Seismic Risk Analysis –
Process & Methodology

A Seismic Risk Analysis involves the estimation of potential loss due to any given earthquake or series of earthquakes. There are basically four steps to performing a seismic risk analysis:

1. Defining the earthquake hazards
2. Evaluating the structural vulnerabilities
3. Determining the structural performance
4. Estimating the losses

1. Earthquake 2. Built Environment 3. Damage 4. Loss Ground Shaking Ground Failure Building Stock Lifeline Systems Essential Facilities Material Response Structural Performance Physical Damage Operability Economic Physical Casualties Loss of Function

As we gain more knowledge (through testing, research, and investigation of past events) about the effects of earthquake hazards, the structural performance of the built environment to these hazards, and the resulting losses, engineers are able to refine their methodologies for predicting earthquake losses. There are uncertainties associated with each step of the analyses so the methodologies have combined both deterministic and probabilistic approaches.

Seismic risk analyses and earthquake loss studies are important and very useful to both the public and private sector. They are useful to government officials in stimulating earthquake mitigation efforts and in preparing emergency response and recovery plans. They are useful to building officials and engineers to improve the development of seismic codes and guidelines. They are useful to companies and owners to encourage the protection of assets, life-safety, and to minimize business interruption. They are useful to insurance and financial institutions to help them in determining insurance premiums and loan criteria.

Earthquake hazards consist primarily of ground shaking and ground failure. For the analysis, shaking is often expressed in terms of intensity (MMI), peak ground acceleration (PGA), or Spectral Acceleration (Sa). Ground failure consists of fault rupture, liquefaction, lateral spreading, or ground settlement. Determining the vulnerability of an existing building depends upon a number of parameters, including its building and construction type, the code to which it was designed/constructed, size of the earthquake, distance from the earthquake source, and soil conditions of the site. Vulnerability (fragility) curves can be generated for specific building types relating ground motion to expected damage state. Once the hazards are defined and the building characteristics and site location are determined, an engineering analysis can be performed to calculate the expected performance (i.e., damage). Finally, loss estimations can be determined using various approaches (i.e., empirical, statistical, etc). The previous figure shows the general steps of the analysis.

This approach can be used for an individual building (single-site) analysis or a portfolio analysis, which can consider numerous earthquake events, site locations, buildings, and structural types. The single-site analysis is often associated with the “PML” (Probable Maximum Loss) Study, mostly used in the financial and insurance industry. A portfolio type analysis is best performed using a computer software program, several of which are available in the industry. The Federal Emergency Management Agency (FEMA), through the technical guidance of the National Institute of Building Sciences (NIBS), has developed such a program (HAZUS), which is publicly accessible. For a more in-depth discussion on the use of the latest technology to manage risk, refer to an article co-authored by Stephen Hom (shom@soha.com), “Catastrophe Modeling,” published in Risk Management magazine, May 1999 issue

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