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.

Vocabulary

• amplitude
• body waves
• crest
• earthquake
• elastic rebound theory
• focus
• seismology
• surface waves
• trough
• tsunami
• wavelength

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).

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.

In an earthquake, the initial point where the rocks rupture in the crust is called the focus. The epicenter is the point on the land surface that is directly above the focus. 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).

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.

Earthquake Zones

Nearly 95% of all earthquakes take place along one of the three types of plate boundaries, but earthquakes do occur along all three types of plate boundaries.

• 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.

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. 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).

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.

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).

New Zealand also has strike-slip earthquakes, about 20,000 a year! Only a small percentage of those are large enough to be felt. A 6.3 quake in Christchurch in February 2011 killed about 180 people.

Convergent plate boundaries

Earthquakes at convergent plate boundaries mark the motions of subducting lithosphere as it plunges through the mantle (Figure below). Eventually the plate heats up enough deform plastically and earthquakes stop.

Convergent plate boundaries produce earthquakes all around the Pacific Ocean basin. The Philippine Plate and the Pacific Plate subduct beneath Japan, creating a chain of volcanoes and as many as 1,500 earthquakes annually.

In March 2011 an enormous 9.0 earthquake struck off of Sendai in northeastern Japan. This quake, called the 2011 Tōhoku earthquake, was the most powerful ever to strike Japan and one of the top five known in the world. Damage from the earthquake was nearly overshadowed by the tsunami it generated, which wiped out coastal cities and towns (Figure below). Two months after the earthquake, about 25,000 people were dead or missing, and 125,000 buildings had been damaged or destroyed. Aftershocks, some as large as major earthquakes, have continued to rock the region.

A map of aftershocks is seen here: http://earthquake.usgs.gov/earthquakes/seqs/events/usc0001xgp/.

Here is an interactive feature article about the earthquake: http://www.nytimes.com/interactive/2011/03/11/world/asia/maps-of-earthquake-and-tsunami-damage-in-japan.html.

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 earthquake in India was responsible for about 20,000 deaths, and many more people became injured or homeless.

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 the result of 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.

Seismic Waves

Energy is transmitted in waves. Every wave has a high point called a crest and a low point called a trough. The height of a wave from the center line to its crest is its amplitude. The distance between waves from crest to crest (or trough to trough) is its wavelength. The parts of a wave are illustrated in Figure below.

The energy from earthquakes travels in seismic waves, which were discussed in the chapter “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 “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).

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 were about 4 meters (13 feet) high in Somalia.

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.

The 2011 Tōhoku earthquake in Japan created massive tsunami waves that hit the island nation. As seen in Figure below, waves in some regions topped 9 meters (27 feet). The tsunami did much more damage than the massive earthquake (Figure below). Worst was the damage done to nuclear power plants along the northeastern coast.

As a result of the 2004 tsunami, an Indian Ocean warning system was put into operation in June 2006. Prior to 2004, no one had thought a large tsunami was possible in the Indian Ocean. In comparison, a warning system has been in effect around the Pacific Ocean for more than 50 years (Figure below). Why do you think a Pacific warning system has been in place for so long? The system was used to warn of possible tsunami waves after the Tōhoku earthquake. People were evacuated along many pacific coastlines although the waves were not nearly as large as those that struck Japan shortly after the quake.

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 travel faster than 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

• The U.S. Geological survey earthquake site is found here: http://earthquake.usgs.gov/.
• Fault line discusses seismic science: http://www.exploratorium.edu/faultline/index.html.
• How the geography of the Pacific Northwest reflects the plate tectonic features is found here: http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/interactive/2.NWplateRollover.swf.

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?