Definition

Seismic zonation. The process of subdividing the territory into regions with respect to the level of seismic hazard. The result of seismic zonation is usually presented as a map, which is based on seismic hazard map.

Introduction

Seismic zonation is useful for hazard reduction such as earthquake-resistant design of structures, risk analysis, land-use planning, etc. Many earthquake-prone countries developed seismic zonation maps. Seismic zonation map is usually revised or updated periodically with the progress in methodology and accumulation of new data. Seismic intensity (see Earthquakes, Intensity ) or ground motion parameters such as peak ground acceleration (PGA), peak ground velocity (PGV), and spectral acceleration at specific natural period are mostly adopted in seismic zonation map. In the early stage, most seismic zonation maps were in terms of intensity, but since the 1980s, ground motion parameters have become popular. More commonly, such maps take the exceeding probability of 10% within 50 years (return period 475 years) as standard.

Methodology

The basic method to develop seismic zonation map is the seismic hazard analysis (see Seismic Hazard ) approach. Both deterministic and probabilistic approaches are adopted. Although some new deterministic approaches based on the computation of synthetic seismograms are used in recent years in developing seismic zonation maps (Parvez et al., 2003), nevertheless probabilistic approaches are more popular. The seismic zonation maps, based on seismic hazard maps (see Seismology, Global Earthquake Model ), of many countries are developed by the use of probabilistic approach. The well-known USGS National Seismic Hazard Maps (Algermissen and Perkins, 1976; Algermissen et al., 1990; Frankel et al., 1996; Frankel et al., 2002; Petersen et al., 2008) and the Global Seismic Hazard Assessment Programme (GSHAP) are the typical (see Seismic Hazard ) ones. In this article, only probabilistic method is briefly introduced.

The probabilistic method (usually referred to as PSHA – probabilistic seismic hazard analysis) was first introduced by Cornell (1968) and since then widely adopted and modified (McGuire, 1978; Bender and Perkins, 1982). There are four basic steps for assessment of PSHA (Figure 1):

  • Step 1: Definition of seismic sources. Sources may range from small faults to large seismotectonic provinces with uniform seismicity. The type of seismic source can be both line or area sources.

  • Step 2: Definition of seismicity recurrence characteristic for the sources, where each source is described by an earthquake probability distribution, or recurrence relationship. A recurrence relationship indicates the chance of an earthquake of a given magnitude to occur anywhere inside the source during a specified period of time. An upper bound earthquake is chosen for each source, which represents the maximum event to be considered.

  • Step 3: Development of ground motion attenuation relationships. This is usually done empirically from strong motion records (see Earthquakes, Strong-Ground Motion ).

  • Step 4: Determination of the hazard at each site. In this case, the effects of all the earthquakes of different sizes occurring at different locations in different earthquake sources at different probabilities of occurrence are integrated into one curve that shows the probability of exceeding different levels of ground motion level (such as PGA) at the site during a specified period of time.

There are two basic assumptions in the seismic hazard analysis method at present: (a) Seismicity in the region around the site in the past indicates that in future, the recurrence rate of given site is the same as that of historic period. (b) Seismicity of the region can be expressed by tectonic earthquakes in the region, i.e., the seismic activities distribute homogeneously in a certain tectonic area or an active fault.

Seismic Zonation, Figure 1
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Approaches of probabilistic seismic hazard analysis.

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The two assumptions accord with the two principles of historic earthquake repeatness and tectonic extrapolation. The difference is that the PSHA adds a new concept of magnitude interval recurrent rate and the hazard is evaluated with probabilistic analysis method. The method can provide the exceedence probability of different ground motion extent (intensity, acceleration, etc.) at the site in specific time intervals, so that the earthquake resistant parameters can be selected with different exposure period, risk level, and various engineering structures.

Example: seismic zonation map of china (2001)

The first seismic zonation map of China was compiled by Wong (1921) after the Haiyuan earthquake, which occurred in 1920. After that, three versions of seismic zonation map were developed in 1957, 1977, and 1990 (Shi et al., 1992). The 1957 version demonstrated the maximum affected intensity of China. The 1977 version was provided by using the methodology of long-term and middle-term earthquake prediction. This version of seismic zonation map demonstrated the maximum encountered intensity in the forthcoming 100 years. This map was adopted by the building code. The 1990 version used probabilistic method. The seismic intensity with exceeding probability of 10% within 50 years was given in the map. It was used in the building code and other regulations related with seismic design. In 2001, a new seismic zonation map of China was issued. This map also used probabilistic method. The PGA and characteristic period of response spectrum Tg with exceeding probability of 10% within 50 years were given in the map. The probabilistic method used in developing seismic zonation of China (2001) was a little bit different from PSHA.

Most earthquakes in China are intraplate earthquakes (see Seismicity, Intraplate ). Their non-homogeneity in space, non-stationarity in time were shown in the historical earthquake catalogs. The PSHA method applied in China can reveal non-homogeneity in space, non-stationarity in time. The approaches applied differ from PSHA in two aspects:

  1. 1.

    Evaluating seismicity and determining magnitude distribution, total annual occurrence rate of the province are on the basis of the seismic tendency estimation and the seismic characteristics analysis.

  2. 2.

    Annual rates of sources in all magnitude intervals are determined by spatial distribution functions, which describe relative risk among sources in the province.

The technical approach developing the national seismic zonation map of China (2001) is shown in Figure 2 (Gao, 2003).

Seismic Zonation, Figure 2
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Technical approach of developing national seismic zonation map of China (2001).

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The key scientific problems in compiling this map are: (a) how to treat the uncertainties in the evaluation of seismicity parameters and the delineation of potential sources as well as the attenuation relationship; (b) how to select the suitable parameters in the zonation map to fit the need of seismic design; and (c) how to use the domestic data and the data from the world to get the attenuation of the ground motion parameters.

The logic tree method is adopted to treat the uncertainties in seismic hazard analysis. The multi-set of potential source delineations was used. Four groups of scientists participated in the work of potential sources delineations based on the independent background and database. There are four sets of potential sources adopted in the logic tree analysis. Figure 3 is one of the potential source set.

Seismic Zonation, Figure 3
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One of the seismic potential source set used in developing national seismic zonation of China (2001).

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Site-related seismic response spectrum is the base of seismic design for ordinary industry and civil construction. The site-related response spectrum is not only closely related to the earthquake environment, but also related to the soil condition. It is very difficult to decide site-related response spectrum by a single parameter such as intensity or peak acceleration. According to the results form seismic hazard assessment and the building code, the basic parameters used in the seismic zoning map to provide site-related response spectrum are PGA and the characteristic period of the response spectrum.

The attenuation relationships for the acceleration response spectrum platform value Sa and the velocity response spectrum platform value Sv were developed by modifying corresponding attenuation relations in the western United States according to the differences of intensity attenuation relations. Then the PGA and the characteristic period of response spectrum of acceleration Tg were defined as:

$$ \eqalign {PGA = & {{Sa}\over{2.5}} \cr Tg = & \,2 \pi {{Sv}\over{Sa}}}$$
(1)

The country was divided into 40,000 grids. The probabilistic seismic hazard analysis for every grid was performed. The ground motion parameters with exceeding probability 10% within 50 years (return period 475 years) were determined.

The new zoning map includes two specific maps. One is PGA in gravity unit g (Figure 4) and the other is Tg in second (Figure 5). The scale of the maps are 1:4,000,000. The soil condition is medium hard soil. From these two parameters, the design response spectrum can be determined easily by (Equation 2):

$$ Sa(T) = 2.5a\left\{ \matrix{{6T +0.4} & {0 \leq T < 0.1} \cr {1} & {0.1 \leq T < Tg} \cr {Tg / T} & {T \geq Tg}}\right. $$
(2)

where a is PGA in g, T is the natural period in second.

Seismic Zonation, Figure 4
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Illustration of acceleration zonation map of China.

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Seismic Zonation, Figure 5
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Illustration of characteristic period of seismic response spectrum zonation map of China.

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In the acceleration map (Figure 4), the territory is divided into seven zones. The acceleration for the seven zones are 0.05, 0.05, 0.10, 0.15, 0.20, 0.30, and ≥0.40 g, respectively.

In the characteristic period zoning map (Figure 5), the territory of China is divided into three zones. Zone 1 is the region with Tg = 0.35 s, zone 2 with Tg = 0.40 s, and zone 3 with Tg = 0.45 s.

For different soil conditions, the Tg value should be adjusted as Table 1, but the PGA value does not change with soil conditions.

Seismic Zonation, Table 1 Tg value for different soil conditions (unit: second)
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The new national seismic zonation map of China serves as the obliged state standard, which took into effect in August 1, 2001. For the ordinary new constructed buildings, the standard must be followed.

Summary

Seismic zonation is a process of estimation of the seismic hazard in terms of parameters of ground motion for a certain area. Assessment results in seismic zonation map compilation, which reflects territorial distribution of the seismic hazard (see Seismic Hazard ). Seismic zonation map is useful for hazard reduction such as earthquake-resistant design of structures, risk analysis, land-use planning, etc. Many countries apply seismic hazard maps in anti-seismic codes. Recently, the probabilistic seismic hazard analysis method is more commonly used in compiling seismic zonation map. The seismic zonataion maps take the exceeding probability of 10% within 50 years (return period 475 years) as standard. Seismic zonation will develop with the development of seismic hazard assessment methodology and anti-seismic policy.

Cross-references

Earthquakes, Intensity

Earthquakes, Strong-Ground Motion

Seismic Hazard

Seismic Microzonation

Seismicity, Intraplate

Seismology, Global Earthquake Model