prize9-4, Exercise 1: Distance from the P-S gap

Model answer (check the details):

P-waves travel faster than S-waves, so the gap between their arrivals grows with distance. A longer gap means a farther source. Using a travel-time chart or a rough rule (for the crust, about 8 km of distance for every second of gap, check), a 40-second gap gives roughly 300 km.

One station gives only a distance, so the quake could be anywhere on a circle of that radius around the station. Two circles intersect at two points; a third circle picks the one correct point. This is triangulation.

Care: the rule of thumb varies with depth and rock type; real networks use detailed velocity models.