Mechanism of the Occurrence of Earthquakes and Tsunamis
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Session I Disaster Preparedness and Response: Earthquake & Tsunami Mechanism of the Occurrence of Earthquakes and Tsunamis JMAJ 50(1): 55–71, 2007 Yoshinobu TSUJI*1 Thank you, Mr. Chairman. Good morning, every- boundaries have pushed into each other and body. accumulated stress, and once in 100 years on Two years ago, at the end of 2004, a great earth- average a great earthquake will happen. Next quake with a magnitude of 9.0 happened near the slide please. south tip of Sumatra Island, Indonesia. Due to This is the configuration of the earth’s surface. this earthquake, a great tsunami happened, and You see here the Pacific Rim—the Japanese Is- in total, nearly 250,000 people were killed. This lands, Taiwan, the Philippines, Indonesia, Papua is the biggest tsunami event in our history. New Guinea, and New Zealand—and the South Today I will give a lecture on the mechanisms American Continent are the plate boundaries. of the generation of earthquakes and tsunamis, And here the Pacific Plate is running westward, and then we will consider how to survive from 9 cm per year, and sinking down below the Japa- these natural disasters. The first slide please. nese Islands, Taiwan, the Philippines, and the In- donesian Archipelago. Once every 100 years or Mechanism of earthquakes and tsunamis several hundred years, a great earthquake has This figure shows a view of the 1896 Great happened near this rim area. Remember that Sanriku Earthquake Tsunami in the northern the medical doctors have come here today from part of Honshu Island, Japan. You can see that Pacific Rim countries. All our countries, all your once the tsunami happened, all peacefully living countries, could possibly be attacked by a huge coastal people would have been killed by the tsunami. This is because all the countries of the covering sea water. As a result of this event, in Pacific Rim are very close to the plate bound- total 22,000 people were killed due to the tsunami aries, and over 80% of tsunamis happen around only, not the earthquake. Next slide please. the Pacific Rim. Next slide please. The earth’s surface is covered with about 25 This is a schematic drawing of plate motion. plates. These so-called plates are like conveyer The plate was generated here and the magma belts. One plate is generated somewhere and flow is coming from deep within the earth and is sinks down like a belt conveyer below the neigh- like a belt conveyer sinking down at the Pacific boring plate. Here are the Japanese Islands, and Rim Archipelago. Here is the plate boundary, and here is the American coast, and the Pacific Plate stress is accumulated here; and finally, disloca- is moving westward—9-cm pike here. And here tion will happen on the surface of this boundary. the plate is sinking down because the plate That is the mechanism of gigantic plate bound- *1 Associate Professor, Earthquake Research Institute, University of Tokyo, Tokyo, Japan ([email protected]). JMAJ, January/February 2007 — Vol. 50, No. 1 55 Tsuji Y ary tsunamis, gigantic tsunamis. Next slide please. plate boundary. On this back side another group This figure shows the distribution of earth- of earthquakes happened. We call this group of quakes of magnitudes exceeding 6.0 in the 20th earthquakes “backed-up earthquakes”, but their century up to now. As we can see, the black frequency is not as frequent as this kind of earth- mark shows earthquakes generated at the plate quake, say a ratio of four-to-one. Next please. boundary. We see here the Japanese Islands, To the north of Papua New Guinea, the Paci- Taiwan, the Philippines, Indonesia and Papua fic Plate moves down from the north, and here a New Guinea—these countries facing the Pacific group of earthquakes happened, and here—the coast in particular have experienced a series Irian Jaya area of Indonesia—is very unstable, of earthquakes. Another group of earthquakes and several times huge earthquakes have happen- happened on the south coast of the Indonesian ed, and these have been accompanied by tsunami Archipelago. This is another plate boundary attacks on this coast. Here the plate boundary is where the Indian/Australia plate moves from very close to the coast, and once an earthquake south to north at a speed of 5 cm per year and has happened, a tsunami attacks this coast very sinks down below Sunda Island in the Indo- rapidly. For example, a tsunami will attack this nesian Archipelago, and here another group of coast within ten minutes or so—only 10 minutes earthquakes occurred. Next slide please. or so. Next slide please. If a plate is sinking down underneath the neighboring plate, the stress will be accumulated, Past earthquakes and tsunamis in Indonesia and finally the plate boundaries will shift. For ex- Next we’ll look at the size of earthquake/tsunamis ample, once every 100 years, suddenly the upper in Indonesia. Around Indonesia, not the Philip- plate will be pushed up and the sea bottom will pine sea plate but the Indian/Australian plate is suddenly move upward, then the sea water be- sinking down at the Sunda Bench, and the great comes disturbed, and a wave will be generated. earthquake of 2004 took place here, and the most Next slide please. recent event, in 2005, took place here, and on Once the plate boundaries have shifted—for July 17th of 2006—this year—another earthquake example, 5 m or 9 m—then the sea bottom will happened here in the central part of Java Island, be pushed up by 2 or 3 m at a depth of 4,000 to here, off Chillachapel Port, and a tsunami was 6,000 m in the deep sea and a wave will be gen- generated and around 500 people were killed due erated and it will begin to approach the coast to the tsunami. In 1994, a serious group of earth- line. At the coast, on the continental shelf, water quakes, gigantic earthquakes, took place at the depth is only 200 m. Here the depth is 4,000 m plate boundary and the trench is here. Another and here it is only 200 m. The energy of the exceptional case was in 1992, the Flores Island wave is contained within 4,000 m of thickness; event, which was a backed-up event here. and then it will be condensed to only 200 m of In the Indonesian case, the epicentral area of water thickness. Therefore, the energy will be the earthquakes was a distance from the main- condensed and the amplitude of the wave will be land of Java Island or Sunda Island, of course, amplified at the coast. Consequently, even if the and there was enough time after the earthquake wave height of the tsunami is only 2 m in the in this case, the East Java event of 1994. A tsu- deep sea, it will be amplified three or four times nami attacked the coast 14 minutes or 15 minutes at the coast, and here, say 6 or 8 m of water, after the earthquake. In 2006, the tsunami struck a rising wave, will appear at the coast and the after 15 minutes, and in the Sunda event, it struck coastal town will be submerged by sea water. nearly one hour after the earthquake—enough Next slide please. time for people to escape to a higher place. But There is one example of the Philippines Archi- in backed-up earthquake types, such as the 1992 pelago, with Fiji on the plate boundary, and here Flores event, the epicentral area is very close a group of earthquakes happened in this century, to the coast, and once it has happened, the tsu- up here, in the sea west of Mindanao Island; and nami attacks the coast in only two minutes or near the Philippines there is a plate boundary three minutes. around here that crosses to the East China Sea, Now, the meteorological agency of Indonesia and here another group of earthquakes hap- has made efforts to establish a system for the pened. And here there is a sunken trench and the issuance of tsunami warnings. Therefore, in the 56 JMAJ, January/February 2007 — Vol. 50, No. 1 MECHANISM OF THE OCCURRENCE OF EARTHQUAKES AND TSUNAMIS near future they will be able to successfully see the columns of this building were reinforced broadcast tsunami warnings through television steel. Not a weak building, but a strong building, programs to the people as the earthquake hap- but even this strong building was broken at the pens and the objective information about a tsu- root of this column. Next please. nami attack will be successfully broadcast. But This is a high school, Banda Aceh Fifth Middle for backed-up event, the warning may not reach High School. You can see that before the tsu- people in time. Next slide please. nami, the second floor of the building was here, but due to the tsunami, this part—the second 2004 Sumatra earthquake (Banda Aceh) floor of a concrete building—was carried away. Here we see the 2004 Sumatra earthquake event. The sea water was up to 56 m above the ceiling The simultaneous damage to places in Banda of the second floor, 8.8 m above mean sea level. Aceh city at the north point of Sumatra Island. Next please. OK. Next slide please. We measured the tsunami height at the court Banda Aceh city is here at the north point —12.2 m. Under the innermost area, the height of Sumatra Island.