The earthquake seismogram can be thought of as a cascade of effects starting with source properties, transmission properties, and receiver-dependent effects, including receiver response and local site effects. We have already looked at receivers to some degree, and can review the transmission behavior as a way to get velocity vs depth and tomographic anomalies, but the main issue is learning about the earthquake source from the seismograms. Locating an earthquake Local arrival times of P and S waves can be used to estimate distance for typical crustal models. The linear relationship is identified via a Wadati diagram, as in the text.
Several stations define a region on the surface where the event took place.
Poor station geometry surrounding the station raises the uncertainty, as does a lack of calibration getting a better handle on local velocities. This describes a circle on the earth, and with several stations, you can locate the origin. Epicentral distance is measured in km or degrees. Also Seismic arrays clusters of seismometers, can look in particular directions selectively by beam forming adding delayed signals. Today, globally distributed broad-band instruments provide real time monitoring of events for defense as well as natural hazard and science purposes.