The 2011 Tohoku Earthquake — from Disaster to Knowledge a Presentation by Professor Jonathan Stewart of UCLA Chengdu, July 1, 2011 1 Preface These are the notes I took during Professor Stewart’s presentation. I give no guarantee for accuracy or correctness. Any mistakes and errors in these notes are my own and probably result from my misunderstanding what Professor Stewart said. Please send any corrections to ute.platzer@durham.ac.uk . Ute Platzer, Chengdu, July 2, 2011 2 Introduction Professor Stewart is a professor of Civil engineering at the University of Cali- fornia in Los Angeles. He is working on geotechnical engineering, especially seismic engineering. He is part of the GEER team, a team of scientists that is based in the United States and goes from there to those parts of the world where disasters happen, to study them and learn from them. The first group from the team went to Japan 2 weeks after the earthquake, and the last group only left a few weeks ago. They always work together with local people and local experts. They also did that during then Wenchuan earthquake. They never go into the field on their own. 1
3 The earthquake The Tohoku earthquake happened on March 11, 2011. There was foreshock activity: on March 9 there was one foreshock with M W 7.2, and on March 11 there were more, three of them with magnitudes > 6. Not all earthquakes have foreshocks, and you know only after the mainshock happened whether an earthquake is a foreshock or a mainshock. The Pacific plate moves north-east, subducting beneath the North-Ameri- can plate (which actually extends from North America and the Arctic towards Japan, see figure 1). The convergence rate/movement is up to 8 cm year − 1 , which is very much. Due to the earthquake, there was very high horizontal displacement of up to 4.5 m eastward movement on the east side of Japan, while the west side of Japan moved only about 1 m. Therefore, Japan got wider by 3.5 m during this earthquake. Vertical displacement was only observed in the east of Japan; it moved down by about 1 m, making the effect of the tsunami worse. Along the slip there was horizontal displacement of up to 32 m, which is really huge. 4 Anticipation and Prediction Could this earthquake and its consequences have been foreseen? Since 1973, the beginning of instrumental measurement, there were 9 earthquakes on the Japan trench with M W > 7. There are also well-docu- mented events in recent geologic history in the Sendai area: • 1896, a M W 7.6 event created a tsunami with a run-up height of 38 m. • 1933 a M W 8.6 event resulted in a tsunami with 29 m run-up height. Both of these events happened north of Sendai, and Sendai itself was pro- tected from the full force of the tsunamis by a small peninsula. Therefore, the expected tsunami height in Sendai was only 4 m. This was obviously a misinterpretation of the available data. • In the year 869, a M W 8.3 event created a tsunami with similar extents to the one observed in 2011. See the paper “Entire coastal plane dev- astation in Sendai city” (Okumura 2011). Tsunamis always leave de- posits because they carry a lot of mud and debris. Shortly after the 869 tsunami, there was a volcanic eruption, covering the deposits with ash 2
A North−American plate North Japan America Pacific Ocean B West East North−American Pacific Plate Plate Hanging wall Footwall Figure 1: A, location of the North-American and Pacific plates. B, sub- duction zone of the Japan trench. 3
tsunami wave inundation depth run−up height normal sea level inundation zone Figure 2: Run-up height, inundation depth and inundation zone of a tsunami. and thereby protecting and preserving them. That is why we can use them today to determine the extent of that tsunami. • In 1611, there as another big earthquake which is not well-documented. It created a 6-8 m tsunami in Sendai and Fukushima. Experts concluded that the worst earthquake to be expected in the region was M W 7.4 – 8.2. Historical data was not taken into account. 5 Tsunamis 5.1 How is a tsunami generated? It is created when the sea floor (the hanging wall of a fault) moves upward. This movement happens in several small steps, and that is why a tsunami always consists of several waves and not only one single wave. The speed of the waves is comparable to that of a Boeing 747 airplane. Some definitions (see figure 2): • The run-up height is the elevation at the inundated point which is furthest away from the shore, minus the sea level elevation. • the inundation depth is the water level at a specific point. • the inundation zone is the area which is inundated. In contrast to earthquake damage which is spotty, tsunami damage is com- plete: a tsunami destroys everything in its path. 4
5.2 The effects of the tsunami The tsunami reached a run-up height of 10 m in Sendai, in contrast to the predicted 2–4 m. In the north, the tsunami reached a run-up height between 34–38 m. In the south near Tokio, it was only 2–5 m, and Tokio itself did not experience any tsunami. It was believed that reinforced concrete buildings on piles are tsunami-safe, and it was even suggested that people who cannot reach higher ground seek refuge on top of this kind of building instead. However, in this tsunami, many reinforced concrete buildings were thrown off their foundations and toppled over. They ended up on their side (see figure 3 A). In one case, such a building was moved 23 m by the wave. Perhaps the existing theory on tsunami effects is wrong because experiments are made with clear water, while the water of the tsunami is obviously dirty and laden with debris. 6 The Fukushima nuclear power plant The plant was built in 1971. It has 6 reactors. In the US, nuclear power plants usually have only 1 or a maximum of 2 reactors per site. The first reactor, built in 1971, was designed to withstand peak ground acceleration (PGA) of 0.18g, based on an earthquake that happened in California in the 1950s. Reactors 3 and 6 were built to newer standards to withstand 0.45g. This plant was protected by seawalls, because the tsunami risk was known. The design tsunami was 5.7 m, which was more than the worst expected tsunami of 2–4 m. It seemed to be very, very safe. 6.1 Pre-event state of the reactors Reactor 5+6 were down for maintenance, i.e. not producing electricity but they still required cooling. Unit 4 was de-fuelled, and 1–3 were operating normally. 6.2 Safety systems at the plant Then the earthquake hit at 14:46 local time, it hit with a PGA between 0.4 and 0.5 (the exact values have not been released). This triggered an automatic shutdown in reactors 1–3. After that, external energy was required for cooling and to run the control electronics (usually the energy for this is generated by the reactor itself). 5
Figure 3: Effect of a tsunami on reinforced concrete structures. They topple over. They had a plan B for this, and this is to use the National Power Grid to get electricity from somewhere else. However, the grid was damaged by the earthquake and there was no electricity. Plan C then were a set of Diesel generators to produce the electricity. There were 3 sets of redundant generators in case one set failed. At 15:41 however, the tsunami hit and destroyed all of the Diesel generators. The tsunami reached a height of 12–14 m, more than double the expected 5.7 m. For that case, there was Plan D, the use of backup batteries to provide electricity for 8 hours, but 8 hours were too short to repair the Diesel gen- erators. Therefore they called for backup generators to be brought in, which the got there after 13 hours. But they could not be connected because of the damage to the plumbing etc. After that, nuclear meltdown happened in units 1–3 etc. Question: why did the cooling system work with the backup batteries, but not with the generators? The problems at the plant remain unsolved. The reactors still require con- stant cooling. It’s a difficult situation because the radiation makes it nearly impossible to do any repairs. 7 Risk assessment 7.1 Types of uncertainty • Aleatory uncertainties: known unknowns. Example: PGA values. We know there’s some uncertainty when predicting PGA values, but we know how large this uncertainty is. • Epistemic uncertainty: we just don’t know. Such as forecasting BIG earthquakes: we have not had enough big earthquakes yet to know enough about them to make predictions. We cannot know how big the errors are in predicting them. 6
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