Earthquake Distance Calculator

Earthquakes generate two body waves that travel at different speeds through the crust — the faster P-wave (compressional) and the slower S-wave (shear) — and the gap between their arrival times at a seismograph, the S-P interval, is a direct proxy for distance to the epicenter. This calculator uses distance = (Vp × Vs / (Vp − Vs)) × Δt, which falls out of the fact that both waves start at the same instant but arrive at different times because they travel at different constant speeds over the same distance. The defaults, 6.0 km/s for P-waves and 3.5 km/s for S-waves, are typical continental crust averages, but actual velocities vary with rock type and depth, so a single-station reading only gives a distance, not a direction — real epicenter location requires combining readings from at least three stations and triangulating where the distance circles intersect.

The calculator enforces that the P-wave velocity must exceed the S-wave velocity by at least a small margin, since P-waves are always faster in any real medium; if you enter values that violate this, they're silently adjusted to remain physically valid. It also back-calculates individual P- and S-wave travel times at the computed distance and re-derives the S-P interval from them as a consistency check — if your inputs are self-consistent, this "verified" value should equal what you originally entered. The most common misunderstanding is treating the output as a compass bearing rather than a radius: with one seismograph, the epicenter could be anywhere on a circle at that distance.