Microzonation

Microzonation is a relatively young work area of seismology which is strongly interlinked with both engineering seismology and engineering geology. The term 'micro' in its name was introduced to distinguish it from the zonation of the regional earthquake hazard as it is stated e.g. for Germany in the German building code DIN 4149.

For microzonation studies the local hazard of direct earthquake impact is subdivided into the three categories seismic amplification (e.g. due to impedance contrasts of soft sediments on hard rock), liquefaction (loss of rigidity and stability of ground layers) and terrain instability (landslide rock fall, sagging, etc.). The three types of hazard are essential and integral parts of microzonation maps. Today, in countries with high earthquake risks microzonation studies are common elements of urban and regional planning. Thereby, an important contribution to the mitigation of the expected earthquake damages is achieved.

Fig.1: Scheme showing site effects Source: BGR

The subject of seismic microzonation is the determination of variations of the ground motion amplification due to local differences in the geological conditions and topographical characteristics (site effects, Fig.1). Such differences in the local geological characteristics, especially in the distribution of shear wave velocity with depth, can provoke severe changes of the duration, the frequency spectrum and the strength of an earthquake tremor. These changes in the earthquake impact can in turn cause serious differences in the grade of damage even at sites at short distance to each other.

Prominent examples for the great differences in the grade of damage in nearby areas caused by the same earthquake (local site effects) are the earthquakes in Mexico (1985) (Fig. 2), Loma Prieta (1989) in Californina (Fig. 3), USA, or the historical earthquake of Basel (1356) in Switzerland. Hence, the objective of seismic mircozonation is the determination of the spatial distribution of the amplification of ground motion (e.g. ground acceleration, response spectrum) resulting from local soil structure and, if necessary, from the topography of the rock basement (2D and 3D effects of valleys and mountain ridges).

Thereby the previously determined hazard for hard rock site conditions from regional hazard studies can be modified for the actual local conditions using the site specific amplification function valid for the sites investigated in the microzonation study (e.g. multiplication of response spectrum and amplification function). Thus particularly endangered sites can be separated from less vulnerable ones. Examples for the application of the methodology of seismic microzonation and the techniques used (H/V spectrum, array microtremor measurement, reference station method) can be found in the projects 'TC-Project Georisk Indonesia' and 'Seismic hazard associated with deep geothermal energy production'. Here, the emphasis is on the determination of the distribution of shear wave velocities in near surface sediments and rock using seismic methods.