Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Earth Systems 3209 Unit: 4 The Forces Within Earth Reference: Chapters 4, 15, 16, 19; Appendix A & B

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Causes and Locations of Earthquakes Unit 4: Topic 4.2 Focus on...  describing the causes of an earthquake.  discussing the relationship between earthquakes and the elastic rebound theory.  identifying the environments where earthquakes occur and relating them to locations along plate boundaries.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Cause of Earthquakes  What is an Earthquake?  An earthquake is the vibration of Earth produced by the sudden, rapid release of energy.  Seismology  When an earthquake occurs, the elastic energy is released and sends out vibrations that travel throughout the Earth. These vibrations are called seismic waves. The study of how seismic waves behave within Earth is called seismology. Text Reference: Pages

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Focus and Epicenter  Earthquake Focus and Epicenter?  Focus:  The exact location within Earth were seismic waves originate and are generated by sudden release of stored elastic energy. Most often located on a pre-existing fault.  Epicenter  The point on the surface of Earth directly above the focus. Earth’s Surface Fault Plane Epicenter Focus

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Cause of Earthquakes  Earthquakes occur when energy stored in elastically strained rocks is suddenly released.  This release of energy causes intense ground shaking in the area near the source of the earthquake (Focus) and sends waves of elastic energy, called seismic waves, in all directions throughout Earth.  Earthquakes can be generated by bomb blasts, volcanic eruptions, and sudden slippage along faults.  Earthquakes are definitely a geologic hazard for people living in earthquake regions, but the seismic waves generated by earthquakes are invaluable for studying the interior of the Earth.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador  How does an Earthquakes Originate?  Movement in areas along the fault plane stops (fault sticks).  Elastic energy is stored in the rock as the rock becomes deformed and bends, much like a bent stick.  When the elastic strain built up along the fault exceeds the elastic limit, the rock will break or slip at its weakest point which we call the the focus.  This slippage along the fault allows the rock to “snap back” and the vibrations sends out waves of energy in all directions called seismic waves, or earthquake waves.  The springing back of the rock is called “elastic rebound”. Cause of Earthquakes

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Elastic Rebound Theory  The mechanism that cause earthquakes was not understood until H. F. Reid proposed his idea of “Elastic rebound.”  Reid suggested that most natural earthquakes are caused by sudden slippage along a fault zone. Original position of rocks before any strain

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador  The elastic rebound theory suggests that if movement along a fault gets stuck, elastic strain energy builds up deforming rocks on either side of the fault. A feature across the fault bends as the rocks on both sides of the fault pushes in opposite directions and elastic strain builds up Fault Elastic Rebound Theory

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador  When the rocks along the fault can no longer hold the strain slippage occurs at the weakest point along a fault (focus) which causes rock to start moving on both sides of the fault.  Energy is released in all directions causing an earthquake. The fault ruptures releasing elastic energy Fault Energy Released Elastic Rebound Theory

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador  When the elastic strain is released the rocks on both sides of the fault will “snap-back” to their unstrained positions. Elastic strain on rocks released Fault Elastic Rebound Theory

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Location of Earthquakes Earthquake Belts:  About 95% of the energy released by earthquakes originate in a few narrow zones that wind around the globe.  The greatest concentration of earthquakes are located along the outer edge of the Pacific Ocean in an area known as the “Pacific Ring of Fire.”

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Location of Earthquakes Earthquake Depths:  Seismic records show that earthquakes originate at depths ranging from 5km to 700km and can be classified according to depth of occurrence.  90% of all earthquakes occur at depths less than 100km. Depths are classified as follows: 1)Shallow Focus (less than 70 km) 2)Intermediate Focus (70km to 300km) 3)Deep Focus (greater than 300km) Text Reference Pages

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Location of Earthquakes Earthquake Depths:  Earthquakes can occur at any of the three plate boundaries. Divergent Plate Boundary:  Earthquakes generated along oceanic ridge systems always have a shallow focus and are not very strong.  Example: Mid Atlantic Ridge.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Location of Earthquakes Earthquake Depths:  Earthquakes can occur at any of the three plate boundaries. Transform Plate Boundary:  Earthquakes generated along transform plate boundaries, have shallow and very powerful earthquakes.  Example: San Andreas Fault.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Location of Earthquakes Earthquake Depths:  Earthquakes can occur at any of the three plate boundaries. Convergent Plate Boundary:  Earthquakes generated along convergent plate boundaries range in depths and are very powerful.  Focus depth increases with distance away form the trench ranging from shallow to deep. This earthquake zone is called the Wadati-Benioff Zone.  Example: Japan Trench.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Example 1: The most common cause of earthquakes is; (A)the collapse of limestone caverns on the borders of continents. (B)the increasing stress between tectonic plates. (C)energy released by volcanic eruptions. (D)the gravitational pull of the moon and sun on the continental landmasses.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Example 2: At which of the following locations would you least likely find earthquakes? (A) transform boundary (B) divergent boundary (C) subduction zone (D) nuclear testing site

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Your Turn... Take the time and complete the following questions... (Solutions to follow) Question: Earthquakes commonly occur at plate boundaries, with reference to elastic rebound, what causes an earthquake?

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Solutions... Question: Earthquakes commonly occur at plate boundaries, with reference to elastic rebound, what causes an earthquake? Answer: There is a build up of stress (energy) within the tectonic plates, where it reaches the elastic limit. The plates then rupture (snap) and the stored energy is released. Plates then move (snap) back to unstressed positions releasing the stored energy as seismic waves. This is what we call an earthquake.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Summary... Overview of Points covered:  Elastic rebound can explain the cause of earthquakes  Earthquakes are limited to narrow areas around the globe. Example: Pacific Ring of Fire  Earthquakes exist at all plate boundaries.  Earthquake depths vary from shallow, intermediate, to deep.