Within a microzonation study there is the need in some cases to pay attention to additional aspects of the sediment layers besides the major values like shear wave velocity, H/V frequency or amplification. When we study structural failures after earthquakes we can note that quite frequently local variations in soil structure and condition cause extensive damages.

Ground water levels and streams can be a major factor strengthening the effect of an earthquake in certain often isolated localities. Ground water flows can indicate the state of the ground body in their vicinity (fissures, discontinuities), and point out possible weak spots where failure of the ground is likelier or could be increased. A map of ground water flow is therefore intended.

Nevertheless, the main danger of groundwater presence in sediments is that it can easily lead to liquefaction. Soil liquefaction describes the behavior of loose saturated cohesionless soils, i.e. loose sands, which go from a solid state to acquire the consistency of a thick liquid. Liquefaction occurs only in saturated soils, that is, soils in which the space between individual particles is completely filled with water. This water exerts a pressure on the soil particles that influences how tightly the solid frame is connected. Prior to an earthquake, as water pressure is relatively low there exists a good connection between soil particles. During strong motion, shaking can cause the water pressure to increase to the point where the soil particles can readily move, so to say flow, with respect to each other. The occurrence of this phenomenon is strongly bound to the type of soil. Liquefaction is more likely to occur in loose to moderate granular soils with poor drainage, such as silty sands or sands and gravels capped or containing seams of impermeable sediments. Depending on the initial void ratio of the material it can act under loading in a strain-softening or a strain-hardening behavior. In the first case the material can easily be triggered into a collapse while in the latter it can suffer cyclic liquefaction or cyclic mobility depending on a possible stress reversal. The resistance of a specific soil to liquefaction depends primarily on the density of the soil, confining stresses, soil structure (fabric, age and cementation), the magnitude and duration of the cyclic loading, and the extent to which shear stress reversal occurs.

Earthquake liquefaction is a major contributor to urban seismic risk hence it plays an important role in a global map of seismic hazard.