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