text: 7,500,OOO (in the U.S.) to 1: 1,000 - 2,000, in many other parts of the world in all kinds of en- vironments . 6.4 Seismic microzonation Seismic hazard analysis provides probabilities of occurrence or exceedance of a certain ground r-notion parameter related to a reference ground for which the attention functions are compiled. However, for planning or construction on the scale of a town, the hazard estimates should be modified because of the strong influence of the ground on the frequency and ampli. tude of ground motion. The procedure of determining the corrections is called seismic microzonation. The existing methods use: 0) recordings of weak shocks or aftershocks at locations with different ground conditions and the process of extrapolation of the data to records of large shocks; (ii) theoretical calculation of ground response using information on thickness of layers and on elastic parameters of underlying rocks, and assuming a given input at the soil-bedrock boundary; (iii) simultaneous recordings of seismic noise (with periods of 0.1 second up to 1.0 second) at different points of an area and comparison of the amplitude; (iv) measurement of seismic impedance using the propagation velocity of P and S waves along profiles crossing the area. The above techniques are listed in the order of preference and the application of two or three such methods is desirable. The results may substantially change the average hazard figures. The above techniques provide information only within the elastic range of deformation of the foundation materials and should be used with caution in deposits of low shear strength or in materials which may lose their shear strength with increasing intensity of shaking. The scientific uncertainties which beset micro-zoning techniques demand that extreme caution be taken when trying to carry out micro-zoning of recent alluvial deposits. 22 VII. VOLCANOES 7.3. Introduction 
analysis: The text appears to be discussing the application of scientific techniques to assess seismic hazard and microzonation in various environments, with a focus on the influence of ground conditions on ground motion. The language used is technical and specialized, suggesting that the text is intended for an audience with a background in geology, engineering, or a related field.