Investigation of ULF magnetic anomaly before moderate earthquakes

Abstract

Electromagnetic anomalies covering a wide range of frequencies from ultra low frequency (ULF), very low frequency (VLF) up to very high frequency (VHF) have been observed before earthquakes. However, the ULF range emissions provide a greater source of information regarding the earthquake precursor. One of the main techniques of investigating such a precursor is by using a magnetic sensor. In this paper, we have carried out a study of spectral density (magnetic field intensity) and polarization ratio methods to extract earthquake precursory signatures of the ULF data for moderate earthquakes (magnitude Mb = 3.7–4.8), using a three-component induction coil magnetometer installed at Shivaji University, Kolhapur (16.40°N, 74.15°E), India. We have applied a Fast Fourier Transform (FFT) procedure to calculate the spectral density of the ULF time series. We have found enhancement in ULF magnetic field intensity 3 to 5 days before the main shock and this specific enhancement appeared ±3 h around the main shock time in the 1–5 Hz frequency range. We have examined ULF variations with polarization values and Kp index data. Magnetic field intensity of ULF data can give important information about earthquake preparation processes and it can be involved in the development of earthquake prediction methodology.