163 groups of strong ground motion records are picked up from Euro databank which are sorted according to earthquake magnitude and site classification.Based on these records

the symmetry of amplitude envelope lines of ground motion in positive and negative directions is discussed

and their effects on site classification and focus mechanism are investigated.From the statistical results

it can be seen that the envelope line is asymmetrical

and the envelope line in the positive direction is more plump than that in the negative direction.In addition

the degree of smoothness of the configuration of envelope line in the positive direction is different from that in the negative direction.In general

the envelope line in the positive direction is smoother than that in the negative direction.The effects on site classification of amplitude envelope line of ground motion in both positive and negative directions is rather complex

and the effect on focus mechanism of amplitude envelope line of ground motion in the positive direction differs from that in the negative direction.Moreover

the envelope lines in both positive and negative directions can be generally represented by single peak curves.