A magnitude 4.8 earthquake rocked the East Coast midmorning on Friday, sending high rises swaying in New Jersey, New York, Pennsylvania, and Delaware.

Californians and other West Coasters may scoff at the alarm expressed by their East Coast counterparts from a seemingly small quake. But this tremor struck the most densely populated region of the US, rocking some of the oldest buildings in the country, and alarming people who often take it for granted that the earth doesn’t move very much. So it’s worth taking seriously.

The East Coast is indeed much less geologically active than the Western US, with fewer active fault lines that cause major earthquakes. There are, however, frequent smaller tremors. The US Geological Survey notes that the eastern part of the country has experienced more than 400 earthquakes with a magnitude greater than 3.5 over the past 50 years. As recently as 2011, a magnitude 5.8 quake that struck Virginia caused shaking to be felt along the East Coast and some damage to buildings in Washington, DC.

© Vox