Seismic Zoning

the evaluation of the potential seismic danger in a seismically active region. The identification of seismically dangerous regions is based on the results of combined analysis of instrument and macroseismic data on past earthquakes and the geological characteristics of the region. The data on past earthquakes include the intensity of oscillations at the earth’s surface, the spatial distribution of earthquake foci, and the size, magnitude, energy, and frequency of the earthquakes. The geological characteristics of the region include the history of geological development, the intensity of recent and modern tectonic movements and the contrast between them, and the age and character of tectonic dislocations and their activity.

The goal of seismic microzoning is the precise determination of the degree to which seismic activity can affect structures. The determination takes into account local, conditions of the particular sector in a seismically dangerous region. Such conditions include the physical and dynamic properties of the ground and basement, the thickness of the tipper layers of the earth’s crust, the presence of permafrost, tectonic and relief characteristics, and the spectral properties of approaching seismic waves.

Seismic zoning is used in making maps that show the intensity of tremors for any geographic point with average ground conditions. According to the Construction Code, “average ground conditions” refers to clays, loams, sands, and sandy loams where the water table is deeper than 8 m from the earth’s surface, and also to coarse ground where the water table is 6–10 m from the earth’s surface. In the USSR the total area of seismically dangerous regions is 28.6 percent of the country’s territory; this includes 2.4 percent for regions in category nine and 3.2 percent for regions in category eight. Regions liable to earthquakes of intensity nine are located in Middle Asia, the Baikal Region, Kamchatka, the Kuril Islands, and elsewhere; regions liable to earthquakes of intensity eight are in Moldavia, the Crimea, the Caucasus, Southern Siberia, and elsewhere.

In the USSR, seismic zoning maps are official documents directly related to the norms and rules of earthquake-resistant construction. The present normative seismic zoning map was ratified by the State Committee for Construction of the Council of Ministers of the USSR in 1969 (Construction Code, part II, section A, chapters 12–69). This map identifies regions liable to earthquakes of intensity six through nine for average ground (according to the GOST 6249–52 seismic scale) and also regions where earthquakes stronger than intensity nine are possible. Seismic zoning maps are also available abroad in Bulgaria, Rumania, Mongolia, the USA, Japan, and several other countries.