Geotechnical Earthquake Engineering

Transcription

1 Geotechnical Earthquake Engineering by Dr. Deepankar Choudhury Humboldt Fellow, JSPS Fellow, BOYSCAST Fellow Professor Department of Civil Engineering IIT Bombay, Powai, Mumbai , India. URL: Lecture 24

2 Module 7 Seismic Hazard Analysis IIT Bombay, DC 2

3 Magnitude indicators M = f(e) E should increase with increasing dimensions of rupture surface Where, M w = earthquake moment magnitude, A = rupture area in km 2 IIT Bombay, DC 3

4 Empirical relationships between Moment magnitude (M w ), Surface rupture length (L in km), Rupture area (A in km 2 ) and Maximum surface displacement (D in m) Reference : Kramer (1996) IIT Bombay, DC 4

5 Tectonic Evidence M w = T V (cm/year) Heaton and Kanamori (1984) IIT Bombay, DC 5

6 Segmentation Segment length (or area) can constrain magnitude Segments bounded by discontinuities Geometric discontinuities - abrupt changes in strike, stepovers, gaps Structural discontinuities - fault bifurcations, zones of increased structural complexity, intersections with other structures Behavioral discontinuities - changes in slip rates, senses of displacement, creeping vs. locked behavior IIT Bombay, DC 6

7 Earliest approach taken to seismic hazard analysis Originated in nuclear power industry applications Still used for some significant structures Nuclear power plants Large dams Large bridges Hazardous waste containment facilities As cap for probabilistic analyses IIT Bombay, DC 7

8 Corps of Engineers Regulation (1995), Sec. 5.h.2.a Deterministic seismic hazard analysis (DSHA). The DSHA approach uses the known seismic sources sufficiently near the site and available historical seismic and geological data to generate discrete, single-valued events or models of ground motion at the site. Typically one or more earthquakes are specified by magnitude and location with respect to the site. Usually the earthquakes are assumed to occur on the portion of the site closest to the source. The site ground motions are estimated deterministically, given the magnitude, source-to-site distance, and site condition. IIT Bombay, DC 8

9 Consists of four primary steps: 1. Identification and characterization of all sources 2. Selection of source-site distance parameter 3. Selection of controlling earthquake 4. Definition of hazard using controlling earthquake IIT Bombay, DC 9

10 DSHA: methodology IIT Bombay, DC 10

11 Identification and characterization of all sources Identification All sources capable of producing significant ground motion at the site Large sources at long distances Small sources at short distances Characterization Definition of source geometry Establishment of earthquake potential IIT Bombay, DC 11

12 Identification and characterization of all sources Establishment of earthquake potential To be estimated by seismologists, geologists, engineers, risk analysts, economists, social scientists and government officials together. Terms use to describe earthquake potential are, Maximum credible earthquake (MCE) Design basis earthquake (DBE) Safe shutdown earthquake (SSE) Maximum probable earthquake (MPE) Operating basis earthquake (OBE) Seismic safety evaluation earthquake (SSEE) IIT Bombay, DC 12

13 Maximum credible earthquake (MCE) is the maximum earthquake that appears capable of occurring under the known tectonic framework. Maximum probable earthquake (MPE) is the maximum historical earthquake and the maximum earthquake likely to occur in a 100-year interval. Many DSHA used both the MCE and MPE for evaluating hazards. IIT Bombay, DC 13

14 Identification and characterization of all sources Which sources are capable of producing significant motion at the site of interest? What is significant motion? Parametric definition Peak acceleration Spectral acceleration - at fundamental period, if known Other parameters Use predictive (attenuation) relationship to determine distance of interest IIT Bombay, DC 14

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18 Establish earthquake magnitude - typically M max Empirical correlations Rupture length correlations Rupture area correlations Maximum surface displacement correlations Theoretical determination Slip rate correlations IIT Bombay, DC 18

19 Slip rate approach Recall seismic moment Mo = μ A D where μ = shear modulus of rock A = rupture area D = average displacement over rupture area IIT Bombay, DC 19

20 Slip rate approach If average displacement relieves stress/strain built up by movement of the plates over some period, T, then D = S x T where S is the slip rate IIT Bombay, DC 20

21 Slip rate approach Then M o = μ A S T and the moment rate can be defined. as M o = M o /T = μ A S IIT Bombay, DC 21

22 Slip rate approach Knowing the slip rate and knowing (assuming) values of, A, and T, the moment rate can be used to estimate the seismic moment as, M o = M o T Then,. M w = log M o / IIT Bombay, DC 22

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28 Select controlling earthquake Decision based on ground motion parameters of greatest interest Consider all sources Assume M max occurs at R min for each source Compute ground motion parameters based on M max and R min Determine critical values of ground motion parameters IIT Bombay, DC 28

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32 Comments DSHA produces scenario earthquake for design (design earthquake) As commonly used, produces worst-case scenario DSHA provides no indication of how likely design earthquake is to occur during life of structure Design earthquakes may occur every 200 yrs in some places, every 10,000 yrs in others DSHA can require subjective opinions on some input parameters Variability in effects not rationally accounted for IIT Bombay, DC 32

33 DSHA calculations are relatively simple, but implementation of procedure in practice involves numerous difficult judgments. The lack of explicit consideration of uncertainties should not be taken to imply that those uncertainties do not exist. IIT Bombay, DC 33

34 Example Problem IIT Bombay, DC 34

35 A site for proposed construction of a nuclear power plant is shown in the figure with coordinate as (0,0). In local vicinity, three independent seismic sources were identified as source 1, 2 and 3 respectively with relevant input data like maximum earthquake magnitude and coordinates of various relevant points as shown in the figure. Using DSHA, compute the design value of PHV for the proposed site using Joyner and Boore (1988) attenuation relationship. Assume the shortest distance for the larger component from the site to the vertical projection of the earthquake fault on the surface of earth is 20 km. IIT Bombay, DC 35