Simultaneous electric, magnetic and ULF anomalies associated with moderate earthquakes in Kumaun Himalaya

Plain language summaryThe search for EM precursors of earthquakes continues to be a promising one. In this pursuit, we have chosen a seismologically active region of Kumaun Himalaya and established geo-electromagnetic data acquisition systems. Scanning through about 5 months of data, we were able to detect anomalous variations in data through three independent approaches, which can be reliably ascribed to a seismogenic source. We present here the analysis of the patterns of these anomalous signals vis-a-vis the occurrences of about 15 earthquakes within a 250 km radius of our sites. Variations in electric and magnetic (EM) fields were examined at two sites: Dhanachuli near the Main Boundary Thrust (MBT) and Patiyasar near the Main Central Thrust (MCT) in Kumaun Himalaya, India. Anomalous EM signatures are defined based on medium-term data recordings. The patterns of anomalous EM signatures are then linked to the occurrence of moderate earthquakes (3.5 <= M <= 5.0), enhanced along the MCT. The vertical component of the magnetic field is due to microtremors prior to the earthquakes; electric fields also show positive anomalies. Surface waves due to microtemors associated with the earthquakes is reflected in the polarization ratios of geomagnetic data in the Pc4 range (0.01 Hz), a few days prior to each earthquake. In the case of one earthquake, during which suitable data could be retrieved, we observe a frequency change in ULF (Pc1 range) in addition to the enhancement of the Pc4, prior to the event.