Figure 3. Synthetic three-component-averaged ground velocity spectra for a vertical force acting on the surface of a two-layer model. The red and blue curves indicate spectra computed for two-layer models whose upper layer thickness is 20 and 100 m, respectively. The receivers are located at epicentral ranges of 50 to 60 km, approximately the ones between the IS stations and Gaza. Spectra are normalized by their maxima.
The IS seismograms are analyzed after applying a 4th order Butterworth filter in the three target frequency bands. Visual inspection reveals distinct amplitude fluctuations appearing from about 20 minutes before the attack (Figure 2a), which can be observed on all three stations after averaging their Fourier amplitude spectra in the target frequency bands (Figure 3a-c). The amplitude fluctuations are most well observed at YATR, the quietest of the three stations. YATR records a sequence of 5 strong bursts, each lasting for about 30 to 60 seconds. The first burst, occurring at about 06:15:30, and the last burst, occurring at about 06:29:30, exceed the noise levels of all three stations.
Given the large inter-station separation and the time-frequency attributes of the pre-attack signals, any correlation among the three stations would require waves induced by a large traffic source (in volume or mass). Because the traffic in Israel on the morning of October 7 (or other Saturday mornings before October 7, 2023; see below) was too sparse to give rise to correlated signals at the station triplet, inter-station seismic correlation is a strong indication that the signal observed on the IS stations is related to motions inside Gaza. To identify this correlation, I computed the three-component-averaged spectral amplitude for 30-second-long windows during the hours leading to the attack. The Fourier spectral amplitude was averaged over the three target frequency bands shown in Figure 2a-c. These time series are indicated by the black curves in Figure 4. The background noise amplitudes computed from Saturday mornings between 2021 and 2023 are indicated by the grey curves, and their median value by the blue curves. The October 7 data show a sharp increase starting at about 06:10 local time, 20 minutes before the attack, especially at YATR and AMZNI. The seismic energy increase during the 20 minutes preceding the attack is substantial, averaging about 150% of the median background level, with multiple windows in YATR and KZIT carrying energy of up to 200% of the median background signal energy level.