## 2 Seismic Monitoring  
The tectonic setting of the Northern and central parts of the Lesser Antilles arc is characterized by the convergence and subduction of the North and South American plates under the Caribbean plate at a rate of about 2 cm per year [12; 13]. Seismic activity is mainly confined to a zone of a width of about 300 km that is parallel to the subduction arc with earthquake depths down to about 200 km (Figure 3). Both Saba and St. Eustatius are located in the seismic zone that is dominated by the occurrence of intermediate-depth (100-150 km) earthquakes. Just west of Saba and St. Eustatius is the Angela Passage, a tectonic feature that is dominated by east-west extension [14; 15]. Figure 3: Epicentres and depths of earthquakes (M 2.0–6.1) in the region around Saba and St. Eustatius between 1 January 2010 and 15 March 2018 (data from ISC). Plate boundaries for the North American plate (NAP), South American plate (SAP) and the Caribbean plate (CP) are taken from Bird [16]. Tectonic faults [17] are indicated in black. The brown arrows represent the motion of about 20 mm per year of the NAP relative to the CP [18]. The blue rectangle on the map is enlarged on top. The white star indicates the magnitude 4.3 earthquake on 14 March 2017 at 18:35 UTC at a depth of 87 km (USGS) just east of St. Eustatius for which the waveform data are presented in Section 2.1. Early seismic monitoring on the islands of Saba and St. Eustatius was carried out by the Lamont-Doherty Geological Observatory, U.S.A., who operated a single one-component seismometer on Saba from 1978 to 1983 [2]. From 1992 to 2004, the Seismic Research Center Trinidad acquired data with one seismometer on the summit of Mt. Scenery [2]. A swarm of 60 earthquakes, many of which were felt by the local population, occurred in the vicinity of Saba in June 1992. Their magnitude ranged from 1 to 4.5, and the bigger events were recorded by multiple instruments of the Eastern Caribbean Seismic network located on nearby islands. The epicenters of the 15 earthquakes which could be located have focal depths of 10-16 km and exhibit a SW-NE trend, parallel to a SW-NE fault underlying the island. A detailed study by [PERSON] and [PERSON] [19] suggests a tectonic rather than volcanic origin of the swarm. In 1994, one magnitude 4.8 earthquake at a depth of 14 km and six aftershocks were detected and located at the extension of the SW-NE alignment of the 1992 swarm in the NE direction [2]. From May 1995 to April 1997, micro-seismicity increased with a peak in December 1996 when 64 earthquakes were recorded by the single seismometer on Saba. At the same time, the temperature of the hot springs at Ladder Bay and opposite Green Island, increased significantly by 7-12 degrees Celsius [2]. The 1995-1997 activity has been attributed to a mild volcano-seismic crisis [2]. Since 2006, KNMI has operated the Caribbean Netherlands Seismic Network with the International Federation of Digital Seismograph Networks (FDSN) network code NA (www.fdsn.org/networks/detail/NA/ or doi/10.21944/dffa7a3f-7e3a-3b33-a436-516a01b6 af3f), a permanent network of broadband seismometers on Saba, St. Eustatius and St. Maarten. The NA seismic network is designed to monitor both (a) seismicity in the Caribbean Netherlands region and (b) seismic signals preceding or accompanying the volcanic activity of Mt. Scenery and/or The Quill. Furthermore, the network fills the gap in seismic station coverage in the Northeastern Caribbean next to the Anegada Passage (Figure 4). Figure 5 shows an example of the recordings of a local earthquake near St. Eustatius. Typically, recordings of this type of earthquake correlate well around the P phase between the different NA stations, but their codas correlate poorly, which may be indicative of a complex velocity structure within/under the volcanoes. Figure 4: Operating seismic stations around Saba and St. Eustatius. Station metadata provided by IRIS (blue; www.iris.edu) and by ORFEUS/EIDA (yellow: www.orfeus-eu.org/data/eida). NA stations are marked in red.