ESTIMATING SLIP RATES AND RECURRENCE INTERVALS FOR STRONG EARTHQUAKES ALONG AN INTRACONTINENTAL FAULT: EXAMPLE OF THE PAMBAK-SEVAN-SUNIK FAULT (ARMENIA)

Abstract

North of the Arabian plate, active tectonics is characterised by both N–S compression and E–W extension associated with strike-slip faults. The Pambak–Sevan–Sunik fault (PSSF) zone in Armenia is one of the major active structures of the region. The fault is comprised of four main segments and it displays morphological evidence for dextral movement during the Holocene. However, no large earthquake (M>7) has occurred in the northern or central parts of the fault during the last 2000 years.We undertook a geomorphological and paleoseismological investigation along the Pambak–Sevan–Sunik fault with the aim of estimating the long-term slip rate and recurrence interval of strong earthquakes.Trenches were excavated at three sites. Detailed studies of trench cross sections and dating (radiocarbon and ceramics) show three faulting events that occurred in the Vanadzor–Artanih segment (Fioletovo and Semionovka areas), whereas a single event took place in the Artanish–Sunik segment (Khonarhasar area).In both areas, we estimated the average slip rate using (i) the offset of rivers along the Vanadzor–Artanish segment (2.24±0.96 mm/year over an interval of 120–300 ka), and (ii) the offset of volcanic cones along the Artanish–Sunik segment (0.53±0.04 mm/year over an interval of 1.4 Ma).These results suggest that a greater slip rate characterises the Vanadzor–Artanish segment (Fioletovo site) from the Artanish–Sunik (Khonarhasar site) segment. Division of the Pambak–Sevan–Sunik fault zone into two main branches, east of the Artanish peninsula (Sevan Lake) could explain the difference in slip rate. In addition to its segmented seismic behaviour, the Pambak–Sevan–Sunik fault is a well-documented example of a fault that generates strong earthquakes with long recurrence time intervals (about 3000–4000 years).