Approximately 10,000 earthquakes large enough to be felt by humans occur every year as tectonic plates below the earth’s surface slide past one another to relieve stress. The seismic activity from these earthquakes is recorded at thousands of seismic stations around the world. Using data from these stations, scientists can learn more about the geology inside of the earth, including things like earthquake location and magnitude.
Even with all those seismic stations, though, and more than 100 years of earthquake records, there are still significant holes in the data because seismic stations are not sampling the earth evenly or completely.
This happens for two reasons. First, large earthquakes do not occur uniformly across the globe. Most happen at the edges of tectonic plates, which means that scientists have more data from those areas than from areas with less seismic activity.