The Seismic Hazard Assessment is a quantitatively estimative of the seismic vibration level (Peak Ground Acceleration - PGA) that can occur in a particular interest site in a specific interval of time. This evaluation can be performed in a deterministic way, using specific seismic standards in the region of study, or in a probabilistic way.
This kind of study needs a deep evaluation of seismicity of the Seismic Influence Region of the project, descriptive characterization of the seismic parameters in the region, and preparation of the seismic hazard study with the determination of the maximum seismic acceleration (PGA, Peak Ground Acceleration) that can affect the project in a determined period of time.
The final report of the seismic hazard assessment includes:
- Study of the Seismic Catalog;
- Presentation of the Seismicity with different graphics;
- Determination of Seismic Sources;
- Selection of the seismic attenuation law for the area;
- Characterization of each Seismic Source with the determination of Frequency-Magnitude Relationship (Gutenberg and Richter, 1954);
- Calculation of the Seismic Hazard Assessment using the Probabilistic Seismic Hazard Analysis (PSHA) or Deterministic Seismic Hazard Assessment (DSHA);
- Preparation of the Technical Report for the site.
The two methodologies that VERACRUZ can use in the calculation of the Seismic Hazard are:
(1) Probabilistic Seismic Hazard Analysis (PSHA)
This method considers all major uncertainties in the earthquake process, including the size, location, and time of occurrence. It's based on statistical relationships between earthquakes and ground motion. The description of PSHA can be found in the work of Cornell (1968).
(2) Deterministic Seismic Hazard Assessment (DSHA)
This method defines seismic hazard as the maximum ground motion from a single earthquake or set of earthquakes. It's based on simple statistics of earthquake and ground motion.
The probabilistic method for seismic hazard assessment is the one most in use today, mostly because it is flexible, and takes into account as much data as you can throw at it. However, it takes a lot of time and it is more expensive than the DSHA.
On the other hand, PSHA is often criticized for offering PGA (Peak Ground Acceleration) values that are typically lower than those obtained with the DSHA method. Since DSHA relies heavily on the experience of the seismologist, it is largely based on the occurrence of a high magnitude earthquake close to the construction site, while PSHA does not carry this subjectivity, but smooths out the distribution of the relevant event at different distances.
VERACRUZ has experienced seismologists who have performed dozens of seismic hazard assessments in different countries, including the United States, Guyana, Trinidad and Tobago, Brazil and South Africa. Contact us for a specific quote for your project.
In some cases, for a better result it is important to study the seismic amplification effects of surface layers, which not only depend on the geotechnical profile (e.g. layer thickness, seismic velocities, and attenuation coefficients), but also on the frequency of interest, which can have its amplitude increased by several times with relation to the amplitude determined for the basement due to the phenomenon of seismic resonance. When using maximum horizontal acceleration as the design parameter, it is convenient to give priority to effects at high frequencies (above 1 Hz), which are amplified by thin layers. To illustrate this phenomenon, Reiter (1990) and Bernreuter et al. (1989) showed that layers of soil and alluvium (VS = 250 m/s) with a thickness of 15–20 meters on the basement have significant amplification for frequencies above 3 Hz, and amplification below 2.0 for frequencies close to 1 Hz. Because of this, in some cases it is indicated that a complementary investigation called Seismic Microzonation, which is another service of VERACRUZ to be performed. Click here to know more about Seismic Microzonation.
References
Bernreuter, D.L.; Savy, J.B.; Mensing, R.W. & Chen, J.C. (1989), Seismic hazard characterization of 69 nuclear plant sites east of the Rocky Mountains, Division of Engineering and System Technology, Office of Nuclear Reactor Regulation, U.S. Nuclear Regulatory Commission, Washington-DC.
Cornell, C.A. (1968), Engineering Seismic Risk Analysis, Bulletin of the Seismological Society of America, 58(5), 1583–1606.
Reiter, L. (1990), Earthquake Hazard Analysis: issues and insights, Columbia Univ. Press, 254p.