## 3 Expansion of the Monitoring Network  
The earthquake detection and location performance of a seismic network strongly depends on the network configuration and the background noise levels affecting the seismic recordings. The ability to automatically detect and locate small- and medium-sized earthquakes requires a seismic network with a sufficient number (usually a minimum of 6) of geographically distributed seismic stations with relatively low noise seismic background levels. The NA network is an important contribution to the regional earthquake and tsunami warning monitoring systems. The small number and the geographical distribution of the sites at each island, however, are also challenging for the development of automatic volcano monitoring systems based on seismicity. Considering the relatively high noise levels at each site, the automatic detection and location capabilities for small, local earthquakes can only be improved by extending the seismic network. This serves the purpose of (a) redundancy by continuously acquiring data for earthquake detection during times of data drops or quality issues at some stations; and (b) to decrease the azimuthal gap of the current network to detect earthquakes on the islands. For example, the north-west of Saba and the south-east of St. Eusattius are poorly monitored by the current network configuration.  
Volcanic eruptions do not always occur following typical seismic indicators. In 2016, Pavlof Volcano, Alaska, erupted a sustained ash plume, reaching 5-9 km in altitude for \(\sim\)40 h without seismic precursors [27]. Also, it is not uncommon for the onset of an eruption to follow only hours after seismicity increases, as happened at Calbuco, Chile in 2015 [28] and at Hekla, Iceland in 2000 [29], allowing little time for precautionary measures. Similarly, not all volcanoes deform before erupting ([30] and references therein), or at least this precursory deformation remains undetected. The monitoring of a volcano is thus best done using multiple techniques to assess the state of activity. KNMI is, therefore, developing the monitoring system at Saba and St. Eusattius towards a multiparameter network with continuous Global Navigation Satellite System (GNSS) measurements and temperature measurements of the hot spring at Saba, both implemented in early  
Figure 10: Four examples of the out-of-balance boom positions resulting in clipping of the recordings (different components). The amplitudes on the \(y\)-axis are given in counts.  
2018. Also, KNMI collects weather measurements at the airports of Saba, St. Eustitius, and St. Maarten. Such data are important to investigate and monitor correlations between environmental data (e.g., barometric pressure, air temperature, wind speed and direction, precipitation) and data from our volcano monitoring network.  
Data collected by monitoring networks are often multi-purpose and can serve not only the initial goal of volcano monitoring and regional earthquake detection but can also be used outside the direct scope of that work. An example of this is the existing real-time data feed of seismic data to the Caribbean Tsunami Warning Program (CTWP; www.weather.gov/ctwp/), where the data are used for tsunami warning services. Furthermore, KNMI opted to install GNSS receivers, capable of ionospheric scintillation observations, which can be used for space weather monitoring. Space weather can influence everyday life, for example, by impacting the Earth's climate and by disturbing GNSS observations and satellite communications, and the importance of space weather observations has been recognised worldwide [31].