Arrival times from an ensemble of discrete earthquakes independently contain information on hypocenter locations and jointly provide information on the velocity model. A properly formulated least squares estimation procedure can be used to determine simultaneously both hypocenter and velocity model parameters. By means of this procedure a sufficiently well distributed set of earthquakes provides information on crustal structure without some of the inherent difficulties associated with explosion refraction interpretation. Useful error estimates and resolving information are provided, and tests on artificial data indicate that successful inversions are obtainable, even for models with low-velocity zones, by using only P wave first arrivals at a number of stations. The method may have application for detecting P wave low-velocity regions resulting from dilatancy and is directly extendable to modeling shear wave velocity structure when S arrivals are used in place of P arrivals. $TA STA STA ß ß . . . ß / ,,, ,,-r ,-,. / :0-' HYPOCENTER \ ,

Tomographic inversion is applied to 17,659 P phase observations at 21 stations from 2023 earthquakes in the vicinity of Mount St. Helens to study the three‐dimensional velocity structure. Block size for the inversion is 2 km horizontally and 2 km or more vertically. Locations of hypocenters are assumed known and are based on a reference one‐dimensional, layered velocity structure. A conjugate gradient technique (LSQR) is used to invert the large sparse system of equations, augmented by regularization with a Laplacian roughening matrix. Resolution is estimated by computing the impulse response of the inversion for various critical locations, and uncertainties of the estimates are determined by a jackknife approach. The results of the inversion show a remarkable correlation with known geological and geophysical features. The Spirit Lake and Spud Mt. plutons are characterized by high‐velocity regions extending to approximately 9 km depth. The St. Helens seismic zone, a band of diffuse seismicity extending NNW from the volcano is evident as a prominent low‐velocity lineation. The change in character of the velocity anomalies south of St. Helens corresponds well with the near cessation of seismic activity there. A low‐velocity anomaly beneath the crater from 6 to 16 km depths may represent modern magma accumulations.

We simultaneously invert travel times of refracted and wide-angle reflected waves for three-dimensional compressional-wave velocity structure, earthquake locations, and reflector geometry in northwest Washington state. The reflector, interpreted to be the crust-mantle boundary (Moho) of the subducting Juan de Fuca plate, separates intraslab earthquakes into two groups, permitting a new understanding of the origins of intraslab earthquakes in Cascadia. Earthquakes up-dip of the Moho's 45-kilometer depth contour occur below the reflector, in the subducted oceanic mantle, consistent with serpentinite dehydration; earthquakes located down-dip occur primarily within the subducted crust, consistent with the basalt-to-eclogite transformation.