When is an aftershock of the earthquake going to hit? And how big will it be?

These are the two questions that Americans up and down the eastern seaboard are asking after an earthquake that registered 5.9 on the Richter Scale rattled houses, schools and skyscrapers. Millions of Americans in the most heavily populated corridor of the United States were affected.

Major financial markets remained open in late day trading. Hundreds of traders poured out onto Wall Street and Broad Street in lower Manhattan's financial district following the quake.

Geologists say that large earthquakes the likes of the one that hit the eastern seaboard today often are followed by so-called aftershocks, geological tremors that are echoes of the initial quake.