Tomographic inversions for mantle <i>P</i> wave velocity structure based on the minimization of <i>l</i>² and <i>l</i>¹ norms of International Seismological Centre Travel Time Residuals

Abstract

We use International Seismological Centre (ISC) P arrival data (1964–1987) and iterative algorithms which minimize the l 1 and l 2 residual norms to solve simultaneously for three‐dimensional P velocity variations in Earth's mantle, source mislocations, and station corrections. We find that the maximum velocity perturbations produced by the l 1 minimization (approximately ±4% in our final model) are relatively insensitive to smoothing and damping constraints. Therefore, using an l 1 norm criterion allows us to keep the bias introduced to the inversion to a minimum. Among the well‐resolved features contained in both the l 1 and the l 2 velocity models are a fast anomaly in the lower mantle beneath the Tonga‐New Hebrides subduction zone to a depth of 1670 km and another fast anomaly beneath the Japanese island arc and eastern Asia. Continuity between these anomalies and shallower fast anomalies is not clear. A fast anomaly extending from 670 km to 2070 km depth appears beneath eastern North America, the Caribbean, and north central South America. A broad, fast anomaly appears beneath eastern Asia just above the core‐mantle boundary as well as several slow anomalies under the Pacific basin of comparable size. Both models contain a circum‐Pacific ring of 2% lower velocities in the depth range 0–200 km, associated with back arc basins. High velocities (over 2%) associated with the continental shields tend to disappear below 400 km, though a significant region of high velocity remains beneath the Siberian platform in the 400 to 670 km depth interval.