6. Seismicity along the Mosha fault from 1996
Reliable earthquake locations by the IRSC seismic network in East of Tehran started in 1996. However, the IRSC network was relatively sparse until 2005 so that there are only 47 well-located earthquakes in the distance of 5 km from the MSH (Fig. 5 a, b). The selected earthquakes have location errors of < 5km, RMS of < 0.5 s, azimuthal gaps of < 180°, and are located by at least six stations. The mentioned events are mostly distributed on the Central segment of the MSH with a considerable concentration near the DMV. As the IRSC seismic network has improved from 2006, in both terms of magnitude completeness and location accuracy, 60 M>=2.5 earthquakes were located by them in the distance of 5 km from the MSH until the May 7th mainshock (Fig. 5 a, b, Table S4). This group of earthquakes has a smaller location error of <3 km, azimuthal gap of < 180°, and RMS of < 0.5 s. This seismicity is also concentrated on the Central segment of MSH, near the DMV. While, the Eastern segment also shows seismicity on its eastern termination, and the Western one is almost silent.
After the 2020 mainshock, 27 M>=2.5 aftershocks were located by the IRSC network, 28 of them were situated in the same distance range of 5 km from the central segment of Mosha (Fig. 5 b, Table S5). It is evident from Figure 5 c, d that the M5.1 mainshock rupture and its early aftershocks (45 days) occurred in a part of MSH that had a much lower seismic activity compared to its neighboring segments. As the located earthquakes after 2006 have better locations, we decided to compute the cumulative scalar seismic moments of the earthquakes that occurred after 2006, to investigate the seismic energy release behavior along the fault. The cumulative scalar seismic moment plot shows three peaks of seismic energy: two of them on the East and West of the M5.1 rupture on the central segment, and one near the eastern termination of the fault (Fig. 5 b, c). These peaks are mostly related to three events that occurred on 20/12/2006 Mw4.2 (#1), 26/02/2007 Mw3.6 (#2), and 4/04/2008 Mw3.6 (#3), and their moment tensors were inverted by Momeni, (2012) (Fig. 1b).
The early aftershocks were surrounded by the two peaks of cumulative scalar seismic moment release of earthquakes from 2006 until the M5.1 mainshock (Fig. 5c). After the mainshock, the cumulative scalar seismic moment plot shows a big peak in the mainshock slip area and its related aftershocks (Fig. 5d) that has a comparable amplitude to the scalar seismic moment release from the 1930 M5.2 earthquake.