Lecture-06: Disasters of Bangladesh: Earthquakes

Transcription

1 URP 4243: Hazards and Disaster Management Lecture-06: Disasters of Bangladesh: Earthquakes 50 1 Course Teacher: Md. Esraz-Ul-Zannat Assistant Professor Dept. of URP, KUET January 18, 2018

2 ACKNOWLEDGEMENT These slides are aggregations for better understanding of the topic mentioned in the previous slide. I acknowledge the contribution of all the authors and photographers from where I tried to accumulate the info and used for better presentation. 2

3 TOPICS TO BE COVERED BY THIS PRESENTATION Earthquakes The Focus and Epicenter of an Earthquake Elastic Rebound Theory? Where Do Earthquakes Occur and How Often? Impacts of Earthquakes Can Earthquakes be Predicted? Can Earthquakes be Controlled? Seismic Waves Body Waves Surface Waves Factors contributing to earthquake hazard Ground Shaking Earthquake Scales Surface Faulting Tectonic Uplift and Subsidence Faulting 3

4 TOPICS TO BE COVERED BY THIS PRESENTATION Factors contributing to earthquake hazard Landslides Liquefaction EQ Information for Planning EQ Assessment and Mapping List of Major Earthquakes Affecting Bangladesh Recent Earthquakes in Bangladesh Historical Seismicity and Plate Boundary Regional Potential Fault Lines Seismic Zoning Map BNBC 1993 Proposed Seismic Zoning Map For Bangladesh (BNBC 2014) Institutional Framework Organizational Structure for DRR Recent Developments Concluding Remarks 4

5 EARTHQUAKES Sudden motion or trembling of the ground, produced by abrupt displacement of rock masses. Usually within the km of the Earth s crust When the strain of forces pulling or pushing adjacent areas of rock exceed the strength of the rock, the rock break along pre-existing or new fracture plane called a fault The rupture extends outwards along the fault plane from its point of origin, or focus 5

6 EARTHQUAKES The epicentre of the earthquake is the point on the Earth s surface directly above the focus During the rupture the sides of the fault rub against each other creating tremors The seismic energy is emitted from the rupture as seismic waves 6

7 WHERE DO EARTHQUAKES OCCUR AND HOW OFTEN? ~80% of all earthquakes occur in the circum-pacific belt most of these result from convergent margin activity ~15% occur in the Mediterranean-Asiatic belt remaining 5% occur in the interiors of plates and on spreading ridge centers more than 150,000 quakes strong enough to be felt are recorded each year 9

8 IMPACTS OF EARTHQUAKES Collapse of Buildings and other structures Fire Ground failure, landslide, liquefaction Tsunami 10

9 SEISMIC WAVES Response of material to the arrival of energy fronts released by rupture Body Waves Travel through the earth's inner layers P and S Surface Waves Move along the surface of the Earth like ripples on water R and L 13

10 Rayleigh Waves SEISMIC WAVES Rolls along the ground just like a wave rolls across an ocean. Because it rolls, it moves the ground up and down, and side-toside in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves. Love Waves The fastest surface wave. Moves the ground from side-to-side. 15

11 SEISMIC WAVES Body and surface waves propagate in all directions away from the focus Frequency range: Hz Body waves cause high frequency (>1 Hz) vibrations which are more effective than lower frequency waves in vibrating low buildings First to strike are P waves, then S waves Surface waves strike last mainly cause low frequency vibrations which are more effective than high frequency waves in vibrating tall buildings As amplitude of low frequency waves decay less rapidly with distance, tall buildings away from the focus are more susceptible than low buildings 16

12 FACTORS CONTRIBUTING TO EARTHQUAKE HAZARD Factors contributing to earthquake hazard Ground shaking Surface faulting Tectonic uplift and subsidence Landslides Liquefaction 17

13 GROUND SHAKING Intensity and character of ground shaking depends on Earthquake source parameters (magnitude etc.) Distance from fault Surface geological material Damage potential from ground shaking depends on Amplitude of seismic waves Frequency content of seismic waves Duration of shaking Hazard Assessment must take into account these factors Estimation of ground motion Estimation of bedrock motion Estimation of response of surface geological units to bedrock motion 18

14 GROUND SHAKING Damage tends to increase with amplitude Surface geological material and structure may resonate with frequency of seismic waves, therefore, frequency content is a critical factor Failure mechanisms in structures and unconsolidated surface deposits depend on duration of shaking For seismic hazard assessment and zoning, ground shaking is characterized by, for example, three physical parameters: Maximum ground acceleration (in % of g) Maximum ground displacement Duration of shaking above a given threshold amplitude 20

15 GROUND SHAKING 21

16 EARTHQUAKE SCALES Earthquakes are described in terms of Magnitude (M) Intensity (I) 22

17 EARTHQUAKE MAGNITUDE A measure of the strength of an earthquake, or the strain energy released by it, as calculated from instrumental record on a seismograph Different scales are used ; most common is Richter scale Introduced in 1935, the Richter scale is a numerical scale for quantifying earthquake magnitude; CF Richter defined local magnitude as the log (base 10) of the maximum amplitude in micrometers of seismic waves at about 100 km from epicenter Arabic (decimal) numbers are used e.g. 6.5; open ended; maximum recorded 9.5 (Chile, 1960) Surface wave magnitude scale (Ms) measures the amplitude of surface waves with a period of 20 seconds The body wave magnitude scale (Mb) measures the amplitude of body waves with periods usually from 1 to seconds

18 EARTHQUAKE MAGNITUDE Although magnitude scale is logarithmic, the energy associated with an increase of one degree of magnitude is not 10 times, by thirty times. Earthquake magnitude is not an adequate planning/mitigation tool, unless magnitude-intensity relationship can be established 24

19 EARTHQUAKE MAGNITUDE Magnitude is a number that characterizes the relative size of earthquakes & is proportional to energy released 25

20 EARTHQUAKE INTENSITY A measure of the effects of an earthquake at a particular place produced by shaking during an earthquake. (not to be confused with Magnitude ) Same earthquake will have a single magnitude but different intensities at different places Two scales of intensity used today Modified Mercalli scale (MM) intensity scale adopted in 1931 that divides the effects of an earthquake into twelve categories, from I (not felt by people) to XII (damage total). Medvedev-Sponheur-Karnik scale (MSK) Japan uses a separate scale 26

21 1897 EARTHQUAKE DAMAGE Masonary building suffer extensive damage around 500 km away from the epicentre 27

22 SURFACE FAULTING The offset or tearing of the Earth s surface by differential movement across the fault line Differential movement can be from a fraction of an inch to tens of feet Hazard to structures built across the fault line (along a narrow strip-like zone) 28

23 TECTONIC UPLIFT AND SUBSIDENCE FAULTING Tectonic deformation of the Earth s surface Horizontal or vertical distortion within a few to a few hundred feet from fault line Usually accompanies surface faulting 29

24 LANDSLIDES Earthquakes can trigger rock falls, avalanches and landslides on steep slopes 30

25 LIQUEFACTION During strong ground motion, areas with clay-free sands and silts and ground water within 30 feet of surface can temporarily lose strength and behave as viscous fluids Structures on such soil may settle, tip or ripped apart as the ground spreads laterally Tilting of building during Nigata (Japan) Earthquake

26 EQ INFORMATION FOR PLANNING Geological maps of urban areas showing distribution of surface deposits Intensity of historical earthquakes and estimates of maximum ground accelerations, combined into single isoacceleration and seismic intensity map 32

27 EQ ASSESSMENT AND MAPPING Seismic Zoning Dividing a national territory into several areas indicating progressive levels of expected seismic intensity for different return periods Described in terms of expected intensity, PGA (Peak ground acceleration) or any other ground motion parameter 33

28 EQ ASSESSMENT AND MAPPING Seismic Micro-zoning Recording in detail all seismological, geological and hydrological parameters that may be needed in planning and implementing a given project area at an appropriate scale for physical planners Detailed mapping showing all possible EQ and EQinduced hazards 34

29 LIST OF MAJOR EARTHQUAKES AFFECTING BANGLADESH Date Name of Earthquake Magnitude (Richter) Intensity at Dhaka (EMS) Epicentral Distance from Dhaka (km) 10 January, 1869 Cachar Earthquake 7.5 V July, 1885 Bengal Earthquake 7.0 VII June, 1897 Great Indian Earthquake 8.7* VIII July, 1918 Srimongal Earthquake 2 July, 1930 Dhubri Earthquake 7.6 VI V January, 1934 Bihar-Nepal Earthquake 8.3 IV August, 1950 Assam Earthquake 8.5 IV

30 RECENT EARTHQUAKES IN BANGLADESH Date Epicentre of Earthquake Magnitude Remarks (M b ) 8 May,1997 Sylhet (Jaintiapur) November, 1997 Bandarban (Myanmar Border) people killed in Ctg 22 July, 1999 Moheshkhali people killed 19 December, 2001 Dhaka (Manikganj) injured 22 July, 2005 Rangamati (Barkal) killed 27 October, 2005 Jessore (Narail) July, 2008 Rajshahi (Tanore) July, 2008 Mymensingh (Haluaghat) December, 2009 Chittagong (Chandanaish?) September, 2010 Chandpur (Matlab Bazar) May, 2011 Comilla

31 HISTORICAL SEISMICITY AND PLATE BOUNDARY 37

32 REGIONAL POTENTIAL FAULT LINES 38

33 SEISMIC ZONING MAP BNBC

34 PROPOSED SEISMIC ZONING MAP FOR BANGLADESH (BNBC 2014) There are 4 Zones with Z = 0.36, 0.28, 0.20 and 0.12 Z represents the maximum considered earthquake (MCE) ground acceleration (g) in these zones. Probabilistically it is considered to have a return period of 2475 years i.e., 2% probability of exceedance in 50 years. The Design Basis motion is taken as 2/3 of maximum considered motion. 40

35 INSTITUTIONAL FRAMEWORK Ministry of Food and Disaster Management DM and Relief Division Disaster Management Bureau Standing Orders on Disasters (April, 2010) National Plan for Disaster Management ( ) National Disaster Management Council Inter-ministerial Disaster Management Coordination Committee National Disaster Management Advisory Committee Earthquake Preparedness and Awareness Committee National Platform for Disaster Risk Reduction 41

36 SAR (Search and Rescue) Equipment procured under Partnerships for Enhanced Engagement in Research (PEER) (US AID-funded) SAR Equipment procured by GoB ( Tk. 590 million +1,640 million) Army and FSCD (Bangladesh Fire Service and Civil Defense) responsible for maintenance and training 62,000 volunteers to be trained Training for doctors and nurses Training for government officials Training for Engineers, Building Professionals ( by BES-ESC, IEB), Masons (CDMP) Masters Programmes in Disaster Management BRAC University Dhaka University RECENT DEVELOPMENTS 43

37 RECENT DEVELOPMENTS Research : BUET, DU, CUET, SUST, UAP Joint Workshops with Developed Countries (e.g. US Armed Forces) CDMP Phase 2 : should be utilized properly World Bank EMI Project (Urban Resilience) JICA-PWD Retrofitting Project 44

38 CONCLUDING REMARKS Considerable seismic hazard exists for major parts of the country About 26 million people, representing one sixth of the current total population, live in Zone 3, i.e. areas which may be classified as "liable to severe damage ; (viz. Sylhet, Mymensing, Rangpur) another 78 million (i.e. approx. 50% of the population) live in Zone 2, i.e. areas "liable to moderate damage". (Dhaka, Chittagong) Bangladesh National Building Code(BNBC93) officially adopted in November, 2006; needs to be enforced Considerable seismic hazard exists for Dhaka city lack of enforcement of building code and lack of preparedness means that Dhaka is among the most vulnerable megacities in 45 the world

39 CONCLUDING REMARKS Recent initiatives by Government, City Corporation, academia and civil society organizations should lead to considerable reduction in vulnerability Earthquake Management Plans must be prepared for Dhaka City, Chittagong City, Sylhet, Mymensing and Rangpur towns Initial studies have been done under CDMP (phase 1); reports should be made available to public (online) EPP (Emergency Preparedness and Planning) volunteers must be trained in the vulnerable areas Existing weak buildings must be identified and retrofitted/replaced in phases 46

40 Earthquakes WHAT WE HAVE COVERED TODAY The Focus and Epicenter of an Earthquake Elastic Rebound Theory? Where Do Earthquakes Occur and How Often? Impacts of Earthquakes Can Earthquakes be Predicted? Can Earthquakes be Controlled? Seismic Waves Body Waves Surface Waves Factors contributing to earthquake hazard Ground Shaking Earthquake Scales Surface Faulting Tectonic Uplift and Subsidence Faulting 47

41 Factors contributing to earthquake hazard Landslides Liquefaction WHAT WE HAVE COVERED TODAY EQ Information for Planning EQ Assessment and Mapping List of Major Earthquakes Affecting Bangladesh Recent Earthquakes in Bangladesh Historical Seismicity and Plate Boundary Regional Potential Fault Lines Seismic Zoning Map BNBC 1993 Proposed Seismic Zoning Map For Bangladesh (BNBC 2014) Institutional Framework Organizational Structure for DRR Recent Developments Concluding Remarks 48

42 WHAT WE LEARNT Understanding of the concept and issues of Earthquakes, and its associated issues in Bangladesh. 49

43 What Next? Lecture 7: Technology for Disaster Reduction 50

44 Sample Questions 1. What do you understand by the Focus and Epicenter of an Earthquake? 2. Can Earthquakes be predicted? Explain. 3. Differentiate between body waves and surface waves. 4. Write down the factors contributing to earthquake hazard. 5. Define Earthquake Magnitude and intensity. How magnitude and intensity is measured? 51