1. Introduction
On May 7th, 2020, an M5.1 earthquake occurred in South-Central Alborz mountains just 4 km North of the Mosha fault (hereafter MSH), 10 km South of Damavand Volcano (hereafter DMV), which is situated 45 km East of Tehran city, capital of Iran with over 15 million population (Fig. 1). In spite of its moderate magnitude, it caused two fatalities and several injuries (IRNA news agency). The peak ground acceleration (PGA) of the mainshock was measured 141 cm/s2 at the close station of the Iranian National Strong Motion Network (ISMN) situated just 12 km Southwest of the epicenter (RDH1, Fig 1b, 6). In terms of mechanism, it exhibits an almost pure strike-slip faulting (Fig. 1, reported by the Iranian Seismological Center (IRSC); Table S1). This event did not produce any surface rupture.
Tectonic Settings
The Alborz mountain range is part of the northern boundary between Iran and Eurasia, located South of the Caspian Sea (Fig. 1). It accommodates about 30% of the total 25 mm/y of shortening between Arabia and Eurasia, the remaining being accommodated both by the shortening of the Zagros mountains and by long strike-slip faults in Central Iran (Vernant et al., 2004). The motion between Central Iran and the South Caspian basin is oblique to the belt and involves roughly ~ 5 mm/y of shortening and ~ 4 mm/y of left-lateral strike-slip motion (Vernant et al., 2004, Khorrami et al, 2019). This oblique motion is due to the clockwise rotation of the South Caspian basin (Djamour et al., 2010).
Alborz mountains have deformed during several tectonic episodes. The first corresponds to the collision of the Iranian microplate with Eurasia that occurred during the Late Triassic (i.e., Asserto, 1966; Berberian and King, 1981; Stocklin, 1974). The second was the collision of Arabia with Iran that had the main contribution to the deformation of Alborz. This collision either began ~12 My ago according to the thermochronology of exhumated rocks (Guest et al., 2006 a, b) or before, ~20 My ago based on the sedimentary studies of Ballato et al. (2008, 2011). The second episode is associated with partitioning. It may be started 10 My ago for Hollingsworth et al. (2008), 5 My for Allen et al. (2003), and even younger for Ritz et al., (2006).
The mainshock occurred near the MSH, the longest fault in South-Central Alborz with a length of ~175 km and a left-lateral strike-slip faulting mechanism (Fig. 1), which mostly accommodates the strike-slip component of the Caspian Sea clockwise rotational relative motion (Djamour et al., 2010). This earthquake is the only well-recorded M>5 seismic event near the MSH by the IRSC network.
MSH consists of three segments: The western MSH segment located North of Tehran strikes WNW and is parallel to the eastern segment of the sinistral-reverse Taleghan fault (Guest et al., 2006a, b). The western MSH could be part of a local partitioning system with the Taleghan fault (Guest et al., 2006 a, b) or deactivated in favor of the Taleghan fault (Nazari et al., 2009).
The central MSH strikes WNW with a length of ~80km and branches to the West to the North Tehran fault (NTF, Solaymani et al., 2011). This segment is also a left-lateral strike-slip fault that accumulated ~35 km of total displacement (Guest et al., 2006 a, b). Abbassi and Farbod (2009), however, believe that the NTF is not presently active and suggests instead that the motion occurs on several smaller faults situated southward. A paleoseismological study by Ritz et al., (2012) on a segment of NTF revealed its shallow dip thrusting toward the North (N115°E) and interpreted between 6 and 7 surface-rupturing events with estimated magnitudes between 6.5 to 7.2 that occurred during the past 30 kyrs.
The eastern segment of MSH has a WNW strike and connects to the left-lateral/normal ENE–WSW Firuzkuh fault to the East. It is situated along the Mosha valley and is almost parallel to the Sorkhe fault on its South. This segment has a left-lateral strike-slip motion and dips to the North (Allen et al., 2004; Bachmanov et al., 2004) but with a slight normal component (Ritz et al., 2006). The total sinistral offset is ~35 km (Allen et al., 2003) and the slip rate ~2 mm/y (Ritz et al., 2006). The recent GPS measurements also estimated 1-2 mm/y of left-lateral strike-slip motion on the MSH (Djamour et al., 2010).
It is estimated that the Firuzkuh fault with a late Quaternary estimated slip-rate of 1.1–2.2 mm/yr have had a maximum magnitude of 7.1, involving 1.2 m average displacement, that is expected to occur every 1100– 540 years (Nazari et al., 2014). As the last earthquake on the Firouzkuh fault may be up to 700 years in age, it can be considered as one of the major hazards for future earthquakes.
Tehran is built on a thick sedimentary basin consist of Quaternary alluvial deposits which are the main cause of wave amplification in that area (i.e., Majidnejad et al., 2017; Kamranzad et al., 2020). These sediments have four units: 1-Hezardarreh formation as oldest deposit in Tehran with a thickness of 1200 m that forms an anticline through the northeast-east of Tehran; 2-Kahrizak formation unconformably overlies on the eroded surfaces of Hezardarreh formation with 10-60 m in thickness and maximum dip of 15°; 3-Tehran formation, formed mainly of conglomeratic young alluvial fan deposits. Its thickness can reach 60 m and its bedding is almost horizontal; 4- The last one is the recent alluvium is the youngest stratigraphic unit within the region and its age reaches to Holocene epoch. Its thickness is 10 m. This unit composes of poorly consolidated to unconsolidated cementation with alluvial and fluvial origin (Kamranzad et al., 2020).
Inside Tehran, several active faults have been recognized like Qeytarieh, Lavizan, Pardisan, Chitgar, Garmdare (Talebian et al., 2016, Ritz et al., 2012; Fig. 4c), among which some have relatively fast slip rates that are estimated > 1mm/yr (e.g., Pardisan). This can have a strong impact on the earthquake hazard assessment of Tehran city and the surrounding region.
DMV is the highest and largest volcano of the Middle-East with an altitude of 5670 m, situated just 50 km Northeast of Tehran city. It is a young, dormant strato-volcano, which is a large intraplate Quaternary composite cone of trachyandesite lava and pyroclastic deposits overlying the active fold and thrust belt of the Central Alborz Mountains. Isotope dating, geological and tomography studies have revealed that the present cone (young Damavand) has been constructed over the last 600 Ky with a dimension of ~80 km3, a little to the South-Southwest and on an older, eroded edifice of the old Damavand (Davidson et al., 2004, Mostafanejad et al, 2011, Shomali and Shirzad, 2014). Damavand had an average uplift rate of 3 mm/y between the years 2003 to 2010 (Vajedian et al., 2015) which was almost uniformly distributed on the area proposing its sill-like magma chamber (Yazdanparast and Vosooghi, 2014). Thermal areas exist near the MSH (Eskandari et al., 2018), confirming the extension and presence of the DMV Magma chamber toward the MSH (Figs. 4a, 8).
Historical earthquakes of Mosha fault
Three M>6.5 historical earthquakes are related to the MSH (Ambraseys and Melville, 1982; Berberian, 1994; Berberian and Yeats, 1999; Tchalenko et al., 1974): The 07/06/1665 AD (M6.5) on the eastern segment, 27/03/1830 AD (IX 7.1) on its central segment, and 23/02/958 AD (X 7.7) on its western segment which is also referred partly to the Taleghan fault (Fig. 1a).
Several moderate magnitude earthquakes have also been reported after 1800 AD on the central segment of MSH near DMV: The 1802, 20/06/1811, 1815, and the 02/10/1930 AD Ms 5.2 and 24/11/1955 AD Mb 4.0 earthquakes that occurred just South of the DMV, very close to the 2020 mainshock (Fig. 4a) (Berberian et al., 1993; Nazari et al., 2009).
Instrumental earthquakes of Mosha fault and region
The instrumental seismicity is widely spread in the region. The EHB catalog (Engdahl et al., 2006) locates most of the seismicity near the Mosha, Firuzkuh, Sorkhe, and Garmsar faults (Fig. 1a). While recent seismic activity recorded by the IRSC network shows a broad distribution of seismicity in the region. Figure 1b shows selected earthquakes of the region that were recorded by the IRSC network since 2006. They are located by at least five stations, have a location error of <3 km, RMS of <0.5 s, and azimuthal gap of <180°. The eastern and central segments of the MSH show more microseismic activity compared to its western segment. A seismic cluster to the East of Tehran city is mostly related to mining activities in that area. The rest of the seismicity is related to the Sorkhe, Eyvanakey, Pishva, Garmsar, Zirab-Garmsar, Firuzkuh, and Robat-Karim faults. Detailed microseismic monitoring on the MSH by a local dense seismic network confirmed its left-lateral strike-slip mechanism with an East-Southeastward oriented fault plane (Tatar et al., 2012; Fig. 4a). Tatar et al. proposed an average dip of 70° to the North for this fault.
Three moderate magnitude earthquakes with strike-slip mechanisms have been inverted by Momeni, (2012) on the central and eastern segments of the MSH; two of them (#1 and #2) were located South of DMV (Fig. 1b; #1: 20/12/2006 Mw4.2, #2: 26/02/2007 Mw3.6, and #3: 24/04/2008 Mw3.6). However, there was no seismic activity reported for the western MSH segment neither from 1900 to 1996 (Berberian et al., 1993), nor in the recent IRSC catalog (Fig. 1b).
In this study, we first invert the local broadband displacement full waveforms of the 7 May 2020 M5.1 mainshock for its moment tensor. Then, we relocate the mainshock and largest aftershock hypocenters. After, we invert the near field strong-motion displacement wave-fields of the mainshock recorded in the Iranian Strong Motion Network (ISMN), for the Spatio-Temporal evolution of the slip, and investigate its relation to the distribution of early aftershocks, and the seismic history of the MSH. Then, we analyse the frequency content of the mainshock rupture, the fmax in the Tehran area, and the stress drop due to the mainshock to better investigate the impact of such earthquakes on the seismotectonics of Tehran city. Finally, we discuss the relation between MSH seismic activity and DMV, and the plausible cause of triggering.