A new scheme for hypocenter calculation is proposed for volcanic regions. The scheme finds the global minimum of the travel time residual, and the resulting solution is more stable than that from the conventional Geiger┼fs method, especially when the number of observations is small or the station distribution is unsuited. In the first step (Step 1) of this scheme, the target area is parameterized by the node. The node distance depends on the heterogeneity of the velocity structure; usually about 0.3┼`1 km is sufficient. Travel times between nodes and stations are stored.┼@In the first half of the second step (Step 2A), a node is sought that minimizes the sum of squares of the travel time residuals for the event. By changing the depth of this node, a set of initial hypocenter candidates is generated. In the following step (Step 2B), with these candidates for the initial hypocenter, precise locations of the event are obtained by nonlinear calculations using the simplex method. The hypocenter with minimum travel time residual is then selected as the most probable hypocenter. For the travel time calculation, the 3-D robust seismic ray tracer known as Fermat (Nishi, 2001) is used. If the velocity structure and the locations of the observation stations do not change, the results of Step 1 are effective for all events. Consequently, repetitions of only Step 2A and 2B are sufficient for every event. Successful outcomes of calculation using synthetic and actual data are obtained with practicable CPU times.
Keywords: nonlinear hypocenter calculation, initial hypocenter, simplex method