Earthquakes are caused by sudden slip on faults where blocks of the Earth's crust move past one another. The hypocenter (or focus) is the point below the surface where the rupture starts; the epicenter is the point directly above it on the surface. Stress can build up at plate boundaries for hundreds of years before being released in a single event (USGS Earthquake Hazards Program).

Magnitude measures the size of an earthquake and the seismic energy released at the hypocenter. The Richter scale (1935) is logarithmic: each whole number increase means about ten times larger wave amplitude and approximately 31 times more energy release. Earthquakes are recorded by seismographs; agencies such as the USGS use the moment magnitude scale for large events. Intensity describes the observed effects of shaking (on people, buildings, and the ground) and varies with distance from the epicenter (USGS; BSSA).

Foreshocks are smaller earthquakes that sometimes precede a mainshock; aftershocks follow the main event and can continue for weeks, months, or years depending on the mainshock size. The data above comes from the European-Mediterranean Seismological Centre (EMSC).