FORMAL INVERSION OF ISC ARRIVAL TIMES FOR MANTLE P-VELOCITY STRUCTURE

Seismic traveltime data are used to determine and assess a weighted le ast-squares model of the 3-D P-velocity structure of the Earth's mantl e. A total of 682 090 summary rays constructed from over three million ISC traveltime observations are used to constrain velocity perturbati ons in 124966-degrees by 6-degrees cells. An iterative block LU decomp osition procedure, applied on the massively parallel Connection Machin e 2 (CM-2), is used to compute a formal weighted least-squares model o f P-velocity variations as well as the full resolution and covariance matrices. It is found that the shallow mantle beneath the continents o f the northern hemisphere is well resolved. However, the majority of m id-oceanic features, such as spreading ridges and hot spots, are not w ell constrained. As a function of depth, the resolution is highest in the upper to mid-mantle, due to variations in ray coverage. Primarily because of ray geometry, single blocks are not resolved below 1870 km. The estimates of spatial resolution, constructed from the complete re solution matrix, are useful in judging whether subducting lithosphere extends into the lower mantle. The results indicate that possible exte nsions of subduction zones in the northern hemisphere are imaged relia bly down to at least 1470 km. However, areas beneath most subduction z ones in the southern hemisphere are averaged over scale lengths of 800 km or more below a depth of 1070 km. Velocity estimates for the mid t o lower mantle beneath Hawaii are the result of averaging over 1900 km or more. Generally, vertical and lateral averaging of 600 to 1000 km is occurring in the depth range 2270 to 2670 km. P-wave velocity value s in the region just above the Core-Mantle Boundary (CMB) are large-sc ale spatial averages (1500 km or more) and individual cells are poorly resolved. The model parameter standard error remains moderate through out the lower mantle due to smaller traveltime errors associated with rays that bottom in this region. These standard errors reach no more t han 0.3 per cent of the Jeffreys-Bullen average velocity. The greatest standard errors, 0.9 per cent of the average velocity, are found in t he upper mantle underlying the Pacific basin. These large parameter er rors are due primarily to the poor ray coverage in the Pacific coupled with the large arrival-time uncertainties for P recordings in the epi central distance range 0-degrees to 20-degrees. The upper 200 km of th e weighted least-squares velocity model correlates strongly with known tectonic features. A circum-Pacific ring of low velocity, with a maxi mum of 2 per cent, dominates the surface anomalies. The most likely or igin of this feature are the high temperatures associated with back-ar c magmatism. The most conspicuous mid-ocean velocity anomaly is Icelan d, which appears as a strong low velocity region extending over many c ells. Continental cratons appear as positive velocity (1-2 per cent) a nomalies extending to depths of 200 to 400 km. Low velocities underly the continents of Eurasia and North America at a depth range of 400 to 670 km. In the mid-mantle, 670 to 1270 km deep, a 1-2 per cent fast v elocity anomaly encircles the region below the Pacific basin. A fast a nomaly also underlies India, in the depth range 870-1470 km, perhaps a ssociated with the convergence of India and Eurasia.