These discrepancies can be explained by different edge effects of the methods used, but they can hardly discredit the final conclusion that can be made from this study about existence of two epochs of deep CW amplitude decrease around 1850 and 2005, which are also accompanied by a large phase jump, like the well-known event in the 1920s.
However, LiDAR images reveal linear "corrugations" (> 5 m amplitude), deep thermo-erosional gullies, and features resembling lake drainage channels, suggesting that highly dynamic surface processes have previously shaped the landscape.
To quantitatively constrain the dynamic model, we derive in Section 2 an estimate of the ratio of shallow versus deep HF source amplitude from the back-projection source imaging of Meng et al. (2011).
These studies suggest that although the loss of the excitation amplitude deeper into the medium can be severe, the corresponding broadening of the focal excitation volume is generally modest [ 25, 49].
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In this representation, one can observe a deep fade in the amplitude at around 10 μs.
In both CW series, three deep minima of the amplitude below 0.05 mas around 1850, 1925 and 2005 are unambiguously detected.
The observation of a deep earthquake's infrasound amplitude being lower than that of a shallow earthquake of similar magnitude supports the concept that infrasound can be used to determine depth, as suggested by Arrowsmith et al. (2011).
Negative responses (i.e. decreases) were observed in the deeper layers, with maximal amplitude in layer IV, within 40 ms of stimulus onset.
With deep breathing, the mean amplitudes were decreased across the six harmonics compared to normal breathing with a 12%, 17 %, 25 %, 15 16%, and 16% reduction in the 1st to 6th handonics, respectively.
