Transcript Document

2. Internal Structure
of the Earth
Earth’s Internal Structure:
Compositional Layers

Crust:
 Continental
crust (20-70 km)
 Oceanic crust (~6 km)

Mantle
 Upper
mantle
 Lower mantle (660km 2900 km)

Core
 Outer
core (liquid)
 Inner core
Earth’s Internal Structure:
Mechanical Layers
Lithosphere
 Asthenosphere (weak layer)
 Mesosphere (mantle)
 Outer Core (liquid)
 Inner Core

Earthquake Seismology
I.
II.
III.
Earthquake descriptors
Seismic waves
Earthquake location
What is an earthquake?

An earthquake is the vibration of
Earth produced by the rapid release of
energy
 Energy
released radiates in all
directions from its source, the focus
(or hypocenter)
 Energy propagates in the form of
seismic (elastic) waves
 Sensitive instruments (seismometers)
around the world record the event
What causes an earthquake?
 Earthquakes
are usually
caused by sudden movement
on faults
Basic terminology
(hypocenter)
Earthquake Descriptors
Epicentral angle
How are earthquakes generated ?

Elastic rebound

Earthquake mechanism
Stick: stress builds up on rough surfaces that is
locked
 Slip: sudden slip on the locked surface (focus)
when stress becomes too high
 Vibrations (earthquakes) occur as the deformed
rock “springs back” to its original shape (elastic
rebound)


Earthquakes most often occur along existing
faults whenever the frictional forces on the
fault surfaces are overcome
Foreshocks and aftershocks
 Adjustments
that follow a major
earthquake often generate smaller
earthquakes called aftershocks
 Small earthquakes, called foreshocks, often
precede a major earthquake by days or, in
some cases, by as much as several years
Seismology
The study of earthquake waves,
seismology, dates back almost 2000
years to the Chinese
 Seismographs, instruments that record
seismic waves

 Records
the movement of Earth in
relation to a stationary mass on a
rotating drum or magnetic tape
Basics of waves
frequency(1/T), period(T), amplitude (A),
velocity (V) and wavelength (V/f)
Seismic waves:
Body wave
P wave
S wave
Surface wave
Reyleigh wave
Love wave
Body waves
 Travel
through Earth’s interior
 Two types based on mode of travel
 Primary (P) waves
 Push-pull (compress and expand) motion,
changing the volume of the intervening material
 Travel through solids, liquids, and gases
 Generally, in any solid material, P waves travel
about 1.7 times faster than S waves
 Secondary (S) waves
 shear motion at right angles to their direction of
travel
 Travel only through solids
Body Waves
P and S waves Particle Motion
Surface Waves
Surface waves are BAD !!
Wave Velocity
One dimensional wave equation:
x
x+dx
When the hammer hit the bar, a stress imbalance occurs between x and x+dx:
   ( x  x)   ( x)
Using Taylor expansion,

 ( x  x)   ( x) 
dx  o 2
x

 
dx
x
Recall that:
f  ma
f  dA
Where dA is the cross-section area of the bar.

f  dA 
dxdA  ma
x
But:
m  dxdA
u
a 2
t
2
We have:

 2u
dxdA  ( dxdA) 2
x
t

 2u
 2
x
t
For elastic media, stress is related to strain,
which is measured by the gradient of displacement:
 x  E x
u
x 
x
We have:

 2u
 2
x
t


 u
 2u
 ( E )  E ( )  E 2
x x
x x
x
 2u
 2u
E 2  2
x
t
 2u E  2u

2
2
t
 x
Wave equation:
2
 2u

u
2
c
2
t
x 2
E
c
