Local Earthquakes Earthquakes occurring within a distance of tow hundred kilometres, say soo kilometres, from a seismic station are called local earthquakes. Local eartbquakes are often characterised by impulsive onsets and high frequencies (Fkss. 3b and 3e). A local earthquake signal typically has an exponentially decreasing tall. The seismic wave data are very useful for studying the local geological structure velocity structure of the Earth's crust and upper mantle. Earthquake parameters, like epicentre, hypoeenoe, epicentre distance, and hypocrntre distance are explained in Fig. sd. With an average value of Vr/V of i 73m we may estimate the hypocentre distanee of an earthquake, which is approximately eight times the S-P time interval (Fig. 3d). For example, if the S-P time interval is lo seconds (meaning the S phase arrives 10 seconds after the armival of the P phase), then the hypocentre distance of the earthquake is approximately 8o kilometres.