MODELING OF NONLINEAR PASSAGE OF ACOUSTIC WAVES CAUSED BY UNDERGROUND FRACTURING THROUGH THE LITHOSPHERE

Abstract

The nonlinear passage of the acoustic waves through the lithosphere to the surface of the Earth during earthquakes and strong underground explosions is analyzed in this report. The underlying mechanism for this is the nonlinear elastic modulus. The waves are excited at the underground source of the earthquakes. The passage of the acoustic waves propagating almost vertically upward leads to a change of the spectrum. The wide spectrum of the acoustic waves up to the radio wave range is assumed to be produced by the fracturing of the rock at the surface. This has been observed by means of satellite measurements and radio telescope investigation of meteor bombing of the Moon. If the fracture occurs at depth corresponding to high frequencies, the waves transform, through nonlinear interactions, into low and super low frequency waves. Low and super low elastic displacement waves reach the surface and produce a seismograph response. In the report the nonlinear excitation of ultra-low frequency (ULF) acoustic waves caused by low frequency (LF) seismic acoustic burst is also discussed. An analysis of the nonlinear transformation of LF acoustic waves (f∼100 Hz) into ULF (f⩽1 Hz) waves is presented. The LF wave is excited as the burst-like envelope of a finite transverse scale by the underground seismic motion caused seismic activity of the Earth. Then, it propagates upwards and is subject to both a nonlinear process and dissipation. The nonlinearity leads to the generation of higher harmonics and, thus, to a saw-like wave structure, and also to an increase of the ULF part of the wave spectrum. This process takes place underground at a depth of about 50–30 km.