Seismic hazard

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When building a house, regional seismic hazard maps are used to find the best (or the worst) place to locate for earthquake shaking. Although greatly confused with its sister, seismic risk, seismic hazard is the study of expected earthquake ground motions at any point on the earth.

Surface motion map for a hypothetical earthquake on the northern portion of the Hayward Fault Zone and its presumed northern extension, the Rodgers Creek Fault Zone

The calculations for seismic hazard can be quite complex. First, the regional geology and seismology is examined for patterns (using seismometers and earthquake location). Zones of similar potential for seismicity are drawn. For example, the famous San Andreas Fault might be drawn as a long narrow zone. Zones in the continental interior (the site for intraplate earthquakes) would be drawn as broad areas, since causative faults are generally not identified.

Each zone is given properties associated with source potential: how many earthquakes per year, the maximum size of earthquakes (maximum magnitude), etc. Finally, the calculations require formulae that give the required hazard indicators for a given earthquake size and distance. For example, some districts prefer to use peak acceleration, others use peak velocity, and more sophisticated uses require response spectral ordinates.

The computer program then integrates over all the zones and produces probability curves for the key ground motion parameter. The final result gives you a 'chance' of exceeding a given value over a specified amount of time. Standard building codes for homeowners might be concerned with a 1 in 500 years chance, while nuclear plants look at the 10,000 year time frame. A longer-term seismic history can be obtained through paleoseismology. The results may in the form of a ground response spectrum for use in seismic analysis.

More elaborate variations on the theme also look at the soil conditions. If you build on a soft swamp, you are likely to experience many times the ground motions than your neighbour on solid rock. The standard seismic hazard calculations become adjusted upwards if you are postulating characteristic earthquakes.

Areas with high ground motion due to soil conditions are also often subject to soil failure due to liquifaction. Soil failure can also occur due to earthquake-induced landslides in steep terrain. Large area land sliding can also occur on rather gentle slopes as was seen in the "Good Friday Earthquake" in Anchorage, Alaska, March 28, 1964.