8. Conclusion
We infer that the seismicity of the central segment of the MSH in the
South of DMV is excited by the existing young sill-like magma chamber of
DMV between them. The eastern segment shows lower seismic activity and
the western one is almost quiet. We suggest that the transferred heat by
the magma chamber to the fractured area of the MSH raises the pore
pressure that consequently decreases the effective normal stress on this
part of the MSH and triggers the rupture nucleation-expansion. The two
peaks of scalar seismic moment release are observed on both sides of the
7 May 2020 M 5.1 mainshock, suggesting its partial locking. However, the
occurrence of several small to moderate magnitude earthquakes on this
segment of MSH suggests a heterogeneous distribution of stress on it.
This means, both the heterogeneous distribution of fault plane strength
and raised pore-pressure due to temperature may play roles in the
central segment of the MSH near DMV.
The PGA distribution of strong motions in near-field and Tehran and the
obtained rupture model for the mainshock reveal its directivity toward
the West (Tehran). The corner frequency is estimated at 1 Hz which is
consistent with the Brune (1970) model. Fmax is
estimated between 6 Hz to 16 Hz in the Tehran region. The obtained
Fmax is close to the averaged value reported in a study
by Haghshenas and Bard, (2007) (10 Hz). These high-frequency waves may
be due to the amplification caused by the sedimentary basin beneath
Tehran. The Amax frequencies change from 2 Hz to 17 Hz. However, most of
stations inside Tehran show relatively similar frequencies for Amax
between 2 Hz and 6 Hz (Fig. 4c, Table S7). Negligible attenuation of
low-frequency signals (< 1 Hz) (related to the source rupture)
in Tehran region is observed (see Figs. 6, 7).
In terms of rupture dynamics, DMV can work as a fuse and nucleate
earthquakes on the MSH, for which if the rupture grows toward the West,
it will cause a strong directivity effect for that earthquake toward
Tehran megacity, similarly to the 7 May 2020 M 5.1 earthquake (Fig. 4b,
c). The western segment of MSH that is the closest segment to the Tehran
metropolis may be fully locked, as it has almost no seismic activity,
but GPS measurements show a deformation rate of 1 mm/y for it, and a
locking depth of 16 km is suggested. This segment did not rupture since
the 958 AD M~7.7 Taleghan-Mosha historical earthquake.