The quest for reference stations at the National Observatory of Athens, Greece

Abstract. The assumption of reference station conditions is investigated for the first time across 60 rock stations belonging to the broadband and accelerometric networks of the National Observatory of Athens. We include in our assessment all stations that have some probability of lying on rock, based on existing data or beliefs, provided their data have been publicly available for long enough to yield a substantial number of recordings. No studies on site effects have been conducted before for the ensemble of stations. Furthermore, almost no ad hoc field campaigns have been performed to characterise them. The first step is to compile all readily available information per station from publicly available external sources, i.e. geology, topography, housing, Vs30 estimates, and any other known metadata. The second step is to analyse geological maps to derive the geological unit and age; to combine this external information with internal information (namely questioning network staff to access the operator's first-hand experience of the sites); and to better describe geology, geomorphology, and station installation details. The third and largest step is to compile the first Greek ground motion dataset on rock and to perform a detailed analysis of the recordings to estimate site-specific amplification to assess the local site response for each station. A strong-motion dataset of over 7500 recordings is developed and curated for this purpose, dating from 2012 to 2023. It is visually inspected and meticulously processed on a waveform-specific basis in the time and frequency domains, paying special attention to signal quality and strength. Single-station amplification functions using horizontal-to-vertical spectral ratios (HVSRs) are then estimated from the database. Considering that “true” reference sites should have low, flat amplification with no directional dependence, the analysis goes beyond the usual path of combining the two horizontal components into a mean HVSR. It also assesses the directional sensitivity of the HVSR to identify departure from the 1D assumption, corrects the HVSR for the vertical amplification effect, and uses clustering techniques to select groups of stations with different response characteristics. This data-derived characterisation is combined with the previously compiled station metadata to evaluate the stations' overall capacity as reference sites. This results in a qualitative ranking of the stations. The least and most adequate reference stations are showcased to facilitate a better use of seismic data in future seismological and hazard applications.