Seismic study of land subsidence and Vulnerability of Rural Buildings by using geophysics methods, near Shiraz city

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1 International Research Journal of Applied and Basic Sciences 2013 Available online at ISSN X / Vol, 7 (11): Science Explorer Publications Seismic study of land subsidence and Vulnerability of Rural Buildings by using geophysics methods, near Shiraz city Hossein Rahnema 1, Sohrab Mirasi 2 Department of Civil and Environmental Engineering, Shiraz University of Technology, Shiraz, Iran Corresponding Author s.mirasi@sutech.ac.ir ABSTRACT: In recent years, land subsidence due to different reason such as excessive withdrawal of groundwater resources has lead to significant damages to farmlands, residential buildings, roads and transmission lines. Inattention to land subsidence results in the ruining of buildings which in turn causes the migration of people and imposes financial and social costs on the government and also changed the quality of ground water resources; increase Salinization of water, depletion of underground aquifers (the second largest fresh water source in the world) and creating fissures and sinkholes. However, some of the buildings in rural areas have lost utilization capability and their Habitation in Fars Province due to neglect of this phenomenon and propagation of cracks and gaps in the roof, Floors and walls. In this paper field observation was conducted and identified damaged villages in Marvdasht plain to investigate effects of faults and earthquakes on land subsidence and available gaps. And vulnerability index of these areas (vulnerability index of structures) was identified by using seismic method in the range of Shul village. And also soil status was determined by using Geoelectric method. Then, vulnerability status of damaged buildings was presented into future earthquakes and at the end of the research, suitable solutions were offered to prevent this phenomenon and its damages. Keywords: subsidence, microtremor, Geoelectric, cracks and fissures, Marvdasht. INTRODUCTION Land subsidence consists of collapse or settlement of the surface level that can be accompanied by a small amount of horizontal displacement. Land subsidence may occur due to some reasons such as dissolution of subsurface formations, sediment compression, loading, drainage, vibration, hydraulic compression, subsurface mechanical erosion, soil instability on sloping surfaces and etc. The Subsidence phenomenon may be accelerated by natural activities such as volcanic activity, earthquakes and landslides (ngdir, 2006). Land subsidence as a serious crisis, has been observed in several Iranian plains in recent years. Although effect of such phenomena may not be as disastrous as that of the earthquake or flood, but it can impose expensive financial and social costs on the government gradually. Traditional methods for determining soil dynamic properties were time-consuming and very costly that after acceptability of Nakamura method to determining this property, the quantity of using traditional methods was became less. Determining ability of soil and structural strain at any point prior the earthquake with Considering the Seismic waves motion through the earth and create strain in various layers, can be Great help to manage the crisis and determine vulnerability of soil and structures. Nakamura in 1989, proposed at first his theory about determining dominant frequency and dynamic characteristics of ground that much was considered. Seismic response characteristics of structures can be estimated Simultaneous on structure and its ground surface by using spectral ratio of microtremor measurements (Nakamura, 1989). One of the main problems associated with destruction and damage of buildings is Behavior of soil layer on the bedrock during earthquakes. Magnification factor and dominant period of site are affecting important factors that caused by earthquake. Thus, determine effects of soil or in other words determine natural period and resonance of soil under the site have significant importance.

2 Assessments vulnerability of soil and structure, based on the amount of Vulnerability index can be detected poorly available points on ground surface or structures and implemented necessary actions (Nakamura, 1989; Nakamura, 2003). MATERIALS AND METHODS In this paper, related parameters to land subsidence were investigated by observations, analysis of field and regional status data. After determining location of plain and damaged villages, Shul village was studied as an example of other same villages. Initially the effect of faults on cracks and gaps were investigated besides the measurement of damage. Due to entered damages and occurred earthquake in the past few years, Vulnerability of damaged and undamaged buildings was obtained and also Vulnerability of these buildings into future earthquakes was evaluated by using microtremor. Status Of Studied Area Marvdasht plain is located 40 km north of Shiraz between longitudes 52 16' to 53 30' and latitude from 28 54' to 30 26'. The ground water level dropping, structural status of the region and effect of Garmabad fault were the main causes of general settlement in the plain. Near the mountain hillside and the places with variable soil layering, especially under the villages and building areas, the subsidence rate is more noticeable and considerable that continuous cracks with the depth even more than of 10 meters and the width more than 8 cm are observed. These cracks with their adjacent counterparts form a hypothetical strip with about 50 m width in the mountain hillside. Shul village, Kamal Abad, Zangi Abad, Ghasem Abad and ancient monuments of Naqsh-e Rustam are located in this hypothetical strip. In this research Shul village was investigated completely as a sample of aforementioned areas. Figure 1. Location of Marvdasht plain in Fars Province, Iran 719

3 Figure 2. Location of damaged villages near mountain hillside in Marvdasht plain Site Envestigation Land subsidence was reported in eight regions of Fars province, such as Marvdasht, Arzhan, Shiraz, Naghshe Rostam, Kuh Sepid, Safashahr sinkhole and Surmagh. Figure 3. Created sinkhole in Surmaqh with 50 m depth and 16 m diameter Created Cracks due to different settlement caused various problems and damage to rural building, especially those were near the mountains, somehow suffering from subsidence and vertical displacement which opening and closing doors and windows was not possibility. Gradually, utilization capabilities and reside in this building was eliminated and choice but to leave the building or destroy houses and not remain for the residents. Figure 4. Created crack on walls of buildings and yard 720

4 According to field observations the lime layers had outcrops of 30 to 40 cm at Hossein mountain. There were also settlements of 20 to 30 cm for rural building near the mountain which reduced by getting farther from the hillside. This reduction can be attributed to effect of fault and variation of alluvia thickness. Figure 5. Effect of land subsidence on building and Seismic Investigation And Effect Of Faults On Land Subsidence Field studies over the cracked region showed that the direction of main crack was along the side of Hossein mountain. Shul village is also located on Garmabad Fault on hillside of Hossein Mountain. existence of this fault caused anomalies in sloped layering and inversion of slope in the southwestern side of Hussein mountain anticline. The fault appeared as a rupture zone in this domain. By mapping the position of created cracks and gaps into the desired region, based on their geographical locations, it was observed that the direction and vicinity of cracks were along the fault. This indicates that presence of fault accelerates the creation of cracks in conjunction with groundwater level dropping. Figure 6. Area of created cracks and fissures by using geological map Grmabad fault that passes through the Shul village, Caused Dissonance of layer slope and invert slope in southwestern of Hossein mountain. Grmabad fault was a reverse fault based on fault angle as shown in following figures. Formation of Rivers faults is associated with shortening of Layers. Figure 7. Slope Inverting of layers near the village Vulnerability Of Rural Building In Shul Village Microtremors passing through the different layers of soil caused changes in their domain. In addition these changing for different soils are not the same in waves. Nakamura in 1989 presented a hypothesis that Based on it the vertical component of microtremors reached to the ground almost have the same primary characteristics of vertical component on bedrock. In case horizontal component of these microtremors are changed. Under the 721

5 assumption of Nakamura, site response in low strain level can be considered equal to spectral ratio of horizontal waves to spectral ratio of vertical waves (Nakamura, 2002). Instrumens And Devices The device used in this study was EXPLORATION SEISMOGRAPH 16S-N (24 PASI. channels) which includes a device Record of vibrations and three sensors with three-axis and cables that connected sensors to each other along with power source. Figure7. (a) Microtremor Device for recording Vibrations (b) A three components sensor The sampling rate is adjustable from 500 to 2,000 records per second. Each sensor consists of two horizontal axis perpendiculars to each other and a vertical axis. That provides capability of Data harvesting in three directions simultaneously. In this study microtremor device was used for recording microtremors and calculating vulnerability of building. The predominant frequency, amplification, vulnerability indices of damaged or undamaged buildings were determined by microtremor in the vicinity of Shul village. Tests were conducted over the damaged structures located near the mountain hillside and the undamaged structures far away from cracks and gaps domain as shown in fallowing figure. Figure 8. The location of damage and undamaged building The vulnerability index of the rural building can be calculated by the average of vulnerability index, because most of the buildings in these areas had one story. K av A H (2) F 2 (2) Where Kav, H, A and F are respectively the average of vulnerability index, building height (m), magnification factor (i.e H/V) and the predominant frequency of the building (Nakamura, 1989; Nakamura, 2003). As it is obvious from the figures, by considering the ratio of horizontal component at building's roof to vertical component of ground, vulnerability index of damaged structures was about 2.5 times higher than undamaged ones, which shows the critical state of such structures with respect to forthcoming earthquakes and future subsidence. 722

6 a) Damaged building b) Undamaged building Figure 9. Spectral ratio H/V (the ratio of horizontal component of the building's roof to the vertical component) Calculation Of Sediments Thickness By H/V Ratio Ibs-von Seht and Wohlenberg Demonstrated frequency is closely related to thickness of soil layer (h) according to following equation (Ibs-von Seht and Wohlenberg, 1999). h af (1) b r Parolai et al. were performed Microtremor measurements in Kolong zone of German and was derived following equation based on them (Parolai et al, 2002). h f (2) 108 r Which h are in meters and frequency (f r ) is in hertz. According to above equation, thickness of soil deposits near mountain (Ranges of damaged buildings) was between 2 to 4 meters and around the village; far away from the mountains (Ranges of normal structures) thickness of soil deposits was approximately 44 meters. Obtained alluvium thickness from the above equation were almost good agreement with obtained data from logs of agricultural wells and results of Geoelectric tests. RESULTS OF GEOELECTRICAL TEST By using Geoelectric method, resistance measurement operations were conducted in the distances between damaged and undamaged buildings. Results were indicated a surface layer with a thickness varying from 0 to 5 m and with high resistance of 20 ohm m, which represented alluvial layer containing coarse grain and fine particles such as clay and sand. And second layer by reduces resistance compare to first layer was softer than upper layer due to reduce amount of coarse particles and increase in fine sediment Such as clay. This layer was continued to a depth of

7 meters and more. And final contours for lower layer were also appeared with red color which represents harder and higher resistant. Figure 10. Results of resistivity methods by using 2D data analysis of Res2dvin software CONCLUSION Obtaining Results from study of fissures and faults position showed that happened earthquakes in last few years were approximately effective on trend of land subsidence and crack opening. Coincidence the direction of created cracks and fissures with direction of available faults in the studied area was shown as affecting factors on cracks. Along with withdrawal of groundwater and water drops, Faults in this region accelerate emergence of cracks, Contribute to growth and opening cracks, Accelerate land subsidence in these areas and intensify damages to rural buildings. Considering results of microtremor, vulnerability index of damaged structures was high, which in turn shows the critical status of such structures. However by appearing extended cracks in the roofs, bases and walls of such structures, their workability was lost and future financial and residential costs may be imposed. The results of present research show that for prevention and control of damages to structures due to land subsidence and related factors, the extraction of underground water resources should be managed properly, highly water-dependent plants should not be grown, unauthorized wells should not be drilled, farmers should be trained, irrigation systems should be improved and construction in vicinity of cracked zone should be prevented. REFERENCES Geological survey of Iran, A review of the regional land subsidence in Iran, Ibs-von Seht M, Wohlenberg J Microtremor measurements used to map thickness of soft sediments, Bulletin of Seismological Society of America, 89, M. Young, The Techincal Writers Handbook, Mill Valley, CA: University Science, Nakamura Y Seismic vulnerability indices for ground and structures using microtremor Prepared for 5th EQTAP Workshop in Bangkok, Thailand, December Nakamura Y A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface, QR of RTRI 30, no. 1, February, Parolai S, Bormann P, Milkereit C New relationships between Vs, thickness of sediments, and resonance frequency calculated by the H/V ratio of seismic noise for the Cologne area (Germany), Bulletin of Seismological Society of America, 92, Yutaka N, Jun S, Tsutomu S "Development of Vulnerability Assessment Models using Microtremor", prepared for 6th EQTAPW Workshop in Kashikojima, Japan, December 724