**6. Discussion**

#### *6.1. Afterslip for Time Period 1*

Our analysis shows that the post-seismic response through about June 2018 was dominated by afterslip along the up-dip continuation of the coseismic rupture, consistent with Barnhart [18] and Yang [24] who studied shorter time periods. The moment released by afterslip was 1.13 × 10<sup>19</sup> Nm, which was about 12% of that by the coseismic slip. To put these results into perspective, we compiled the location of afterslip and the ratios between afterslip and coseismic moment release found in previous studies of earthquakes, which are listed in the supplementary Table S1.

The afterslip on the up-dip continuation of the coseismic rupture indicates a zone along the fault with velocity-strengthening frictional behavior [41] above a zone with velocity weakening behavior that ruptured during the earthquake. Up-dip afterslip was also observed for the 2003 Mw 6.9 Boumerdes [42] and the 2003 Mw6.8 Zemmouri, Algeria [43] earthquakes. For many other earthquakes, afterslip occurs on the down-dip extension of the coseismic rupture zone. Examples include the 2015 Mw 7.8 Gorkha earthquake [44], the 2011 Mw 9.0 Tohoku earthquake [45–47] and the 2008 Mw 7.9 Wenchuan earthquake [48].

The afterslip duration time of six months is comparable to that of the 2014 Napa earthquake [10] but much smaller than for example for the 2004 Mw6.0 Parkfield, California, earthquake [49] after which it continued for about 12 years. The relative amount of afterslip moment release is at the lower end compared to other earthquakes (10%, 28% and 56% for the Gorkha, Tohoku and Pakistan earthquakes, respectively).

Afterslip is promoted by clay-rich sediments/fault with clay-rich gouge, high temperature and elevated pore-fluid pressure [7]. Therefore, the low heat flow (average of 74 mW/m<sup>2</sup> [50]) and relatively low pore-fluid pressure (indicated by the relative low Vp/Vs ratio about 1.73 [51]) might contribute to the relative short duration time and small moment of Kermanshah afterslip compared to the mainshock moment.

#### *6.2. Viscoelastic Relaxation for Time Period 2*

Our modelling results show that the best-fitting viscosity of the lower crust is 10<sup>19</sup> Pas while the viscosity of upper mantle is 10<sup>19</sup> Pas or larger, i.e., the viscosity of the lower crust is equal or lower than that of the upper mantle. The inferred lower crust viscosity beneath the Kermanshah region equals to that in the regions of the Hector Mine [52], EL Mayor Cucapah [53], Iceland [54], Loma Prieta [55], Irpinia [56], Gonghe [38] and Bam [57] earthquakes (Table S2). It is lower than that of the Central Nevada Seismic Belt [58], and above that of Hebgen Lake [59], Northridge [60], Izmit [61], Chi-Chi [62], Manyi [63] and Mongolia earthquakes [64]. The inferred upper mantle viscosity range is consistent with upper mantle viscosities found for the regions of the Izmit, Northridge, Wenchuan [48] (Tibet), Bhuj [65], Gonghe, Bam, Northridge, Loma Prieta, Chi-Chi, Manyi and Kokoxili [66] earthquakes, but is higher than for the Hebgen Lake, Landers [39], Hector Mine, Denali [67], Central Nevada Seismic Belt, El Mayor Cucapah, Iceland, Parkfiled [68] and Mongolia earthquakes.

Laboratory experiments and previous studies show that lithospheric viscosity is mainly dependent on the composition, temperature and water content [69–71]. Lower temperature and less water lead to higher viscosity. The relatively high upper mantle viscosity of the Kermanshah region compared to the western US (Landers, Hector Mine, Central Nevada Seismic Belt, EL Mayor Cucapah, Heban Lake earthquakes), South central Alsaka (Denali earthquake) and Iceland is consistent with the relatively lower average heat flow (Iran: 74 mW/m<sup>2</sup> [50], Western US: 91 mW/m<sup>2</sup> [50], South central Alsaka [72]: 89 mW/m2; Iceland: 175 mW/m<sup>2</sup> [73]). It is also consistent with the presence of the high-velocity anomalies in the upper mantle beneath the Zagros [74] which mark the presence of dry and cold lithospheric mantle [48]. What is more, the relatively high viscosity of the lower crust is also consistent with the relatively low average Vp/Vs value (Iran: 1.73 [51]; Chi–Chi: 1.9 [75]), which can be an indication of low fluid content [76]. Lower temperatures might also contribute to a higher viscosity in the lower crust.

There was no significant relaxation deformation in the first six months, which can be explained by the relatively high lower crust and upper mantle viscosities. Furthermore, laboratory experiments also sugges<sup>t</sup> that the power–law viscous flow often occurs on the hot lithosphere rocks [77], thus the relatively cold lithosphere rocks in our study region may mean the low possibility of the power–law viscous flow occurring in the first few months, which may also contribute to this no significant relaxation deformation in the first six months phenomenon.
