Lesson Objectives

  • Be able to identify an earthquake focus and its epicenter.
  • Identify earthquake zones and what makes some regions prone to earthquakes.
  • Compare the characteristics of the different types of seismic waves.
  • Describe how tsunamis are caused by earthquakes, particularly using the 2004 Boxing Day Tsunami as an example.

Introduction

An earthquake is sudden ground movement caused by the sudden release of energy stored in rocks. Earthquakes happen when so much stress builds up in the rocks that the rocks rupture. The energy is transmitted by seismic waves. Each year there are more than 150,000 earthquakes strong enough to be felt by people and 900,000 recorded by seismometers!

Causes of Earthquakes

The description of how earthquakes occur is called elastic rebound theory (Figure below).
external image images?id=309300
Elastic rebound theory. Stresses build on both sides of a fault, causing the rocks to deform plastically (Time 2). When the stresses become too great, the rocks break and end up in a different location (Time 3). This releases the built up energy, and creates an earthquake.
Elastic rebound theory in an animation http://earthquake.usgs.gov/learn/animations/animation.php?flash_title=Elastic+Rebound&flash_file=elasticrebound&flash_width=300&flash_height=350
external image images?id=309314
In the vertical cross section of crust, there are two features labeled:
This animation shows the relationship between focus and epicenter of an earthquake: http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::640::480::/sites/dl/free/0072402466/30425/16_04.swf::Fig.%20%2016.4%20-%20Focus%20of%20an%20Earthquake
In about 75% of earthquakes, the focus is in the top 10 to 15 kilometers (6 to 9 miles) of the crust. Shallow earthquakes cause the most damage because the focus is near where people live. However, it is the epicenter of an earthquake that is reported by scientists and the media (Figure above).

Earthquake Zones

Nearly 95% of all earthquakes take place along one of the three types of plate boundaries but all three types of plate boundaries have earthquakes.
  • About 80% of all earthquakes strike around the Pacific Ocean basin because it is lined with convergent and transform boundaries (Figure below).
  • About 15% take place in the Mediterranean-Asiatic Belt, where convergence is causing the Indian Plate to run into the Eurasian Plate.
  • The remaining 5% are scattered around other plate boundaries or are intraplate earthquakes.
external image images?id=325753
This map of magnitude 8.0 and greater earthquakes since 1900 highlights the Pacific Basin and Himalaya regions.

Transform plate boundaries

Deadly earthquakes occur at transform plate boundaries. Transform faults have shallow focus earthquakes. Why do you think this is so? The faults along the San Andreas Fault zone produce around 10,000 earthquakes a year. Most are tiny, but occasionally one is massive (Figure below). In the San Francisco Bay Area, the Hayward Fault was the site of a magnitude 7.0 earthquake in 1868. The 1906 quake on the San Andreas Fault had a magnitude estimated at about 7.9 (Figure below).
external image images?id=309276
The San Andreas Fault zone in the San Francisco Bay Area.
external image images?id=309293
The 1906 San Francisco earthquake is still the most costly natural disaster in California history. About 3,000 people died and 28,000 buildings were lost, mostly in the fire.
Recent California earthquakes
  • 1989: Loma Prieta earthquake near Santa Cruz, California. Magnitude 7.1 quake, 63 deaths, 3,756 injuries, 12,000+ people homeless, property damage about $6 billion.
  • 1994: Northridge earthquake on a blind thrust fault near Los Angeles. Magnitude 6.7, 72 deaths, 12,000 injuries, damage estimated at $12.5 billion.
external image images?id=310804
Although California is prone to many natural hazards including volcanic eruptions at Mt. Shasta or Mt. Lassen, and landslides on coastal cliffs, the natural hazard the state is linked with is earthquakes. In this video, the boundaries between three different tectonic plates and the earthquakes that result from their interactions are explored (9b): http://www.youtube.com/watch?v=upEh-1DpLMg (1:59).

Convergent plate boundaries

Earthquakes at convergent plate boundaries mark the motions of subducting lithosphere as it plunges through the mantle (Figure below).
external image images?id=325749
This cross section of earthquake epicenters with depth outlines the subducting plate with shallow, intermediate, and deep earthquakes.
Convergent plate boundaries produce earthquakes all around the Pacific Ocean basin (Figure below). The Philippine Plate and the Pacific Plate subduct beneath Japan, creating a chain of volcanoes and as many as 1,500 earthquakes annually.
external image images?id=309310
Destruction in Kobe, Japan, from the 1995 Great Hanshin Earthquake in which 6,434 people died.
The Pacific Northwest of the United States is at risk from a potentially massive earthquake that could strike any time. Subduction of the Juan de Fuca plate beneath North America produces active volcanoes, but large earthquakes only hit every 300 to 600 years. The last was in 1700, with an estimated magnitude of around 9.
An image of earthquakes beneath the Pacific Northwest and the depth to the epicenter is shown here: http://pubs.usgs.gov/ds/91/.
Elastic rebound at a subduction zone generates an earthquake in this animation: http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/5/AOTF5_Subduction_ElasticRebound480.mov
Massive earthquakes are the hallmark of the thrust faulting and folding when two continental plates converge (Figure below). The 2001 Gujarat, India, earthquake was responsible for about 20,000 deaths with many more people injured or made homeless.
external image images?id=321255
The 2005 Kashmir earthquake
external image images?id=310804
In Understanding Earthquakes: From Research to Resilience, scientists try to understand the mechanisms that cause earthquakes and tsunamis and the ways that society can deal with them (3d): http://www.youtube.com/watch?v=W5Qz-aZ2nUM (8:06).

Divergent Plate Boundaries

Earthquakes at mid-ocean ridges are small and shallow because the plates are young, thin, and hot. On land where continents split apart, earthquakes are larger and stronger.

Intraplate Earthquakes

Intraplate earthquakes are caused by stresses caused by plate motions acting in solid slabs of lithosphere. In 1812, a magnitude 7.5 earthquake struck near New Madrid, Missouri. The earthquake was strongly felt over approximately 50,000 square miles, and altered the course of the Mississippi River. Because very few people lived there at the time, only 20 people died. Many more people live there today (Figure below). A similar earthquake today would undoubtedly kill many people and cause a great deal of property damage.
external image images?id=309286
The New Madrid Seismic Zone is within the North American plate. Around 4,000 earthquakes have occurred in the region since 1974.

Seismic Waves

Energy is transmitted in waves. The parts of a wave are defined in the figure (Figure below).
external image images?id=315494
Every wave has a high point called a
The energy from earthquakes travels in seismic waves, which were discussed in the chapter on Plate Tectonics. The study of seismic waves is known as seismology. Seismologists use seismic waves to learn about earthquakes and also to learn about the Earth’s interior. The two types of seismic waves described in the lesson about Plate Tectonics, P-waves and S-waves, are known as body waves because they move through the solid body of the Earth. P waves travel through solids, liquids and gases. S-waves only move through solids. Surface waves travel along the ground, outward from an earthquake’s epicenter. Surface waves are the slowest of all seismic waves, traveling at 2.5 km (1.5 miles) per second. There are two types of surface waves (Figure below).
external image images?id=309285
P-waves move material forward and backward in the direction they are traveling. The material returns to its original size and shape after the P-wave goes by. S-waves move up and down, perpendicular to the direction the wave is traveling. This motion produces shear stresses.
In an earthquake, body waves produce sharp jolts. The rolling motions of surface waves do most of the damage in an earthquake.
Interesting earthquake videos are seen at National Geographic Videos, Environment Video, Natural Disasters, Earthquakes: http://video.nationalgeographic.com/video/player/environment/ Titles include:
  • Earthquake 101
  • “Inside Earthquakes” looks at this sudden natural disaster.

Tsunami

Tsunami are deadly ocean waves from an earthquake. The sharp jolt of an undersea quake forms a set of waves that travel through the sea entirely unnoticed. When they come onto shore, they can grow to enormous heights. Fortunately, few undersea earthquakes generate tsunami.
How a tsunami forms is shown in this animation: http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::640::480::/sites/dl/free/0072402466/30425/16_19.swf::Fig.%2016.19%20-%20Formation%20of%20a%20Tsunami
The Boxing Day Tsunami of December 26, 2004, was by far the deadliest of all time (Figure below). The tsunami was caused by the 2004 Indian Ocean Earthquake. With a magnitude of 9.2, it was the second largest earthquake ever recorded. The extreme movement of the crust displaced trillions of tons of water along the entire length of the rupture. Several tsunami waves were created, with about 30 minutes between the peaks of each one. The waves that struck nearby Sumatra 15 minutes after the quake reached more than 10 meters (33 feet) in height. The size of the waves decreased with distance from the earthquake and in Somalia were about 4 meters (13 feet) high.
external image images?id=309308
The countries that were most affected by the 2004 Boxing Day tsunami.
About 230,000 people died in eight countries (Figure below), with fatalities even as far away as South Africa, nearly 8,000 kilometers (5,000 miles) from the earthquake epicenter. More than 1.2 million people lost their homes and many more lost their ways of making a living.
external image images?id=309275
The Boxing Day tsunami strikes a beach in Thailand.
As a result of the 2004 tsunami, an Indian Ocean warning system was put into operation in June 2006. A warning system has been set up around the Pacific Ocean for more than 50 years. Why do you think that is the case? (Figure below).
external image images?id=309302
This sign indicates a tsunami hazard zone in California.

Lesson Summary

  • During an earthquake, the ground shakes as stored up energy is released from rocks.
  • Elastic rebound theory states that rock will deform plastically as stresses build up until the stresses become too great and the rock breaks.
  • Earthquakes occur at all types of plate boundaries.
  • The Pacific Ocean basin and the Mediterranean-Asiatic Belt are the two geographic regions most likely to experience quakes.
  • Surface waves do the most damage in an earthquake.
  • Body waves travel through the planet and arrive at seismograms before surface waves.
  • Tsunami are deadly ocean waves that are caused by undersea earthquakes.

Review Questions

  1. What is an earthquake’s focus? What is its epicenter?
  2. Why do most earthquakes take place along plate boundaries?
  3. Using elastic rebound theory, describe what triggers an earthquake.
  4. Why are there far more earthquakes around the Pacific Ocean than anywhere else?
  5. What causes intraplate earthquakes?
  6. Besides the San Andreas Fault zone, what other type of plate boundary in or near California can produce earthquakes?
  7. Using plate tectonics and elastic rebound theory, describe why Juan de Fuca plate subduction produces so few earthquakes. What will happen in the future?
  8. What type of faulting is found where two slabs of continental lithosphere are converging?
  9. What are the characteristics of body waves? What are the two types?
  10. What types of materials can P-waves travel through and how fast are they? Describe a P-wave’s motion.
  11. What material can S-waves travel through and how fast are they? Describe an S-wave’s motion.
  12. How are surface waves different from body waves? Which are more damaging?

Further Reading \ Supplemental Links

Vocabulary

wavelength The distance from crest to crest or trough to trough between two waves. tsunami A deadly set of waves caused by an undersea earthquake in which large amounts of seawater are displaced. Tsunami rise high on a beach and can travel far inland. trough The lowest point of a wave. surface waves Seismic waves that travel along the ground surface; they do the most damage. seismology The study of seismic waves including earthquakes and the Earth’s interior. seismic wave Seismic waves transport the energy released during an earthquake. focus The point where rocks rupture during an earthquake. epicenter The point on the Earth’s surface that lies above an earthquake’s focus. elastic rebound theory How earthquakes are generated. Stresses cause strain to build up in rocks until they can no longer bend elastically and they break, causing an earthquake. earthquake Ground shaking caused by the release of energy stored in rocks. crest The highest point of a wave. body waves Seismic waves that travel through the body of a planet; e.g. primary or secondary waves. amplitude The height of a wave from the center to the top of the crest (or the bottom of the trough).

Points to Consider

  • Do the largest earthquakes cause the most deaths and the most damage to property?
  • The last time there was a large earthquake on the Hayward Fault in the San Francisco Bay area of California was in 1868. Use elastic rebound theory to describe what may be happening along the Hayward Fault today and what will likely happen in the future.
  • Why is California so prone to earthquakes?
  • How could coastal California be damaged by a tsunami? Where would the earthquake occur? How could such a tsunami be predicted?