Reply to "Comment on 'Attenuation in West Bohemia: Evidence of High Attenuation in the Nový Kostel Focal Zone and Temporal Change Consistent with CO2 by M. Wcisło, L. Eisner, J. Málek, T. Fischer, J. Vlček, and G. Kletetschka" by Morozov
Abstrakt
Here, we respond to the comment of Morozov (2019) on Wcisło et al. (2018). We use this opportunity to describe in more detail the principles of the method used in Wcisło et al.(2018) and clarify the points discussed in the earlier debate of Eisner et al. (2013, 2014) and Morozov (2014).
In microseismicity, the influence of attenuation on observed seismic waveforms is significant. For stronger earth-quakes (e.g., felt by humans at a surface), the seismic waves are usually dominated by relatively low frequencies (below 10 Hz).
Microseismic monitoring networks, by definition, record seismic signals at near distances, with signals domi-nated by higher frequencies because receivers are close to the sources. Let us consider propagation that would imply accumulation of the global absorption factor (Gajewski and Psencik, 1992) to value 0.01 that would correspond to the propagation of a seismic wave through homogeneous medium with Q factor equal 100 for 1 s.
In such a medium, the amplitude of the signal is reduced due to attenuation for 10 Hz by 27%, for 25 Hz by 54%, and for 50 Hz by 79% when compared with to the pure elastic case. Obviously, this differ-ence further increases for longer travel paths, hence strong and weak attenuation can be only defined for fixed distance, ideally in the number of wavelengths traveled by the observed waves between the source and the receiver.
Our study of western Bohemia microseismicity uses a dedicated local array at near epicentral distances resulting in observed waveforms of high frequency with dominant frequencies exceeding 10 Hz, in general. Morozov (2019) suggested that the peak frequencies of Wcisło et al. (2018) were not linked to attenu-ation (that characterizes the medium in West Bohemia) and stated that the peak frequencies could be rather due to rever-berations from shallow strata beneath the receiver or source spectra variations.
This judgment is based on arguments explained and addressed in the following paragraphs. In part, the discussion repeats the earlier debate between Eisner et al.(2013, 2014) and Morozov (2014), and underlines the bench-mark study of Wcisło and Eisner (2016).Wehope the inde-pendent readers of this study may find our observations relevant and interesting as we do.