| Tomographic inversion techniques have been applied to local earthquake travel times to delineate lateral crustal velocity variations in Western Washington. Shallow earthquakes from the early 1970's to the present, ranging to depths of 40.0 km, are used as sources. Two methods for inverting large sparse matrix systems are compared: an iterative back-projection method, ART, using relaxation and smoothing to attain regularization versus a conjugate gradient method, LSQR, which uses constraints to regularize. These techniques are compared with synthetic examples that simulate the characteristics of real data inversion. The resolution is approximated by calculating impulse responses at blocks of interest and estimates of standard errors are calculated by the jackknife. Initial reference models are one dimensional layered velocity structures derived by least squares analysis.; In the Puget Sound region the target consists of a 150 x 250 km area divided into blocks of 5 km per side. In depth the model extends to 40 km divided into 10 layers. A total of 4387 earthquakes gave rise to 36,865 raypaths which had good resolving power in the 2-16 km depth range. High correlations with known features are apparent in the shallow structures and evidence for accretionary underplating dipping to the east appear at depth beneath the low velocity sediments of the Puget Sound. In the Mt. St. Helens region, a separate inversion was performed with 17,659 rays using an 80 x 80 km grid divided into cells 2.0 km per side. Major structural features such as the Spirit Lake and Spud Mountain plutons are evident in the shallow layers. The Saint Helens Seismic zone is characterized by a prominent low velocity feature. A low velocity anomaly beneath the crater at depths of 4-9 km may indicate the presence of magma accumulations. |
