General

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^TM 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^TM considers financial losses from direct damage to the structure and from loss of business revenue.

Seismicity

ST-RISK^TM bases its seismic hazard calculation on reports produced by the California Division of Mines and Geology (CDMG) and the United States Geological Survey (USGS). ST-RISK^TM uses the same methodology and interpretations, therefore obtaining results that are consistent with the US National Seismic Hazard Map.

ST-RISK^TM contains both fault sources 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^TM 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.

Site Conditions

ST-RISK^TM can determine the NEHRP soil category for any site in California, using maps distributed by the CDMG, providing a reasonable estimate of the likely soil conditions at the site. No regional database is error-free, however ST-RISK^TM 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^TM to determine the susceptibility to liquefaction.

Given the soil conditions, ST-RISK^TM applies soil amplification measures based on the NEHRP recommendations. NEHRP provides amplification factors for only two periods, therefore ST-RISK^TM 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^TM user.