An abnormally high number of earthquakes has shaken up Yellowstone National Park in the past week. Since Friday, a “swarm” of more than 250 low-magnitude, shallow quakes has repeatedly rattled an area under Yellowstone Lake, with the highest-magnitude tremor — a magnitude-3.9 quake — on Saturday. The seismic activity has raised fears that the quakes may foreshadow a larger earthquake, or a volcanic eruption — but scientists say there isn’t yet reason to fear an eruption.