Andreas Krietemeyer

In the Caribbean Netherlands, the islands of Saba and St. Eustatius host the active but quiescent volcanoes Mt. Scenery and The Quill. To mitigate volcanic risk to the islands, robust monitoring is essential. Therefore in the past five years the Royal Netherlands Meteorological Institute (KNMI) significantly expanded the volcano monitoring network on both islands.

The seismic monitoring network was expanded from seven to 11 broadband seismometers located across the islands. Seismic data are sent to and stored at KNMI and Observatories and Research Facilities for European Seismology (ORFEUS). Eight permanent continuous Global Navigation Satellite System (GNSS) stations were newly installed, where possible co-located with the broadband seismometers. GNSS data are sent to and stored at KNMI and UNAVCO. On a daily basis we run an automatic earthquake detection system and coincidence trigger to identify seismic events and create GNSS time series using both network and Precise Point Positioning (PPP) solutions.

The installation of new instruments was challenging due to the remoteness of the envisioned locations which were needed to monitor all sides of the volcanoes.  Local governmental and military assistance was key to the success of the mission. At the most remote locations instruments are operated on solar power and data are transmitted using  Very-Small-Aperture Terminal (VSAT) technology. Ensuring the operability of the monitoring network remains demanding due to the harsh tropical conditions (hurricanes, UV-radiation, sea spray, lightning) as well as network and power outages. 

Apart from seismic and GNSS instruments, we also deploy three temperature sensors and four cost-effective GNSS units to extend our monitoring network. Furthermore, in collaboration with Delft University of Technology (TU Delft) we test the feasibility of the use of Interferometric Synthetic Aperture Radar (InSAR) for the monitoring of these islands.

In the Caribbean Netherlands, on Saba, St. Eustatius and St. Maarten, the Royal Netherlands Meteorological Institute (KNMI) deploys the seismic network NA (Caribbean Netherlands Seismic Network) to monitor local tectonic earthquakes and volcanic seismicity. Saba and St. Eustatius are part of the Lesser Antilles volcanic arc and each host an active but quiescent volcano: Mt. Scenery on Saba and The Quill on St. Eustatius. The network comprises 11 broadband seismometers of which data are a) transmitted to KNMI by DSL, cellular connection and VSAT, b) processed in real-time at KNMI using SeisComP and a coincidence trigger, c) forwarded in real-time to the Pacific Tsunami Warning Center (PTWC) and d) openly available to research and monitoring communities through ORFEUS/EIDA and EPOS via standardized services. 

In the past six years we detected and located more than 350 earthquakes with magnitudes ranging from 0.4 to 6 within a 150 km radius from the center of the network. About 230 of these earthquakes were exclusively reported by KNMI as they were probably too small to be detected by, or too distant from, seismic networks operated by other agencies in the region. A previously unnoticed shallow (5-10 km depth) swarm of 22 tectonic earthquakes was detected and located through reanalysis of data from before 2017. This swarm took place in 2008, in the same area as the tectonic swarm of earthquakes in 1992, less than 15 km west of Saba, with magnitudes ranging between 2.3 and 4. One of the challenges for our network is building a reliable detection, identification and location system for volcanic earthquakes, which is hampered by the quiet state of both volcanoes. Another challenge is decreasing the hypocenter uncertainties, which are caused by the complex seismic velocity structure underneath the volcanoes, the large lateral velocity inhomogeneities in the subduction zone and the elongated set-up of the regional seismic networks.

We present initial positioning results obtained by analyses of data from four cost-effective Global Navigation Satellite System (GNSS) units installed on the island of Saba. The island hosts the active but quiescent stratovolcano Mt. Scenery which reaches an elevation of 887 metres and was last active around 1640. The cost-effective GNSS units were installed around the volcano in February 2022 and house all necessities for autonomous, continuous monitoring. The overall equipment cost per unit is about 1000 Euros, a fraction of the material costs of a conventional, permanent continuously monitoring GNSS station. Furthermore, the typical installation time of permanent stations takes multiple days whereas the installation time required for our cost-effective units can be undertaken within a few hours. We demonstrate that the performance of the cost-effective GNSS units for daily positioning estimations is comparable with the performance of permanent stations. We investigate the precision and accuracy of the time series of kinematic and static positioning solutions using geodetic positioning estimation algorithms. For direct comparison we placed one cost-effective GNSS unit next to a permanent, conventional GNSS station. Furthermore, we investigate if results improve after applying a minimum-effort calibration of the cost-effective antenna using a permanently installed GNSS station. We demonstrate that cost-effective GNSS units are i) well-suited to extend an existing volcano monitoring network of permanent GNSS stations and ii) can potentially even be used independently for basic volcano monitoring when funding is limited. We also envisage the use of cost-effective GNSS units for rapid deployment in hazardous or risk-prone areas where installations of conventional GNSS stations could be deemed too costly.