## 4 Results
### Electromagnetic Surveys  
Four surveys used electromagnetic techniques, specifically VLF-EM and low-frequency MTS, for the geometry and structure of Holocene to late Pliocene formations resulting from the action of first-order thrusts and strike-slip faults [18; 39; 45; 52]. In general, electromagnetic techniques infer subsurface ER from measurements of natural geomagnetic and geoelectric field variations at the ground surface [73; 74]. In particular, VLF-EM and low-frequency MTS use a fixed grounded dipole or horizontal loop as an artificial signal source to determine ER [75; 76]. Both natural and controlled-source electromagnetic signals are used to obtain 1D ER models beneath the measurement site [77]. The ER dataset at the corresponding depths and signal-source distances are mathematically inverted to produce a 2D ER model [75; 77]. One survey used VLF-EM (<10 Hz) in 13 profiles with a prospecting length in the 160-750 m range, a maximum prospecting depth of 130 m, and ER in the 15-280 \(\Omega\) m range [39]. Three surveys used low-frequency MTS (>10 Hz) [18; 45; 52] with a total prospecting length of 33.3 km, a maximum prospecting depth of 4000 m, and ER in the ranges of 10-220 \(\Omega\) m for geological formations catalogued as aquifers and 220-27,100 \(\Omega\) m for geological formations catalogued as aquitards (Table 2). The low-frequency MTS survey labelled 01 in Figure 1b and Table 2 was selected (Figure 4). This survey was performed in 2016 and included 13 measurement sites aligned in an NNE-SSW profile perpendicular to the Saguanchi Gorge strike-slip fault in the southern border of the MDQ (Figure 4b) [18]. Strike-slip faulting produces additional extensional areas disposed perpendicular to the primary shortening tectonic component evidenced by the Quito Fault System, which has implications for the drainage network and the extension and thickness of aquifers in the IAV [18]. The ER data were acquired using the StrataGem EH-4 four-channels Hybrid Source with a TxIM2 transmitter and electric BE-26 and magnetic G100k sensors by Geometrics, Inc., San Jose, CA, USA. The configuration was as follows: 13 measurement sites were set up using a variable 40-170 m array spacing and applying three frequencies in the 10-50 Hz range to record ER from depths of 0.6 to 1.8 km. The recorded ER data were mathematically inverted to obtain a 2D ER model. See [PERSON] et al. [18] for further methodological details. Figure 4b shows the location of the selected low-frequency MTS profile, here called MTS1 (Figure 4c). The profile features included a prospecting length of 1300 m, a prospecting depth of 1800 m, ER in the 10-8010 \(\Omega\) m range, and an average RMSE of 14.32. The ER values were similar to that reported for similar volcano-sedimentary rocks with variable degrees of fissuring, fracturing, and saturation [52; 56]. The ER model displayed (i) high-resistivity southern and northern sectors with an average ER around 1000 \(\Omega\) m and several anomalies higher than 3000 \(\Omega\) m, and (ii) a low-resistivity central sector bounded by strike-slip faults with an average ER lower than 1000 \(\Omega\) m. In detail, ER values in the 10-50 \(\Omega\) m range are associated with (i) strongly fractured volcanic rocks of different ages and high saturation degrees within the strike-slip fault zone, and (ii) sub-horizontal contacts within the Pleistocene and late Pliocene formations inferred by first-order thrusts observed at other sites (Figure 4c). ER values in the 50-200 \(\Omega\) m range are associated with Holocene and Pleistocene volcano-sedimentary formations with moderate degrees of fissuring, fracturing, and saturation. ER values in the 10-50 \(\Omega\) m range are associated with (i) strongly fractured volcanic rocks of different ages and high saturation degrees within the strike-slip fault zone, and (ii) sub-horizontal contacts within the Pleistocene and late Pliocene formations inferred by first-order thrusts observed at other sites (Figure 4c).