01 - Earthquake Zones
The first step in any natural hazards assessment process is to determine the hazard level. Based on the location of the site, site conditions and structural characteristics (for existing or yet-to-be built structures) the shaking levels, expressed either as peak ground accelerations or intensities can be assessed. It is to be noted that different parts of the structure respond differently to the same shaking level. As such, a distinction is to be made between structural elements, ie. those parts of the structure that resist the lateral forces, and the non-structural elements, eg. contents, machine and equipment anchorages, piping bracing, etc. Damage to these elements can occur at much lower shaking levels than those which could cause damage to the structural elements.
Structural design codes are the most reliable tools to determine the expected shaking levels as well as structural design requirements for load-bearing as well as non-load bearing elements. Local conditions in terms of soil conditions, topographic features, historical events, predominant structural types, etc. are considered in defining the design force levels.
Some of the issues to take into consideration in determining the site shaking levels are listed below (list is not exhaustive).
- For existing buildings the age of the building determines the conformity of the design and detailing to state-of-the-art requirements. Design codes are revised regularly to consider modern construction methods, construction materials, recent events, etc. Older buildings may not meet the requirements of the most recent code version. This is to be considered when any changes are done to an existing structure. This should be checked by a qualified structural engineer. Based on the analysis, which should take the effective condition and material properties of the structure and its contents into consideration, a retrofit strategy defining the nature and level of strengthening of specific elements is to be developed by the engineer.
- Type of structure in terms of occupancy, structural system, material of construction defines the design force level. The impact of these parameters differs based on country. However, the common philosophy is that those buildings, which must sustain minimum damage levels after an earthquake, either due to the post-earthquake function as a rescue center, national significance of the contents, danger to occupants and surrounding community, etc. are given the highest priority factor and are thus designed to more stringent requirements.
- Local soil conditions determine the level of shaking at ground level. Softer soils, their level of saturation (moisture content), topographic conditions (presence of water body in the vicinity and slope of the ground) and the presence of ground water are just some factors that can lead to soil failures and consequent building damage. Some manifestations of earthquake-induced seismic damage are lateral spread, liquefaction, seismic amplification, etc. Geotechnical investigations are always conducted before construction to determine local soil conditions and to make recommendations regarding the most suitable foundation types. Such investigations can also give an indication of the impact of local conditions on expected shaking levels. Microzonation and liquefaction studies can also be conducted to determine expected response of the site to ground shaking. Even though such studies can be expensive, they provide reliable information to be used in the design of thee site (foundations, structures and non-structural elements).
- If the location is on a hill then topographic amplification can occur, depending on the faulting conditions.
- Caution is to be exercised when using resources other than seismic zonation maps of the national structural design codes to determine the hazard level. The assumptions behind the development of such maps in terms of the historical "depth" of the database, soil and topographical conditions considered, unit of measurement (moment magnitude, Richter magnitude, body wave magnitude, European Macroseismic Scale EMS Intensity, Modified Mercalli Intensity MMI, etc.) is to be understood to allow interpretation of the results.
- This risk factor deals with hazard levels only. Controls are covered by the other risk factors.