Figure 11: Electrical resistivity structure (in ohm-m) of a subduction zone, using the central Andes as an example. The volcanic axis (VA) and high and very high conductivity zones (HCZ and VHCZ) are regions of enhanced conductivity associated with the magmatic arc and back-arc regions. Region a is the 10 km thick upper part of the subducting plate to 40 km depth, while regions b, c, and d represent different parts of the mantle wedge, e is an extension of the upper subducting plate to 80 km depth, and f is the asthenospheric corner. If region a is ten times more conductive than its surrounding (as is observed in Cascadia), the offshore MT anomaly relative to a model without this feature is very large (about 20 times the standard error with which the MT response can be measured). The figure shows the differences between the phase of the two independent modes for a 2D structure at offshore distances of 70 and 140 km, straddling the trench at 100 km, as a function of period in seconds. The differences are relative to those for a model with enhanced conductivity in region a only. The solid line represents hydration (and hence ten times larger conductivity) in the entire mantle wedge (regions b-d), while the dot-dash line is for enhanced conductivity in region e extending to the putative asthenosphere at 80 km, and the dashed line is for enhanced conductivity only in the thin middle part of the wedge (region c).