Dr. Christine Jurasinski
LabLearner Staff Scientist
On Saturday February 27th, the world was once again reminded of the awesome forces that are at work on our planet as an earthquake awakened Chileans in the early morning. The earthquake, which was centered just off the coast of Chile, measured an 8.8 on the Richter scale and generated enough force to generate tsunamis across the Pacific Ocean. The quake came only a day after a 7.0 earthquake in Ryukyu, Japan and a month and a half after the 7.0 earthquake that devastated Haiti.
What is happening here? Are we experiencing more earthquakes than in previous periods of history? While the answer is likely “No, we just are a more populated and therefore a more aware, more affected and more technologically savvy planet than in the past.” these are questions that scientists are actively researching. Their research takes them into the fields not only of geology, but also physics, fields that LabLearner students explore as they investigate principles in CELLs such as Forms of Energy, Earth’s Forces, Potential and Kinetic Energy, and Sound Waves and Pressure and in GAP Units such as Earth’s Changing Surface, The Changing Earth, and Geologic Time.
The earthquake that struck Chile occurred along the boundary that separates two tectonic plates: the Nazca Plate and the South American Plate.
As LabLearner students learn in the The Changing Earth GAP, this is a type of convergent boundary known as a subduction zone. In this particular boundary the Nazca plate moves below or beneath the South American Plate at a rate of 8.9 cm per year.
As this occurs, there are places in which the plates encounter resistance and get stuck. Tension builds between the plates until it becomes so great that the plates “rupture.” To understand rupture, think of a safety pin with the needle of the pin tucked safely under the protective casing. When the needle is depressed and moved out of the area of the casing, it springs open. There is a recoil associated with the release of the needle. This is similar to what happened on Saturday. The two plates were stuck and the tension was so great that the crust on the South American plate suddenly “sprung” above the Nazca plate. As a result, the Earth’s crust was lifted and then settled. This type of movement is called elastic rebound and results, as you can imagine, in the displacement of the Earth’s surface.
However, what seems relatively mild in description produces devastating results. Much of the damage in an earthquake is due to this elastic rebound. However, seismic waves, which are generated as the rupture occurs, also contribute to the devastation. Seismic waves are essentially sound waves that originate from the earth as the rupture is occurring. As the rupture occurs, the waves move the particles of earth in different directions. P waves are seismic longitudinal waves that alternately compress and expand particles of the solid and liquid portion of the Earth through which they move. LabLearner students studying the Sound Waves and Pressure CELL should be familiar with these types of waves as longitudinal waves are responsible for the generation of sound in solids, liquids and gases. In addition to P waves, the rupture causes another type of deep wave within the Earth’s surface called S waves and two types of waves on the surface of the Earth. It is the surface waves that cause additional horizontal and vertical movement of the Earth’s crust and much of the damage from an earthquake.
But as illustrated by Saturday’s event in Chile, the damage caused by the earthquake itself may not be the only concern. Often earthquakes can set off tsunamis- huge masses of moving water that when coming ashore produce their own destruction to both coast and inland areas. Tsunamis occur when there is a significant enough displacement of water to generate waves that have incredibly long wavelengths. They can be caused by earthquakes such as the one in Chile, by volcanic eruptions, and by landslides. Much of what is understood about tsunamis is still evolving.
One of the most devastating tsunamis occurred on December 26, 2004 off the coast of Sumatra and much of what we understand and how we predict tsunamis has come from that event. Luckily the tsunamis that resulted from Chilean earthquake on February 27th were nowhere near the same power. So, how were the tsunamis this past weekend generated?
Again, imagine the events that happened between the two converging plates. The build up of tension resulted in the plates that were “stuck” to suddenly move. In doing this, the South American plate was vaulted upward momentarily and then came back down. What then happened was the column of water above the plate was also pushed upward and then pulled back down by gravity. The result was a wave with a crest and trough that then spread outward like a ripple in a pond creating multiple waves extending from the epicenter of the quake. The energy that drove those waves came from the potential energy of the water as it was lifted. As gravity acted on the uplifted water, the gravitational potential energy was transferred into the kinetic energy of a moving wave. The higher the lift, the greater the potential energy and the greater the kinetic energy of the wave, a concept that LabLearner students explore in the Forms of Energy and Kinetic and Potential Energy CELLs.
What makes tsunami waves different from other types of ocean waves are their potentially crippling power. This power stems in part from their incredibly long wavelengths. As students learn in the Sound Waves and Pressure CELL, the wavelength is the distance between the crest or trough of one wave to the crest or trough of another. The ocean waves generated by wind that we are used to seeing have wavelengths of about 150 m. Tsunami waves are longitudinal waves that have wavelengths of 100 km. This means that normal ocean waves occur with a wave passing approximately every 10 seconds where as the time between successive waves in a tsunami can be more on the order of one every hour. In addition, tsunami waves move at incredible speeds such as 700 km/hr, which means they can travel huge distances across an ocean without losing much energy.
But one of the most intriguing things about tsunamis is the relationship between the height of the wave, its wavelength and the depth of the water. Tsunami waves generated in deep water tend to have relatively small amplitudes (heights) and long wavelengths. This is because the mass and energy of the wave is spread over a large and deep column of water. Thus, when these waves are in the open ocean they are only a few centimeters high and not easily detected by ships or cameras. As a result, tsunami waves are considered shallow water waves. As the tsunami wave moves closer to shore, the rising land of the continental slope or shore decrease the depth of water. Because of the Law of Conservation of Energy, the wavelength of the wave decreases and the height or amplitude of the wave increases. Although the wave slows from 700 km/hr (500 mph) to 50 km/hr (30 mph) as a result of friction with the land and as a result of a decrease in water depth, the incredible energy of the wave is “compacted” into a series of waves closer together and with much greater heights. Waves that impact the shore can be up to 9 meters (30 ft) in height. Thus, these waves can contain an enormous amount of energy and may move water inland for miles with great force. In addition, tsunami waves often do not break like normal ocean waves and dissipate energy. Rather, they move in like tides as great walls of water.
The tsunamis of December 26, 2004 were such waves. Luckily, the tsunamis of February 2010 were not. Tsunamis did reach Hawaii, Japan, the Philippines and the west coast of the United States. However, the amplitude of the waves as the reached shore ranged from several centimeters to approximately 1.8 meters (6 feet). Why these tsunamis differed from those that occurred in Sumatra will be heavily investigated for months to come.
However, what is becoming clear is that at this time in our species history, we can benefit from engaging our students’ interest in the forces that drive and govern our planet. Enhancing their understanding of the geology, physics, chemistry and biology of our planet can ultimately lead to better predictive models, warning systems, preparedness and even solutions in our planet’s future.
Find out more about both earthquakes and tsunamis by investigating the links below.
Tsunamis
http://walrus.wr.usgs.gov/tsunami/basics.html
http://www.ess.washington.edu/tsunami/index.html
http://www.pbs.org/wgbh/nova/tsunami/anatomy.html
http://www.tsunami.noaa.gov/
http://www.tsunami.noaa.gov/tsunami_story.html
http://www.pbs.org/wnet/savageearth/tsunami/index.html
http://faculty.gvsu.edu/videticp/waves.htm
Earthquakes
http://www.pbs.org/wnet/savageearth/index.html
http://www.seismo.unr.edu/ftp/pub/louie/class/100/seismic-waves.html
http://www.geo.mtu.edu/UPSeis/waves.html
http://pubs.usgs.gov/gip/dynamic/dynamic.html
