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History Channel's How the Earth Was Made is a wonderful earth science documentary series. The episode titled "Tsunami" is the story of the history of tsunamis, and the relationship between these cataclysmic ocean waves and earthquakes. The episode is set up as a type of detective story, with evidence shown to back up a hypothesis. I haven’t found a viewable version of the video on YouTube. An acceptable version is posted on DailyMotion. I watched the video on the Season 1 DVD.
The video worksheet is a one-page, double-sided handout consisting of 45 multiple choice questions that track the progress of the video. This format enables the students to pay attention to the video while quickly recording their answers. In this way, the students are not bogged down in writing long responses, and they can better enjoy watching the video. A key is included, and the files are provided in both MS Word and PDF formats.
History Channel’s How the Earth Was Made: Tsunami Overview
Tsunami waves are associated with underwater earthquakes and represent some of the earth’s most destructive natural disasters. The energy released by motions in the earth’s crust is transformed into monstrous waves that can quickly ravage coastlines and consume everything in their path. Tsunami danger is present all around the Pacific Ocean’s Ring of Fire, and also in the Indian Ocean and Atlantic Ocean. Tsunamis are of great concern due to the increase in coastal populations. The 2004 Indian Ocean event was the most apocalyptic tsunami of recent times. In an instant, an earthquake released the energy equivalent of one year of United States energy consumption, and the resulting tsunami was responsible for the deaths of 250,000 men, women, and children.
Tsunami waves have been present since the beginning of earth's oceans. When newly formed, the moon was much closer and filled the sky. The gravitational pull of the infant moon created mile-high tsunamis that ravaged the primeval earth. Today’s tsunamis are not related to the moon’s gravity, instead they are the product of disasters such as earthquakes, landslides, and meteor impacts.
In Japanese, the word tsunami means “harbor wave”, and Japan is the world’s tsunami hotspot. The island nation has suffered several devastating tsunamis in its history, with the most recent occurring in 2011 (which occurred after this video was made-see my NOVA: Japan’s Killer Quake Video Questions). According to records dating to 684 AD, Japan has been struck by a tsunami nearly every seven years. The video documents the destructive tsunamis of 1896 (Honshu) and 1993 (Okushiri).
Hawaii is also plagued by tsunamis, and the city of Hilo has been called the “tsunami capital of the world”. Hawaii is home to the Pacific Tsunami Warning System (PTWS), which monitors predictive changes in sea level. In 1960, the PTWS forecasted the arrival of a tsunami generated by a 9.5 magnitude earthquake in Chile, which is still the largest earthquake ever observed. This event demonstrated that tsunami waves could travel thousands of miles from their source, and that they can retain nearly all of their destructive potential.
The Pacific Northwestern United States also shows evidence of cataclysmic tsunamis. Native American folklore speaks of gigantic waves, and such stories prompted geologist Dr. Brian Atwater to look for evidence of past tsunamis. His excavations near the coast revealed deposits of sand representing tsunamis that date back to 5,000 years in the past. A recent sand deposit was found positioned above the remains of a Native American fishing camp, which supported the veracity of the Native American legends. Dr. Atwater also investigated a stand of dead trees, a “ghost forest”, which had perished due to inundation by salt water. Tree ring analysis indicated that the trees had died sometime between 1680 and 1720 AD, and that the ground itself had sunk during an earthquake, which allowed the ocean waters to flood the land. Intriguingly, Japanese records tell of a massive tsunami that struck in the year 1700 AD. The Japanese called this event an “orphan tsunami” because a strong earthquake had not preceded it. (The Japanese had learned to associate tsunamis with earthquakes, which were usually a clear indication that disaster was imminent.) The 1700 orphan tsunami was likely generated near the Pacific Northwestern coast of the United States by a magnitude 9.0 earthquake. This earthquake struck along the submerged Cascadia Fault, which is located too far away to be felt in Japan. The tsunami waves generated by the earthquake traveled thousands of miles and struck Japan, their virulence hardly affected by the long distance of travel.
Dr. Atwater’s research has led not only to a confirmation of the 1700 tsunami, but also to an increased awareness and preparation for tsunamis along the Pacific Northwestern coast of the United States. Similar work in the Indian Ocean by Dr. Kerry Sieh has led to some relatively precise forecasts of future tsunamis. Prediction is dependent upon recognizing the cycles of earthquakes and where they will likely occur. Dr. Sieh does research in the Mentawai Islands, which are located along the Sunda Megathrust, the undersea fault line that produced the cataclysmic 2004 Indian Ocean tsunami. He studies coral reefs, which are built from the skeletons of millions of tiny sea creatures, and which are only able to thrive underwater. Coral exposed to air will stop growing, and this feature has allowed Dr. Sieh to study the upheavals of the earth due to movement along the Sunda Megathrust. His work has demonstrated the elastic nature of the ocean floor crust under the Mentawai Islands, where the crust is slowly forced downward by the motion of an adjoining section of earth’s crust, a tectonic plate. Eventually, the downward motion of the crust halts and suddenly releases and springs up in a manner similar to the action of a diving board. The shapes of corals near the Metawai Islands document this up and down cycle of motion. By studying “supercycles”, cycles within cycles, Dr. Sieh predicted an earthquake that occurred in 2007, and predicted that another would occur in the next 30 years. (In 2010, after this video was produced, a magnitude 7.8 earthquake occurred in the Mentawai Islands, and also generated a destructive tsunami.) Dr. Sieh’s work has enabled the island residents to prepare for future tsunamis. School children and other inhabitants are instructed to seek high ground after an earthquake, and roads were constructed from the shore to higher ground for easier access.
Tsunamis can also be generated by landslides and meteor impacts. A landslide-generated tsunami, termed a “megatsunami”, occurred in remote Lituya Bay, Alaska in 1958. Part of the bay’s shoreline collapsed due to a small earthquake, and the resulting landslide created a massive wave taller than the Empire State Building! This destructive wave scoured the shorelines of trees leaving behind a rind of bare rock. According to the video, the potential for a megatsunami also exists in the Atlantic Ocean’s Canary Islands. These volcanic islands are located near the western shore of Africa. The largest island, La Palma, is composed of two massive volcanoes joined together into a single island. The active side of La Palma, named Cumbre Vieja, erupted in 1971. The existence of a large fault, and coincident volcanic craters from the 1971 eruption, has led Dr. Simon Day to propose that part of the island could collapse under the stress of a large earthquake. Dr. Day claims that 1/6th of the island could crash into the ocean in a single massive landslide. Such an event would generate titanic waves that would quickly travel across the ocean and inundate the coastal cities of Europe, the United States, and other North American countries. Upon reaching the United States, the waves would have grown to a staggering height of 3,000 feet! The video points to the 1980 eruption of Mt. St. Helens as an example of a similar volcanic collapse. Dr. Day has also documented a past La Palma landslide and tsunami that occurred 65,000 years ago.