# What is an Earthquake?

```Earthquakes and Earth’s Interior
1. What are Earthquakes
2. Locate and Measure EQ
3. Use EQ to understand
Earth’s Structure
What is an Earthquake?
• Earthquake = ground shaking by waves
• Waves are caused by the passing of ENERGY
1
Definition of Earthquake:
Motion of earth caused by transmission of energy (wave)
released from breaking strained rocks (in lithosphere)
EQ focus vs. epicenter
Focus = location in Earth where breaking occurs
Epicenter = projection of Focus onto surface
2
Earthquakes &amp; Faults
• EQs generated on faults
• Faults = failure by stress (more on faults later)
• The rupture/failure of a fault is caused by stresses
Earthquakes and Plate Tectonics:
• Thus, most Earthquakes are located
at plate margins where tectonic
stresses are greatest.
• Recall: EQs define lithospheric
plate boundaries
3
The Elastic Rebound Theory
• How faults create earthquakes
• Rocks deform or strain due to tectonic stresses
(elastic deformation)
• Failure occurs (rupture strength exceeded) at one
spot/point (focus) in rock when elastic deformation
can no longer accommodate stress.
• Strained rocks around focus rebound, releasing
energy in the form of waves (Earthquake!)
• Draw on board…
4
Elastic Rebound Theory
Types of Seismic Waves
P-wave
animation
S-wave
5
Locating Earthquakes:
Seismographs and difference in wave speeds
Principle ideas behind locating earthquakes
P-wave velocity &gt; S-wave velocity
Thus there is a lag time in the arrival of the P and S waves at a
seismometer.
Given the velocity of these waves through the Earth and the lag
time, one can calculate a distance.
Seismograph
Animation
6
How many seismic stations are required to locate
the focus of an earthquake?
7
Local or Richter Magnitude: ML = log10 (A/T) + f(d,D)
Or
Moment magnitude: Mw=2/3log10&micro;Au – 6.0
A = wave amplitude, T = wave period, d =
distance, D = depth
Note ML is function of depth and Distance
to Focus - not desirable.
Mw is ‘complex’ but really just = amount of
slip * length of rupture * depth of rupture *
strength of rock
Mw better than Richter because Mw does
not ‘saturate’ (give low values) at high
magnitude – BETTER measurement of
energy.
Note these are log10 scales:
For one unit increase, 10x wave amplitude
increase
For one unit increase 32-time energy
increase
December, 2005 Earthquake and Tsunami: M = 9, fourth largest since
1900. Earth rang like a bell for more than a month afterward.
EQ lasted 7 minutes, displacement of up to 20 m along a 1200 km rupture.
Released strain = to sum of all EQs between 1976 and 1990.
&gt;220,000 people killed by Tsunami (most lethal disaster in human history).
Bbc Animation
8
The Stress Map
• Stresses focused at plate boundaries
• Types of plate boundary and related stresses:
– Convergent = compressional stress
– Divergent = tensional stress
– Transform = shear stress
9
Stresses and their resultant Faults
•Tension = Normal Fault
•Compression = Reverse
and Thrust Faults
•Shear = Strike-slip Fault
Faulting
Animation 1
Animation 2
10
11
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