پهنه‌بندي شدت آرياس و اثر رسوبات كواترنري بر تغييرات آن مطالعه موردي: گستره خوي

Arias intensity Macrozonation and effect of Quaternary Deposits on its propagation Case Study Khoy region

زلزله يكي از خطرات طبيعي است كه طي سال‌ها باعث تلفات و خسارات مالي زيادي در سراسر جهان شده است. به همين دليل است كه تجزيه‌وتحليل خطر زلزله بايد با جدّيّت بيشتري مطالعه شود. ايران در يكي از مناطق لرزه‌خيز جهان، كمربند هيماليا‌ـ آلپ، قرار دارد كه هر ساله زلزله‌هاي زيادي در آن رخ مي‌دهد. تابع شدت آرياس، به منزله يكي از پارامترهاي مهم زمين‌لرزه، به تجزيه‌وتحليل خطر لرزه‌اي كمك مي‌كند و مي‌تواند براي برآورد عملكرد سازه، پايداري دامنه ها، و روان گرايي در طول زلزله مورد استفاده قرار گيرد. در اين مطالعه، ضمن تحليل زمين‌ساختي و لرزه‌زمين‌ساختي گستره خوي در شمال غرب ايران، پهنه‌بندي شدت زمين‌لرزه بر اساس روش آرياس صورت گرفت. مقادير عددي شدت آرياس براي دوره بازگشت‌هاي 475 و 975 و 2475 ساله به صورت نقشه‌هاي هم‌پتانسيل تهيه شد. در تحليل‌ها، اثر تغييرات رسوبات كواترنري نيز لحاظ شد. به طور كاملاً مشهود، شدت زمين‌لرزه، جداي از فاصله از گسل، از تغييرات سرعت موج برشي تأثير مي‌پذيرد؛ به نحوي كه با فاصله از چشمه لرزه‌زا، در مناطقي با ضخامت بالاي رسوبات كواترنري، حتي افزايش نيز مييابد. اين موضوع بازگوكننده اثر شرايط زمين‌شناسي در برآورد پارامتر‌هاي وابسته به زمين‌لرزه است.

Introduction Earthquakes are one of the natural hazards that have caused many casualties and financial losses around the world. This is why earthquake risk analysis studies need to be conducted more seriously. Iran is located in one of the seismogenic regions of the world, the Himalayan-Alpine belt, which is subject to many earthquakes every year. The Arias intensity, as one of the important seismic parameters, helps in seismic hazard analysis, and can be used to estimate structural performance, slope stability, and liquefaction during an earthquake. The region under study is in Khoy area in the northwest, near the Iran-Turkey border. The geographical area covers 38.5°N, 43.5°E to 39°N, 45°E. The main features of the region are the presence of the major lakes of Van and Ercek in the west, and also the Iran-Turkey border in the middle of the map.Arias (1970) created an equation for measuring the intensity of an earthquake based on the time-integral of the of the square of the ground acceleration, which was later used by some researchers to evaluate the damage potential. Harp and Wilson (1995) found out that the Arias Intensity is reliably linked with the distribution of landslides caused by an earthquake. Later on, Kayen and Mitchell (1997) proposed a method for evaluating the soil liquefaction during an earthquake’s strike with the help of the Arias Intensity. In another work (Cabañas et al., 1997), a noticeable correlation was found between the Arias Intensity and MSK scale. It was also revealed that for certain structures, such as the ones found in villages and adobe and brick buildings, the damage could be linked to the Arias Intensity. More recently, Borja et al. (2002) used the Arias Intensity as a measure for comparing the results of two seismic response analyses. Additionally, for determining damage indexes, such as the value of destructiveness potential factor , where la is the Arias Intensity, and V0 is the amount of the zero crossings of the acceleration–time history, this parameter is used (Araya and Saragoni, 1984). Method The Arias Intensity is one of the seismic parameters which is usually used in seismic hazard analysis to shed light on the potential damage earthquakes may cause. The Arias Intensity, as a scale of the shakings linked with an earthquake in terms of the amount of cumulative energy, is defined as an infinite set of single degree-of-freedom oscillators of unit weight with the frequency of zero to infinity (Elnashai and Sarno, 2004). This parameter correlates with the integral of the square of the module of ground acceleration over the time history of an earthquake. The Arias Intensity is a parameter which includes characteristics such as the domain and duration of ground motion for a wide range of recorded frequencies. Thus, compared to the parameters depending on the maximum value of ground motion, it is far more effective for evaluating earthquake impacts on engineering targets. In fact, evidence (Khademi, 2002; Mamseyedeh et al., 2021) supports that the Arias Intensity is to correlate proportionately with the damage caused by an earthquake, making it a reasonable choice when describing shakings capable of causing instability in structures, landslides (Chousianitis et al., 2014; Travasarou and Bray, 2003), and liquefaction (Kayen and Mitchell, 1997; Orense et al., 2015). Result The a and b values were calculated using the Gutenberg and Richter law (Gutenberg and Richter, 1956) in MapSeis software (Figure 3.). To complete the information about the regions seismic sources, the iso-potential maps shown in Figure 3 were used. As is seen in the figure, the a-b values of the center of the region of the study were higher. These higher values indicate more seismic activity in the area (Kanamori, 1981; Kanamori, 2013; Wyss, 1975; Wyss et al., 2001). On the other hand, the accumulation of energy in the areas with a low b value shows their potential for future earthquake events. Of course, along with the consideration of this variable, other variables such as a and λ, in addition to the seismotectonics of the region, must be observed to locate future earthquakes with minimized error (Jarahi, 2017). Observing Figure 2 and Figure 3, along with noting the location of seismic sources in the region, reveals that three areas (A, B, and C) are to be expected to be subject to future earthquake events. Altogether, the mentioned revelations prove that the region is highly seismic. The information formed the basis for the analyses done by Ez-Frisk for further calculations. Figures 4 to 6 show the Arias Intensity iso-potential maps for probabilities of 20%, 10%, and 5%, respectively in 100 years. Conclusion The Arias Intensity map can be used as one of the most important indexes for measuring the destructiveness of an earthquake. In this study, for the three return periods of 475, 975, and 2475 years, the Arias Intensity was examined and analyzed. During the study, the main seismic sources of the region were identified and their seismic parameters were calculated. Three areas, marked A, B, and C, were introduced as the areas with potential for future earthquake events. In the Arias Intensity of the region, some specific points were noticed. In general, the further the area is from the seismic source, the less is the intensity of the earthquake there. Nevertheless, it is not always true. That is because another important parameter, known as shear wave velocity, can control this pattern or even reverse it. In various regions, such as the land surrounding the Shkriazy fault, this reverse pattern was detected and that displays the effect of shear wave velocity on the Arias Intensity distribution. Considering the history of liquefaction during earthquakes in this region, the results of the current study could serve as the basis for liquefaction and landslide studies conducted there. From among the three recommended areas, area B is adjacent to a relatively large sedimentary basin (northeast of Salmas), where the very low shear wave velocity makes it prone to liquefaction. Therefore, the potential for the occurrence of future earthquakes, on the one hand, and the conditions increasing the likelihood of liquefaction and resonance, on the other hand, make area B the most dangerous one in the region. Moreover, it is recommended that in the three areas discussed in this study, measures be taken for reinforcing and strengthening structures to prepare them for future earthquake events.