An earthquake hazard is a multi-hazard, which includes the following main hazards for a large dam project
- Ground shaking
- Movements along faults or discontinuities in the footprint of the dam and/or the reservoir.
- Mass movements into the reservoir causing impulse waves and increase in reservoir level, damaging transmission lines, blocking access roads, and so forth, and
- Project-specific and site-specific hazards (i.e., grounddeformations, seepage, liquefaction, etc.).
Ground shaking is usually considered to be the main seismic hazard. However, movements in the footprint of a concrete dam are more critical than ground shaking, as any such movements would, for example, cause a complicated crack pattern in highly statically indeterminate arch dams, which cannot be reliably predicted by numerical models. The dynamic behavior of the dam would become very complex, as cracking in the dam due to foundation movement and ground shaking would occur at the same time. Therefore, the possibility of foundation movements must be studied carefully. Even if no seismogenic fault crosses the dam foundation, a strong earthquake at a nearby fault can cause movements along discontinuities in the footprint of a dam. These discontinuities are faults, shear zones, fissures, joints, and bedding planes. Such movements are hard to estimate because they depend on the site conditions, the distance from the seismically active fault, and its maximum surface movement. Some faults may also splay near the surface and reactivate discontinuities.
Therefore, it can be concluded that input is needed from seismologists and geologists on ground shaking, movements in the footprint of a dam (this is most important if a monolithic concrete dam is planned), and critical slopes in the dam and reservoir region.
Usually, seismic hazard analyses are only concerned with the estimation of ground motion parameters such as peak ground acceleration (PGA) and response spectra. Ground motion parameters can be determined by a probabilistic and/or deterministic seismic hazard analysis.In the probabilistic analysis, the mean values of the ground motion parameters shall be used, whereas in the deterministic analysis, the mean-plus-one sigma values are used. If both a probabilistic and deterministic analyses are done—which is recommended—the maximum values of the ground motion parameters shall be used.