Why a huge tsunami warning was issued for California but quickly rescinded

A magnitude 7 earthquake off the coast of Humboldt County triggered a tsunami warning, prompting residents along the Northern California coast to seek higher ground and leave immediately Thursday morning. According to the advisory, which was sent shortly before eleven in the morning, "a tsunami with damaging waves and powerful current is possible." However, the alarm was canceled around an hour later. Some others thought it was emergency whiplash. Others were left perplexed.