EARTHQUAKE ENGINEERING
MODULE 1
Earthquake engineers asked the following :
1. What is the mechanical explanation for damage or lack thereof when structures are subjected to seismic
strong motion?
2. What are the essential characteristic properties of the seismic wave inputs to the affected structures?
3. What is the seismicity, i.e., a specified region’s earthquake source characteristics?
Seismic sea waves, or TSUNAMIS, are long water waves generated by sudden ground displacements under
oceans. The most common cause is the impulsive displacement along a submerged fault associated with a large
earthquake, although large submarine landslides are often the direct cause of major tsunamis. Because of the
great earthquakes that occur around the Pacific, this ocean is particularly prone to seismic sea waves.
The geological model of plate tectonics provides the most coherent global explanation of the occurrence of
the majority of earthquakes. The basic concept is that the earth’s outermost part, known as LITHOSPHERE,
consists of several large and fairly stable rock slabs called PLATES. Each plate extends to a depth of about 80
km and include the earth’s outermost rigid rocky layer, called the CRUST.
The moving plates of the earth’s surface also provide an explanation of the various mechanisms of seismic
sources. Collisions between adjacent lithospheric plates, destruction of slab-like plates as they descend or
SUBDUCT into a dipping zone beneath island arcs and tectonic spreading along mid-oceanic ridges produce
significant straining and fracturing of the regional crustal rocks. The earthquakes in these tectonically active
boundary regions are called PLATE-EDGE EARTHQUAKES.
Many large earthquakes are produced by slips along faults connecting the ends of offsets in the spreading
oceanic ridges and the ends of island arcs or arc-ridge chains. In these regions, plates slide past each other along
what are called TRANSFORM FAULTS. The Himalayas, the Zagros ( Iran ) and the Alpine regions are
examples of mountain ranges formed by CONTINENT-to-CONTINENT COLLISIONS.
INTRAPLATE EARTHQUAKES can be found on nearly every continent, recent example was the
disastrous Bhuj ( M = 7.7 ) earthquake in western India in the seismically active Kutch province.
During an earthquake, seismic waves radiate from the earthquake source below the ground surface as
opposite sides of a slipping fault rebound in opposite directions thus decreasing the strain energy in the rocks.
Consequently, the seismic source is spread out through a volume of rock. Nevertheless, it is often convenient to
model a simplified earthquake source as a point from which the waves first emanate, this is called the
EARTHQUAKE FOCUS. The point on the ground surface directly above the focus is called the
EARTHQUAKE EPICENTER.
Although many foci are situated at shallow depths, in some regions they are hundreds of kilometers deep,
such regions are the PLATE SUBDUCTION ZONES. On average, the frequency of the occurrence of
earthquakes in these regions declines rapidly below a depth of 200 km, but some foci are as deep as 680 km.
Rather arbitrarily, earthquakes with foci from 70 – 300 km deep are called INTERMEDIATE FOCUS and those
below this depth are termed DEEP FOCUS.
