1. UGRC 144 Science and Technology in Our Lives/Geohazards
Session 3 – Understanding Earthquakes and Earthquake Hazards
Lecturer: Dr. Patrick Asamoah Sakyi
Department of Earth Science, UG
Contact Information:

2. Session Overview
Earthquakes are natural ground motions caused as the Earth releases energy.
The science of earthquakes is seismology, "study of shaking" in scientific Greek. Earthquake energy comes from the stresses of plate tectonics.
As plates move, the rocks on their edges deform and take up strain until the weakest point, a fault, ruptures and releases the strain.
The occurrence of earthquake also results in various hazards that may bring about disasters.
Dr. Patrick A. Sakyi, Dept. of Earth Science

3. Session Outline
The key topics to be covered in the session are as follows:
Topic One – What is an earthquake
Topic Two – Mechanism for earthquake
Topic Three – Seismic Waves
Topic Four - Earthquake instruments and seismic records
Topic Five – Earthquake Measurement & Size
Topic Six - Earthquake Hazards
Dr. Patrick A. Sakyi, Dept. of Earth Science

4. Reading List
Chapter 4 of Environmental Geology 4th Edition – by Carla W Montgomery (1995)
Unit 1, Section 3 of UGRC 140 II Geohazards – Institute of Continuing and Distance Education
Dr. Patrick A. Sakyi, Dept. of Earth Science

5. What is an earthquake Topic One
Dr. Patrick A. Sakyi, Dept. of Earth Science

6. What is an Earthquake
An earthquake is the vibration of Earth produced by the rapid release of energy
This energy radiates in all directions from its Source, the FOCUS, in the form of SEISMIC WAVES.
Dr. Patrick A. Sakyi, Dept. of Earth Science

7. What is an Earthquake Where do earthquakes occur?
Globally, earthquakes occur in well-defined zones marked by tectonic plate boundaries and faults .
Dr. Patrick A. Sakyi, Dept. of Earth Science

8. What is an Earthquake Where do earthquakes occur?
Earthquake epicenters are not randomly distributed over the earth.
Areas that have no history of earthquakes are not likely to experience earthquakes in future.
Dr. Patrick A. Sakyi, Dept. of Earth Science

9. What is an Earthquake Focus and Epicenter
Focus is the point within the Earth where the earthquake starts. It is also called the Hypocenter
Epicenter is the location on the surface directly above the focus.
Faults
Faults are fractures in the Earth where movement has occurred.
Dr. Patrick A. Sakyi, Dept. of Earth Science

10. Mechanism for earthquake
Topic Two
Mechanism for earthquake
Dr. Patrick A. Sakyi, Dept. of Earth Science

11. Mechanism for Earthquake
Elastic Rebound Hypothesis
The mechanism for earthquake generation is termed Elastic Rebound
Most earthquakes are produced by the rapid release of elastic energy stored in a rock that has been subjected to great forces.
When the strength of the rock is exceeded, it suddenly breaks, causing vibrations of the earth, leading to an earthquake
Dr. Patrick A. Sakyi, Dept. of Earth Science

12. Mechanism for Earthquake
Elastic Rebound Hypothesis
It has been noticed that earthquakes occur along large Faults and plate boundaries.
Dr. Patrick A. Sakyi, Dept. of Earth Science

13. Mechanism for Earthquake
Elastic Rebound Hypothesis
Tectonic Forces over tens or hundreds of years slowly deform the crustal rocks on both sides of the fault.
Dr. Patrick A. Sakyi, Dept. of Earth Science

14. Mechanism for Earthquake
Elastic Rebound Hypothesis
Under these conditions, the rocks bend and store elastic energy much like a wooden stick would if bent.
Dr. Patrick A. Sakyi, Dept. of Earth Science

15. Mechanism for Earthquake
Elastic Rebound Hypothesis
Eventually, the Frictional resistance holding the rocks together is overcome.
The vibrations we know as an earthquake occur as the rock elastically snaps back to its original shape.
Dr. Patrick A. Sakyi, Dept. of Earth Science

16. Mechanism for Earthquake
Elastic Rebound Hypothesis
Summary of causes and mechanism for earth quake
Dr. Patrick A. Sakyi, Dept. of Earth Science

17. Mechanism for Earthquake
Aftershocks and Foreshocks
An aftershock is a small earthquake that follows the main earthquake.
Foreshock is a small earthquake that often precedes a major earthquake.
The study of seismic waves related to earthquakes is called Seismology
Scientists who study seismic waves produced by earthquakes are called Seismologists
Dr. Patrick A. Sakyi, Dept. of Earth Science

18. Topic Three
SEISMIC WAVES
Dr. Patrick A. Sakyi, Dept. of Earth Science

19. What is Seismic Wave
Seismic Wave is the energy released when a fault ruptures, causing a vibration of earth in an earthquake
This energy radiates in all directions from its source called the focus.
Seismic waves spreads from the focus in three different forms:
Compression waves, exactly like sound waves (P waves)
Shear waves, like waves in a shaken jump-rope (S waves)
Surface waves resembling water waves.
Dr. Patrick A. Sakyi, Dept. of Earth Science

20. What is Seismic Wave
Surface waves are seismic waves that travel along Earth’s outer layer.
Surface waves are slower still and cause the majority of damage associated with earthquake disasters.
Dr. Patrick A. Sakyi, Dept. of Earth Science

21. What is Seismic Wave
Body Waves, identified as P and S waves are seismic waves that travel in the interior of the earth
P waves
Are push-pull waves that push (compress) and pull (expand) in the direction that the waves travel
Travel through solids, liquids, and gases
Have the greatest velocity of all earthquake waves
P waves always arrive first and do little or no damage.
Dr. Patrick A. Sakyi, Dept. of Earth Science

22. What is Seismic Wave S waves
They are seismic waves that travel along Earth’s outer layer
The shake particles at right angles to the direction that they travel,
Travel only through solids,
Slower velocity than P waves
They may cause damage
Dr. Patrick A. Sakyi, Dept. of Earth Science

23. Earthquake instruments and seismic records
Topic Four
Earthquake instruments and seismic records
Dr. Patrick A. Sakyi, Dept. of Earth Science

24. Earthquake Instrumentation and Seismic Records
Seismographs are instruments that measure and record seismic waves in the earth during an earthquake.
Dr. Patrick A. Sakyi, Dept. of Earth Science

25. Earthquake Instrumentation and Seismic Records
Seismograms are traces of amplified, electronically recorded ground motion made by seismographs
A seismogram shows all three types of seismic waves— surface waves, P waves, and S waves
Dr. Patrick A. Sakyi, Dept. of Earth Science

26. Earthquake measurement and size
Topic Five
Earthquake measurement and size
Dr. Patrick A. Sakyi, Dept. of Earth Science

27. Earthquake Measurement and Size
Historically, scientists have used two different types of measurements to describe the size of an earthquake, namely: intensity and magnitude.
Intensity classifies the degree of shaking and magnitude is a measure of the amount of energy released
Intensity is gauged from inspection of the damage and other effects of an earthquake. It is greatest close to the epicentre, diminishing with distance Intensity
The 12-point Mercalli Scale is used to measure intensity.
Dr. Patrick A. Sakyi, Dept. of Earth Science

28. Earthquake Measurement and Size
Earthquake magnitude is determined from the amplitude of body waves recorded by sensitive seismographs. Earthquake magnitude measures how big an earthquake is, that is, how much energy is released in seismic waves
The Richter Scale is used to measure earthquake magnitude
Based on the amplitude of the largest seismic wave
Each unit of Richter magnitude equates to roughly a 32-fold energy increase
Does not estimate adequately the size of very large earthquakes
Dr. Patrick A. Sakyi, Dept. of Earth Science

29. Earthquake Measurement and Size
Momentum Magnitude
Derived from the amount of displacement that occurs along the fault zone
Momentum magnitude is the most widely used measurement for earthquakes because it is the only magnitude scale that estimates the energy released by earthquakes.
Measures very large earthquake
Dr. Patrick A. Sakyi, Dept. of Earth Science

30. Earthquake Measurement and Size
Dr. Patrick A. Sakyi, Dept. of Earth Science

31. Earthquake haZARDS Topic Six
Dr. Patrick A. Sakyi, Dept. of Earth Science

32. Earthquake Hazards
Possible hazards from earthquakes can be classified as follows:
Ground Motion
Fire
Landslides
Changes in Ground Level
Tsunami
Flooding
Liquefaction
Dr. Patrick A. Sakyi, Dept. of Earth Science

33. Earthquake Hazards
Ground Motion - Shaking of the ground caused by the passage of seismic waves, especially surface waves, near the epicenter of the earthquake are responsible for the most damage during an earthquake and is thus a primary effect of an earthquake.
The intensity of ground shaking depends on
Local geologic conditions in the area.  In general, loose unconsolidated sediment is subject to more intense shaking than solid bedrock.
Size of the Earthquake.  In general, the larger the earthquake, the more intense is the shaking and the duration of the shaking.
Distance from the Epicenter.  Shaking is most severe near the epicenter and drops off away from the epicenter.  The distance factor depends on the type of material underlying the area.
Damage to structures from shaking depends on the type of construction.
Dr. Patrick A. Sakyi, Dept. of Earth Science

34. Earthquake Hazards
The structural damage include buildings and other structures such as railways, roads, dams, telephone and power transmission poles and lines
Factors that determine structural damage;
Intensity and duration of the vibrations
Nature of the material upon which the structure is built
The design of the structure
Concrete and masonry structures are brittle and thus more susceptible to damage
wood and steel structures are more flexible and thus less susceptible to damage.
Dr. Patrick A. Sakyi, Dept. of Earth Science

35. Earthquake Hazards Fire - Fire is a secondary effect of earthquakes.
Because power lines may be knocked down and because natural gas lines may rupture due to an earthquake, fires are often started closely following an earthquake.
The problem is compounded if water lines are also broken during the earthquake since there will not be a supply of water to extinguish the fires once they have started.
Dr. Patrick A. Sakyi, Dept. of Earth Science

36. Earthquake Hazards
Faulting and Ground Rupture - Ground rupture generally occurs only along the fault zone that moves during the earthquake, and are thus a primary effect.
Thus structures that are built across fault zones may collapse
whereas structures built adjacent to, but not crossing the fault may survive
Dr. Patrick A. Sakyi, Dept. of Earth Science

37. Earthquake Hazards
Landslides - In mountainous regions subjected to earthquakes ground shaking may trigger the following, all of which are secondary effects; 
Landslide
Rock and debris falls
Rock and debris slides
Slumps
Debris avalanches
Dr. Patrick A. Sakyi, Dept. of Earth Science

38. Earthquake Hazards
Tsunami - Tsunami a secondary effect that are giant ocean waves that can rapidly travel across oceans.
Earthquakes that occur beneath sea level and along coastal areas can generate tsunami, which can cause damage thousands of kilometers away on the other side of the ocean. 
A tsunami, triggered by an earthquake occurs where a slab of the ocean floor is displaced vertically along a fault.
Dr. Patrick A. Sakyi, Dept. of Earth Science

39. Earthquake Hazards
A tsunami also can occur when the vibration of a quake sets an underwater landslide into motion
Tsunami is the Japanese word for “harbour wave.”
Dr. Patrick A. Sakyi, Dept. of Earth Science

40. Earthquake Hazards
Changes in Ground Level - A secondary or tertiary effect that is caused by faulting.
Earthquakes may cause both uplift and subsidence of the land surface.
This causes changes in ground level
Dr. Patrick A. Sakyi, Dept. of Earth Science

41. Earthquake Hazards
Flooding - Flooding is a secondary effect that may occur due to rupture of human made dams and levees, due to tsunami, and as a result of ground subsidence after an earthquake
Dr. Patrick A. Sakyi, Dept. of Earth Science

42. Earthquake Hazards
Liquefaction (soil) describes a phenomenon where by a water-saturated or partially saturated soil or unconsolidated sediment substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid.
In areas underlain by such material, the ground shaking causes the grains to lose grain to grain contact, and thus the material tends to flow.
Dr. Patrick A. Sakyi, Dept. of Earth Science

43. Summary
Dr. Patrick A. Sakyi, Dept. of Earth Science

44. END
Dr. Patrick A. Sakyi, Dept. of Earth Science