Procedure of Seismic Hazard Assessment

There are two approaches for seismic hazard assessment:

1. Deterministic Seismic Hazard Assessment (DSHA)
2. Probabilistic Seismic Hazard Assessment (PSHA)

The DSHA approach will involve following four-steps:

The first step consists of demarcating earthquake sources. This step involves identification and characterisation of all earthquake sources capable of producing significant ground motion at the site. The individual sources can be modelled as point, line, area or volume depending upon the distribution of seismicity and its possible relationship with the tectonics. The second step involves selection of the controlling earthquake that will produce strongest level of shaking. This step also takes into account the selection of parameter that defines the source to site distance for each source zone associated withcontrolling earthquake. Generally, the shortest distance between the source and the site of interest is chosen. The third step consists of determination of earthquake effect by means of some suitable ground motion attenuation model. Theground motion attenuation model provides an estimate of ground motion for an earthquake of a given magnitude at different distances. Final step comprises the estimation of hazard at the site in terms of peak ground acceleration, velocity, response spectrum or the other measure which describes the effects of controlling earthquake.

Whereas PSHA includes following four steps:

Based on the synthesis of available information on the geological, tectonic and seismicity (available from earthquake catalogues) various types of seismotectonic sources are characterized. In most cases, uniform probability distribution is assigned to eachseismotectonic source. This implies that earthquakes are equally likely to occur at any point within the source zone. The second step involves estimation of seismicity recurrence rate for each source zone. A recurrence relationship, which specifies the average rate at which an earthquake of some size will be exceeded, is used to characterise the seismicity of each source zones. This step consists of selecting a ground motion prediction model for a region or each source zone. Using this model, the ground motion produced at the site by earthquakes of different sizes occurring at different distances in each source zone is determined. The uncertainties in the attenuation model are also considered in PSHA. Final product in PSHA is the estimation of seismic hazard based on the combining effects of all the earthquakes of different sizes, occurring at different locations in different source zones at different probabilities of exceedances. A probability value is obtained showing thatthe ground motion parameter will be exceeded during a particular time period.