Moderate earthquakes striking Tehran metropolitan area: a case study of 2017 Malard and 2020 Damavand seismic sequences

The 2017 Malard and 2020 Damavand moderate crustal earthquakes (M_w 4.8) occurred about 40 km west and about 55 km northeast of Tehran, the capital and economic heart of Iran, with a metropolitan population of over 15 million. Seismic hazard assessment in the region has been affected by few historically documented destructive earthquakes with magnitudes around 7.0 (e.g., 312–280 B.C, 958, 1177, and 1830 A.D.); however, in the absence of large contemporary earthquakes, a detailed analysis of moderate earthquakes is essential. In this study, seismic sources of the two earthquakes are characterized in terms of focal mechanism, fault geometry, and rupture directivity through waveform inversion, hypocenter relocation, and empirical Green’s function methods. The eastern segment of the well-known Mosha fault is responsible for the 2020 Damavand earthquake, with a left-lateral strike-slip mechanism ruptured unilaterally westward where Tehran is situated. The 2017 Malard earthquake is a peculiar case in a poorly studied region. For this event, we propose a left-lateral strike-slip mechanism corresponding to E-W trending Mahdasht fault. This event was preceded by a swarm of events, 12 km northward, that started a few months earlier and terminated right before the mainshock. The energy released due to this precursory activity was higher than the Malard mainshock and its aftershocks. The events seem to align along an N-S transverse basement fault that, further southward, may intersect with the Mahdasht fault system.