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.