SEISMIC HAZARD ANALYSIS
Seismic hazard is defined as the probability that the value of the parameters of the ground characteristics and the magnitude of the earthquake at the construction site exceed a certain level in the estimated time. The purpose of this analysis is to determine the probability values foreseen in the project area and the appropriate earthquake parameters that will be the basis of the project in the closest manner within a certain system with catalog data of previous earthquakes, their geological features, seismological, statistical and other factors. For this purpose, by calculating with probabilistic and deterministic methods, it is possible to select the earthquake parameters to be taken as basis during the design of all units to be built within the scope of the project.
Deterministic Seismic Hazard Analysis
A specific seismic scenario is developed and the assessment of ground motion hazard is made accordingly. In the scenario earthquake, there is a precondition for an earthquake of a certain size and location to occur. Typically, they can be defined as four procedures (Reiter, 1990):
These features include determining all earthquake sources that can create significant ground motion in the project area and revealing their properties, and defining the geometry and earthquake potential of each source.
Selection of source-field distance parameter for each source. In most DSHAs, the shortest distance between the source and the project site is chosen. Depending on the distance criteria in the decrement relations used in the next step (s), the epicenter distance or the hypocenter depth can be used.
The selection of the determinant earthquake (that is, the earthquake that will produce the strongest shaking), which is usually indicated with a certain ground motion parameter in the project area, is made by comparing the shaking levels to be caused by the earthquakes detected in the first step and assumed to occur at the distance in the second step. The determinant earthquake (usually expressed as moment magnitude) is defined by its size and its distance to the project area.
Hazard in the project area is generally defined in terms of ground motion that a determinant earthquake will cause on the site. Hazard characteristics are defined by one or more ground motion parameters derived from decrement relationships. Parameters that are most commonly used to characterize a seismic hazard are peak acceleration, peak velocity and response spectrum ordinates.
Probabilistic Seismic Hazard Analysis (PSHA)
With the Probabilistic Seismic Hazard Analysis (PSHA), which has been used for nearly 20 years and which aims the seismic assessment of the uncertainties encountered in the changes in earthquake size, its location, its return interval, its magnitude and its local characteristics, the uncertainties are determined, quantitatively evaluated and rationally combined in order to reveal the seismic hazard in a healthier way (Kramer, 1996). In order to understand the concept and mechanics of this analysis, it is necessary to know the basic concepts of the probability theory.
PSHA can be defined as a four-step process, each step of which is somewhat similar to the steps in DSHA (Reiter, 1990).
The first step, which is to identify the earthquake sources and determine their characteristics, is identical to the first step of DSHA, except for the requirement to characterize the probability distribution of potential tearing locations within the source. In most cases, a uniform probability distribution is applied to each source zone. This means that the probability of an earthquake in the source area is the same every location. These distributions are then combined with the resource geometry to obtain the probability distribution of the source-field distance. On the other hand, DSHA accepts the probability of earthquake occurrence at the closest point of each source to the project area (1); it accepts zero at all other points.
In the next step, the temporal distribution of seismicity or earthquake return is presented. The return relation, which defines the average rate at which a given magnitude will be exceeded, is used in the evaluation of each source zone.
The ground motion to be generated in the project area by an earthquake of any magnitude that may occur at any point of the source area should be determined using decrement relations. In the PSHA, uncertainties specific to the decrement relationship are also taken into account.
Finally, by combining the uncertainties associated with the earthquake location, the magnitude of the earthquake and the estimation of the ground motion, the probability of exceeding the ground motion parameter in a given time interval is obtained.