UGRC 144 Science and Technology in Our Lives/Geohazards

Transcription

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: pasakyi@ug.edu.gh Dr. Patrick Asamoah Sakyi Department of Earth Science, UG, Legon College of Education School of Continuing and Distance Education 2014/ /2017

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 Slide 2

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 Slide 3

4 Reading List Chapter 4 of Environmental Geology 4 th 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 Slide 4

5 Topic One WHAT IS AN EARTHQUAKE Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 5

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 Slide 6

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 Slide 7

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 Slide 8

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 Slide 9

10 Topic Two MECHANISM FOR EARTHQUAKE Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 10

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 Slide 11

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 Slide 12

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 Slide 13

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 Slide 14

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 Slide 15

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

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 Slide 17

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

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 Slide 19

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 Slide 20

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 Slide 21

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 Slide 22

23 Topic Four EARTHQUAKE INSTRUMENTS AND SEISMIC RECORDS Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 23

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

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

26 Topic Five EARTHQUAKE MEASUREMENT AND SIZE Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 26

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 Slide 27

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 Slide 28

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 Slide 29

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

31 Topic Six EARTHQUAKE HAZARDS Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 31

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 Slide 32

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 Slide 33

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 Slide 34

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 Slide 35

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 Slide 36

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 Slide 37

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 Slide 38

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 Slide 39

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 Slide 40

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 Slide 41

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 Slide 42

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

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