Fault Movement Potential of Marzanabad Area, North Alborz, Iran

The major Quaternary faults of Marzanabad in the north Alborz can be classified based on their strikes into two sets: northwest and eastwest. In this paper, we use a model to evaluate their movement potential. Their theoretical model is based on the relationship between fault geometrical characteristics and regional tectonic stress field. The results show that Taleqan, Kandovan Chitan-Dozbon and Makaroud-Dalir fault zones are of the highest movement potential in the area. Also, the region where the fault zones have been intersected (northeastern part of study area) is prone to high seismicity; however, these fault zones don’t have high movement potentials.


Introduction
Seismicity is closely related to active Quaternary faults.This attracts many researchers to investigate the quantitative relationships between them.As a new parameter, FMP is defined to quantify earthquake risk along active faults by [1].Therefore, we use it for evaluation of earthquake risk Marzanabad in the north Alborz, Iran.The landforms in this area are mainly controlled by two sets of Quaternary faults, striking northwest and eastwest, respectively (Figure 1).
The questions to be addressed in this paper are: 1) what are the activity levels of these faults?And 2) will these faults cause destructive earthquakes?Previous work regarding these topics was mainly based on field works [2].In this paper, we use a new method to evaluate fault activity by considering the mechanical relationships between fault geometry and regional tectonic stress field.This method has been used to evaluate the fault movement potentials of all the major Quaternary faults Marzanabad in the north Alborz.

Fault Sets
Quaternary faults are well developed in Marzanabad area.They were classified into two sets based on their strikes: northwest and east -west.The northwest striking fault set comprises Mosha-Fasham, Taleqan, Varangeh Roud, Kandovan, Dona-Siahbisheh and Valiabad fault zones in the southern part of study area (Figure 1).
The east-west striking fault set are well exposed and can be traced intermittently for a long distance in nearly east-west direction.In this set, there are Nessen, Chitan-Dozbon, Sama-Majlar, Makaroud-Dalir, Dashte Nazir-Nater, Marzanabad, Azadkooh, Siahbishehfault zone in the northern part of study area (Figure 1).
In summary, all of these fault zones are active in current tectonic regime (CTR) and characterized by seismic events (Table 1).These events have shown that, seismic layer is in 20 -30 km depth.This area has low to moderate earthquakes with intermediate frequency and repeat time.
In the following sections, we will evaluate the earthquake risk along these faults, and discuss which fault is most favored to move under the influence of present-day tectonic stress field.We make this evaluation based on the relationships between tectonic stress orientation and fault geometric properties.

Theoretical Model for Analysis of Fault Movement Potential
The fault movement potential (FMP) is closely related to tectonic stress (σ), fault plane geometry (G) and the physical property of the medium within and on both sides of the fault (P).FMP is the function of these factors [1]: Although a geological medium is generally heterogeneous and very complicated, however it can be taken as homogeneous and isotropic in statistical view of our case.Based on this consideration, and for the purpose of simplification in the theoretical derivation, they also take the geological medium containing the faults as a homogeneous, isotropic and elastic material.Therefore fault movement potential can be simplified as: Finally, according to [3] and [4] researches, [1] defines FMP to quantify the relationship between fault movement potential as a normalized factor by the following equations: θ is the angle between the regional maximum principal compressive stress orientation (σ 1 ) and the normal line of fault plane.

Regional Tectonic Stress Orientations
Tectonic stress means an additional stress to lithostatic stress state, in the other words, the part of stress deviated from lithostatic stress.Earthquake focal mechanism solution is one of the commonly used methods in the study of contemporary tectonic stress field.Therefore, we use results of [5]- [7] and our field study to estimate the regional maximum principal compressive stress orientation (σ 1 ).The statistical result shows that the average attitude of σ 1 is 15˚, 040˚.This area belongs to West-Central Alborz and lesser Caucasus hinterland [8] [9] that formed on the inverted back arc intra-continental rift since Oligocene.Dominant structural trend in West-Central Alborz and lesser Caucasus province is NW-SE (Figure 1).From tectonics view, it contains deformed zone (fold and thrust belt) of Cimmerian miniplate that formed in northern active margin until late Triassic.Then it has rifted by tension in a back arc basin of Neotethyian subduction zone in the south margin of Cimmerian miniplate.
Development of that rift stopped in the late Cretaceous and then, renewed in the Eocene by spreading in submarine arc basin of Neotethyian subduction zone.In the other word, this hinterland is result of a magmatic arc system spreading in the evolutional back arc basin.After that, this region has converted to back arc regime again and West-Central Alborz and lesser Caucasus hinterland has formed by its deformation and regional uplift from SW part of Caspian Sea to Black sea.Recently, Damavand and Sebalan cones have formed by late volcanism that related to final subduction of oceanic slab (Neotethys) toward north and northeast [10].This area has an active tectonics regime [11]- [25] in compared to the Central Iran [26]- [35] and Zagros in the southern Iran [36]- [43].Also, some concepts of its metal mineralization, have been investigated by [44]- [48].

Results and Discussion
The fault movement potential of the major Quaternary faults of Marzanabad have calculated using the equations (3) and the regional stress orientation as well as the fault plane attitudes.The results are shown in Table 2 and Figure 2. Also, seismic events have 25 Km depth as average of focal depth.The earthquakes are related to two sets of Quaternary faults with northwest-southeast and east-west strikes.
Taleqan, Kandovan, Chitan-Dozbon and Makaroud-Dalir fault zones have large angle between the normal to the fault planes and the compressive principal stress along these fault zones.The fault movement potential of this fault set ranges from medium to high, suggesting that this fault set has the sufficient potential for generating destructive earthquakes.This situation has been shown by contoured map, too (Figure 3).In Figure 4, Epicenter of earthquakes (Table 1) on FMP contoured map of Marzanabad area have shown and it implied that high FMP values are not always coincident to previous seismic history.

Conclusion
Based on this research, the contemporary movements potential along fault zones of various orientations are different under the action of present-day regional north-northeast compressive stress field in study region.Taleqan, Kandovan, Chitan-Dozbon and Makaroud-Dalir fault zones have medium to high movement potentials.Also, the region where the fault zones have been intersected (northeastern part of study area) is prone to high seismicity; however these fault zones don't have high movement potentials.

Figure 1 .
Figure 1.Stations of fault attitude measurements on Marzanabad area.

Figure 2 .
Figure 2. Stations of FMP measurements on Marzanabad area.

Figure 4 .
Figure 4. Epicenter of earthquakes on FMP contoured map of Marzanabad area.

Table 1 .
Seismic events of study area.

Table 2 .
The calculation of fault movement potential in Marzanabad in the north Alborz.