# The Contribution of Near-Surface Geophysics for the Site Characterization of Seismological Stations
## 1 Introduction  
The permanent network of the Seismological Laboratory of the National and Kapodistrian University of Athens is the Athenet network (SL-NKUA), and its code is HA [1]. It currently comprises 24 weak- and 7 strong-motion sensors, operating in real-time. The stations are mainly deployed in Central Greece, covering the Gulf of Corinth, Euboea Island, and, partly, Saronikos Gulf (Figure 1). All seismological stations of the Athenet network transmit data to the central server of SL-NKUA in real-time, at a sampling rate of 100 sps, through 4G or GPRS telemetry. Since June 2008, SL-NKUA has been one of the institutes that contribute to the Hellenic Unified Seismological Network (HUSN) [2]. HUSN is a virtual network of seismological and accelerometric stations, which has unified the individual networks of different institutes in Greece, namely the Geodynamic Institute of the National Observatory of Athens [3], the Department of Geophysics of the Aristotle University of Thessaloniki [4], the Seismological Laboratory of the University of Patras [5], the Hellenic Mediterranean University Research Center [6], and the Institute of Engineering Seismology and Earthquake Engineering [7]. Waveform data are shared between the partners of HUSN in real-time and are also freely available to the public through the European Integrated Data Archive (EIDA), hosted at GI-NOA [2]. The past few years have witnessed a significant rise in the number of permanent seismic network stations globally. This increase has resulted in a surge of earthquake recordings and their real-time data applications. The availability of vast seismic datasets has brought to the forefront the intricate nature of ground-motion prediction and its direct correlation to the characteristics of the site where the recording instrument was installed. The local subsurface characteristics have significant impact on surface ground motions, and their determination is beneficial in seismological studies. For example, they are usually determined in the context of seismic hazard assessment [8; 9], estimation of Ground Motion Prediction Models (GMPMs) [10; 11], and classification of soil for microzonation studies [12]. For that reason, the identification of the peak frequencies which exhibit significant amplification is crucial. In this paper, we present the geophysical investigation that has been carried out at selected seismological stations of the Athenet network (Figure 1) in order to calculate essential parameters for their site characterization. For that reason, we applied a multidisciplinary geophysical survey, including the Horizontal to Vertical Spectral Ratio (HVSR) technique, Seismic Refraction and Multichannel Analysis of Surface Waves (MASW) measurements [13; 14; 15; 16; 17; 18], and Electrical Resistivity Tomography (ERT) measurements, near the selected stations (Figure 1). These methods are fast, reliable, and have been used in several similar cases for the site characterization of seismological stations in the past [19; 20; 21; 22; 23]. Compared to geotechnical methods, which involve collecting data from boreholes, the geophysical methods are of significantly lower cost and, therefore, are often the preferred choice. Credible site characterization for a seismic station requires the determination of several key parameters. According to [19], the most important ones are the fundamental frequency (\(f_{0}\)), shear-wave velocity distribution (\(Vs\)), average shear-wave velocity in the upper 30  
Figure 1: Locations of the investigated areas and selected seismological stations (red circles). m of the site (\(V_{S30}\)), depth of seismic (\(H_{seis}\)) bedrock, surface geology, and soil-type classification at each site. These parameters are crucial in estimating the potential effects that can be induced in the seismic recordings, such as distortion of the amplitude, frequency spectrum, and duration of the signal.