Complex Wave Propagation Revealed by Peak Ground Velocity Maps in the Caucasus Area by C. P. Legendre, T. L. Tseng, H. Mittal, C.-H. Hsu, A. Karakhanyan, and B. S. Huang ABSTRACT In 2013, a new network of broadband seismic stations was de- ployed in the Caucasus region. Broadband displacement data are inverted for the seismic moment tensor of two medium- sized earthquakes occurring in northeast Georgia (17 Septem- ber 2013, M w 5.1) and northern Armenia (26 May 2014, M w 3.6), respectively. The peak ground velocity (PGV) is esti- mated for each station that recorded the seismic events. We then construct shake maps for those seismic events. Those time-dependent shake maps can provide not only the informa- tion on the initial rupture, the ground response as well as the nature of the soil (soft or stiff ), but also the potential direc- tivity of the rupture. The goal of the study is to provide a comprehensive image of the spatial distribution of the PGV in the Caucasus region from small-to-moderate earthquakes, as well as to estimate the rupture directivity for the considered seismic events. Our re- sults demonstrate a good consistency between the source mechanism and the regional tectonics, and a directivity of the rupture perpendicular to the faults. INTRODUCTION Tectonic Background The Caucasus region is at the junction between eastern Europe, central Asia, and the Middle East (Fig. 1). The major tectonic features and earthquakes in west Asia are greatly associated with the continental collision of the Eurasian and Arabian plates that began sometime in Oligocene after the closure of Neotethys ocean (Stampfli and Borel, 2002; Agard et al., 2005; Boulton and Robertson, 2007; Keskin et al., 2008; Hatzfeld and Molnar, 2010). The continuous north–south compression of the collision not only produced the Caucasus mountain belts but also the westward extrusion of the wedged Anatolian block that is offset by North and East Anatolian faults (e.g., McKenzie, 1972; Sengör, 1979; Barka, 1992; Stein et al., 1997; Armijo et al., 1999). The North Anatolian fault is part of an Alpine–Himalayan collision belt running from the French Alps toward Tibet (Molnar and Tapponnier, 1978; Jackson and McKenzie, 1984; Legendre, Deschamps, et al., 2015) and turning to Red River fault zone (Searle, 2006; Legendre, Zhao, Huang, et al., 2015). The Caucasus region is at the junction of several tectonic plates (Bird, 2003) which can be observed on tomographic models at local (Šílenỳ et al., 1992; Triep et al., 1995), regional (Legendre et al., 2012; Legendre, Zhao, and Chen, 2015), and global scales (Mon- tagner and Tanimoto, 1991; Trampert and Woodhouse, 2003; Lebedev and Van Der Hilst, 2008). Because of significant crustal shortening and thickening since mid Miocene, the East Anatolian plateau (part of Turkish–Iranian plateau) has reached its current elevation of around 2 km (Dewey et al., 1986; Koçyiğit et al., 2001). The influence of collision propa- gated to the Caucasus region approximately in early Pliocene, evidence to its rapid exhumation (e.g., Philip et al., 1989; Per- chuk and Philippot, 1997; Avdeev, 2011; Avdeev and Niemi, 2011; Vincent et al., 2011). Seismic Risk In the Caucasus region, the seismicity is relatively high, with the occurrence of moderate-to-strong earthquakes (Nowroozi, 1971; Kaila et al., 1974; Westaway, 1990; Smirnov, 1995; Matcharashvili et al., 2000; Tan and Taymaz, 2006; Ekström et al., 2012; Mumladze et al., 2015). In the last decades, several seismic events with magnitude greater than 7 (and up to 8.4) occurred in the Caucasus region (Ekström et al., 2012). Those very large earthquakes have a destructive impact on human society and the risk of casualties is very high. Therefore, antici- pating the regions with potentially high seismic risk or shaking is of great importance. Unfortunately, before time of the deployment of our seismic network, those events were only re- corded by seismic networks outside the Caucasus region. The determination of focal solution was available, but investigations of the near-field effects were not possible. A newly deployed seismic network in the Caucasus area (Tseng et al., 2016) allows further understanding of the seis- motectonics of the region, as well as improved knowledge of the potentially damaged regions following an earthquake. This network can also be used to extract information on seismic hazard, propagation of the ruptures and to make a link with the well-known geological structures in the region. Recent studies (Wald et al., 2006; Papanikolaou, 2011; Ebel and Kim, 2012) showed the importance of seismic-hazard maps for the assessment of effects of earthquakes in urban areas. doi: 10.1785/0220160178 Seismological Research Letters Volume 88, Number 3 May/June 2017 1 SRL Early Edition