Four hundred forty-seven well-recorded earthquakes at Mount St. Helens during the late 1990s are relocated with high precision using a combination of a cross-correlation technique and then a procedure to obtain a one-dimensional velocity structure solution together with station corrections. The resulting high-resolution pattern of the spatial-temporal evolution of the earthquake activity, along with focal mechanisms of events provide additional evidence for the presence of a magma reservoir in the depth range 5.5--10 km, connected to the surface by a thin vertical conduit. Moreover, the distribution of the events below 5.5 km yielded a much clearer view of the structures that generate the seismicity at that depth. Some of the deeper events occur along a sequence of faults under the influence of a radial stress field; however, a large number of the events occur on a NNE-SSW striking steeply dipping fault with slip consistent with magma being periodically injected into a truncated dike on the northwest side of this fault.