Indexed on: 12 Jul '13Published on: 12 Jul '13Published in: Geomagnetism and Aeronomy
Spectral density estimates for distant whistlers in subranges of 8–12, 12–20, 20–30, and 30–40 kHz, based on data from stations located at distances of 250–300 km between each other, during the preparation of M = 5–6.2 earthquakes are obtained, and spectral density ratios are analyzed. It is shown that a stable decrease in spectral density ratios is observed for high frequency bands for a couple of stations which are the closest to and the farthest from the epicenter during a period of several days before an event (up to 2–3 weeks before it). Another peculiarity found is an increase in spectral density ratios several hours before an earthquake. These features can be used as short-term precursors.