Earthquakes Module 5

Tectonics & Earthquakes
Vanessa Helland 
Part of Waves of Devastation, a class website on the Indian Ocean Tsunami & Global Environmental Injustice, produced by students of Geography 378 (International Environmental Problems & Policy) at the University of Wisconsin-Eau Claire, USA, Spring 2005.


    The theory of plate tectonics is that the Earth's outer layer, the crust, is made up of plates. These plates have moved throughout Earth's history and continue to move as much as a few inches a year. This theory explains the creation of mountains, the existence of earthquakes and volcanoes, and the existence of similar animals living on continents that are separated by the seas.

    Source: Environmental Physical Geography

The world has nine major plates and many smaller plates. The nine major plates are the North American, South American, Eurasian, African, Indo-Australian, Antarctic, Pacific, Nazca, and Cocos. The relatively smaller size of the other plates does not diminish their significance or impact.

There are three types of plate boundaries: divergent boundaries, convergent boundaries, and transform boundaries.

Plate Boundaries

Divergent Boundaries: This type of boundary is created when two plates are pulling away from each other. Divergent boundaries can form in the middle of a continent, but eventually the rift that is created from the separation forms ocean basins. It is considered a constructive boundary as the space it creates is filled with molten magma that forms below. An example of this is the Great Rift Valley in Africa.

Convergent Boundaries: This type of boundary is created when two plates collide, and is known as a subduction zone. Mountains and/or volcanoes are generally found along these boundaries. There are three types of convergent boundaries: Oceanic-Continental Convergence; Oceanic-Oceanic Convergence; and Continental-Continental Convergence. The subduction of one plate is decided by the density of the plates. Generally the denser plate will subduct beneath the other. An example of continental-continental convergence would be the Himalayas.

Source: Introduction to Plate Tectonics

Transform Boundaries: This type of boundary is formed by two plates sliding horizontally by each other. These boundaries are more commonly known as faults. They generally offset active spreading ridges on the ocean floor with a few occurring on land. An example of this type of plate boundary is the San Andreas Fault in California.

Source: Introduction to Plate Tectonics



Plate boundaries are important as they are the major source of earthquakes. Merriam-Webster Online defines an earthquake as the shaking or trembling of the earth that is volcanic or tectonic in origin. Because of friction, the plates do not move easily past each other; instead stress builds until it reaches the slipping point of rocks on either side of the fault. The energy that is released results in an earthquake. Earthquakes can happen any where in the world, but more than 90% of them happen along plate boundaries. The map below shows the epicenter of earthquakes in 1963-1998. The earthquake locations effectively delineate the plate boundaries.

Source: Digital World Tectonic Activity Map (DTAM)

Earthquakes are an everyday occurrence all over the world, but most are minor and cause no damage. Large earthquakes, on the other hand, can result in massive damage and loss of life. Earthquakes not only cause the ground to shake and sometimes rupture, but the effects of the shaking and rupturing can cause other events to occur such as inundation (tsunami, seiche), ground failure (liquefaction, landslide), fire, or a release of hazardous materials.

Social Impacts of an Earthquake

Two earthquakes in December 2003 demonstrate the uneven effects of earthquakes on different economic groups. The San Simeon earthquake of California on December 22 and the Bam, Iran earthquake on December 26, are two earthquakes of equal strength which had disproportionate social impacts. Both earthquakes were similar in style and intensity but culture and geography left Iranians worse off than the Californians.

When the earthquake hit San Simeon it had been the first of its size in the area since 1952. The earthquake was measured at a 6.5 magnitude and was caused by reverse faulting. Located near several faults, most of the city was prepared for an earthquake. The buildings that took the most damage were old masonry buildings that had not been retrofitted to withstand an earthquake, however many of them had been retrofitted and survived the earthquake. The California Seismic Safety Commission reported 2 deaths, 47 seriously injured, 290 homes damaged, and 191 commercial structures damaged. This earthquake caused enough damage to be declared a disaster by the president and the rebuilding of San Simeon is well underway with $20.1 million of federal relief money.

Area of heaviest damage in San Simeon

When the first tremors came to Bam, Iran at 4 am Friday the 26 of December, some people came out of their houses into the streets but shortly thereafter went back into their houses as tremors were not that uncommon for the area. At 5:27 am, an earthquake of 6.6 magnitude hit. The cause of the earthquake was reverse faulting and strike slip faulting within the zone of deformation. The city of Bam is an ancient city, most of the buildings were made of adobe brick and mud; none of the buildings were prepared to handle an earthquake of this magnitude. After the dust had settled, according to the International Federation of Red Cross and Red Crescent Societies, there were 43,000 dead, 30,000 injured, and 75,000 left homeless with over 85 percent of the buildings destroyed. Rescue workers reported that the collapsing mud-brick structures had completely disintegrated and buried people in piles of earth, rather than trapping them in air pockets between building slabs, as would happen in a concrete building collapse. Even though the relief phase is now over it was thought unlikely that the rebuilding of Bam would be completed within the next two years (as of August 2004). Bam is slowly being rebuilt due largely to the International Federation of Red Cross and Red Crescent Societies which have posted an appeal for $42 million.

Click on the Photo's for larger Before and After pictures of Bam, Iran
Bam before the earthquake
Bam after the earthquake

The Recent Earthquakes of Sumatra

On December 26, 2004, exactly one year after the Bam earthquake, an earthquake of 9.0 magnitude occurred 160 km off the western coast of Sumatra along the Burma and Indian plate boundary. Here the Indian plate is subducting under the smaller Burma plate. It is estimated by the U.S. Geological Survey (USGS) that the resulting rupture was 1200 km in length along the Sunda Trench and a width of over 100 km. A large portion of the ocean sea floor was thrust upwards, resulting in a tsunami. The earthquake itself caused initial damage in northern Sumatra, Indonesia, and the Nicobar Islands; however the tsunami was much more devastating and affected a larger area. The height of the tsunami reached 30 meters in some areas.

Another earthquake that occured on March 28, 2005 was a 8.7 magnitude earthquake and occured 100 miles southeast of the December 26, 2004 Sumatra quake. It has been questioned as to whether or not this earthquake is simply an aftershock of the December earthquake, but it occurred on a different fault line. Initially there was fear that this earthquake would generate a tsunami similar to the 2004 earthquake, but fears were put to rest as the largest tsunami seen was only approximately 3 meters in height. The map below shows the general location of both earthquakes and the resulting aftershocks. Click on it for a larger version.

Source: USGS Earthquake Hazards Program - Latest Earthquakes

USGS researchers recently published an article titled "USGS Research Sheds Light on Why the March Tsunami was Smaller than the December Tsunami." The article gives four main reasons for the smaller tsunami in March. The first was that the smaller magnitude of the March earthquake. The magnitude is a function of the rupture area and the amount of slip. The second was the depth of the water where the earthquakes occured. March's earthquake was about six-tenths of a mile underwater and December's earthquake was 1-2.5 miles under water. The deep water of December's quake resulted in larger amplification of the tsunami as it traveled from the source region of the quake to shore. The third reason given by USGS was the depth under the earth where the fault slip occured. The majority of the falut slip was 12 to 25 miles below surface on March 28; December's slip may have extended all the way to the sea floor. This factor along with the greater magnitude of the December earthquake "resulted in greater vertical movement of the sea floor," according the USGS. The last factor for the smaller tsunami in March was the difference in the primary direction of the tsunami wave. December's tsunami was focused in an east-west direction where as March's tsunami was focused southwest, away from nearby land. The March earthquake occured mainly under the island shelf of Sumatra, so the island itself blocked most of the wave activity that would have been produced.

Quake Shook Entire Planet

Scientists: Sumatra quake longest ever recorded:
Temblor big enough to 'vibrate the whole planet '

Marsha Walton, CNN, Thursday, May 19, 2005

Dramatic new data from the December 26, 2004, Sumatran-Andaman earthquake that generated deadly tsunamis show the event created the longest fault rupture and the longest duration of faulting ever observed, according to three reports by an international group of seismologists published Thursday in the journal "Science."
"Normally, a small earthquake might last less than a second; a moderate sized earthquake might last a few seconds. This earthquake lasted between 500 and 600 seconds (at least 10 minutes)," said Charles Ammon, associate professor of geosciences at Penn State University.
The quake released an amount of energy equal to a 100 gigaton bomb, according to Roger Bilham, professor of geological sciences at the University of Colorado. And that power lasted longer than any quake ever recorded.
The quake, centered in the Indian Ocean, also created the biggest gash in the Earth's seabed ever observed, nearly 800 miles. That's as long as a drive from northern California into southern Canada.
Scientists estimated the average slippage (ground movement up and down) along the entire length of the fault was at least 5 meters (16.5 feet) -- with some places being moved nearly 20 meters (50 feet).
Scientists have also upgraded the magnitude of the quake from 9.0 to between 9.1 and 9.3, a dramatically more powerful event. As a comparison: the ground shook 100 times harder during December's earthquake than what was felt in the 1989 Loma Prieta quake in California. That 6.9 magnitude quake caused extensive damage from Santa Cruz to San Francisco.

Monster quake
The stunning power of Asia's earthquake and tsunamis last December has left even veteran scientists in awe.
"I think it was humbling for everyone that analyzed the earthquake," said Thorne Lay, professor of earth sciences and director of the Institute of Geophysics and Planetary Physics at the University of California, Santa Cruz.
"We're sitting in our laboratories working on the signals from this earthquake, trying to understand what happened scientifically, and then watching TV at night and seeing the death toll rising for weeks," he said.
The enormous human toll from the natural disasters spurred Lay to organize dozens of scientists from all over the world to share their data and analysis of the quake. The long-term goal is to try to get more, and more accurate tsunami warning systems in place.

Whole planet vibrated
A wide array of instruments were used for the first time to study the earthquake, and its many aftershocks.
Global broadband seismometers recorded the ground in Sri Lanka, a thousand miles from the epicenter, moved up and down by more than 9 centimeters (3.6 inches), according to the report.
But no place on Earth escaped movement.
"Globally, this earthquake was large enough to basically vibrate the whole planet as much as half an inch, or a centimeter. Everywhere we had instruments, we could see motions," Ammon said.
Much of that information came from digital broadband seismometers, a new era of instruments that the National Science Foundation and the U.S. Geological Survey began deploying around the world several years ago.
Lay says the equipment is sensitive enough to pick up the motion of wind blowing through trees, or cows walking in a field, or the massive motions produced by this earthquake.
"We'd never seen signals from an earthquake of this size, and the availability of this instrumentation was a real breakthrough in being able to see the complete rupture process of one of these truly monstrous events," Lay said.
Other tools added to the scientists' understanding. Underwater cameras documented the huge crack in the ocean floor. Tsunami buoys, and sonar from the British Navy helped with the analysis.
And a fortunate bit of timing enabled researchers to get a view of the tsunami they have never seen before.
"Two hours after the earthquake has occurred, the wave is spreading out from the Bay of Bengal," Lay said. "Two satellites went over, with the capability of measuring the elevation of the ocean surface. The satellites saw the south-going wave and the north-going part of the wave. "It was just good luck that the passage of the satellites caught the tsunami in motion," he said.
Crunching numbers, and creating maps and models is taking on a new urgency for some of the scientists involved in this research.
"There will be more earthquakes of this type, and with more humans exposed to the hazard there will be more devastating losses of life. What we hope to do is develop technologies that can minimize that loss," Lay said.


For more information on this topic:

Plate Tectonics - Wikipedia :

Plate Tectonics:

A Science Odyssey:

USGS Earthquake Hazards Program:

Red Cross Red Crescent - Iran:

Vingenuity Incorporated:

BAM - Iran's Ancient City:

California Seismic Safety Commission:

USGS Release:

2005 Sumatran Earthquake:

2004 Indian Ocean Earthquake: