The method used to achieve the purpose of the present study, that is, revealing the characteristics related to the seismic hazard of the studied Iranian blind reverse-fault earthquakes, consists of two investigation approaches as follows:
(1) As the seismic hazard in the immediate vicinity of an earthquake strongly depends on the rupture extent in the seismogenic layer, we compared the depth of the bottom of the sediment layer obtained from the CRUST1.0 model and previous local studies with the InSAR-derived depth of the top of the rupture.
(2) A larger stress drop leads to the radiation of strong short-period ground motions. The stress drop is a key parameter for the estimation of strong ground motions caused by earthquake ruptures (e.g., Miyakoshi et al. 2020; Soghrat et al. 2012). We compared the stress drops of the studied Iranian earthquakes with those of the Japanese earthquakes obtained from our earlier study (Ghayournajarkar and Fukushima 2022) and other studies.
Tables S1 and S2 list the fault parameters of the preferred models derived from the inversions of InSAR data for the studied Iranian and Japanese earthquakes (Ghayournajarkar and Fukushima 2022). The InSAR-derived depths of the upper limit of the fault ruptures ranged between 4.5 and 14.8 km. The comparison between the InSAR-derived depth of the top of the rupture and the depth of the bottom of the sedimentary layer shows that there is a systematic gap between the depth of the top of the rupture and that of the bottom of the sedimentary layer for all the studied Iranian earthquakes (Table 3 and Fig. 3).
Table 3 Comparison of the InSAR-derived depth of the top of the rupture of the preferred fault model and the thickness of the sedimentary layer for the studied Iranian earthquakes
Earthquake
|
This study
Top Fault
|
CRUST1.0
Sediments
|
Previous local studies
Sediments
|
Depth
(km)
|
Thickness
(km)
|
Thickness
(km)
|
Type of the study
|
Qeshm 2005
|
6.0
|
5.0
|
3.0
|
Rayleigh wave dispersion
(Shirzad and Yaminifard 2020)
|
Dargahan 2008
|
5.7
|
5.0
|
3.0
|
Rayleigh wave dispersion
(Shirzad and Yaminifard 2020)
|
Sefid Sang 2017
|
5.4
|
0.9
|
1.2
|
Stratigraphy
(Poursoltani 2017)
|
Kermanshah 2017
|
14.8
|
0.5
|
8.0
|
Stratigraphy
(Nissen et al. 2019)
|
Hojedk 2017
|
4.5
|
2.0
|
1.0
|
Stratigraphy
(Poole and Ataabadi 2005)
|
The uncertainties in the InSAR-derived depth of the top of the rupture range between 0.2-1.0 km, which is smaller than the gap for most cases. Although the uncertainties in the sedimentary layer thickness from previous studies vary and are not necessarily estimated, the consistent gap exists for results derived from both the CRUST1.0 model and previous local studies, supporting the actual presence of the gaps.
Next, we calculated the stress drops of our analyzed Iranian and Japanese earthquakes, according to the following relation (Kanamori and Anderson 1975):

where M0 is the seismic moment in Nm, calculated assuming a shear modulus of 30 GPa, and S is the rupture area in M2. Strictly speaking, the factor
is used for circular faults. For rectangular faults, such factors depend on the aspect ratio and have only been numerically calculated for some specific aspect ratios. Hence, we opted for Equation (1) in this study. According to Noda et al. (2013), the factors for rectangular faults are slightly larger than the factor for circular faults, and consequently our estimates of stress drop when using Equation (1) are expected to be slightly smaller.
Table 4 lists the parameters derived from the fault models of the studied Iranian earthquakes (Ghayournajarkar and Fukushima 2022), including their seismic moment, rupture area, stress drop, and moment magnitude. The calculated stress drops range from 9.6 to 17.1 MPa.
Table 4 Parameters derived from the fault models of the Iranian earthquakes
Iranian Earthquakes
|
Seismic moment
(Nm)
|
Rupture area
|
Stress drop
(MPa)
|
Magnitude
|
Qeshm 2005
|
1.22E+18
|
34.5
|
14.6
|
5.99
|
Dargahan 2008
|
1.28E+18
|
47.1
|
9.6
|
6.00
|
Sefid Sang 2017
|
1.21E+18
|
36.4
|
13.5
|
5.99
|
Kermanshah 2017
|
7.79E+19
|
629.1
|
12.0
|
7.19
|
Hojedk 2017
|
1.17E+18
|
30.3
|
17.1
|
5.98
|