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Intra-trench variations in flexural bending of the subducting Pacific Plate along the Tonga-Kermadec Trench
Fan Zhang, Jian Lin, Zhiyuan Zhou
We conducted a detailed analysis of along-trench variations in the flexural bending of the subducting Pacific Plate at the Tonga-Kermadec Trench. Inversions were conducted to obtain best-fitting solutions of trench-axis loadings and variations in the effective elastic plate thickness for the analyzed flexural bending profiles. Results of the analyses revealed significant along-trench variations in plate flexural bending: the trench relief (W0) of 1.9 to 5.1 km; trench-axis vertical loading (V0) of –0.5×1012 to 2.2×1012 N/m; axial bending moment (M0) of 0.1×1017 to 2.2×1017 N; effective elastic plate thickness seaward of the outer-rise region (TeM) of 20 to 65 km, trench-ward of the outer-rise (Tem) of 11 to 33 km, and the transition distance (Xr) of 20 to 95 km. The Horizon Deep, the second greatest trench depth in the world, has the greatest trench relief (W0 of 5.1 km) and trench-axis loading (V0 of 2.2×1012 N/m); these values are only slightly smaller than that of the Challenger Deep (W0 of 5.7 km and V0 of 2.9×1012 N/m) and similar to that of the Sirena Deep (W0 of 5.2 km and V0 of 2.0×1012 N/m) of the Mariana Trench, suggesting that these deeps are linked to great flexural bending of the subducting plates. Analyses using three independent methods, i.e., the TeM/Tem inversion, the flexural curvature/yield strength envelope analysis, and the elasto-plastic bending model with normal faults, all yielded similar average Te reduction of 28%–36% and average Te reduction area SΔTe of 1 195–1 402 km2 near the trench axis. The calculated brittle yield zone depth from the flexural curvature/yield strength envelope analysis is also consistent with the distribution of the observed normal faulting earthquakes. Comparisons of the Manila, Philippine, Tonga-Kermadec, Japan, and Mariana Trenches revealed that the average values of TeM and Tem both in general increase with the subducting plate age.
key words: Tonga-Kermadec Trench, horizon deep, axial vertical force, axial bending moment, effective elastic thickness, flexural curvature analysis

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