A GRAPHICAL METHOD FOR LOCATING THE HYPOCENTER (FOCUS) OF AN EARTHQUAKE

A commonly used exercise in schools today is the location of the epicenter of an earthquake. This exercise usually includes the determination of the distance to the earthquake from three stations and the triangulation of the distances to locate the point of intersection of the distances to locate the epicenter. The following is a graphical method for locating the focus or hypocenter of an earthquake.

The method consists of the construction of a wavefront chart in a two-dimensional cross section based on the model chosen for the epicentral region. An array of depth arcs based on the S-P times of each of the three stations is drawn. Intersections of the depth arcs will give rise to three lines, which intersect at a point that locates the hypocenter of the earthquake. The focal depth is obtained by the depth labeled on any depth arc passing through the location of the event.

The method is tested on aftershocks of the 5.2 magnitude Goodnow earthquake of October 7, 1983 in the central Adirondacks. The epicentral locations and focal depths of the aftershocks were determined. This graphical technique gives epicentral locations and depths that compare favorably with Lamont-Doherty Earth Observatory depths.