High Resolution 1:10,000 scale Mapping Strategy of Multi ... - NDMA

of the macroseismic approach, i.e. that part of seismology dealing with noninstrumental
data. The shape and size of the isoseismal regions can be used to help
determine the magnitude, focal depth and focal mechanism of an earthquake. The
isoseismal maps give very important information about hazard created by an
earthquake at places away from earthquake. The intensity in MM scale and PGA
generated at epicenter by earthquake of different magnitude are given Table App
4.2.
It is seen from isoseismal map that the intensity is highest at the epicenter and at
every 50 km distance from the epicenter the MM scale intensity reduces by one
scale. For example the intensity at epicenter of a magnitude 7 on Richter scale is 10.
The intensity in 100 km circle around the epicenter will be 8 in MM scale. This
estimate is correct to the first order. However the local intensity may be modified due
to under line geo technical and seismotectonics features of the site.
INFLUENCE OF SEISMOTECTONICS ON PGA
The underlying faults play important role in generating PGA on the site. Normally the
faults amplify the earthquake intensity. Similarly the Geotechnical features of the
sites have major effect on PGA. However both these features locally modulate the
isoseismals. The general effect of the earthquake hazard undergoes only minor
modification. We can observe that isoseismal map contours are never circular
centered at epicenter. The distribution of intensity is affected by the intervening
seisomotectonics features and geotechnical features. Both these features can
amplify or attenuate the Seismic Intensity at site.
EFFECT OF GEOTECHNICAL FEATURES ON EARTHQUAKE
Geotechnical features may have a profound influence on the ground motion
characteristics, namely, amplitude, frequency contained and duration. The most
35