Abstract
Seismic history of the Mosha fault, the largest and most active fault of
Eastern Tehran metropolis (>15 million population) with
three documented 6.5<M<7.7 historical earthquakes,
and its relation to the Damavand active Volcano, the highest mountain of
the Middle East, is investigated. We infer that the seismic activity of
the Central Mosha, close to the Damavand, is quite high compared to its
western and eastern segments. On May 7, 2020, an Mw5.1 earthquake struck
the Central Mosha, some 40 km East of Tehran, 10 km SSW of Damavand
crest, and was strongly felt in Tehran. Its rupture is imaged as a patch
that nucleates at the depth of 14 km and mostly grows toward the up-dip
and to the WNW with an average speed of 2.75 km/s that lasts for
~2.8 s. The rupture is located in a region that
presented a relative seismic quiescence compared to its Eastern and
Western parts, during the last 14 years, suggesting its partial locking
and a heterogeneous distribution of fault frictional strength on this
segment of Mosha. Early aftershocks were mainly distributed toward the
West and up-dip of the slip model, confirming a strong directivity
effect that is also observed from the PGA distribution of the mainshock
recorded in the ISMN network. A relatively small stress drop of 2.6 bar
is estimated from the extended rupture model, consistent with the
relatively large rupture dimension of 9 km for an Mw5.1 earthquake, and
suggests the easy rupture expansion on the central segment of the Mosha
fault near the Damavand Volcano.
The occurrence of 1930, 1955, and 1983 earthquakes on the Central Mosha,
its high microseismic activity, and the 2020 seismic sequence, strongly
suggest an effect of Damavand Volcano on the seismicity of the Central
Mosha. This is supported by the observed extension of a sill-like
Damavand young Magma chamber until this segment of Mosha fault in
tomography studies, and, by the observation of thermal areas on the
Mosha fault. We suggest that the existing heat raises the pore-pressure
on the fault, which decreases the effective normal stress (and ease the
rupture nucleation-expansion) unclamping the fault. Damavand may work as
a fuse and nucleate earthquakes in which if the rupture grows toward
West, will have a strong directivity effect of low-frequency seismic
waves that arrive without attenuation in Tehran and affect tall
buildings. Besides, high site amplifications for frequencies up to 16 Hz
due to the deep sedimentary basin, mainly in the mid-city of Tehran,
will be remarkable for short buildings.
Keywords: Induced seismicity; Earthquake source observations; Earthquake
hazards; Volcano seismology; Waveform inversion; Permeability and
porosity