4. Focus & Epicentre
• The point from which an earthquake originates
is called the ‘focus’ and the point on the earths
surface, directly above this is called the
‘Epicentre’.
7. P - WAVE
• Primary(P) waves cause the particles of rock
to vibrate backwards and forwards in the
wave direction.
8. S - WAVE
• Secondary(S) waves cause the particles to
oscillate at right angles to the wave direction.
9. L - WAVE
• When P and S waves reach the surface they are converted into long(L)
waves.
• They cause the particles to travel in transverse horizontal motion,
perpendicular to the direction of propagation and generally parallel to the
Earth’s surface.
• It is the L waves that produce the most violent shocks and the resultant
destruction.
10. RAYLEIGH WAVE
• Motion is both in the direction of propagation and
perpendicular (in a vertical plane), and “phased” so
that the motion is generally elliptical – either
prograde or retrograde.
11. SEISMOGRAPH
• The instrument used to detect and record the seismic
waves is called “Seismograph” and the record, which
appears like a graph is called “Seismogram.”
12.
13. Richter Scale
• Richter scale is used to measure the magnitude of
the earthquakes. A magnitude of 2 is rarely felt,
while a magnitude of 7 indicates the lower limit of an
earthquake that has devastating effect over a large
area.
