7. Discussion
The seismicity of the MSH is investigated from documented historical
earthquakes previous to 22 June 2020. Three M>=6.5
historical earthquakes cover all the MSH segments. However, instrumental
seismicity is relatively poor. The EHB catalog (Engdahl et al., 2006)
shows three earthquakes near the Eastern and Central segments of MSH,
close to DMV. The GCMT catalog shows two 5<M<5.3
earthquakes on the Central and Eastern segments of the MSH. The IRSC
network earthquake catalog has improved from 2006 in terms of
completeness. They show 67 M>=2.5 earthquakes within a
distance of 5 km from the fault before the 2020 mainshock. Most of this
seismicity concentrated on the Central segment of MSH, South of the DMV.
Three peaks are observable in the cumulative Scalar Seismic Moment chart
of these earthquakes (Fig. 5c). Interestingly, the central peaks that
are mostly related to the 2006 and 2007 earthquakes, coincide with the
estimated rupture areas during the Ms 5.2 1930 and Mb 4.0 1955
earthquakes. A possible explanation is that they are late aftershocks of
these earthquakes. The Western one is close to thermal areas reported by
Eskandari et al. (2018). A low-velocity region has obtained Southwest of
DMV that extended until the MSH down to the depth of 15 km in a
tomography study by Mostafanejad et al. (2011) (Fig. S1a). The observed
thermal activities in the same area are probably due to the existence of
some branches of the DMV magma chambers in that area which was also
suggested by Eskandari et al. (2018).
The rupture process and the fault geometry of the 7 May 2020 M5.1
Damavand earthquake was investigated by inverting both the local
broadband seismic data for the moment tensor and the near-field
strong-motion displacement time series for its extended rupture model.
The mainshock occurred on the central segment of the MSH: It nucleated
~15 km SSW of the DMV crest and at a depth of
~14 km. The rupture is estimated in an elliptical patch
with a major-minor axis of 5 km-3.6 km. It evolves mostly toward the
Northwest along strike and to the up-dip direction at a sub-shear speed
of ~2.75 km/s for 2.8 s. The estimated geometry is
~WNW (292°) strike and ~60° dip to the
North. The obtained scalar seismic moment by point-source moment tensor
inversion is 4.8 e+16 Nm while using an extended rupture model, this
value reduces to 4.04 e+16 Nm, suggesting the release of some of the
scalar seismic moment at relatively lower frequencies between 0.03 Hz to
0.08 Hz.
The interpolated PGA from 33 recorded stations of the ISMN network
suggest a west-northwestward directivity, which is to some extent
consistent with our source model showing a westward directivity (Fig.
4a). For each station, peak values of the geometric average of the two
horizontal components of strong motions are considered as horizontal
PGA. The damping observed in PGA in the center of Tehran is interpreted
as attenuation due to the deepest part of the sedimentary basin (see
Majidnejad et al., 2017). The Fourier spectra of the strong motion data
for stations FRK3 and LVS1 that have negligible site effect, show the
low-frequency content of this event with a corner frequency of 1 Hz.
Far-field Brune models for an M5.1 earthquake is estimated in Tehran
region (Brune, 1970) in which source slip patch radius is roughly 4.3
km, S-wave velocity is 3.5 km /s, and \(\rho\) is equal to 1.5 (Figs. 6
and 7). The fmax is obtained from the smoothed spectra
(see Konno & Ohmachi, 1998; Figures 6, 7, S12, S13) in the ranges
between 6 Hz to 16 Hz in the Tehran region. Such difference is mainly
related to the site attenuation (i.e., scattering and dissipation) (see
Gomberg et al., 2012; Hanks, 1982).
The stress drop has obtained 2.6 bar from the extended rupture model,
posing roughly a circular slip patch with a radius of 4.3 km, and a
scalar seismic moment of 4.8 e+16 Nm (see Madariaga, 1977). An empirical
relation between scalar moment and stress drop by Ide and Beroza, (2001)
suggests a stress drop of ~10 bars for an M5.1
earthquake. The obtained relatively low stress drop is consistent with
the relatively large rupture length. We note that the obtained rupture
length (estimated between 7 km to 9 km) is relatively large for such a
magnitude earthquake (for example see Momeni et al., 2019 for a rupture
length of ~12 km estimated for an Mw6.5 earthquake; and
Vicic et al., 2020 with rupture length of ~4 km for an
Mw5.1 earthquake).
The mainshock exhibits a left-lateral strike-slip mechanism (Rake=14°)
the same as the general mechanism of MSH proposed by Tatar et al.,
(2012), a geodetic study of Djamour et al., (2010), and
geological-paleoseismological studies by Nazari et al., (2009) and
Solaymani-Azad et al., (2011). A maximum slip of ~3 cm
was estimated between depths of 12 km and 11 km. The rupture stopped at
a depth of 8 km.
The mainshock rupture and the early aftershocks occurred between the two
peaks of cumulative scalar seismic moments on the MSH, proposing that
this part of the fault was somehow locked compared to two other
neighbors that experienced the 1930 and 1955 earthquakes.
The aftershocks were distributed toward the West and up-dip, consistent
with the main rupture direction and general orientation of the MSH. The
largest aftershock with M4.1 occurred 20 days after the mainshock with a
left-lateral strike-slip mechanism, the same as the mainshock.
Aftershocks surrounding the mainshock slipped area (Figs. 4a, 5), is a
consistent feature of large earthquakes (see Henry and Das, 2002).
The 2020 seismic activity occurred at a depth range between 15 km to 8
km, where Tatar et al. (2012) also detected most of the
microearthquakes. This range is almost the same as the upper-crystalline
layer of the velocity model obtained by Abbasi et al. (2010) for the
region. This relatively thick and deep part of the seismogenic layer may
have the potential for the production of large earthquakes with
low-frequency contents that can reach Tehran with less damped seismic
energy and affect the tall buildings, the same as the
7th May 2020 M5.1 mainshock.
The smooth geometry of the central segment of MSH may facilitate the
rupture expansion on it. Occurrence of the 1930 (Ms 5.2), 1955 (Mb 4.0),
1983 (Mw 5.3), and 2020 (Mw 5.1) earthquakes in the South of the DMV,
together with its seismic activity from 2006, suggest a strong
relationship between the volcanic activity of DMV and relatively high
seismicity rate of the central segment of the MSH. Also, most of the
microseismic activity and larger microearthquakes were reported by Tatar
et al. (2012) on the central segment of MSH, just to the South of DMV
between longitudes from 51.75 E to 52.2 E, while their seismic network
was well-distributed on the two other segments of MSH.
Previous studies suggested the existence of a hot young sill-like magma
chamber of DMV in the Southwest of its current crater (i.e. Mostafanejad
et al., 2011; Shomali and Shirzad, 2014; Yazdanparast and Vosooghi,
2014; Eskandari et al., 2018). While the old magma chamber of Damavand
is detected toward the North-Northeast of the crater and is detected as
a cooled high-velocity dike-like structure (Mostafanejad et al., 2011).
The existing young magma chamber may increase the pore pressure on the
left-lateral strike-slip MSH which consequently decreases the effective
normal stress on it and facilitates the rupture nucleation-expansion
(Fig. 8). Such phenomena have been widely observed and reported mostly
for Strike-slip and Normal faulting mechanisms (i.e. Saar and Magna,
2003, Goebel et al., 2017, Scuderi et al., 2017, Johann et al., 2018,
Eaton and Schultz, 2018, Benson et al., 2020). On the other hand, such a
mechanism may not allow considerable accumulation of strain on this part
of the MSH near DMV (i.e. Yagi et al., 2016).
The 2020 M5.1 earthquake is the largest well-recorded event on the MSH
after the 1983 event. This segment of the MSH has experienced the 1830
IX 7.1 historical earthquake. All of the evidences indicate that the
2020 M5.1 mainshock and recent seismicity of the central segment of MSH
are related to the existence/activity of the magma chamber of DMV. We
also stress that 1930, 1955, and 1983 earthquakes on the South of DMV
might have happened as a result of the same unclamping mechanism due to
the existing high pore pressure.
Compared to the Central segment of Mosha, the Western segment that is
closer to Tehran city is silent. However, GPS studies confirm its lower
deformation rate (1mm/y, Djamour et al., 2010). The occurrence of
earthquakes like the 2012 Ahar-Varzaghan doublet (Mw 6.5 and Mw 6.3)
with almost no detected seismic activity in the IRSC network before the
mainshock and low deformation rate (i.e., Momeni et al., 2019)
highlights the importance of a detailed seismic-geodetic study on the
Western segment of MSH that will affect the seismic hazard of that
region, and especially Tehran city. Also, the Eastern segment of MSH
shows seismic activity which highlights its importance as another
potential segment of the MSH for future large earthquakes.