# Strong Ground Motion Sensor Network for Civil Protection Rapid Decision Support Systems
## 2 Research Area, Sensors, and Data  
The monitoring area consists of a large basin with sediments that vary in thickness and stiffness which is ideal for deploying a dense strong ground motion network with sensors installed on different geologic formations to record the ground shaking variations. Crete is located in the southern part of Greece and depicts diversified geological setting (Figure 1a). As described by [10,11], the whole island is a complex structure with continuous nappe units such as Paleozoic to Triassic Phyllite-Quartzite (brown color in Figure 1a) along with the lowest formations comprised by Jurassic white-grey recrystallized limestones and marbles with considerable thickness (blue color in Figure 1a) along with Permian schist type rocks and shallow sea carbonates (pink color in Figure 1a). Superincumbent the bedrock formation, are the large sedimentary basins with Neogene formations, mainly consisted of marles, sandstones (bronze color in Figure 1a) and limestones (beige color in Figure 1a) with Terra rossa weathering (magenta color in Figure 1a). To the extent of the area under study, they are visible in the northern and eastern part of the city, as well as in the neighboring Akrotiri peninsula and at the foot of the mountain range. The Quaternary deposits (white color in Figure 1a) are loose to very loose material, such as sands, clays and gravels with considerable thickness that is estimated about 150 m [12], covering the bigger part of the southern Chania suburban area. In Figure 1a, the yellow lines depict possible active faults while the green ones are inactive. The prevalent geological formations in each SGM-station location and the sensor type installed are summarized in Table 1.  
Figure 1: (**a**) The geological setting of Chania metropolitan area and the SGMN topology (see text). (**b**) Epicenters and magnitudes of the studied earthquakes. SGMN utilizes two-generation sensors of the 24 bit integrated tri-axial accelerometer produced by REF TEK [13]. The first type, the more recent one, is the 130-SM, equipped with a 131-8019 sensor that works on a full-scale range \(>\pm\)3.5 g with \(\pm\)10 V full scale voltage, while the second one is the ANSS/02 that has a 0131-02/03/1 sensor which works with \(\pm\)6.9 V full scale voltage and has \(\pm\)3.5 g clip level. Both models are equipped with three microelectromechanical systems (MEMS), one for vertical and two for the horizontal components. Si-Flex MEMS have silicon proof mass and springs, specifications derived from the REF TEK sensor manuals [13]. A switched capacitor integrated circuit is used to calculate the mass position changes and a compensation circuit is used to center the mass with electrostatic forces. The MEMS accelerometers have no need for calibration except a periodically dc offset remove. Maintenance options, performed from time-to-time, are: firmware upgrade, replacement of internal battery or flash card, and check of the connectors. Four boards are assigned for different modules such as A/D conversion, signal processing, in-situ data storage on memory cards, and data transfer through internet telemetry. All sensors are telemetrically connected via ADSL lines to a central server running the REF TEK Protocol Daemon (RTPD) software which provides remote control of the SGM-stations, data error-correction, archiving and storage. As the SGMN is part of the HC network, the strong ground motion recordings with 250 Hz sample rate are stored in REF TEK PASSCAL archives [14] and in miniSEED format [15] at the Hellenic Seismological Network Crete (HSNC) data storage units [16; 17; 18; 19]. In the framework of HELPOS (Hellenic Plate Observing System, Greece) action that aims to homogenize and unify the seismic networks in Greece, all the waveforms are stored and are available for distribution through the National European Integrated Data Archive (EIDA) node ([[http://eida.gein.noa.gr/](http://eida.gein.noa.gr/)]([http://eida.gein.noa.gr/](http://eida.gein.noa.gr/))) (accessed on 2 February 2021).