Hundreds of small earthquakes have rumbled under northwestern Nevada like a seismic drumroll since midsummer , and in recent days , they have built to a crescendo . Hundreds of mainly low-magnitude quakes have been recorded . Such groups of quakes are called swarms . They are happening in and around a national wildlife refuge in Nevada . The swarm is caused by the movement of magma under the surface of the Earth, which has created an area where it can move more easily than normal.
Hundreds of mainly low-magnitude quakes have been recorded . Such groups of quakes are called swarms . They are happening in and around a national wildlife refuge in Nevada . The swarm is caused by the movement of magma under the surface of the Earth, which has created an area where it can move more easily than normal.