Probablistic seismic hazard analysis is the process of determining the probability of ground shaking intensity at a site. Many factors contribute to this process but they reduce to four important contributors; the proximity to faults, the size of the earthquakes that can be generated by these nearby faults (and how often they occur), the resulting base-level ground motion at the site, and the effects of local site conditions.
ST-RISK™ analyzes all "possible" events that can effect a site. Each event is assigned a probability of occurrence. If the effects of all events are aggregated, the hazard can be determined, that is, the probability that a ground motion intensity (A) will be exceeded. The hazard measures the likelihood that a building will encounter certain levels of ground shaking. The risk to the building then represents a probabilistic measurement of the hazard and the building's response to certain levels of shaking, or damage. Damage can then be translated into financial loss to the owner, casualty to occupants, operational loss to a business (including loss of market share), and other losses. ST-RISK™ considers financial losses from direct damage to the structure and from loss of business revenue.
ST-RISK™ bases its seismic hazard calculation on regional seismic models. In the USA, the calculations are based on the USGS 2008 National Seismic Hazard Map seismic model, and uses the same methodology and interpretations as used by California Division of Mines and Geology (CDMG) and the United States Geological Survey (USGS). In Canada, the calculations are based on the 2005 Geological Survey of Canada seismic model. In other areas, ST-RISK™ uses recent proprietary seismic models developed by Fugro Consultants, Inc.
ST-RISK™ uses fault sources, area seismic sources (also know as seismic zones), and variable seismicity background sources. The background source represents all events that can occur where faults are not known to exist. Using this methodology ST-RISK™ accounts for "unexpected" earthquakes.
To check the level of shaking that is possible you can take a look at the spectral response curve generated by the program. This curve indicates the ground shaking amplitudes that are predicted at user-specified return periods.
ST-RISK™ can determine the NEHRP soil category for any site providing a reasonable estimate of the likely soil conditions at the site. In California, maps distributed by the CDMG are used. No regional database is error-free, however ST-RISK™ allows you to override the default soil condition if you have more accurate geotechnical information. Liquefaction maps for the San Francisco Bay area and Los Angeles are utilized by ST-RISK™ to determine the susceptibility to liquefaction.
Given the soil conditions, ST-RISK™ applies soil amplification measures based on the NEHRP recommendations. NEHRP provides amplification factors for only two periods, therefore ST-RISK™ contains supplemental factors for other periods based on our expertise and past experience. These amplification factors are vitally important to assessing the overall site-specific hazard, and having them resident within the program makes the calculation of site-specific spectra straightforward for the ST-RISK™ user.