SHEAR-WAVE VELOCITY STRUCTURE IN NORTH-AMERICA FROM LARGE-SCALE WAVE-FORM INVERSIONS OF SURFACE-WAVES

A two-step nonlinear and linear inversion is carried out to map the la teral heterogeneity beneath North America using surface wave data, The lateral resolution for most areas of the model is of the order of sev eral hundred kilometers. The most obvious feature in the tomographic i mages is the rapid transition between low velocities in the tectonical ly active region west of the Rocky Mountains and high velocities in th e stable central and eastern shield of North America. The model also r eveals smaller-scale heterogeneous velocity structures, A high-velocit y anomaly is imaged beneath the state of Washington that could be expl ained as the subducting Juan de Fuca plate beneath the Cascades. A lar ge low-velocity structure extends along the coast from the Mendocino t o the Rivera triple junction and to the continental interior across th e southwestern United States and northwestern Mexico, Its shape change s notably with depth. This anomaly largely coincides with the part of the margin where no lithosphere is consumed since the subduction has b een replaced by a transform fault, Evidence for a discontinuous subduc tion of the Cocos plate along the Middle American Trench is found, In central Mexico a transition is visible from low velocities across the Trans-Mexican Volcanic Belt (TMVB) to high velocities beneath the Yuca tan Peninsula. Two elongated low-velocity anomalies beneath the Yellow stone Plateau and the eastern Snake River Plain volcanic system and be neath central Mexico and the TMVB seem to be associated with magmatism and partial melting, Another low-velocity feature is seen at depths o f approximately 200 km beneath Florida and the Atlantic Coastal Plain. The inversion technique used is based on a linear surface wave scatte ring theory, which gives tomographic images of the relative phase velo city perturbations in four period bands ranging from 40 to 150 s. In o rder to find a smooth reference model a nonlinear inversion based on r ay theory is first performed, After correcting for the crustal thickne ss the phase velocity perturbations obtained from the subsequent linea r waveform inversion for the different period bands are converted to a three-layer model of S velocity perturbations (layer 1, 25-100 km; la yer 2, 100-200 km; layer 3, 200-300 km). We have applied this method o n 275 high-quality Rayleigh waves recorded by a variety of instruments in North America (IRIS/USGS, IRIS/IDA, TERRAscope, RSTN). Sensitivity tests indicate that the lateral resolution is especially good in the densely sampled western continental United States, Mexico, and the Gul f of Mexico.