Java applet, developed by Anthony Lomax from an applet originally written by Yann Foucard
Find the epicentral distance by moving the vertical cursor (with the mouse button pressed) until the desired S-wave - P-wave time (Ts-Tp) is obtained.
Ts-Tp is displayed on the upper left; the corresponding epicentral distance
is displayed below the cursor.
This applet is used on the French high school site: sismo des écoles (http://www.edusismo.org/)
...on that site, the students obtain Ts-Tp times from analysis of digital seismograms (with SeisGram2K Seismogram Viewer), use this Hodochrone applet to calculate the epicentral distance, and then triangulate the distances from several stations (using Interactive map) to obtain the earthquake epicentral location.