I read the mini apocalypse in Mark, the author was simply saying the same again.
The dark period of the decades surrounding 70, inspired the eschatological anticipations similar to those of the century before when they faced Antichus and then Pompey. Similar to the many times previous that they faced times of great suffering by war, food shortage, disease.
Earthquakes are possibly explainable by the earthquakes that had happened in the Mediterranean region, but I'm suspecting the author had in mind earthquakes such as supernaturally caused. Matthew specifically added an earthquake in the death scene. It is accompanied with a mass resurrection. That suggests to me that that redactor of Mark thought the earthquakes mentioned by the author of Mark were assumed supernatural.