A Possible Mechanism of the Generating of the Unusually Long Lunar Seismic Oscillations

Abstract

One of the possible reasons of the long duration of lunar seismograms are multiple reflections in the upper part of lunar cross-section, similar to those generating under Earth conditions. While registration of seismic oscillations in a region with stratified low-velocity sediment rocks of 6–7 km thickness the authors observed long seismic low-decaying oscillations. The observed decay was about 40 db during 30 min.

The long duration records are obtained only on the distances where the intense overcritical reflections exist from the M-boundary situated at a depth of 50 km.

These waves generated numerous multiple reflections, longitudinal, shear and converted waves in the upper part of the cross-section. These body waves are responsible for the generation of the surface waves, recorded in the subsequent parts of the seismograms. Periods of body waves are about 1–1.5 s., those of surface waves — about 1.5–3 s.

Extrapolating similar mechanism of the wave generation to lunar media, one should assume the presence of a layered upper part of the cross-section and the possibility of the sufficiently intense wave incidence on its lower boundary.

It is possible if either reflecting boundaries or a vertical velocity gradient are present in the underlying media. The first variant is accompanied with overcritical reflections, the second one — with intense refracted waves.