Engineering seismology
Probabilistic Seismic Hazard Analysis (PSHA) is defined as the frequency of occurrence of an event at some place (usually given in terms of peak ground acceleration).
(EM 1110-2-6050, 1999)
PSHA is using to represent some level of ground motion parameter for some specified frequency of exceedance. This parameter could be peak ground acceleration (PGA) or a spectral acceleration for a specified oscillator period. The frequency may be expressed as the mean annual rate of exceedance for some time interval (e.g. 475 years).
Fundamentals of earthquake hazard assessment
The parameters that would describe the seismicity of the specified area (number of events n with magnitude mmmin within specified time interval T) are:
- Mean activity rate for mm= λ
- Parameter (Gutenberg-Richter) = b (or equivalently)
- Maximum possible earthquake magnitude =mmax
In seismology, the cumulative distribution function (CDF) and the probability density function (PDF) of earthquake magnitudes allows us to find out the probability of occurrence of an earthquake. The CDF will be derivate from the Gutenberg-Richter relation, while the PDF describes how the CDF changes with respect to magnitude.
A seismic hazard analysis is given in terms of:
- Seismic hazard curve,
- The mean return times of earthquake magnitudes,
- The maximum possible magnitude mmax.
For a specified site, distribution of peak ground acceleration and acceleration response spectra are also needed. These parameters are more useful to engineers for the optimal design of critical structures. So, we need to calculate the conditional probability that earthquakes of random magnitudes M at a random distances R will cause a PGA equal to, or greater than, an acceleration of engineering interest amin. At any given site the logarithm of the PGA is distributed according to the same type of distribution as earthquake magnitude. The two distributions differ only in the value of their parameters.
The PSHA Procedures, fall into different categories and approaches. One of our basic studies in ZAP Consulting Engineers is earthquake hazard. We have extensive experience in identification, monitoring and analyzing of all earthquake sources which are essential for a seismic hazard assessment.
Our seismologists using up to date softwares to determine the potential, characteristics and behavior of seismic sources and calculating required parameters. These parameters are the main input for seismic microzonation, which is one of our main tasks in ZAP Consulting Engineers.
We are also involved with the risk assessment which is the probability of harmful consequences or expected losses (deaths, injuries, property, livelihoods, economic activity disrupted and environmental damages) resulting from interactions between seismic hazard and vulnerable conditions.
