GB/T 17742-2020The Chinese seismic intensity scale (English PDF)

1 Scope

China's national seismic intensity scale - the twelve-degree scale on which the severity of an earthquake at a given place is assessed after it has happened. It specifies the terms, definitions, symbols and abbreviations, the intensity degrees and the categories of building for which intensity is assessed, and the procedure for evaluating seismic intensity. Intensity is not magnitude, and the difference is the reason this document exists. Magnitude is one number for the whole earthquake, computed from instruments, describing the energy released at the source. Intensity is a different number at every location, describing what actually happened there - and it is what determines relief allocation, damage compensation, the revision of the seismic hazard map and the retrospective assessment of building codes. The same magnitude produces wildly different intensities depending on distance, on the depth of the source, on the soil, on the topography and on how the buildings were built. Assessing it means reading evidence: what people felt and how they reacted, how furniture and objects behaved, and above all the pattern of damage across buildings of different construction. That is why the scale is anchored to defined building categories - a wall crack means one intensity in an unreinforced masonry house and quite another in a reinforced concrete frame, and a scale that ignores the distinction gives a false map. The 2020 edition also brings the scale closer to instrumental measurement, providing the correspondence between the intensity degrees and recorded ground motion parameters, so that an intensity map can be drawn quickly from strong-motion records and refined afterwards by field survey. Issued on 21 July 2020 and in force since 1 February 2021, it replaces GB/T 17742-2008.

This standard specifies the seismic intensity level and the category of buildings for evaluating the seismic intensity, as well as the seismic intensity evaluation method. Evaluation indicators include seismic damage to buildings, human feelings, response of utensils, lifeline engineering seismic damage, other seismic damage phenomena and seismic intensity measured by instruments. The evaluation method is a multi-index method that comprehensively uses macro-survey and instrumental measurement. This standard is applicable to seismic intensity evaluation.

2 Terms, definitions, symbols and abbreviations

2.1 Terms and definitions The following terms and definitions apply to this document.

2.1.1 Seismic intensity The magnitude of ground motion caused by an earthquake and its impact.

2.1.2 Damage index A quantitative indicator of the degree of seismic damage to buildings.

Note: A number between

0.00 and

1.00 indicates the degree of damage from light to severe.

2.1.3 Mean damage index The weighted average of the damage index of similar buildings.

Note: The mean damage index is the sum of the product of the proportion of buildings damaged by the earthquake at all levels and the corresponding

b) "Minority" is 10% ~ 45%;

c) "Majority" is 40% ~ 70%;

d) "Most" is 60% ~ 90%;

e) "Overwhelming majority" is more than 80%.

3.3 Types of buildings for assessing intensity The buildings used to assess the intensity include the following five types:

a) Category A1: Civil, brick, stone and wood buildings without seismic fortification;