## 2 Seismic Monitoring
### Data Processing and Results
Figure 6: Seismic noise Probability Density Function (PDF) from the vertical component 40 sps recordings of the NA seismic stations in dB relative to ground acceleration between 1 January 2013 and 1 June 2018. The black lines are [PERSON]’s high and low noise models [24]. The red lines in the figures for SABA and SEUT are related to recordings that are affected by out-of-balance boom positions. The horizontal bar below the PDF plots shows the data used from each station between 1 January 2013, or the starting date of the station hereafter, and 1 June 2018. Note: because of the different starting dates of the seismic stations, the horizontal bars differ in time span. The green color in the top row represents the available data, and the red color represents data with gaps that were used in the PSD calculations. The bottom row, in blue, shows data that were used in the PDF plot. Data gaps show up as empty segments, where segments have a length of 30 min [25]. Over the years, we have managed to minimise data loss and succeeded in optimising the waveform data quality by implementing various improvements in the installation and the infrastructure at the sites. For example, the use of local data storage devices, the protection of cables against UV radiation, the use of controlled UPSs, and efforts towards local capacity building to increase local awareness and support, all decreased the overall data loss and improved the timing quality. Figure 8 shows the data availability percentages per month for the NA network since 2013,  
Figure 7: (**a**) Temporal variations of PSD (vertical component, dB rel. to m/s\({}^{2}\)) for 0.01 Hz (blue) and 1.0 Hz (red); (**b**) seismometer mass position for each triaxial, oblique component: U (green), V (red) and W (blue) [26]. Data gaps are visible as interruptions of different time lengths in both PSD and mass position graphs. as well as its weighted percentage. The weight factor is used to value the impact of the network size and equals the number of deployed stations in the network for each year compared to the current configuration (eight stations). The percentage of network data availability has increased over time and reached a stable level of about 85% in the first 6 months in 2018, which is significantly higher than the percentage for the overall time window of 70.3% in the years before. Currently (July 2018), only station SABV is not operating (in repair after possible lightning damage), while the other stations show very high data availability percentages in 2018: SABA 99.7%, SABQ 99.8%, SABW 99.7%, SEUG 98.3%, SEUS 80.2% SEUT 98.6% and SMRT 99.4%. Long data gaps, due to lightning damage, for example, have a severe impact on the data availability of the overall network and often require local support to solve. By putting efforts into local capacity building, we aim to further reduce data loss and prevent severe damage to the equipment. For example, during the passage of Hurricane Irma in September 2017 the power supply was pro-actively switched off at most sites to prevent possible damage by potential electrical breakdown. Only station SEUG operated throughout the passage of the hurricane and provided ground velocity data in real-time via a Satellite Internet connection. Also, simple maintenance tasks like the rebooting of the datalogger can be completed by local contacts. The loss of data quality due to an out-of-balance mass position in the seismometer remains to be solved. In general, the seismometer performance is strongly influenced by tilt (mainly affecting the horizontal components), pressure variation (e.g., barometric-induced tilt and deformation of the suspension system), and temperature fluctuations (air convections, change of geometry of the suspension; [26]). In order to protect the seismometer against temperature changes and human-induced tilt, the sensors were packed with dry sand in early 2018 (Figure 9). Unfortunately, this did not prevent a new case in which the mass position (or boom position) went beyond the operational range.