Fig. 1. Map illustrating the location of the seismometer and the path of the fibre connected to the DAS instrument (courtesy of Sandy Croatto).

Fig. 2. Spectrogram made from the entire DAS dataset. There is a prominent seasonal amplitude contrast between summer and winter. The yellow/green horizontal stripes are caused by earthquakes due to their broad frequency spectrum, high intensity and short duration. Blank areas indicate periods with no data available.

Fig. 3. The rms of the ocean-generated microseism frequency band [0.05 Hz, 0.5 Hz] in September 2023, based on the data shown in Figure 2. It can be seen that earthquakes have frequency components within the ocean-generated microseism frequencies. The spikes without annotations are most likely caused by lower-magnitude earthquakes.

Fig. 4. Comparison of BAM and DAS frequency spectra for 1 h of data. For this visualization, the DAS data is not high-pass filtered. The scales are aligned, so that 1 nm/m corresponds to 5 fs as in Equation (1).

Fig. 5. Correlation of the BAM with DAS data using a time window of 300 s (3000 samples each). Firstly, the cross-correlation (shown on the left) is calculated to correct for time offsets. Secondly, the correlation plot (shown on the right) was created and linear regression was performed. The correlation factor resulting from the regression is 9.922 fs/(nm/m).

Fig. 6. Comparison of the BAM and DAS data using the calculated shifting and scaling factors. The lower figure is a zoomed version of the upper one. The DAS data is scaled with the correlation factor calculated by linear regression, as shown in Figure 5. Both signals are dominated by ocean-generated microseism.

Fig. 7. Seismometer data of two periods in September with strong microseism: the days around 4 and 21 September 2023. On the left is an averaged spectral density of each component (north, east, vertical). The spectrogram on the right shows the vertical component over a period of several days.