Soil Classes and Microzonation

As an earthquake wave travels from the crust to the surface, it amplifies or dampens depending on the soil layers it passes through. In an earthquake of the same magnitude, a building on loose, water-saturated soil is put under noticeably more stress than a similar building on solid rock.

Soil classes

The Turkish Building Earthquake Code classifies soils from ZA to ZF based on shear-wave velocity and other properties. ZA denotes hard rock, while ZE and ZF denote loose or soft soils, or ones that require special study. Older studies and microzonation maps also use other scales, such as Z1 to Z4.

Microzonation

In major cities, microzonation studies map ground behavior at the scale of districts and neighborhoods. These studies map hazards such as ground amplification, liquefaction risk, landslides and tsunamis. Microzonation studies carried out for Istanbul show significant differences between areas.

Why does it matter?

• In areas with high ground amplification, the earthquake force acting on a building increases.
• In areas with liquefaction risk, the foundation system requires a special solution.
• A soil survey is mandatory for new buildings; for older buildings, soil information is often missing.
• Ground conditions can vary at the scale of a single street, even within the same neighborhood.