Investigation of ULF magnetic anomaly before moderate earthquakes

摘要

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 M_b = 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 K_p 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.