Decrease of intensity of ELF/VLF waves observed in the upper ionosphere close to earthquakes: A statistical study
Abstract
We present results of a systematic study of intensity of VLF electromagnetic waves observed by the DEMETER spacecraft in the upper ionosphere (altitude 700 km). We focus on the detailed analysis of the previously reported decrease of wave intensity shortly before the main shock during the nighttime. Using a larger set of data (more than 3.5 years of measurements) and a newly developed data processing method, we confirm the existence of a very small but statistically significant decrease of wave intensity 0-4 hours before the time of the main shock at frequencies of about 1.7 kHz. It is shown that the decrease does not occur directly above the earthquake epicenter but is shifted about 2°in the westward direction. Moreover, it is demonstrated that the decrease occurs more often close to shallower earthquakes and close to earthquakes with larger magnitudes, as it is ''intuitively'' expected, representing an additional proof of validity of the obtained results. Finally, no dependence has been found on the occurrence of the earthquake below the ocean or below the continents.
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