LOW FREQUENCY SEISMIC SIGNALS AS REGIONAL EARTHQUAKE PRECURSORS

Abstract

Regional precursors of near earthquakes can possibly be identified in low-frequency seismic signals preceding a main shock. A special software package was used to examine seismic records of more than 200 earthquakes. More than 30 events were found to contain low-frequency precursory signals (LFPS) with typical periods of 3 to 200 s and advance times of 20 s to 1.5 h before P-wave arrivals. Features of these signals and their classification are described. A great variability of these signals was found to be related to the location of the future earthquake. Earthquakes in any particular region are preceded by low-frequency seismic motion involving insignificant scatter in the signal parameters. The signals identified were examined for comparison with the results of other researchers in this line. A hypothesis is advanced that relates the size of the precursory area to the period of the signal radiated. An analysis of experimental laboratory studies and field observations showed that the empirical relation between the period and wavelength of signals radiated during the earthquake precursory period can be described by the law of dispersion for parametric waves in a medium under stress.