Spectral characteristics of seismic events off the coast of the Levant

Seismic events which occur off the coast of the Levant may be grouped into two categories: probable earthquakes and probable underwater explosions (UWE). Spectral analysis of these events, recorded by the Israel Seismic Network (ISN) helps to characterize these two types of events. The observed spectral modulation of the seismic waves is compared with the expected spectra due to source mechanism of UWE and possible effects of the liquid layer (i.e. bubbling, reverberation and channelling). A remarkably complete harmonic series of maxima up to the fifth order, due to the bubbling phenomenon, was found for all UWE recorded at stations far inland. The same form of modulation and maxima at the same frequencies are present at different stations for both P and coda waves. These effects were not observed in the case of earthquakes. It is suggested that the UWE spectra are mainly induced by the source effect rather than by the path effects. A distinct split of the fundamental mode was revealed in the observed spectra of the coda waves in some UWE from the Tyre region. This phenomenon may be the result of the asymmetry of the resonating water layer contributing to the energy of the first spectral maximum. The results of the study demonstrate that spectral analysis of seismograms can be used to discriminate between off‐coast earthquakes and underwater explosions recorded in Israel.