Local Hurst Exponent Computation of Data from Triaxial Seismometers Monitoring KAGRA

The Hurst exponent was computed locally for three triaxial seismometers monitoring the KAGRA corner station and the two end stations of the interferometer allowing to estimate variability in the persistent behaviour of the seismometer time series. Results show that, in all the three cases considered, the vertical component of the seismometer has lower persistency compared to the horizontal degrees of freedom, as also confirmed by the low frequency region of the amplitude spectral density of the data. Furthermore, data of the horizontal components of the EXV end station, located in the direction of the X arm of the interferometer, exhibits lower values of seismic noise and of its Hurst exponent. This is possibly due to the lack of an exit point at this location, and hence to a reduced human activity, to the presence of water flow in the mine hosting the KAGRA detector or to differences in seismometers' self noise.