Multiparametric geophysical monitoring of apparent resistivity and gas variations associated with seismic and volcanic events: rgunal network design and preliminary results

The Geophysical Network of the Universidad Nacional de Colombia (RGUNAL, Spanish acronym) was established to test whether transient, time‐varying anomalies in observed magnetotelluric (MT) apparent resistivity ( \rho_a ), and, where available, soil‐gas indicators, can precede or accompany episodes of seismic or volcanic unrest. Because MT resistivity is particularly sensitive to fluids and melts, our working interpretation emphasizes fluid‐related conductivity changes; however, resistivity and gas variability are not uniquely diagnostic and can also reflect stress‐induced cracking, temperature/hydrology effects, cultural noise, or space‐weather contamination. We describe the design and deployment of RGUNAL, a multiparametric monitoring network that integrates continuous MT measurements with low-cost soil-gas screening sensors and real-time telemetry. Subsequently, we present three illustrative case studies. In central Colombia, a transient decrease in \rho _a sensitive to depths of ~10–15 km was observed ~8 h prior to the 24 December 2019 M_W6.0 Mesetas earthquake. At an Antarctic station, \rho _a variations occurred concurrently with the onset of the 2020 Bransfield Strait seismic swarm (near the Orca submarine volcano), which independent studies have attributed to a magmatic intrusion. Near the Nevado del Ruiz volcano (Colombia), a ~20% decrease in \rho _a and disturbances in soil‐gas time series were observed ~2 days before a volcano‐tectonic seismic burst and subsequent ash emissions in late September 2023. However, these preliminary observations do not constitute an operational forecasting method. To enable reproducibility, we provide an explicit anomaly definition, quality‐control criteria (multi‐component coherence and K_P index evaluation), and a framework for prospective statistical evaluation for the growing dataset. Overall, the results of this study indicate that MT data and gas indicators can be used to detect seismic and volcanic unrest prior to associated events, suggesting the potential of RGUNAL as an early-warning system, subject to prospective validation.