The sites we describe in this manuscript are testing sites for the Austrian part of the international AlpArray seismic network . AlpArray is a unique European transnational research initiative in which 43 research institutes from 15 countries join their expertise to advance our knowledge about the structure and evolution of the lithosphere beneath the entire Alpine area. AlpArray will shed light on the detailed geological structure and geodynamical evolution of the Alps to answer outstanding questions e.g. on slab geometry and subduction polarity under the Eastern Alps. While the primary scope of AlpArray is fundamental research the unique dataset will also improve our knowledge about near-surface geologic structures and help to assess the seismic hazard in the Alpine area. The scientific goals of the AlpArray project are manifold and among others include e.g. Alpine geodynamics, crustal and mantle imaging, seismic anisotropy, as well as regional and local seismic activity. Hence, temporary seismic stations installed in the framework of AlpArray should be multi-purpose stations that perform reasonably well both for frequencies above and below the microseism peaks.

The AlpArray temporary seismic network is designed to complement existing permanent seismic stations. In Austria the Austrian Central Institute for Meteorology and Geodynamics operates 15 permanent stations (Fig. ). The additional temporary seismic stations densify this network to achieve a uniform coverage with approximately 40 km inter-station spacing. Central coordinates for all temporary AlpArray stations were computed to obtain homogeneous coverage throughout the entire array and all stations must be installed within a 3 km radius around the central coordinates. This constraint usually limits the choice of potential installation sites.

One seismic station comprises the following components (Fig. ): a 60 s broadband sensor Reftek 151 “Observer” together with a Reftek 130 or Reftek 130S 24 bit digitizer with > 136 dB dynamic range (at 100 Hz sampling rate), a continuous mode Reftek 130 GPS, a Digi WAN 3G mobile router for telemetry and a 100 Ah battery. For stations not connected to the power grid 100 W solar panels charge the supply battery. Especially in Alpine regions snow coverage during winter may, however, prevent power supply through solar panels. In this case fuel cells can act as backup power source when the batteries are drained below a given threshold. For a temporary deployment of approximately three years, our specific requirements for seismic sites inside the 3 km radius were the following:

Seismic noise: Average noise levels should be 20 dB lower than the New High Noise Model (NHNM) on all components within the 1–20 Hz frequency range. For long periods (30–200 s range) average noise levels on the vertical component should be 20 dB lower than the NHNM while on horizontal components noise levels should only be 10 dB less than the NHNM. This accounts for the strong sensitivity of horizontal components to e.g. long-period surface tilt from atmospheric pressure fluctuations. Consequently, for near-surface stations, noise on horizontal components is usually stronger than on the vertical. Avoiding long-period noise on horizontal components requires advanced site preparation which is usually out of scope for temporary deployments.

Station 01 is located in an old bunker near the village of Falkenstein, Lower Austria and power supplied by a solar panel only. It is approximately 600 m from inhabited houses, 300 m from a minor road with little traffic and surrounded by vineyards (Fig. ). The bunker that hosts the seismic station is built upon and likely connected to a 200 m long outcrop of bedrock which used to be a quarry. Cables for the GPS and mobile antennas are lead to the surface through ventilation pipes, which were sealed with expanding foam. The location inside the bunker provides good ground contact and minimizes daily temperature changes. Consequently, site 01 is by far the most seismically quiet station in the network to date (Fig. ). Long-period (30–200 s) noise levels on the vertical component are close to the New Low Noise Model (NLNM) . On the E–W component, long-period noise is substantially higher than on the vertical and approximately 20 dB less than the NHNM. We note that in the same period range, noise on the N–S component is 10 dB stronger than on the E–W, which may be a sensor leveling effect. The elevated long-period noise on horizontal components compared to the vertical is likely due to ground tilt. In the higher frequencies (> 1 Hz), noise levels are comparable on all components and approximately 10 dB higher than the NLNM. This shows that anthropogenic noise from the nearby roads and village is small.

Station 05 is located in a small stone shelter inside the village Schmida adjacent to the floodplain of the Danube river in Lower Austria (Fig. ). It is supplied from the power grid. The site is few meters away from a road, 600 m from a highway and 5 km from the Danube. The concrete base of the shelter provides good ground contact, yet ground in the entire area is made of loose sediments dominated by the nearby river floodplain. Consequently, noise levels on Station 05 are poor (Fig. ). Long-period vertical noise levels are in the order of the NHNM while horizontal components suffer from substantially stronger noise. We suspect that the site experiences strong long-period tilt from passing cars. Traffic is most likely also responsible for the elevated high-frequency noise in the order of the NHNM on all three components. In fact, during night times (8:00 p.m. to 6:00 a.m.) noise levels are generally 10 dB lower on all components than at daytime (6:00 a.m. to 8:00 p.m.) which confirms anthropogenic noise as the main noise source. At night, noise levels on the vertical component are close to NHNM -20 dB for both long-period and high-frequency bands. Despite the generally high noise levels at this site (especially during daytime), recordings could still be used for identification of both teleseismic and local seismic events (Figs.  and ).

Station 06 is placed inside an abandoned wine cellar approximately 5 m below the surface (Fig. ) outside the village Kleinriedenthal in Lower Austria. Power supply is from the grid. Since the ground consists of loose soil, we built a concrete base to put the sensor on. The site lies 200 m outside a small village, 300 m from a road and is surrounded by vineyard agriculture. Despite the underground location noise levels are high (Fig. ). Long-period vertical noise is steadily well below NHNM -20 dB while horizontal noise levels strongly vary in amplitude and on average fall around the NHNM. This behavior is unexpected since the underground installation should minimize both temperature fluctuations and effects of surface tilt. One reason for the strong susceptibility of the horizontal components to long-period noise might be sensor mis-leveling. During its installation the sensor was put on the concrete base which might not have been entirely solid at the time (two days after construction) and thus the sensor or the base may have tilted. Tilted sensors will show much stronger horizontal noise. In fact, upon inspection about 2 months after installation, the sensor leveling did considerably change since the time of installation. The high frequency noise also distributes around the NHNM but also down to -20 dB less. The high frequency noise appears to split into a major (close to NHNM) and a minor (close to NHNM -20 dB) branch, which indicates that the elevated noise levels may be due to anthropogenic noise. While we did not expect huge impact of traffic for this site, a comparison of day to night data in fact confirms very clearly that high frequency noise is primarily anthropogenic and as such probably due to traffic. Both on vertical and horizontal components, nighttime noise levels are more than 20 dB less than during daytime for higher-frequencies and 10 dB less for longer periods. In addition to traffic from the closest road (300 m) a huge commercial and recreation facility about 3 km NE of the site may contribute to the observed noise levels.

Station 08 is placed near a flood protection facility inside a several meter deep artificial trough next to the village Tiefenfucha in Lower Austria (Fig. ). It is supplied from the power grid. The station is 200 m from the village, 1.5 km from a highway and the Danube river and surrounded mostly by vineyards. Trees (approximately 5–10 m high) surround the site. The sensor is placed atop a base made of concrete which fills the foundation of a small wooden hut, that serves as shelter for the station. An artificial water stream may flow several m from the station after periods of strong precipitation but is almost non-existing during dry conditions. Long-period noise levels (Fig. ) are well below NHNM -20 dB on the vertical component but significantly higher on horizontal components. High-frequency noise splits into two branches on all three components which likely reflects the effect of day and night anthropogenic noise or workday to weekend variations. Despite the comparably small distance between the station and the closest houses, high-frequency noise levels are reasonably well below NHNM -10 dB or even NHNM -20 dB.

Station 09 is located inside a rarely used storage cellar that is built into the slope of a hill (Fig. ) and supplied from the power grid. The site is close to a secluded family house and 800 m from any larger settlement (Hafning, Lower Austria), 1.5 km from a highway and mainly surrounded by forest. The installation inside the cellar reduces surface effects which results in low long-period noise levels (Fig. ). While the vertical component long-period noise level is approximately only 10 dB higher than the NLNM, horizontal noise-levels fall around NHNM -10 dB but are still within the desired range. High-frequency noise on all components separates into two branches which, however, fall below NHNM -20 dB. This likely reflects the little anthropogenic noise originating from the nearby house. Among the sites where continuous data is available for more than one week, Site 09 performs second best to the installation inside the bunker (Site 01).

Seismic data used for this manuscript is not publicly accessible by decision of the AlpArray working group. Waveform data from the preliminary station tests may be available upon request directed to the corresponding author of this manuscript (florian.fuchs@univie.ac.at).