In international standards (ASHTO, FHWA, etc.) and many reports of engineering companies, for deep underground structures (deeper than 30 meters), when PGA is transferred to the depth, a reduction factor is multiplied in PGA. However, this reduction factor is probably applied when there is some layers of soil or alluvium on the bedrock. Is this statement true? if there isn't the soil layer, Is it necessary to multiply this reduction factor in PGA with increasing depth for the rock environment (with high shear wave velocity as seismic bedrock)? Does PGA decrease with increasing depth in the bedrock?