The Earth is a dynamic planet. Every year, there are about 5 million earthquakes of various sizes on Earth, which works out to over 10,000 earthquakes per day. With earthquakes happening so frequently, why don't we feel them?
This is because the vast majority of earthquakes are minor ones with a magnitude of less than 3 on the Richter scale, which don't cause strong ground shaking that humans can feel. Even if some earthquakes have a high magnitude and cause significant ground shaking locally, as the earthquake waves spread outward (the larger the spread area, the less energy distributed to each region), and the energy further attenuates during transmission, the ground shaking becomes weaker in areas farther from the earthquake epicenter, so people naturally won't feel any obvious shaking. Before the advent of digital seismometers, old-fashioned large seismometers were equipped with drum recorders. Therefore, studying earthquakes requires the use of highly sensitive seismometers. Seismometers are humanity's "ears to the wind", capable of recording even the slightest vibrations. The earthquake magnitude is a human-defined scale, with a magnitude of 0 indicating that there is no shaking at all, and magnitudes below 0 indicating even weaker shaking. Seismometers can record earthquakes with magnitudes of 0 or even below 0. To put it another way, if a book "thuds" to the ground, humans won't feel the earthquake, but from the waveform chart recorded by the seismometer, a clear earthquake wave signal will be generated when the book falls to the ground, which can be considered a minor earthquake.
In addition to seismometers, studying earthquakes also requires a well-distributed and rational network of seismic stations. Only by setting up a "comprehensive monitoring system" can we capture as many earthquakes of various sizes occurring around the world as possible, study the propagation characteristics of earthquake waves across the entire Earth, and subsequently conduct detailed research on the Earth's internal structure.