CHAPTER 3 METHODOLOGY

Transcription

1 32 CHAPTER 3 METHODOLOGY 3.1 GENERAL In 1910, the seismological society of America identified the three groups of earthquake problems, the associated ground motions and the effect on structures. Indeed these are still the fundamental elements in evaluated earthquake risks. Reducing these risks requires absolutely knowledge, planning and resource: earthquakes are the only remaining type of great, naturally occurring catastrophe that can rarely be predicted. Resources for earthquake safety compete with resource for reducing risk from the hazards and decision on reducing earthquake risk are made in the context of the size and likelihood of that risk. Hence, seismic risk analysis is important (Robin Mc Guire 2004). The Earthquake damage basically depends on three groups of factors: earthquake source and path characteristics, local geological and geographical site conditions, structural design and construction features. Seismic hazard analysis addresses the assessment of the first two groups of factors. Hazard analysis is the initial base of earthquake risk mitigation and requires multidisciplinary approach with major contributions from geology, seismology, geotechnical and structural engineering (Ansal 2004). An Earthquake study falls into the category of applied research and need information. It requires more information regarding site specific geological condition, ground response to earthquake motions and their effects on the safety of the constructions taking into the consideration of desire aspects of

2 33 buildings (Anbazhagan et al 2009). The following are three major events involved in this study. Evaluation of expected input motion Local site effects and ground response analysis Preparation of hazard maps The maps prepared with scale of 1:50000 and incorporated ground motion are gained from historical earthquakes and excising information of geological and geomorphological map. 3.2 FLOW CHART FOR SEISMIC HAZARD ANALYSIS Even though Seismic Hazard Analyses is grouped into three major categories, it needs to adopt step by step procedure to arrive at final map of Hazard Index (HI). The steps followed in seismic hazard analysis for Coimbatore corporation in the present study is bear and show the form of flowchart in Figure 3.1. The first step illustrates the assessment of expected ground motion in deterministic seismic hazard analysis. The second step explains about the site characterized for the study area at local scale of 1:50000 using geotechnical and shallow subsurface geophysical data. Third step tells about the local site effects using first and second part outputs and producing the ground level hazard parameters. As the site comes under class D (as per NEHRP recommendation and SPT N value is more than 50), the assessment of liquefaction effects is not to be considered. Finally, the hazard index maps are prepared in terms of ground motion parameters using the GIS platform.

3 34 Figure 3.1 Flow chart showing the methodology adopted Methodology Adopted for Deterministic Seismic Hazard Analysis Deterministic Seismic Hazard Analysis (DSHA) identified the particular seismic scenario upon which a ground motion hazard evaluation is based. The scenario consists of the postulated occurrence of an earthquake of specified size at a specific location (Kramer 2007). A typical DSHA described as four step process (Reiter 1990) and illustrated in Figure 3.2.

4 35 Source characterization, which includes identification and characterization of all the earthquake sources which may cause significant ground motion in the study area. The shortest distance selected between the source and the site of interest. Selection of controlling earthquake i.e. the earthquake that is expected to produce the strongest level of shaking. It is described in terms of its size expressed in magnitude and distance from the site. Defining the hazard at the site formally in terms of the ground motions produced at the site by controlling earthquake. Figure 3.2 Different steps for deterministic seismic hazard analysis (after Kramer 1996)

5 DATA COLLECTION Regional geological and seismological details for the Coimbatore corporation were collected from the literature review, study of maps and remote sensing data. Geology of the study area presented in the Seismotectonic Atlas of India (SEISAT, 2000) prepared by Geological Survey of India, (Map number - 33, 34, 38, 39 and 42). The study area with radius of about 350km around Coimbatore corporation has been selected for the seismicity study as per regulatory guide (1997). The seismological study area has the centre point of PSG College of Technology, Coimbatore (11º N and 77º E) with a circular area of 350km radius (which covers the latitude of N to N and longitude of E to E). From this seismotectonic atlas, 51 faults, 4 shear zones and 288 historical earthquake events (recent earthquake data from USGS website) are identified around 350km radius from the center of study area. Topography sheets on 1:50000 scales were collected from the Survey of India and used for the study. Ground water details and lithological data, rainfall data, geological data, geomorphologic data, physiographic, soil types of the study area are obtained from State Ground and Surface Water Resources Data Centre of PWD, Chennai and Central Ground Water Board, Chennai. Building assessment data and population data are gathered from Coimbatore Corporation. Geotechnical data is collected from the published literatures, reputed educational institutions around the study area, geological department- Coimbatore and highways department- Coimbatore. Field investigation is carried out by using electrical resistivity test conducted on 25 locations spreading around the study area of sq.km.

6 DETERMINISTIC SEISMIC HAZARD ANALYSIS Seismic hazard analysis involves the quantative estimation of ground shaking hazard for a particular place, when a particular earthquake scenario is assumed. (Kramer 1996) Anbazhagan (2009) used geology and seismic history to identify the earthquake source in deterministic seismic hazard analysis. The largest earthquake that can be expected is called Maximum Credible Earthquake (MCE). In order to avoid the surprises, all the structures should be designed for maximum credible earthquake. The identification of controlling parameter of earthquake is crucial part of the earthquake hazard analysis. Seismogenic source is identified to calculate peak ground acceleration and develop response acceleration for an area. The design of civil engineering structures essentially need for spectral acceleration of all possible sources of seismic activity and must be taken into account while evaluating seismic hazards for a particular place. One can understand the seismotectonic activity of the region with the help of identified lineaments and faults within the radius of 350kms. The maximum credible earthquake has been calculated using the correlation given by Iyengar and Raghukanth (2004). Lineaments, faults, shear zone (seismic sources) and earthquake events will decide the recent seismic activity of the study area. By considering the seismic source and earthquake events a new seismotectonic map has been prepared. The peak ground acceleration from all seismogenic sources has been calculated to identify the maximum credible earthquake and scenario earthquake.

7 SITE CHARATERIZATION A complete site characterization is essential for the seismic site classification and site response studies, because both of them together can be used for seismic hazard analyze. Site characterization provides data on the following: Site description and location Geotechnical data Soil conditions Geological data Ground water level data Geomorphology Demographic data In respect of the site characterization, experimental data should be collected, interpolated and represented in the form of maps. The site characterize is carried out for the Coimbatore corporation using Geotechnical and geophysical experimental data. About 143 collected geotechnical borehole information with standard penetration test SPT N values data and 25 geophysical data of electrical resistivity field test results are used for site characterization and classification. 3.6 SITE CLASSIFICATION NEHRP recommends provisions for the seismic regulations according to site class (A to E) defined for similar seismic response. Average shear wave velocity (V s 30) is accepted for site classification as per NEHRP classifications and UBC (Uniform Building Code 1997, Anbazhagan 2007).

8 39 The site classes A and B are assigned to hard rock and rock site conditions with Vs 30 > 1500 ms -1, and within ms -1, respectively while site class C is designated to soft rock, hard or very stiff soils or gravels exhibiting Vs30 in the range of ms -1, and stiff soil in the range of ms -1 designated to be in site class D. the above site classifications are shown in Table 3.1. However, site class E is implicated to a soil profile with more than 3m soft clay (Seismic Microzonation Handbook, New Delhi 2011). The rock depth/soil overburden thickness distribution map is generated based on the bore holes close to the electrical resistivity test locations. The calculated average shear wave velocities are grouped according to the NEHRP site classes and map was generated. Table 3.1 Site classification table Site class Range of average shear wave velocity (m/s) A hard rock 1500 < V S 30 B rock 760 < V S C very dense soil and soft rock 360 < V S D stiff soil SPT-N < < V S E soft soil SPT-N <15 V S 30 < 180 Site classification plays a vital role in the construction of spectral acceleration, which will be directly useful to the field engineers, architects and government officials. Local soil profile reflects through a different design spectrum for rock, medium and soft soil. Based on IS 1893:2002, the soil is also classified in to three types, such as rock or hard soils, medium soils and soft soils.

9 SITE RESPONSE The site response study of Coimbatore corporation has been carried by using predominant frequency. Site response analysis aims to determining the response of a soil deposit subjected to the motion of the bedrock beneath it. The overburden plays an important role in determining the characteristics of the ground surface motion. Thus, emphasizing the need for ground response analysis. A number of techniques have been developed for ground response analysis. Site response studies need to determine the frequency of soft soil, amplification which play a vital role to evaluating and characterizing the ground motion for seismic hazard quantification. The peak surface acceleration, ground response spectrum and period of soil column are obtained as outputs from this analysis and these values are used to produce maps indicating zones of amplification potential, spectral acceleration various frequencies and period of soil column for Coimbatore Corporation. Subsurface explorations were conducted in the area to determine the geotechnical properties of strata and to find out the site response characteristic of the ground concerning to expected ground motion due to any earthquake event (Kandpal et al 2007). The SPT N values were converted in to shear wave velocity by following empirical relationship for all soils category (Uma Maheshwari et al 2010). Predominant frequency map for our study area for 30m soil column was prepared using the simple relationship between the shear wave velocity () of the sediment column 30m viz., f = /4H. 3.8 DATA INTERPRETATION USING GIS An integration of all the derived maps are made on the basis of weights and ranks. A final hazard index map for the study area is obtained

10 41 through Analytical Hierarchy Process (AHP) and intergrating the results with GIS (Geographical Information System). Application of GIS for seismic hazard analysis is amply demonstrated by many researchers all over the world. Nath (2004) used GIS as integration tool to map seismic ground motion hazard for Sikkim Himalaya in India. In this study, similar approach of Nath (2004) is followed to develop a hazard index map wherein the seismic hazard parameters are integrated and coupled with ground information. 3.9 WEIGHTED OVERLAY ANALYSIS Many layers of thematic spatial data with attributes data is required to confirm multi hazard assessment. Including both analysis and map of seismic hazard, effectiveness of GIS output can be drawn on the basics of the quality and availability of the relevant data. GIS is certainly an effective tool which creates hazard maps by way of using various inputs together. Overlay operation creates separate natural hazard maps with integration of the various input parameters PREPARATION OF HAZARD ZONATION MAP A frequently used method for assessing and mapping earthquake hazard is seismic hazard zonation map in which area with same hazard level are mapped and classified. Two types of seismic hazard zonation exist: Rough method of microzonation and mapping hazard on small scale. Microzonation allows a local geological and site condition. Hazard index which is an integrated factor depends on weights and ranks of the seismological and geomorphological themes. Theme weight can be assigned based on their rate of contribution to the seismic hazard. Rank can be assigned with in theme based on their values closer to hazards.