Long-term induced seismicity on the Mosha fault by the Damavand Volcano, N-Iran, Implications for the seismic hazard of the Tehran metropolis

Abstract:

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 40 km East of Tehran, on the central segment of the Mosha fault, 10 Km South of Damavand Volcano crest. Its moment tensor obtained by inverting the local broadband displacement waveforms, showed a strike-slip mechanism with N-S and WNW-ESE planes and a centroid depth in 12 km. Its extended rupture was imaged on a single elliptical slip patch that nucleates at a depth of 14 km; we find that 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. Early aftershocks (first 45 days), were mainly distributed in the up-dip direction of the slip model, showing a strong directivity effect of the mainshock.

We observe in the last 14 years a relative seismic quiescence in the ruptured zone of the 2020 sequence on the Mosha fault compared to its East and West segments, suggesting that it was partially locked. The occurrence of several instrumental earthquakes on the central segment of Mosha fault, 10 km South of the Damavand Volcano (1930, 1955, and 1983), its recent high microseismic activity, and the 2020 seismic sequence on it, suggest that the Volcano affects the seismicity of this part of Mosha fault. This idea is supported by the existence of a young Sill-like magma chamber under Damavand to the SSW of its crater in the vicinity of the Mosha fault. The existing heat may raise the pore-pressure on the fault, which decreases the effective normal stress and facilitates the rupture nucleation-expansion. Several hot springs observed in the same area on the Mosha fault confirms the existence of the magma chamber. The proposed mechanism suggests that some parts of the central segment of Mosha fault cannot accumulate considerable tectonic strain, while its seismicity rate would be high.