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

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. 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