earthquakes occurring within a dist

earthquakes occurring within a distance of a few hundred kilometres, say 500 kilometres, from a seismic station are called local earthquakes.
Local earth quakes are often characterised by impulsive onsets and high frequencies (Figa.3b and 3c). A local earthquake signal typically has an exponentially decreasing tail. 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, hypocentre, epicentre distance, and hypocentre distance are explained in Fig. 3d. With an average value of v/v of 1.73, we may estimate the hypocentre distance of an earthquake, which is approximately eight times the s-p time interval (fig. 3d). For example, if the s-p time interval is 10 seconds (meaning the s phase arrives 10 seconds after the arrival of the P phase), then the hypocentre distance of the earthquake is approximately 80 kilometres.