Nanoseismic monitoring of extremely weak failure signals in unstable shallow subsurface cavities and validation of numerical stress predictions

Incipient instabilities, which are presently developing within the chalk of ancient (ca. 1100 years old) bell-shaped caverns at the Bet Guvrin National Park (Israel), have been the aim of multiple investigations for more than a decade. An innovative and independent validation approach is presented here, whereby numerical predictions of material instabilities are compared with the spatial distribution of extremely weak (ML > -4.0) failure signals detected by nanoseismic monitoring. Our data show that it is indeed possible to detect a series of extremely weak spiky broad-band signals. Their characterization and location indicate that they are unequivocally generated by incipient brittle failure within the chalk material of the bell-shaped caves, thus verifying numerical predictions. Furthermore, our investigations suggest that numerical modelling is best used to guide nanoseismic monitoring, which, in turn, can quantify the spatio-temporal distribution of actual incipient failure.