Tectonophysics 712-713 (2017) 557–577 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Mid-European seismic attenuation anomaly Jiˇ rí Málek a , Johana Brokešová b, * , Jiˇ rí Vackᡠr a a Department of Seismotectonics, Institute of Rock Structure and Mechanics, Czech Academy of Sciences, Prague, Czech Republic b Department of Geophysics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic ARTICLE INFO Article history: Received 26 July 2016 Received in revised form 30 May 2017 Accepted 2 June 2017 Available online xxxx Keywords: Seismic wave attenuation Peak ground motion H/V ratio ABSTRACT Macroseismic studies of various historical earthquakes with epicenters in the Eastern Alps region have shown a significant elongation of isoseismals in the North-West direction. Such an anomalous attenuation of seismic waves in Central Europe is investigated on the basis of instrumental records of two moderate- size earthquakes in the Vienna Basin, which occurred in September and October 2013. It has been found that for both earthquakes the peak amplitudes of both velocity and acceleration are considerably higher to the North-West of the epicenters compared to the other directions. The peak ground velocity amplitudes at comparable epicentral distances but different azimuths may vary by as much as one order of magnitude. The inspection of individual seismograms suggests that the phenomenon is associated mainly with the prop- agation of S waves. Significant differences in frequency content of the seismic waves radiated to different azimuths are also demonstrated. The maximum predominant frequency was not observed at stations closest to the epicenters but about 250 km away, in the Bohemian Massif. The possible causes of these observations are briefly discussed on the basis of an elementary data analysis but further research and in-depth analysis is required to elucidate the causes of these phenomena. © 2017 Elsevier B.V. All rights reserved. 1. Introduction From historical findings, earthquakes with epicenters in the East- ern Alps seem to display a strong azimuthal dependence of their amplitude attenuation with distance. The distribution of macroseis- mic observations of the East Alpine earthquakes felt in Bohemia and Moravia on July 25, 1927 (Supplementary Fig. S1), October 8, 1927, November 8, 1938 and September 18, 1939 was studied by Zátopek (1948), who reported that the “macroseismic field asymmetrically elongated in pear-shape to the NNW reaches as far as Dresden”. That study was based on an extensive collection of 2966 reports from the territories of Bohemia and Moravia related to four strong earth- quakes with epicentral intensities greater than 7 MCS. The results of this early study were later confirmed by Procházková and Kárník (1978) and Procházková (1993), who report an asymmetric shape of isoseismals, significantly extended in the North-West (NW) direc- tion for the earthquake of April 16, 1972 (see Fig. 1). These findings suggest that macroseismic intensities of East Alpine earthquakes * Corresponding author at: Department of Geophysics, Charles University, V Holešoviˇ ckách 2, Prague 180 00, Czech Republic. E-mail address: Johana.Brokesova@mff.cuni.cz (J. Brokešová). decay more slowly towards Bohemia in a systematic manner. The intensities reflect the spatial distribution of earthquake impacts in a given region and contain some degree of subjectivity. We can also speculate that ground motion amplitudes may display a correspond- ing asymmetry in their spatial distribution. In this study we call this phenomenon the mid-European attenuation anomaly. Confirma- tion of this anomaly with instrumental data is the main subject of this paper. Seismograms recorded by modern seismic stations in this region give us an opportunity to study this phenomenon in greater detail than isoseismal mapping. We used the recent moderate-size (M L 4.2) earthquakes at Ebreichsdorf, situated in the Eastern Alps region, which occurred on September 20, 2013 (hereinafter Event 1) and on October 2, 2013 (hereinafter Event 2). These two events are located within the Vienna Basin Fault System, one of the most seismically active regions of Austria (Tary et al., 2016). The Vienna Basin separates the Eastern Alps from the Western Carpathians and lies in the northeastern extension of the active Mur-Mürz- Fault between the Bohemian Massive and the Pannonian Basin. Both events are nearly co-located (47.9318 ◦ N, 16.4230 ◦ W for Event 1, and 47.9315 ◦ N, 16.42229 ◦ W for Event 2; Apoloner et al., 2015) in the part of the basin adjoining the Eastern Alps. The depth of both events is about 10.5 km. We collected seismograms from seismic stations distributed across the region within 400 km of the epicen- ters in order to study spatial distribution and azimuthal dependence http://dx.doi.org/10.1016/j.tecto.2017.06.003 0040-1951/© 2017 Elsevier B.V. All rights reserved.