Earthquake locations are fundamental for assessing seismic hazard. To provide these, areas with high seismicity rates and large magnitudes are instrumented with seismic stations. Particularly depth estimation requires a dense seismic network around the suspected epicenter. In contrast our study area, the Vienna Basin in Austria, a region of low to moderate seismicity with a largest instrumentally recorded magnitude around 5, is only covered sparsely with seismic stations. Nevertheless, the area is one of the most densely populated and most developed areas in Austria. In areas like these, estimation of seismic hazard has to be based on location of earthquakes with small magnitudes.

Regional Depth Phases (RDP) like sPg, sPmP and sPn are P phases converted to S at the surface. They develop at different regional distance ranges and the time difference between direct and reflected phase is sensitive to epicentral depth. This property already has been successfully used for calculating depth even if an RDP is observable at least at one station. However, especially in regions with sedimentary basins, these phases prove difficult or impossible to recover from the seismic records. On the other hand, seismic arrays together with appropriate processing can be used to lower signal to noise ratio of seismic recordings and so help detect and identify phases as e.g., describes.

The purpose of this study is to investigate the possibility of using a seismic array for identifying RDP in our region of interest.

In our study area we previously investigated a series of strike-slip earthquakes and relocated them using local stations. From this dataset we select all earthquakes with local magnitudes from 2.0 to 4.2 and analyse the records at the 220 km distant seismological array GERES . According to the literature those events should have a suitable magnitude and distance to recover PmP and sPmP phases. We use array processing on the whole array and polarization analysis on the four 3-component stations. At the same time, we perform similar processing of synthetic data specially modeled for this application. We compare real and synthetic results to assert which phases can be identified.

Ongoing convergence between the Bohemian Massif on the European Plate in the north and the Adriatic Plate in the south lead to lateral extrusion of crustal blocks to the east e.g., into the Pannonian Basin as is shown in Fig. 1. Two main sinistral strike-slip faults show this process: the Salzach-Enns-Mariazell-Puchberg (SEMP) fault and the seismically active Mur-Mürz-Lineament (MML). In the northwestern extension of those faults a pull-apart basin, the Vienna Basin started forming and now is filled with sedimentary layers of a few kilometers. Beneath this basin the MML branches into the Vienna Basin Fault System (VBFS) which produces moderate seismic events with local magnitudes around 4.0. The Bohemian Massif extends beneath the Northern Alpine Transition (NAT) and forms the crystalline basement beneath the Vienna Basin .

To approximate the underground between Ebreichsdorf and GERES we used the model proposed by , which is a 4-layer simplification of the 3-D model by .

For our research, we used the data from the Ebreichsdorf 2013 earthquake series, which was located by with all available data within 230 km and the 3-D velocity model by . For modeling the wave propagation we used the largest event with a local magnitude of 4.2 and a depth of 10.5 km. Also, this is one of the earthquakes in the Vienna Basin where a focal mechanism was published. From this dataset we selected the six earthquakes with local magnitudes above 2.0, which are listed in Table 1. Most important for this study the events were also recorded at the seismic array GERES in Germany . The array consists of 21 1-component and four 3-component seismic stations depicted in Fig. 1 which have been continuously recording since 1991 . The array is at a distance of approximately 220 km from our area of interest and has a backazimuth of 115∘ to the selected events.

Regional depth phases such as sPg, sPmP and sPn in combination with their reference phases Pg, PmP and Pn can be used to estimate focal depths of regional earthquakes, if they can be identified. Figure 2 sketches their wavepaths. In principle, a single station with one phase pair may be sufficient for accurately determining earthquake depth from the difference in their arrival times. Different studies, e.g., and mention that regional depth phases depend mainly on epicentral distance: Between 200 and 300 km waveforms should be quite simple: a weak Pn is followed by a strong PmP and sPmP using a simple 2-layer model and a thrust type mechanism.

The synthetic traces in Fig. 3 as well as the spreading maps in Fig. 4 and 5 show, that PmP and PbP should be visible clearly because of their high amplitudes at GERES. However, S to P conversions like sPmP and sPbP are not visible. As mentioned before array processing can be used to improve SNR of time series by, e.g. creating vespagrams. Figure 6 shows the results for all earthquakes in our dataset in color and for the synthetics of the first earthquake A.

Most elements of GERES are only recording the vertical movement. This is sufficient to identify backazimuth and slowness by array processing. However, GERES does also have four 3-component sensors, which are shown in the top left panel in Fig. 1. In the next step we try to use those stations to identify the phases visible in the vespagrams, using their polarization. For polarization analysis we used the method introduced by . Figure 7 shows the results for earthquakes A to E in grey and for the synthetics of the first event in black. Earthquake F was not used because of low SNR due to the small magnitude.

Lateral analysis of synthetic data shows that regional depth phase propagation is strongly dependent not only on depth but also on the focal mechanism. Therefore, the knowledge of focal mechanism is important and a typical mechanism for the area needs to be used. Also, additional layers in the underground can produce strong reflections not reported by literature like PbP or p5PbP. Although no RDP pair could be identified, the visible regional phases restrict depth of the events in our dataset between a layer above and below the earthquakes, which relate to the bottom of the Vienna Basin and the Conrad discontinuity.

From comparison of the afore mentioned results to our dataset, we conclude that earthquakes from the Vienna Basin develop clear PmP and PbP arrivals at GERES. However, the converted depth phases sPn, sPg, sPmP and sPbP are not visible, not even with the improved SNR of the array. Yet, vespagram analysis can be used to identify phases in the P coda by their slowness even down to magnitudes of 2.1.

Further research will analyse RDP propagation around the Vienna Basin in more detail. One important feature that needs to be addressed is the low-velocity sediment layer, which is reported to weaken converted phases we are looking for. Also, we will investigate data from our dataset recorded at stations, which are in areas where the synthetics indicate high amplitudes for RDPs. Since time difference between RDP pairs is mainly affected by depth, as stated in , it should be possible to use phase readings of single phases at varying distances to estimate depth.

With newly deployed dense seismic network like the AlpArray presented in or profiles like EASI (see wave propagation could be monitored and maybe even tracked across the region. This extended knowledge of RDP behaviour can then be used to locate even small earthquakes more accurately as more information than first picks is available.