How do you know if there was just an earthquake?

The United States Geological Survey keeps a record of all earthquakes around the globe. In addition to providing seismic data from survey sites, the USGS also offers an application that allows users to report earthquake tremors they have felt. The USGS records all geological activity, not just earthquakes.

The majority of earthquakes are small and occur in areas that are too sparsely populated to be felt. In general, earthquakes that are below 4.0 on the Richter scale generally go undetected except by seismographs. An earthquake would have to be several magnitudes larger than 4.0 to cause any significant damage or injury.