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For the Nankai Trough and Japan Trench subduction zones, we examine the combined thermal effects of lateral heat exchange by fluid circulation in an oceanic crustal aquifer and the thickening of that aquifer due to plate‐bending‐related faulting. Faults induced by the bending of a plate entering a subduction zone are hypothesized to increase the depth over which vigorous hydrothermal circulation can redistribute heat in the oceanic crust. Previous 1‐D (vertical) thermal models have examined how aquifer thickening can mine heat from deep in the crust seaward of the trench. Here we construct 2‐D thermal models that include aquifer thickening and lateral heat exchange in the aquifer. We vary the maximum aquifer thickness and the landward extent to which hydrothermal circulation persists within the subducted crust. For the Nankai margin, models most consistent with heat flux data require vigorous fluid circulation extending up to 150 km landward of the trench; aquifer thickening is permitted but not required for models to be consistent with the heat flux observations. Conversely, for the Japan Trench, preferred models include substantial aquifer thickening (maximum aquifer thickness of 1.8–5 km); vigorous circulation extending landward of the trench is permitted but is not required. For hot subduction zones, the thermal effects of aquifer thickening are modest relative to the large lateral advective heat redistribution. For cold subduction zones, where small lateral temperature gradients limit the amount of lateral heat redistribution, aquifer thickening can be the dominant process generating thermal anomalies.

 

Abstract The hybrid design of the Pierre Auger Observatory allows for the measurement of the properties of extensive air showers initiated by ultra-high energy cosmic rays with unprecedented precision. By using an array of prototype underground muon detectors, we have performed the first direct measurement, by the Auger Collaboration, of the muon content of air showers between $$2\times 10^{17}$$ 2 × 10 17 and $$2\times 10^{18}$$ 2 × 10 18 eV. We have studied the energy evolution of the attenuation-corrected muon density, and compared it to predictions from air shower simulations. The observed densities are found to be larger than those predicted by models. We quantify this discrepancy by combining the measurements from the muon detector with those from the Auger fluorescence detector at $$10^{{17.5}}\, {\mathrm{eV}} $$ 10 17.5 eV and $$10^{{18}}\, {\mathrm{eV}} $$ 10 18 eV . We find that, for the models to explain the data, an increase in the muon density of $$38\%$$ 38 % $$\pm 4\% (12\%)$$ ± 4 % ( 12 % ) $$\pm {}^{21\%}_{18\%}$$ ± 18 % 21 % for EPOS-LHC , and of $$50\% (53\%)$$ 50 % ( 53 % ) $$\pm 4\% (13\%)$$ ± 4 % ( 13 % ) $$\pm {}^{23\%}_{20\%}$$ ± 20 % 23 % for QGSJetII-04 , is respectively needed.