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To make a checkerboard in the resolution tests, we assigned positive and negative velocity anomalies with magnitudes of 5% to the 3D grid nodes. Synthetic data were calculated for the checkerboard model. Then we added random errors to the synthetic data and inverted them with the same algorithm that we used for the observed data. The inverted image of the checkerboard suggests where the resolution is good and where it is poor. The checkerboard resolution tests and other synthetic tests we conducted showed that both the high-velocity Tonga slab and the low-velocity back arc and mantle wedge were reliably resolved and that there was no trade-off between them.
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We thank M. Bevis, W. Crawford, K. Draunidalo, S. Escher, T. Fatai, H. Gilbert, S. Helu, K. Koper, M. McDonald, J. McGuire, B. Park-Li, G. Prasad, E. Roth, A. Sauter, P. Shore, and L. Vuetibau for their assistance during the seismic experiment and at the data-processing stage and G. Abers and an anonymous referee for thoughtful reviews, which improved the manuscript. Broadband seismographs were obtained from the PASSCAL program of the Incorporated Research Institutions in Seismology (IRIS). Supported by the NSF under grants EAR-9219675, OCE-9314446, and EAR-9614502. This paper is Southern California Earthquake Center publication 386.
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