Imaging regional electrical structure in the central Chilean subduction zone (35 - 36° S) near the 2010 Maule earthquake using magnetotellurics

The fluid cycle in subduction zones provides insight into the frictional forces which may induce earthquakes and partial melting processes which leads to volcanism. Understanding where dehydration reactions occur and how fluids circulate can give a better idea of the water budget in subduction zones. Broadband and long-period magnetotelluric (MT) data were collected along an ENE profile in central Chile at 35° - 36°S in 2016 and 2017 in an effort to better understand the subduction processes at this latitude. A total of 38 MT sites were included in the analysis and inverse modelling. Initial analysis shows a geoelectric strike of approximately 15° east of north with more scatter near important volcanos such as Tatara-San Pedro (TSP) and the Laguna del Maule Volcanic Field (LdMVF). Induction vectors trend approximately 50° east of north at long periods which is not perpendicular to geoelectric strike. This suggests that anisotropy may be a factor in the lower crust or mantle. These data were inverted to create a regional two-dimensional isotropic electrical resistivity model of the subduction zone.

The regional 2D inversion model images two conductors in the forearc above the slab interface at 35 - 50 km depth which are likely related to aqueous fluids released from dehydration of the down-going slab. Interestingly, both are correlated with intermediate-depth seismicity which may indicate that dehydration-induced increases in pore pressure can trigger earthquakes. In the backarc, a significant conductor is imaged at depths >100 km which may be related to partial melting of the mantle after de-serpentinization. A conductive anomaly is imaged beneath the TSP and LdMVF at 30 km depth which suggests that both volcanos share a common MASH zone. No shallow conductors are imaged west of the volcanic arc which precludes the possibility of continuous eastward arc migration.

Correlations with a previous 3-D seismic tomography survey in the region show a resistive zone on the plate interface which correlates with an anomalous high-velocity zone. This has been interpreted as a large asperity on the plate interface with limited fluid content which may increase friction and provide an explanation for why such large earthquakes, such as the 2010 Maule earthquake, occur in this region.