Short Note Deep Earthquakes beneath the Northern Caucasus: Evidence of Active or Recent Subduction in Western Asia by R. J. Mellors, J. Jackson, S. Myers, R. Gok, K. Priestley, G. Yetirmishli, N. Turkelli, and T. Godoladze Abstract An intermediate-depth earthquake is confirmed at a depth of 158  4 km under the northern foothills of the Greater Caucasus. Separate methods were used to confirm the depth: data from local and regional networks, teleseismic depth phases, and examination of waveforms. Additional examination of global catalogs suggests the presence of a (perhaps remnant) northeast-dipping subduction zone under the Greater Caucasus. The most likely explanation appears to be subduction of oceanic crust with the interface at the northern edge of the Kura Basin. Events at depths of 30– 50 km in the Kura Basin may be related to underthrusting by the South Caspian basin rather than subduction in the Greater Caucasus. Introduction The Caucasus region is an area of active seismicity and high elevation, yet the exact nature of the convergence remains enigmatic (Fig. 1). Global Positioning System (GPS) measurements suggest that up to 60% (13 mm=yr) of the present-day Arabian–Eurasian convergence is accommo- dated in the Greater Caucasus (Reilinger et al., 2006). While the existence of subduction under the Greater Caucasus has long been conjectured (Khalilov et al., 1987; Khain and Lob- kovsky, 1994), previous studies using teleseismic data (e.g., Tan and Taymaz, 2006) were unable to confirm subcrustal events, and direct geologic evidence for subduction is incon- clusive (e.g., Mosar et al., 2010). In contrast, studies based on local network data indicate the existence of one cluster of subcrustal events under the northern foothills of the Greater Caucasus (e.g., Tskhakaya, 1962). Using data from 1972 through 1987, Godzikovsky (1988) and Godzikovskaya and Reysner (1989) located six hypocenters at depths of 80 to 120 km based on local network arrival times (Fig. 2). An additional 26 events were inferred to be subcrustal based on observed waveform characteristics. Khain and Lobkovskiy (1994) interpreted these events as in- dicating a northeasterly dipping subduction zone below the eastern Caucasus. In this paper we reconcile the discrepancy between the local and teleseismic studies. Detailed analysis is provided of a deep (158  4 km) event under the Greater Caucasus using a combination of regional and teleseismic data. The purpose is to refine the depth and establish that it is subcrustal. Ex- amination of additional hypocenters from independent cata- logs indicates a northeasterly-dipping zone, as suggested by Khain and Lobkovskiy (1994). The confirmation of deep earthquakes will assist in understanding the tectonics of this region, as well as in other zones of subcrustal earthquakes along the Tethyan margin such as the Hindu Kush–Pamir and the Vrancea zones. An active subduction zone also may imply an enhanced seismic hazard for the region. Structurally, the Greater Caucasus consists of an actively-propagating double-verging fold and thrust belt with isolated volcanoes in the western Greater Caucasus (Mosar et al., 2010). The timing of the initial uplift appears to range from late Eocene to middle Miocene (Allen et al., 2002) and is presumed to represent the northern edge of deformation from the Arabian–Eurasian collision zone. At the eastern end, the Greater Caucasus mountains taper down to the Cas- pian Sea but are in line with a major structure, the Absheron Ridge, which crosses the Caspian Sea roughly from east to west and forms the northern boundary of the South Caspian basin (Fig. 2)(Jackson et al., 2002). The average crustal thickness as estimated from refrac- tion data in the Greater Caucasus is approximately 50 km (Kondorskaya et al., 1981). Gok et al. (2011) reported Moho depths of 45–50 km along the southern slope of the Greater Caucasus, using a combination of receiver functions and sur- face waves. Subcrustal earthquakes with depths to 80 km occur under the Absheron Ridge, which have been interpreted to indicate incipient subduction (or underthrusting) of the South Caspian basin northward under Eurasia (Jackson et al., 2002). Offshore seismic reflection profiles display a clear se- dimentary wedge and dipping interface on the south side of the Absheron Ridge (Knapp et al., 2004). Events with depths up to 80 km also occur northwest of the Absheron Ridge, 862 Bulletin of the Seismological Society of America, Vol. 102, No. 2, pp. 862–866, April 2012, doi: 10.1785/0120110184