By: Becky Oskin
Before the earthquake struck early this afternoon, the island's volcano monitoring agency, Pelvolca, had raised the volcanic eruption risk for El Hierro to "yellow." This warning means that activity is increasing at the volcano, but no eruption is imminent. A similar burst of activity prompted a yellow warning in June 2012, but the volcano soon quieted down.
Parts of El Hierro have swelled nearly 3 inches (7 centimeters) in the past week, with the growth centered between El Pinar and La Restinga, according to Involcan, the Volcanological Institute of the Canaries.
More than 550 earthquakes rattled the island between Monday and Wednesday, also centered on La Restinga. About 30 of the earthquakes were greater than magnitude 3, Involcan said. The earthquakes are triggered by magma rising underground, fracturing rocks and swelling the surface as the hot rock reaches upward. "The earthquake swarm corresponds to a new magmatic intrusion," Involcan said this morning in a statement.
Today's preliminary magnitude-5.1 earthquake was on the opposite side of the island from the ongoing swarm.
El Hierro was the site of a spectacular underwater volcanic eruption in 2011, which severely affected island fisheries and forced islanders to evacuate.