A 6.4-magnitude earthquake struck Puerto Rico before dawn on Tuesday, killing one man, injuring at least eight other people and collapsing buildings in the southern part of the island. The quake was followed by a series of strong aftershocks, part of a 10-day series of temblors spawned by the grinding of tectonic plates along three faults beneath southern Puerto Rico. Seismologists say it’s impossible to predict when the quakes will stop. The 6.4-magnitude quake cut power as power plants shut down to protect themselves. Authorities said two plants suffered light damage and they expected power to be restored later Tuesday.