Influence of the Effect of Place on the Seismic Intensity expected
This effect is very complex in nature, pues includes: effects of resonance of waves in one, two and/or three dimensions; the effect of the no linear behaviour of the soil for big deformations (which varies the intensity of the effects of place what less linealmente comports the soil); phenomena of liquefaction of sandy layers; split of the soil (slides and agrietamientos) topographical accidents etc. And it requires studies of big detail for his determination, even for zones of small extension.
In works of seismic zonation, when they analyse the effects of place with sights to his application to the calculation of the seismic dangerousness, are used to realise several simplifications. By a part consider that the variation owed to the effects of place is produced only by the variation in the lithology of the soil and that these are constant, that is to say, the no linear behaviour of the soil no incide in the magnitude of the effect besiege. On the other hand, it uses a value average, no necessarily specific of the zone of study, determined from a wide database This is due to the fact that the mechanical properties of a determinate lithology, that control the answer of said material, go to have a margin of variation limited.
Given the similarity in the properties of a lot of lithologies, can agruparlas in function of his mechanical characteristics. In concrete can define four lithologic groups (Lajoie and Helley (1975) and Tinsley and Fumal (1986)).
Group I: Rocks cementadas and consolidated. Hard to very hard. Little altered and/or fracturadas. Speed of shears upper average to 1500 m/s.
Group II: Rocks cementadas and consolidated. Alterables Or very fracturadas. Speed of shears average comprised between 700 and 1500 m/s.
Group III: detrital Sediments of thick grain and rocks weakly cementadas. Speed of shears comprised between 375 and 700 m/s.
Group IV: detrital Sediments of fine and half granulometries: sands, silts and clays. Sediments little compacted and/or consolidated. Speed of low shears, in inferior average to 350 m/s.