Why is it shaking so much in Puerto Rico?

According to the Puerto Rico Seismic Network, in recent weeks more than 2,000 seismic movements shook the island. On December 28, one of magnitude 4.7 was felt and hours later it was succeeded by another of magnitude 5, and this earthquake, in turn, was followed by another 400 replicas of magnitudes greater than 2. This Tuesday there was another earthquake of magnitude 6.4.