Seismic hazard assessment by Analytic Hierarchy Process(AHP) and Relative Tectonic Activity Index(Iat) methods, Southeastern Iran

The sensitivity of the drainage pattern to the active tectonics processes leads to a geomorphic expression that provides a useful tool to evaluate relative tectonic activity (Cox, 1994). Geomorphic indices provide a useful method for assessing the extent of tectonic activity; Especially in areas where earthquakes happen on faults in which there was not enough information in advance. The study area is in the west of Lut block, southeast of Iran, which is in the range of longitude from 55°22ˊ 15˝ to 58° 31ˊ17˝E and latitude from 28°50ˊ4˝ to 31°9ˊ30˝N. This region has a long history of devastating earthquakes, the historical types of which date back to around 1850AD. The occurrence of earthquakes on previously unknown faults (Bam, 2003) and the need to pay attention to areas in this part that have not yet experienced a devastating earthquake despite being surrounded by active faults (Kermen city), are the most important reasons that led us to find the best way of morphotectonic analysis and compare the results to seismic structures and data as a tool to identify structures and their activities also to estimate the seismic risk.

To obtain the drainage network layer, basins area, and water dividing layer in the study area, the hydrological modeling process was done by Arc GIS10.3 software. The basic data used are 30 m resolution DEM and geological maps. Also, to investigate the relationship between seismicity and tectonic activity, seismic data were used for earthquakes greater than 0.4 Mw. In this study, the results of calculating 7 geomorphic indices, including stream length-gradient index(SL), drainage basin asymmetry(Af), hypsometric integral(Hi), the ratio of valley-floor width to valley height(Vf), index of drainage basin shape(Bs), index of mountain front sinuosity(Smf), and Transverse Topographic Symmetry Factor(T), for 51 basins were analyzed by Iat and AHP methods. Finally, by using seismic data and field studies, we discuss our findings to compare the accuracy of AHP and Iat methods in analyzing morphometric indices to assess seismic risk, which is the aim of this paper.

The evaluation of the distribution of relative tectonic activity in the study area according to Iat, which is obtained by averaging the different classes of morphotectonic indices, divided the region into three categories in terms of tectonic activity. Class2, class3 and class4, have allocated 5.9%, 54.9%, and 39.2% of the total study area to themselves, respectively. It should be noted that none of the basins were in class1. 43.2% of epicenters were located in basins with high activity and 17.6% in low ones. Since the Iat index is not based on the weighting of the indices, all the indices were calculated with equal importance ratios. Therefore, there are some inconsistencies in comparing the Iat distribution map with the main structures of the region and the epicenter of the earthquakes. The AHP makes it possible to determine the amount of tectonic activity in the study area base on the weight and importance of each index compared to others, instead of relying solely on the average of each index (Iat). The values of the coefficient of determination(R2) are extracted from the graph of the indicators relative to each other. The correlation coefficient(R) was calculated from the roots of R2. After checking the acceptability of the Inconsistency, the final weights assigned to each of the indicators are (0.015)Bs, (0.012)Smf, (0.025)AF, (0.021)T, (0.104)Hi, (0.116)Vf, (0.708)SL. These results were classified into four categories. Accordingly, class1, class2, class3 and class4, occupy 21%, 24%, 31%, 24% of the total study area, in turn. The basins with very high and high tectonic activity, are all located along the main faults of the region, in addition, the distribution of earthquakes in these basins (81.8.% of earthquakes were in high active basins and 4.4% in low ones), shows how they are consistent with the results of AHP.

In this study, we tried to more accurately assess the impact of active tectonics and determine the most appropriate method for analyzing geomorphic indices, by comparing the results obtained from Iat and AHP methods. Proper matching of geomorphic features, calculated indices, and classification of relative tectonic activity obtained by the AHP, with the main structures, field observations, seismic data and previous studies in the study area, confirms the significant superiority and higher accuracy of AHP over the Iat in the identification of tectonically active areas which subsequently leads us to seismic hazard assessment. Based on the final map derived from the AHP, the study area was divided into four tectonic activity classes. The results show that morphological features in the study area are strongly affected by the main faults. The basins along them, generally show very high to high tectonic activity rates. The existence of many active faults, the epicenter of earthquakes and the location of most cities in the study area within or adjacent to basins with very high to moderate tectonic activity indicate the need for attention and additional studies there.The findings of this paper show how using a suitable way of morphometric analysis can help us to identify areas with the potential for seismic hazards. Due to its low-cost and timesaving, this method can be a good alternative for expensive and complex methods, especially in vast and remote places. As a result, it can be a useful tool for regional planners and decision-makers involved in seismic risk assessment and disaster risk reduction.