## 2 Seismic Monitoring
### Data Processing and Results
In other words, the large relative mass acceleration, with respect to the ground, could not be compensated by the feedback system to keep the mass centered. Although the dataloggers are configured to automatically recenter the mass, sudden large amplitude accelerations cannot be compensated for and result in an unbalanced mass. Such large accelerations are usually an effect of tilting of the sensing system or deformation of the suspension system. Tilting may have local causes, like heavy traffic or groundwater movement, or atmospheric pressure variations leading to (sub)surface tilt over a larger region. The mass position plots (Figure 7) seem to indicate unidirectional  
Figure 8: Data availability for the NA network, expressed as percentage (blue) and as weighted percentage (black) per month since 1 January 2013. The weight is the number of deployed stations in each year compared to the current number (eight) of deployed stations: three stations (SABA, SEUS, SMRT) from 2013 to 2016, and five additional stations (SABQ, SABV, SABW, SEUG, SEUT) from 2016 onwards. tilt in some cases (e.g., SABA, SEUS, SEUT) indicated by the gentle slope of the mass position levels over longer time periods, before the instrument becomes out-of-balance. In some cases (e.g., SABV, SABW), short term (order of months) tilt seems to be approximately transverse to one of the sensor components so that the other two drift away in opposite directions. Figure 10 shows four examples of waveform data prior to and during the out-of-balance event, three (SEUS, SEUT, SABW) before insulation by sand and one after (SABA). The type of signals in Figure 10 are not of seismic origin but correlate with (relatively) large offsets in the mass positions in the seismometer. It is therefore evident that monitoring of the mass positions is required in any quality monitoring system. The four examples show different features in the signals at different scales prior to reaching the operational limits, indicating different causes. For SEUS and SABA, the waveforms show a disruption on all components, while at SEUT and SABW, mainly the horizontal components are affected. Note that both stations equipped with an STS-2 (SEUS, SABA) as well as stations with an STS-2.5 (SEUT, SABW) are affected (Table 2). We assume that local tilt may be responsible for the observations in SEUT and SABW, while for SEUS and SABA, the cause remains unclear. Deterioration issues of the sensors (e.g., corrosion of a spring) cannot be ruled out, as both sensors at SEUS and SABA are of the same type (STS-2) and have been in operation since 2006. Further simultaneous analysis of the mass positions and the broadband waveforms may help to exclude, or find, some explanations. Worth noting is that the passage of Hurricane Irma on 6 September 2017 (Julian day 249) across St. Maarten and of Hurricane Maria on 17 September 2017 (Julian day 260) in the region did not cause ground accelerations resulting in an out-balanced mass at any of the seismometers in our network. The impact of the hurricanes on the seismometer network was limited to two cases of broken GPS connections. Figure 9: Seismic installation at station SABV. The seismometer rests on the tiles on the concrete floor and is completely surrounded by dry sand in a quasi-flexible container. Next to the sensor is the datalogger.