Seyyedmaalek Momeni1,2*, Raul Madariaga3
1. EPFL Geo-Energy Lab – Gaznat Chair on Geo-Energy, Lausanne, Switzerland.
2. International Centre for Theoretical Physics (ICTP), Trieste, Italy.
3. Labratoire de Geologie, Ecole Normale Superieure, 75231 Paris Cedex 05, France.
* E-mail: maalek.momeni@gmail.com
Seismic history of the Mosha fault, the most important active fault of Eastern Tehran metropolis, and its relation to the activity of Damavand Volcano, the highest mountain of the Middle-East, is investigated. Historical earthquakes cover the three segments of the Mosha fault by three 6.5<M<7.7 events. Instrumental earthquake catalogs show that the seismicity of the central segment of Mosha fault, close to the Damavand Volcano, is quite high compared to its Western and eastern segments. On May 7th, 2020, an Mw5.1 earthquake struck in the 40 km East of the Tehran, on the central segment of Mosha fault, 10 Km South of Damavand Volcano crest. Its moment tensor obtained by inverting the local broadband displacement waveforms, showing a strike-slip mechanism with N-S and WNW-ESE planes and a centroid depth in 12 km. Its extended rupture imaged on a single elliptical slip patch that nucleates at a depth of 14 km and its slip grows mostly toward up-dip and to the WNW with an average speed of ~2.5 km/s and lasts for ~2.8 s, releasing a total scalar seismic moment of ~4.8E+16 Nm. It exhibits a left-lateral strike-slip mechanism (rake=14°) as expected for the Mosha fault. Early aftershocks (first 45 days), including an M4.1 event, mostly distributed to the up-dip of the slip model showing a strong directivity effect of the mainshock.
We observe a relative seismic silence for the ruptured part of Mosha during 2020 sequence compared to its East and West parts, in the last 14 years, suggesting its partial locking. The occurrence of several instrumental earthquakes on the central segment of Mosha fault in 10 km South of the Damavand Volcano (1930, 1955, and 1983), its recent high microseismic activity, and the 2020 seismic sequence on it, suggest the effect of Volcano on the seismicity of this part of Mosha fault. This idea supported by the existence of a Sill-like young magma chamber of Damavand on the SSW of its crater toward the Mosha fault. The existing heat raises the pore-pressure on the fault, which decreases the effective normal stress and ease the rupture nucleation-expansion. Several hot springs observed in the same area on the Mosha fault that confirms the existence of the magma chamber. The mentioned mechanism may propose that some parts of the central segment of Mosha fault cannot accumulate considerable tectonic strain, while its seismicity rate would be high. In terms of dynamic rupture simulation, Damavand volcano can work as a fuse and nucleate earthquakes on the Mosha fault, for which if the rupture grows toward the West, it will cause a strong directivity effect for that earthquake toward Tehran megacity.
Vicinity of the Mosha fault to Tehran city with >15 million population, its straight length of >150 km, and reported historical earthquakes together with recent seismic activity on it highlight the importance of detail study of this fault system in terms of assessment of seismic hazard of the Tehran city.