Presentations. Session 1. Slide 1. Earthquake Risk Reduction. 1- Concepts & Terminology

Transcription

1 Earthquake Risk Reduction Presentations Session 1 Slide 1 Earthquake Risk Reduction 1- Concepts & Terminology Welcome to the World Bank Institute s (WBI) Distance Learning (DL) course on Earthquake Risk Reduction. This presentation is the first of three PowerPoint sessions of the course. In this first presentation, we will provide an overview of the entire ERR process, and introduce key concepts and terminology. This session is followed by a session on Hazard, Vulnerability and Risk Assessment, and the third session pulls everything together, to show how to develop a Mitigation, or Earthquake Risk Reduction, program. Slide 2 Earthquakes cause death and destruction this is the fundamental problem. In order to manage this problem, we need to understand how and why earthquakes occur, and how and why they cause damage. This presentation provides an overview of earthquakes and their effects, and how the earthquake problem can be managed. The earthquake problem is managed via the Earthquake Risk Reduction process. This slide shows the biggest earthquakes of the last threedecade. Slide 3 These are some of the key words used in this presentation. All key words are defined in the Glossary provided as part of the Course. Each presentation will begin with a listing of key words. For this introductory session the key terms are: Plate Tectonics, Subduction, Fault (i.e., Earthquake Fault), Seismotectonics, Magnitude, Intensity, Vulnerability and Risk Management Slide 4 We begin with the cause of earthquakes. This is a map of the world, with earthquake activity (or seismicity) for the period (source: USGS). Each dot represents an earthquake, color-coded by depth (shown at the right, in kms) note that most are orange, meaning they are shallow earthquakes, which cause the most damage. Certain blocks of the earth, such as Africa, and the Pacific Ocean, are outlined by earthquakes, with very few earthquakes within the outline. These blocks are termed plates, and the science of studying these plates is called plate tectonics (i.e., plate structure ). The actual plates are shown more clearly on the next slide. Slide 5 This shows the plates more clearly there are large plates, such as the Eurasian, Pacific and North American Plates, and smaller plates, such as the Philippine Sea, Cocos and Nazca plates. These plates are all moving, as shown on the next slide. The arrows on each of the larger plates 1

2 show the relative motions of the various plates the Pacific Plate for example, is moving towards Japan, as is the Eurasian plate, with the Philippine Sea plate caught in between. This forces the Philippine Sea plate to slide along the Philippines, and go under Japan (i.e., is subducted under Japan). Similar situations exist elsewhere, such as for the Philippine archipelago, where the Philippine Sea Plate is subducting under the archipelago on the east, and the Eurasian Plate is subducting under the archipelago on the west. In fact, the Philippines is on a wedge of crust Along the west coast of South America, the Nazca Plate is being forced under the South American Plate, whereas along the west coast of North American, the relative motion of the Pacific and North American Plates is such that the Pacific Plate is generally sliding sideways along the North American Plate. (More detailed information on Plate motions can be found at website) Slide 6 This slide shows the entire plate tectonic process, including how one plate goes under, or is subducted under, another plate. Heat in the center of the earth causes molten lava to rise along a mid-ocean trench. When the lava reaches the surface, it cools, solidifies, and becomes crust material. However, more lava rising beneath, forces the lave to the sides. Over millions of years, the lava moves across the surface of the earth, until it meets another plate, at a plate boundary. At that boundary, the two plates may slide past one another, or one plate may be forced under (be subducted) the other plate. The sliding and subduction of the plates is not smooth as they move, their bumping and grinding causes vibrations to be transmitted within the earth. We feel these vibrations, and call them EARTHQUAKES. Earthquakes occur with the Plate (Intraslab) as it bends under the other Plate; even large earthquakes occur due to the sliding of one Plate past another (Interslab); and smaller shallow earthquakes occur with the crust (Crustal) away from the subduction zone, since the crust there is still stressed by the subduction process. This is the cause of most earthquakes the process is termed SEISMOGENESIS (from seismo, meaning earthquake, and genesis, meaning origin). Slide 7 There are a number of effects of earthquakes, all of which can cause damage these are termed agents of damage. The initial agent of damage is the actual displacement of the plates, along a fault. A fault may be the actual plate boundary, or other cracks in the earth, more or less subsidiary to a plate boundary. When an earthquake occurs, the plates move, and if you have one leg on each side of the fault, your legs are moved relative to each other, just as if you put only one foot on an escalator. If you don t do something you, or the building on the fault, will fall down, or collapse. Fault movement under the ocean floor can displace cubic km of water, causing a massive wave, termed a Tsunami, to occur. The Dec. 26, 2004 Indian Ocean tsunami killed more than 250,000 persons. After faulting, the primary earthquake agent of damage is shaking, which is the vibration in the earth s crust due to the fault movement. Other agents of damage include liquefaction (soft loose soil when shaken becomes like a liquid), fire following earthquake, and landsliding. Slide 8 Measuring earthquakes is a first necessary step in understanding them. A number of methods for measuring earthquakes have been developed, but the most common measure today is the Moment Magnitude scale, denoted Mw. Magnitude refers to the size of an earthquake, and is computed from the total energy of the event. On Moment Magnitude scale, a Mw 6 is a damaging earthquake, Mw 7 is a major earthquake, and Mw 8 a potentially catastrophic earthquake. The Dec. 26, 2004 Indian Ocean earthquake was approximately Mw 9, and the third largest 2

3 earthquake to occur since By the way, the moment magnitude scale is different from the Richter scale. Charles Richter defined the concept of earthquake magnitude in 1935, but now the moment magnitude scale is generally preferred over the Richter or other magnitude scales. Slide 9 While magnitude is a measure of the overall size of an earthquake, any one earthquake will have effects at many locations virtually everywhere. The effects at each location are measured on the Intensity scale there have been many intensity scales developed the most common is the Modified Mercalli Intensity (MMI) scale, but various scales are used in various countries. Slide 10 This shows the PHIVOLCS Earthquake Intensity Scale (PEIS), used to measure seismic intensity in the Philippines. It is similar to the MMI scale, but modified somewhat for Philippine-specific conditions. Note that Roman Numerals are used. Intensity scales like MMI, MSK etc are not instrumentally measured, but rather are estimated based on damage, human reactions and other indicators. Roman rather than Arabic numbers are therefore used, to indicate their subjective nature. MMI is used in the USA and many other countries. Other scales include the Ross-Forel scale, used in Italy and sometimes in the USA; MSK (Medvedev-Sponheuer-Karnik), developed in the Soviet Union, and used in Europe and some other regions; and JMA, only used in Japan, or by Japanese investigators researching earthquakes overseas. Slide 11 The slide shows the MMI scale and as was mentioned Roman Numerals are used to describe the intensity of the earthquakes. The intensity is subjectively measured by damages caused and human reactions, etc. For example, earthquake intensity V on MMI scale is when almost everyone feels the movement, sleeping people are awakened, doors swing open or close, dishes are broken, pictures on the wall move, small objects move or are turned over, trees might shake, liquids might spill out of open containers. Slide 12 Buildings and other structures are vulnerable to earthquakes and measuring the vulnerability of structures is a key step towards managing their vulnerability. Note that vulnerability is used here in a technical sense. The term vulnerability has several meanings in natural hazards, and is sometimes used to refer to the social vulnerability of a population (rather than the physical vulnerability of a structure), and is sometimes used to refer to the overall set of characteristics of a population or community, that make them vulnerable. Slide 13 In order to measure seismic vulnerability, let s first define this term: Seismic Vulnerability is the degree of damage or loss caused by a given level of seismic intensity. Seismic vulnerability depends on the materials, age, condition and structural layout of a building or other structure. Weak brittle materials, such as adobe, unreinforced masonry, and older reinforced concrete buildings, are very likely to be damaged in an earthquake they have high vulnerability. 3

4 Steel, wood and newer reinforced concrete buildings are less likely to be damaged in an earthquake they have low vulnerability. Slide 14 Quantifying, or measuring, seismic vulnerability, can be done in several ways basically, analytically, or statistically. Analytical models involve development of detailed engineering analysis models of a structure, based on detailed data for that structure, which is compiled in equations, analyzed in a computer and presented in terms of displacements as a function of accelerations (Sa). This work needs to be done by an engineer it is rather time-consuming and expensive, but is very accurate. It is justified when the structure is important, such as a school, city hall, high-rise building, dam, powerplant, etc. The other approach is more general, and involves collecting damage data from several earthquakes, for similar types of buildings, such as houses, and analyzing these statistics to develop correlations of the damage. This approach is less expensive, but is rather general, and provides only average damageability or vulnerability information for a specific building, based on its structural type. The two approaches are analogous to a detailed physical exam by a doctor, versus determining a person s likelihood of a heart attack based only on age and weight. Slide 15 Example of seismic vulnerability of three general building types URM is unreinforced masonry, which will have on average about 50% damage at MMI X; RC Shear Wall is a reinforced concrete building with shear walls, which will have on average 30% damage at MMI X, and Wood Frame is a typical stud wall house, which will on average have about 20% damage at MMI X. Sources of information on structural vulnerability include the literature (e.g., the ATC-13 and HAZUS publications), empirical data, and detailed structural evaluations. Structural engineers determine structural vulnerability. Slide 16 Risk or Loss estimation is the quantification of the loss in value of assets as a result of an earthquake. The estimation of potential loss is a basic first step in managing earthquake risk. It begins with the identification of ASSETS at risk these assets may be people, property, profits or anything else of value. DAMAGE is the physical degradation of these assets (e.g., cracks in the wall). LOSS is the reduction in value of these assets, and might be quantified in terms of the cost of repairs. RISK is the uncertainty in the value of that loss the loss might be small, it might be large, one does not immediately really know which it will be. RISK or LOSS ESTIMATION is the quantification of earthquake loss. Please note that the terms damage and loss are used differently than in other courses of the program (in other courses damage is the impact on stock, or assets and loss is the impact on flows) Slide 17 Once earthquake Risk (i.e., potential Loss) has been estimated, it can be determined if the potential loss is Acceptable, or Not. The determination of Acceptable Risk is complex, and usually decided by a group composed of those who are responsible for the buildings or other structures (ie, the owners ), and those who occupy those buildings or other structures (ie, the risk-bearers ). A risk that is usually NOT acceptable is the probable collapse of buildings in an earthquake. If Not Acceptable, then the risk must be reduced, or mitigated. Since loss is the product of asset value times hazard times vulnerability, each of these factors provides an opportunity to reduce potential loss. That is, breaking the chain of loss causation reduces risk. 4

5 Slide 18 Earthquake Risk Reduction is the process of reducing earthquake risk. The process consists of an initial assessment of the problem, including identifying the assets at risk; more detailed analysis of the risk (i.e., the expected loss); development of a mitigation program; implementation of the program; and maintenance of the program. Each of these aspects is covered in more detail in the next Sessions. Slide 19 The goal of Earthquake Risk Reduction is not to find A SOLUTION, but rather to find the best solution. Best implies decision-making. Decision-making consists of two basic steps: quantifying or Estimating the Risk, and Examining Mitigation Alternatives. If the Risk is not acceptable, and the cost of mitigating the risk is less than the potential Loss, then mitigation is beneficial, and warranted. 5