Tomographic inversion for three‐dimensional velocity structure at Mount St. Helens using earthquake data

Abstract

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.