Earthquakefinalnaata.docx

DEFINE RISK REDUCTION Generally speaking, risk reduction is the process of minimizing or mitigating the risk. This can be personal or property risk. It starts with the identification and evaluation of risk followed by optimal use of resources to monitor and minimize the risk.

Some types of risk: 1. Health and safety risk - The potential for harm to come to people. 2. Infrastructure risk - The risk that basic services such as an hospitals of schools will fail. 3. Environmental risk - the potential of a natural caused disaster to damage assets and cause casualties.

Conceptual Diagram of Risk Reduction: RISK IDENTIFICATION

RISK ANALYSIS

JUDGEMENT OF ADOPTATION, PRIOTIZATION

PROPOSAL OF IMPROVEMENT MEASURES

IMPLEMENTATION OF IMPROVMENT MEASURES

RISK REDUCTION

ELEMENTS OF RISK:

RISK

1. Hazard - A dangerous phenomenon, substance, human activity or condition that may cause loss of life, injury or other health impacts, property damage, loss of livelihoods and services, social and economic disruption, or environmental damage 2. Exposure - People, property, systems, or other elements present in hazard zones that are thereby subject to potential losses. **Measures of exposure can include the number of people or types of assets in an area. These can be combined with the specific vulnerability of the exposed elements to any particular hazard to estimate the quantitative risks associated with that hazard in the area of interest. 3. Location - A particular geographical location, independent of the nature of the population. 4. Vulnerability - The characteristics and circumstances of a community, system or asset that make it susceptible to the damaging effects of a hazard.

Classification of Natural Hazards: 1. Hydro-meteorological Hazards - weather related hazards such as storms and droughts 2. Geological Hazards - is an extreme natural events in the crust of the earth that pose a threat to life and property, for example, earthquakes, volcanic eruptions, tsunamis (tidal waves) and landslides. 2.1. Earthquake - The sudden slip on a fault and the resulting ground shaking and radiated seismic energy caused by the slip, or by volcanic or magmatic activity, or other sudden stress changes in the earth. • •

Earthquakes may be highly destructive natural hazards that may occur at any time with practically no warning. They may have sudden impact causing the destruction of buildings and infrastructure in seconds, killing or injuring people.

WHAT IS DISASTER RISK REDUCTION? Disaster Risk Reduction aims to reduce the damage caused by natural hazards like earthquakes, floods, droughts and cyclones, through an ethic of prevention. Disaster risk reduction is the concept and practice of reducing disaster risks through systematic efforts to analyze and reduce the causal factors of disasters. Reducing exposure to hazards, lessening vulnerability of people and property, wise management of land and the environment, and improving preparedness and early warning for adverse events are all examples of disaster risk reduction. (UNISDR) It is expressed as the probability of loss of life, injuries, property damage, loss of livelihood, disruption of economic activity, or environmental damage. Additionally, risk is composed of two factors: Hazard and vulnerability.

R = f (H, V)

The equation implies that risk is a function of the hazard and the level of vulnerability, and it is directly proportional to both of these factors.

WHAT IS ACCEPTABLE RISK? According to International Strategy for Disaster Reduction, acceptable risk is the probability of harmful consequences or expected losses (deaths, injuries, property, livelihoods, economic activity distributed or environment damage) resulting from interactions between natural or human-induced hazards and vulnerable conditions. Under the caption "The Concept of Safety" (Section 5), this appears in ISO/IEC Guide 51: Safety Aspects &endash; Guidelines for its inclusion in standards:

"There can be no absolute safety: some risk will remain, defined in this Guide as residual risk. Therefore, a product, process or service can only be relatively safe.

"Safety is achieved by reducing risk to a tolerable level, defined in this Guide as tolerable risk."

One of the most significant and influential publications on the concept of acceptable risk is Of Acceptable Risk: Science and the Determination of Safety by William W. Lowrance. He wrote:

"Nothing can be absolutely free of risk. One can't think of anything that isn't, under some circumstances, able to cause harm. Because nothing can be absolutely free of risk, nothing can be said to be absolutely safe. There are degrees of risk, and consequently there are degrees of safety.".

A FRAMEWORK FOR ACCEPTABLE RISK

If the risk for a task or operation is never zero, for what risk level does one strive? An additional excerpt from ISO/IEC Guide 51, Section 5, helps in understanding the process:

"Tolerable risk is determined by the search for an optimal balance between the ideal of absolute safety and the demands to be met by a product, process or service,

and factors such as benefit to the user, suitability for purpose, cost effectiveness and conventions of the society concerned."

Based on a study of the concept of acceptable risk, the following are observed:

1. Safety practitioners should accept that zero risk is not attainable for hazards that cannot be eliminated. 2. Where hazards cannot be eliminated, the goal should be to reduce risks so that the residual risks are acceptable. 3. Safety practitioners should debate and consider accepting the proposed definitions for terms defined herein. 4. Risk assessments and the risk decision process should become more structured and documented in accordance with recent guidelines such as ANSI B11.TR3 2000, SEMI S10-1296 and ANSI/RIA R15.06-1999. This process will advance the understanding and acceptance of the concept of acceptable risk and of residual risks. 5. Safety practitioners should recognize that a universal definition of an acceptable risk level cannot be attained because of the many variables in individual risk situations.

EXAMPLES OF ACCEPTABLE RISK: 1. 2. 3. 4. 5.

Structures that can be damaged due to flooding potential loss of school days due to flooding loss of crops due to volcanic eruption coral reef damage due to storm surges psychological damage to citizens

SEISMIC HAZARD Seismic hazard is the probability that an earthquake will occur in a given geographic area, within a given window of time, and with ground motion intensity exceeding a given threshold

It is defined as any physical phenomenon, such as ground shaking or ground failure, which is associated with an earthquake and that, may produce adverse effects on human activities.

SEISMIC HAZARD ANALYSIS It refers to the estimation of earthquake-induced ground motions having specific probabilities over a given time period.

Representation of Seismic Hazard 

The seismic hazard can be expressed in different ways: from simple observed macroseismic fields to seismostatistical calculations for analysing earthquake occurrences in time and space and assessing their dynamic affects in a certain site or region, to sophisticated seismogeological approaches for evaluating the maximum expected earthquake effects on the Earth surface.

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Representation of Seismic hazard ground motion includes 1. The selection and utilization of natural ground maps. 2. The representation of site response effects; and 3. The possible incorporation of other parameters and effects , including energy or duration of ground motions, vertical ground motions , near source horizontal ground motions , and spatial variations of ground motions.

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Seismic hazard can be represented in different ways but most frequently in terms of values or probability distribution of accelerations, velocities, or displacements of either bedrock or ground surface. 1. The peak ground acceleration, ground acceleration time history or response spectral acceleration are useful because the product of a mass and the acting acceleration equals the magnitude of inertial force acting on the mass 2. The peak ground velocity, ground velocity time history or response spectral velocity are useful because the product of square of velocity and a half of mass equals the amount of kinetic energy of the mass 3. The peak ground displacement, ground displacement time history or response spectral displacements of a structure are useful since damage of structures subjected to earthquakes is certainly an expressed in deformations (e.g. Bommer and Elnaashai,1999 )

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Time histories of ground motion are often used in practice for non linear analyses when damage caused by ground shaking can accumulate in time.

Two Measures of Ground Motion Probabilistic (Maximum Considered Earthquake)  

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Is simpler and used in standard building codes The term is used specifically for general building codes, which people commonly occupy; building codes in many localities will require non-essential buildings to be designed for "collapse prevention" in an MCE, so that the building remains standing allowing for safety and escape of occupants - rather than full structural survival of the building. Where an earthquake is expected to occur once in approximately 2,500 years; that is, it has a 2-percent probability of being exceeded in 50 years.

Deterministic (Maximum Credible Earthquake)  

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Is more detailed and incorporated in the design of larger buildings and civil infrastructure like dams or bridges. Is used in designing for skyscrapers and larger civil infrastructure, like dams, where structural failure could lead to other catastrophic consequences. These MCEs might require determining more than one specific earthquake event, depending on the variety of structures included.

Important Factors Affecting Seismic Hazard at a Location 1. 2. 3. 4.

Earthquake Magnitude The Source-to-Site Distance Earthquake Rate of Occurrence Duration of Ground Shaking

Earthquake Magnitude – is the most common measure of an earthquake’s size. Magnitude can be based on: 1. Ml – local magnitude

2. Mb- body wave magnitude 3. Ms- surface magnitude 4. Mw – moment magnitude The Source-to-Site Distance – interprets the distance between the source of an earthquake and the particular site Earthquake Rate of Occurrence – statistical measurement denoting the average recurrence interval over an extended period of time, and is usually required for risk analysis. Duration of Ground Shaking – length of time shaking felt at any given point

DETERMINISTIC SEISMIC HAZARD ANALYSIS (DSHA) Deterministic seismic hazard analysis is the earliest approach taken to seismic hazard analysis. It is originated in nuclear power industry applications. 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. DSHA involve the assumption of some scenario and the occurrence of an earthquake of a particular size at a particular location for which ground motion characteristics are determined. When applied to structures for which failure could have catastrophic consequences, such as nuclear power plants and large dams, DSHA provides a straight forward framework for evaluation of “worst-case” ground motions.

Deterministic Seismic Hazard Analysis 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

PROBABILISTIC SEISMIC HAZARD ANALYSIS (PSHA) Probabilistic seismic hazard analysis is the most widely used approach for the determination of seismic design loads for engineering structures.

The use of probabilistic concept has allowed uncertainties in the size, location, and rate of recurrence of earthquakes and in the variation of ground motion characteristics with earthquake size and location to be explicitly considered for the evaluation of seismic hazard.

CALCULATION FOR SEISMIC HAZARD -

were first formulated by C. Allin Cornell in 1968 and, depending on their level of importance and use, can be quite complex.

Calculation Process: -

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regional geology and seismology setting is first examined for sources and patterns of earthquake occurrence, both in depth and at the surface from seismometer records o Seismometer – is an instrument that responds to ground motions, such as caused by earthquakes, volcanic eruptions, and explosions. Seismometers are usually combined with a timing device and a recording device to form a seismograph. the impacts from these sources are assessed relative to local geologic rock and soil types, slope angle and groundwater conditions. Zones of similar potential earthquake shaking are thus determined and drawn on maps. Each zone is given properties associated with source potential: how many earthquakes per year, the maximum size of earthquakes (maximum magnitude) the calculations require formulae that give the required hazard indicators for a given earthquake size and distance. The computer program then integrates over all the zones and produces probability curves for the key ground motion parameter. The final result gives a 'chance' of exceeding a given value over a specified amount of time. The results may be in the form of a ground response spectrum for use in seismic analysis.

PURPOSE OF SEISMIC HAZARD ANALYSIS -

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To state the probability that something of concern will occur given one or more earthquakes. It provides an estimate of ground motion for a certain probability level. Seismic hazard analysis principally used to produce seismic hazard maps. These maps provide important information using in putting in place mitigation measures against the effects of destructive earthquakes. Seismic Hazard Analysis determines the probability if an earthquake will occur in a given geographic area, within a given time and with ground motion intensity exceeding a given threshold. Predict strong ground motion and involve quantitative estimation of ground shaking for a given site. Seismic hazard analysis determines earthquake ground size, location and time of occurrence. Provides seismic loading parameters over the full range of potential loading to the intervals and provide a complete consideration of site seismic hazard from multiple sources and for appropriate intervals. Widely used by government and industry in application with lives and property hanging in the balance, such as deciding safety criteria for nuclear power plants, making official hazard maps, developing building code requirements and determining earthquake insurance rate.