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IITK-BMTPC Earthquake Tip 2

How the ground shakes? page 2
local soil (Figure 1). They carry distinct information
regarding ground shaking; peak amplitude, duration of
strong shaking, frequency content (e.g., amplitude of
String shaking associated with each frequency) and energy
Magnet content (i.e., energy carried by ground shaking at each
frequency) are often used to distinguish them.
Pendulum Bob Peak amplitude (peak ground acceleration, PGA) is
Pen physically intuitive. For instance, a horizontal PGA
value of 0.6g (= 0.6 times the acceleration due to
Support Rotating
Drum gravity) suggests that the movement of the ground can
cause a maximum horizontal force on a rigid structure
equal to 60% of its weight. In a rigid structure, all
Chart Paper Direction of points in it move with the ground by the same
Ground Shaking Recorded amount, and hence experience the same maximum
acceleration of PGA. Horizontal PGA values greater
than 1.0g were recorded during the 1994 Northridge
Figure 3: Schematic of Early Seismograph
Earthquake in USA. Usually, strong ground motions
One such instrument is required in each of the two carry significant energy associated with shaking of
orthogonal horizontal directions. Of course, for frequencies in the range 0.03-30Hz (i.e., cycles per sec).
measuring vertical oscillations, the string pendulum 1985 Mexico Earthquake (SCT 1A; N90E)
(Figure 3) is replaced with a spring pendulum
oscillating about a fulcrum. Some instruments do not 1940 Imperial Valley Earthquake (El Centro; S00E)
have a timer device (i.e., the drum holding the chart
paper does not rotate). Such instruments provide only 1971 San Fernando Earthquake (Pacoima Dam; N76W)
the maximum extent (or scope) of motion during the
earthquake; for this reason they are called seismoscopes. 0 10 20 30 40 50 60
The analog instruments have evolved over time, Time (sec)
but today, digital instruments using modern computer 0.5g 1991 Uttarkashi Earthquake (Uttarkashi, N75E)
technology are more commonly used. The digital
instrument records the ground motion on the memory Figure 4::
of the microprocessor that is in-built in the instrument. Some typical recorded accelerograms
Strong Ground Motions
Shaking of ground on the Earth’s surface is a net Generally, the maximum amplitudes of horizontal
consequence of motions caused by seismic waves motions in the two orthogonal directions are about the
generated by energy release at each material point same. However, the maximum amplitude in the
within the three-dimensional volume that ruptures at vertical direction is usually less than that in the
the fault. These waves arrive at various instants of horizontal direction. In design codes, the vertical
time, have different amplitudes and carry different design acceleration is taken as 1 to 2 3 of the
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levels of energy. Thus, the motion at any site on
ground is random in nature with its amplitude and horizontal design acceleration. In contrast, the
direction varying randomly with time. maximum horizontal and vertical ground accelerations
Large earthquakes at great distances can produce in the vicinity of the fault rupture do not seem to have
weak motions that may not damage structures or even such a correlation.

be felt by humans. But, sensitive instruments can Reading Material
record these. This makes it possible to locate distant Bolt,B.A., (1999), Earthquakes, Fourth Edition, W. H. Freeman and
earthquakes. However, from engineering viewpoint, Company, New York, USA
strong motions that can possibly damage structures
are of interest. This can happen with earthquakes in Authored by:
the vicinity or even with large earthquakes at C.V.R.Murty
reasonable medium to large distances. Indian Institute of Technology Kanpur
Characteristics of Strong Ground Motions Kanpur, India
The motion of the ground can be described in Sponsored by:
terms of displacement, velocity or acceleration. The Building Materials and Technology Promotion
variation of ground acceleration with time recorded at Council, New Delhi, India
a point on ground during an earthquake is called an
accelerogram. The nature of accelerograms may vary This release is a property of IIT Kanpur and BMTPC New
(Figure 4) depending on energy released at source, Delhi. It may be reproduced without changing its contents
type of slip at fault rupture, geology along the travel and with due acknowledgement. Suggestions/comments
may be sent to: nicee@iitk.ac.in. Visit www.nicee.org or
path from fault rupture to the Earth’s surface, and www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips.

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