Appraisal of contemporaneous application of polarization ratio and fractal analysis for studying possible seismo-electromagnetic emissions during an intense phase of seismicity in and around Assam Valley and the Eastern Himalayas, India

Abstract

Mechanical deformations from within the earthquake preparation zones are believed to cause seismo-electromagnetic (SEM) emission in ultra-low frequency (ULF) band, i.e. between 0.001 and 10 Hz, and these are studied as a precursor for short range earthquake forecast. Data from highly noise-resilient, 3-component ULF induction coil magnetometers, installed at the Multi-parametric Geophysical Observatory (MPGO), Tezpur, which is situated at a very close proximity with the northern end of the Kopili Fault and the Bomdila Fault, the Main Boundary Thrust of the Eastern Himalaya, the Naga and Disang Thrust, the Assam Syntaxis Zone as well as the two tectonically active Precambrian shields – Shillong and Mikir, was used for the first time to study SEM emissions employing both polarization ratio analysis and fractal analysis in the background of an intense phase of seismicity during the campaign period of April 20 – September 3, 2019. Fifty-one events were filtered via strain radius and index of seismicity calculations to nominate credible events which were studied vis-à-vis polarization ratio and fractal dimension parameters of 3-h night time data, centered around local midnight contemporaneous with the 18–21 UT window of the KP index (global geomagnetic activity index) in the 0.03–0.1 Hz frequency band. The findings show candidate SEM emissions, in the form of enhancements in SZ/SH, associated with all the seven credible events, even as nine enhancements could not be attributed to immediately adjacent credible events. The fractal dimensions, calculated with the spectral density method, ambiguously depicted a gradual increase prior to the intense phase of seismicity.