Magma system recharge of Mount St. Helens from precise relative hypocenter location of microearthquakes

Abstract

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.