Causes
Many cities live with the seismic risk of a rare, large intraplate earthquake. The cause of these earthquakes is often uncertain. In many cases, the causative fault is deeply buried, and sometimes cannot even be found. Under these circumstances it is difficult to calculate the exact seismic hazard for a given city, especially if there was only one earthquake in historical times. Some progress is being made in understanding the fault mechanics driving these earthquakes.
Read more about this topic: Intraplate Earthquake