1. MOTHER NATURE’S RUMBLINGS
EARTHQUAKES
MOTHER NATURE’S RUMBLINGS

2. What is that force?
Earthquake – a vibration of the solid earth produced by the very rapid release of energy
The energy that drives these quakes is derived from the earth’s interior
The motions of the earth’s plates are frequently the cause of this rapid energy release

3. Why bother in the first place?
Why do we study these earthquakes?
Pure scientific curiosity
Try and learn how to predict them
Gives us a view of the earth’s interior
So we can try and avoid tragedies like the one shown here
In 1985, an earthquake measuring over 8.0 struck outside of Mexico City
An estimated 25,000 people were killed in this tragedy

4. Where in the world are they?
This map show world earthquake distribution
What familiar pattern do these earthquake locations line up with?
That’s right, the tectonic plates!

5. Where do they happen? Focus – The point of origin of an Earthquake
The earth’s lithosphere ruptures, and energy radiates out in all directions
Most earthquakes do not occur directly at the surface
Epicenter – a location on the surface directly above the focus
A diagram showing a fault with an earthquake focus occurring at depth with the epicenter directly above it

6. How deep?
Although it’s possible, most earthquakes do not happen right at the earth’s surface
The depth that they occur, depends upon the plate boundary
Convergent boundaries produce the largest quakes and also the deepest ones—600 km depth is maximum
Transform boundary quakes can also be large but are generally shallow—less than 80 km below the surface
Divergent boundaries produce weaker, shallower quakes as well
This map and cross section of the Japanese Island Arc shows the range of earthquake depths found at convergent subduction zones

7. Waves on the Water
Just as waves radiate out from a stone being dropped in water, so will earthquake waves radiate out in all directions
EQ waves will radiate out from the focus in 3-D
Energy dissipates very rapidly through solid rock, but sensitive instruments can pick up, that is “listen for” seismic events all around the globe

8. Whose fault is it? Most earthquakes occur along faults or fault zones
Fault – a fracture in the earth’s crust along which there is movement
Some are a few km in length, others 1000s of km long
Faults come in all varieties of angles, depths, and shapes

9. Earthquake Processes
No one knew how earthquakes worked until early in the 20th century
H. F. Reid studied the 1906 San Fran quake and came up with a theory called elastic rebound
Just like a stick under pressure, rocks will bend to a certain point and will then rupture and release stored energy with violent shaking

10. Understand the Physics
Elastic rebound can be explained once the idea of stress and strain is understood
Stress = pressure
Strain = deformation
Rocks in the crust are under incredible amounts of pressure and will slowly deform
Once the rocks’ strength is exceeded they will rupture causing vibrations
There are different kinds of stress as demonstrated by the above diagram

11. The Theory Behind the Shake
Elastic Rebound – sudden released of stored strain in rocks
The result of this rebound is sudden movement along a fault
The rupture of this fault trace in California was formed due to elastic rebound
The strength of the rocks could only hold so much stress and then they gave way

12. What comes before and after?
Foreshocks – small earthquakes preceding a major quake by days or even possibly years
Aftershocks – the settling or adjustment of crustal rocks after a large earthquake

13. Seismology
The entire branch of geology dedicated to studying earthquakes and earthquake waves is called seismology
Seismographs – the instruments used to measure and record seismic waves
Almost 2000 years ago the Chinese made the very first seismograph
The original seismograph was invented by the Chinese 2000 years ago
Metal spheres were delicately balanced in mouths of dragons, and the slightest vibrations would cause them to fall out

14. Modern Seismograph
Modern seismographs incorporate the principle of inertia
Inertia – an object at rest will stay at rest, & an object in motion will stay in motion
Seismographs are placed at depth away from background noise
Always in a group of three to measure all three dimensions of movement (3-D)
A modern seismograph has a pen attached to a weight on a spring; the ground shakes the rotating drum while the pen remains stationary due to inertia and traces out the quake

15. Look at the Data
When seismographs record earthquake activity onto paper it is called a seismogram
Top: simple seismogram showing the arrival of different P & S waves
Bottom: a long track seismogram showing a major earthquake accompanied by both a foreshock and an aftershock

16. To Review:
Earthquakes are the result of a rapid release of stored energy from within the crust
The focus is where the EQ occurs and the epicenter is on the surface directly above the focus
EQ happen along faults which are long cracks in the earth’s crust
Earthquake energy radiates out in all directions like ripples on a pond
H. F. Reid came up with theory of elastic rebound when studying the great 1906 quake of San Fran
This theory is used to explain EQ even today
Stress (pressure) causes strain (deformation)
Seismograph is the instrument used to measure EQ and it operates on the law of inertia
Seismograms are recorded earthquakes on paper