The earthquake described in Revelation would have met or exceeded 8.9, the largest seismic activity scientists have been able to measure since the creation of the Richter scale in 1935. It's magnitude can only be theorized by modern theologians and geologists.
The Richter scale measures the magnitude of an earthquake by recording the motion of the ground with in an interval of .1 to .001 milliseconds. This measurement is described by whole and decimal numbers. Each whole number represents a tenfold increase in magnitude, which is roughly 31 times more energy than the previous number. For example, an earthquake registering at 2 or 3 describes minor effects, whereas those at 7 or 8 describe catastrophic effects.
In Revelation, the earthquake is not only described as bringing down a tenth of a city and killing thousands of people, it also produced blackness in the sky and the moon is described as turning blood red. When taken literally, imagination is key to understanding the effects. One could imagine a similar scenario by investigating the effects of the 1960 earthquake in Chile, the 1964 earthquake in Alaska or even the 2004 tsunami.
It is reasonable to believe that an earthquake by Revelation's description meets or exceeds what humans have witnessed on Earth since the invention of the Richter scale.