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.