Comparison of one dimensional dynamic analysis and microtremor measurement results: Example of microzonation for Guzelyali

Earthquakes affect human life in negatively and cause loss of life and property. Especially, active seismic regions could be very dangerous for people and Turkey is one of the active seismic regions with North Anatolian Fault Line, Western Anatolian Fault Line and Eastern Anatolian Fault Line. Over and above, a big part of Turkey population and many factories are located in this region. Therefore, knowing the characteristic of this region is the most significant to protect people lives from earthquakes effects. Many of the disciplines such as geology, seismology, geophysics, geotechnics, and structural engineering are working together to reduce these effects and take necessary precautions against earthquake. There are three main factors that cause damage to civil engineering structures with earthquake impact. These are earthquake source characteristics, building performance characteristics and local soil conditions. Especially, local soil conditions which have the different characteristic even within the same region cause great damage during earthquakes. For this reason, the effect of strong ground motion on ground dynamic behavior must be determined correctly. When seismic waves which are occurred by strong ground motion, moved through soil layers, they change their own characteristics and soil layers properties. This leads to serious structural damages. For reduce the damage that occurs after earthquakes, responsibilities of geotechnical engineers are to determine dynamic behavior of soil layers and how this dynamic behavior affects civil engineering structures. That's why microzonation studies are carried out in order to reduce the damage that may occur in earthquakes. Microzonation; in the most general sense, to estimate the behavior of the soil layers during the earthquake and to determine the changes which caused by the earthquake in the surface. Microzonation studies which have three different stages are carried out for soil amplification, soil stability and liquefaction in geotechnical earthquake engineering. The first stage of microzonation is reconnoitered in the field and pre-made maps, field experiment data are collected. The second stage is tried to determine the dynamic ground properties of the region by carrying out necessary field studies in the region. In the third and last stage, soil models were constructed using the results obtained in the first two stages and 1, 2 or 3-dimensional dynamic analyzes were performed to determine the characteristics of earthquake effect on the soil surface. In the content of microzonation studies, soil amplification needs to be determined for urban and regional planning and land use decisions. In accordance with this purpose, microzonation studies are carried out considering the fault characteristic in the region. In microzonation, the study area is divided into small sub-parts for defined the effect of eartquake on soil surface such as, soil amplification, predominant period, peak ground acceleration (PGA), displacement and shear strain in soil layers. Microzonation studies consist of three different stages, as similar as microzonation studies. In the first stage of microzonation, the seismic hazard analysis is performed by using the geological, geophysical, tectonic, seismological and geotechnical characteristics of the region and evaluating the seismicity of the region. In the second stage, previously determined field measurements are carried out in the region and these field measurements are supported by laboratory experiments. In the third stage microzonation studies, soil models are prepared by considering local soil conditions and strong ground motion characteristics are determination by using soil dynamic analysis on soil surface. Distribution of these results within the region are evaluated within microzonation and results are mapped by using Geographic Information System (GIS). In this study, microzonation is carried out Bursa-Guzelyalı region which located on the 1. degree earthquake zone. For this purpose, within the scope of first stage microzonation, 68 borehole data and previous geotechnical reports are collected. As a result of these data and reports, soil layers are classified in accordance with Deprem Bölgelerinde Yapılacak Binalar Hakkında Yönetmelik (DBYBHY), Eurocode 8 and National Earthquake Hazards Reduction Program (NEHRP). However, active fault lines are determined and seismic hazard analysis are examined in this region. In the second stage of microzonation studies, working region is divided into cells with dimensions of 200mx200m and each cell is named. Previous 60 single point microtremor measurements, 14 single point microtremor measurements and 7 multi point microtremor measurements which are done by our research group are mapped according to their location within the working area. Microtremor records which are collected as a result of the studies, are analyzed according to H/V method by using Geopsy V2.9.1 and determined predominant period and soil amplification. Shear strain on soil surface and vulnerability indices are calculated with Nakamura Method by using these analysis's findings and previous probabilistic seismic hazard results. Multi point microtremor measurements are analyzed according to Spatial Autocorrelation method (SPAC) and Hybrid Heuristic Method and determined the shear wave velocity (Vs) change with depth and depth of seismic bedrock. The last and third stage of microzonation, determined shear wave velocity change with depth and bedrock are evaluated with 68 borehole data and created soil models for 1D soil dynamic analysis. In order to six different earthquakes records are determined by considering the fault mechanism and past earthquakes in this region. These earthquakes are 1979 Coyote Lake (0.1g), Palm Springs, 2005 Anza (0.2g), Bigbear, 2004 Parkfield (0.3g) and 1999 Kocaeli (0.4g). Soil models are analyzed according to these earthquake records by using Deepsoil V6.1 and determined peak ground acceleration (PGA), displacement and shear strain in soil layers. The predominant period and soil amplification which are obtained from analyzes of single point microtremor measurements, results that are calculated by using Nakamura Method and PGA, displacement and shear strain in soil layers which are determined by one-dimensional soil dynamic analysis are mapped by using ArcGIS that is a a geographic information system software. As a result of this study, the boreholes has about 15 ~ 35 m depths in the researh area and according to geotechnical reports and others, the soil type of region is extensively alluvial. The plain areas in region have coastal sediments and extend from the east to the west and parallel to coastline. The sediments composed of neogene aged conglomerate, sandstone, claystone, limestone and marble are located in the eastern and western directions of the region and in the southern parts of the region. To the south of the research area are volcanic units consisting of Eocene and Neogene Andesites. Triassic aged bedrock outcrops are observed east of the region. Quaternary alluviums, marsh sediments and coastal sediments composed of Neogene aged conglomerate, sandstone, claystone, limestone and marn as geology of the region. The alluviums inside of the region are generally composed of gravelly sandy clay and silty clay fine grained sediments but coastal sediments are generally composed of gravelly silty sandy material. As a result of single point measurements analysis, predominant periods soil amplification are determined range from 0.06 to 0.90 seconds and range from 2.1 to 10.7 respectively. When the distribution of the calculated Vulnerability Indices in the region is examined, these values change between 0-20 in the middle, western and eastern parts of the region. Vulnerability Indices change range from 20 to 30 in soil layers which are formed alluvium and the underlying clay and sandstone. Vulnerability Indices range from 30 to 100 in deep alluvial deposits. Shear strain values are changed as 0.00004-0.02527. As a result of multi point measurements analysis, shear wave velocity changes with depth and seismic bedrock depth are determined for each point. In the north and west parts of the region, the depth of the seismic bedrock is defined about 70-75 meters and the east and south parts of the region the depth of the seismic bedrock determined varies between 45-54 meters. As a result of 1D soil dynamic analysis, PGA, displacement, spectral soil amplification, predominant periods, spectral acceleration and shear strain are determined for research area. PGA values are changed between 0.11-0.73g for six different earthquakes. Moreover, displacements are varied between m, 0.0008 and 0.1101 m for all results of 1D analysis. Spectral soil amplification and predominant period are respectively changed from 1.16 to 4.48, betweeen 0.07-1.40 in research area. However, spectral accelerations are calculated for 0.2, 0.6 and 1.0 s in the region. As a result of calculation of shear strain values using by 1D analysis, values are varied as %0.001- 0.154 for four different accelleration of rock outcrop. As mentioned above, the level of shear deformations occurring in soil layers near the surface due to the earthquakes affect the structural damage distribution. In order to predict this effect, dynamic behavior of the ground should be determined under strong ground motion. In accordance with this objective, in the study are microtremor measurements were performed and examined borehole studies for determine soil layer properties. A result of field studies, microtremor measurements were analyzed according to the Nakamura Method and 1D Dynamic analyzes were performed. The obtained results are examined both in the map and numerical results. When the Nakamura method and 1D analysis results were considered, it was determined that similar results were obtained in the same regions. For reason of these kind of similarities, a exponential correlation was obtained shear strain values between result of microtremor measurements and 1D dynamic analyzes for aluvion soil layers. This correlation seems to be useful estimating deformations of soil layers based on microtremor measurement. Our purpose in future studies are to improve the correlation by increasing the number of data and to make it usable for practical purposes.