Search for Pulsed Ultralow-Frequency Electromagnetic Precursors of Earthquakes

Abstract—Numerous publications have appeared in the literature describing a new seismo-electromagnetic effect—the appearance of magnetic pulses with a duration of 1–40 s, which are detected at distances up to ten thousand km a few minutes before earthquakes of even low magnitude. An assumption was made about the universality of the processes responsible for generation of pulsed precursors and the fundamental possibility of a short-term (a few minutes) warning of an approaching earthquake. In this article, the possibility of the occurrence of ultra-low-frequency pulses that precede seismic events with magnitudes M > 5.0 is studied based on the data from a network of induction magnetometers in the Far East. Pulsed disturbances occur regularly in the records by these highly sensitive magnetometers. Some pulses are observed synchronously at several stations, which excludes their origin from local noise. The spectral maximum of about 7–8 Hz of the oscillatory structure, which appears in many pulses, corresponds to the fundamental frequency of the Schumann resonator. A comparison of magnetic observations with data from the World Wide Lightning Location Network (WWLLN) showed that some of the pulses were caused by lightning discharges in the 650 km vicinity of magnetic station. Although some of the impulses are observed immediately before earthquakes, their connection with seismic activity cannot be claimed with certainty. The calculation of the number of pulses within the 5-min interval before and after the seismic shock shows that they are randomly distributed around the time of the earthquake. Apparently, the pulses are mainly due to the ionospheric response to a distant lightning discharge.