Earthquakes have been occuring for millions of years, and will continue to occur. Just as earthquakes are caused today by the movement of techtonic plates - so too they were 2000 years ago.
For Jesus to specify that the end times would be marked with "earthquakes" is about as useful as him saying that in the last days "the sun will rise one day after another." Earthquakes simply occur (in fact, small earthquakes are constantly occurring hundreds of times per day around the globe) - as they always have - making this 'indicator' useless. Evidently, the bible authors did not understand the real nature of earthquakes, and likely they believed them to be acts of god and/or satan. Because the authors did not know, neither could Jesus... thus he throws out this bogus 'sign' that matches all times past and future. (As is the case with all of Jesus's 'signs')
- Lime