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is at risk. The San Andreas Fault near Los Angeles is a hotspot, as is the Bosporus region, almost all of Italy, Portugal, Iran, China, New Zealand. The list of regions threatened with a major earthquake is long, and some 1500 quakes of magnitude 5 or more occur every year.

No explicit warning of imminent danger

Many of these tremors cause damage only in sparsely populated regions − the ground surface is suddenly raised or a road offset by a few meters. But large numbers of cities with millions of in hab­it ants are located in seismically active zones. More than 6 million people live in Santiago de Chile, metropolitan Los Angeles is home to 18 million. Peking has 18 million, Istanbul 17 million and Tokyo 35 million inhabitants. Seismolo­gists can now simulate the physical ef­fects of single earthquakes pretty well, allowing them to make recommenda­tions as to where one can construct se­cure buildings and what areas to avoid. The simulations can estimate the ex­pected acceleration of the ground sur­face, a particularly important parameter in this context. This tells us how violent­ly, and in what direction, the surface at any given point is likely to move when a quake of a certain magnitude strikes.

Heiner Igel and his team have performed such simulations for various regions, in­

When an earthquake shook the Upper Rhine Valley early this year, the police and the fire brigade were bombarded with anxious phone­calls. The strongest tremor had a magnitude of 4.4 on the Richter scale. “This sort of shock is not unexpected,” says Heiner Igel. “Quakes of up to magnitude 6.5 can occur in the Cologne Basin.” Seismologists are not excitable by nature, and come out quite casually with remarks that make others flinch. The Richter scale is logarithmic, so a quake of magnitude 6.5 is a hun dred times more powerful than one of 4.5. And such an event in the Cologne Basin would hit big cities like Cologne and Düsseldorf, perhaps causing loss of life and damage worth millions of Euros. Then Heiner Igel, Professor of Geophysics at LMU, adds: “It is impossible to reliably predict when such a strong quake might occur.” It might come thousands of years hence, or it might happen tomorrow. We just don’t know.

This is the seismologist’s dilemma. The locations of the endangered zones are known. The magnitude of the next quake in each zone can be estimated – but not the time. So it is impossible to give people explicit warning of imminent danger. If you live in Haiti, you must reckon with the possibility; in Japan and on the west coast of Indonesia perceptible earth­quakes occur every day. The whole Pa­cif ic coast of Central and South America

On the brink

Waiting for the Big One. Many of the world’s megacities are located on the boundaries between two tectonic plates. Using computer simulations, seismologist Heiner Igel and his team are trying to estimate the shaking of future earthquakes in these urbanized areas.

Geophysics

By Hubert Filser

insightLMUinsightLMU / Issue 4, 2011Research

cluding the Cologne Basin, Southern Cali fornia or the Peking Valley. They have analyzed 400 different earthquake sce narios for metropolitan Los Angeles alone, based on the choice of probable epicenters along a 70 km fault. The cru­cial ques tion they want to answer is how exactly the vast amounts of energy re­leased during an earthquake would af­fect the archi tectural fabric of the city. As a basis for their simulations, the re­searchers construct a precise three­di­mensional com puter model of the earth’s crust in the region of interest. To do this they use a lattice consisting of up to a billion points, and calculate how seismic waves would propagate within this 3­D network. The input parameters also in­corporate factors such as the thickness of the crust, the nature of the subsoil, and the speed with which the waves propagate. The more local data is avail­able – measurements taken from bore­holes for example – the more realistic the model.

These models permit mapping of the pattern of oscillations induced by seismic waves over an entire region. In principle, one can compare an earthquake with a crack in a windscreen, which extends in both directions within a fraction of a sec­ond. The exact location of the initial rup­ture is crucial for what follows. The earth­quake simulations also show that it makes a big difference whether the epi center

Source: Tim Clayton / Corbis

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insightLMU / Issue 4, 2011

lies near the middle or on the edge of a fault.

Experts agree that a massive earthquake, a “Big One,” is likely to hit the Los Angeles area soon. The chance that this will occur within the next 30 years is now reckoned to be 99.7%. This makes Igel’s simu la­tions especially valuable. They provide indications of the levels of stress build­ings must withstand, where landslides are likely to occur or gas mains to burst. “When the crunch comes, it will very soon be clear which areas are worst hit,” says Igel. But on the basis of the simulations one can draw up emergency re sponse and evacuation plans today. Engineers can begin to strengthen the structures of houses, offices and power plants to resist the expected stresses; reinforced con­crete is more resistant than brick. In addi­tion to minimizing the potential damage to buildings, urban plan ners and emergency services must consider the consequences for the city’s infrastructure. In and around Los Angeles, the water supply is particu­larly vulnerable. Indeed, it is possible that the entire system would need to be rebuilt in the aftermath of a massive quake. Attending to the injured would also present grave difficulties, given that two­thirds of the hospital beds in the surrounding counties might be put out of action by a “Big One”. Power and tele­phone lines would be interrupted, as would Interstate Highways 10 and 15, where the weak links are the bridges. The overall result would be total chaos and damage amounting to 200 billion dollars, according to a study by the US Geological Survey.

The extent of the threat to life and prop­erty depends not only on the geological conditions but on the structural stability of the buildings in cities at risk. Accord­ing to geoscientists, when both factors are considered, the city at greatest risk is

Kathmandu, the capital of Nepal. One million people live there, most of them in very simple dwellings. For similar rea­sons experts fear that a large earthquake in Delhi or Mumbai, Manila, Islamabad, Karachi, Teheran, Jakarta or Bandung, in Bogotá, Mexico City, Guatemala City, San Salvador, Quito or Lima would claim a huge number of victims. The same holds for one European metropolis − Istanbul.

The threat to the Turkish megacity can be gauged from the effects of the magni­tude 7.6 tremor that hit the city of İzmit, 100 km away, in 1999. Of its 300,000 in­habitants, 18,000 lost their lives. Istanbul has a population of 17 million. Both cities lie on the North Anatolian Fault, which marks the interface along which the Eura­sian and Anatolian Plates slide past one another. The probability of a substantial earthquake here within the next 30 years is calculated to be 60%. “We can only simulate such huge quakes on super­computers,” says Heiner Igel. It is fasci­nating to watch a simulated earthquake on a computer monitor, but current mod­els are not sufficiently realistic. Hence seismologists need ever more complex software and more powerful supercom­puters, such as the unit at the Leibniz

Computing Center in Garching near Munich. Igel persistently em pha sizes this point, because, as he insists: “Our models are still very primitive.” The work of large­scale European re­search networks such as SPICE or QUEST (the latest) indicates how much basic re­search is required to understand just how the Earth ticks or, rather, trembles. Igel plays a leading role in both SPICE and QUEST. Cycling jerseys emblazoned with the emblems of both projects hang in his office on Munich’s Theresienstraße. As a passionate cyclist, he designed them himself. On the left arm of one of them is a complex curve. “This is the only jersey in the world that shows a seismogram of a magnitude 8.0 earthquake,” he says with a laugh.

Researchers are not just counting on better supercomputers. They are develop­ing new instruments, such as rotation sensors, and refining their theoretical models of the processes that give rise to great tremors. They now know that mega­quakes, which have become more fre­quent in recent years, can set off sec ond­ary shocks thousands of miles away. The question is, how? Earthquakes radiate

Source: Patrick Robert / Sygma / Corbis

Japan lies in a highly active seismic zone: The Kobe earthquake of 1995.

3The original article appeared in „Einsichten – das Forschungsmagazin No. 1+2, 2011“, LMU‘s German-language research magazine. Translation: Paul Hardy. Copyright: Ludwig-Maximilians-Universität München, 2011.

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energy in two ways. P waves pass through the Earth at speeds of up to 20,000 km/h, but do little damage. They can be detected by seismometer net­works in time to allow high­speed trains to be stopped or gas pipelines to be shut down before the slower shear (or sur­face) waves arrive. S waves travel only half as fast as P waves, but are far more destructive.

The detailed mechanisms of seismic wave propagation are dauntingly com­plex. As the skyscrapers in Tokyo swayed in March 2011, and the whole of Japan felt the shocks, 9000 km from the epi­center, in Bavaria, the ground surface was shifted horizontally by 2.5 cm, and the groundwater level in a borehole in the Ziller Valley in the Austrian Alps rose by as much as 50 cm. “An absolutely astound ing observation,” says Igel. In fact the earthquake led to a redistribution of the Earth’s mass and changed its rotation period. The day is now 3 microseconds shorter than before, and the whole planet rang like a bell for months.

The intensity of the Japanese quake of March 2011 was a surprise for seismolo­gists, but the tremors in Christchurch in New Zealand really shocked them. “No body expected the quakes in New Zea land,” Igel admits. “Here, we seis­mologists really must apologize for our risk analysis.” Twice within six months, Christchurch, with its 400,000 inhab­it ants, was hit by earthquakes of magni­tudes of 7.1 and 6.3. Moreover, the less powerful one caused the greater damage, with surface acceleration exceeding 2 g at the epicenter. Even apparently earth­quake­proof structures suffered severe damage. Several buildings in the city center, including an office complex and a television station, appear to have been subjected to particularly violent vibration. Seismologists believe that an extinct

volcano nearby may have acted as a seis­mic reflector that amplified the motion induced by the quake itself. But it is too early to be sure. A delegation from New Zealand recently visited Igel’s Institute to discuss their observations, but nobody could come up with a convincing model. To obtain better measurements of earth­quake frequencies, geologists are cur­rently drilling boreholes on coasts at high risk for tsunamis. They are looking for traces of earlier quakes in the layers of sand and mud that make up the sedi­ments. Instrumental records go back about 100 years, and most researchers have felt that this is long enough to allow them to estimate recurrence intervals. However, it is now clear that one must look much deeper. Sediment cores rep­resent an archive of information that ex­tends for thousands of years, and pro­vide the most direct way of identifying the long­term rhythms of seismic activity in different regions.

The method is being used in a region for which Igel may soon be able to perform simulations, the region around Seattle. In most catalogs of earthquake risk, this conurbation in the Northwest of the US, with its 4 million inhabitants, does not figure near the top of the list. But two tectonic plates collide just off the coast here. In the region between Vancouver

Island just over the border in Canada and Northern California, the Juan de Fuca Plate dives under the North American Plate. The magnitude of an earthquake is a function of the length of the rupture and the thickness of the crust at that point. If the crust off the Pacific North­west were to rupture over a distance of 1000 km, the result would be a mega­quake of magnitude 9.0. The last time such a powerful tremor occurred in this area was more than 300 years ago, in Janu ary 1700. The ground surface rose by almost 10 m, and the tsunami that this produced was recorded by Japanese monks in their monasteries on the other side of the Pacific. The geological evi­dence suggests that at least seven earthquakes of this magnitude have here occurred during the past 3500 years, i.e., on average once every 300­400 years. ”The next one is due, and it can happen anywhere between Northern California and the Canadian Southwest,” says Igel, “and maybe right under Seattle.” A huge tsunami could rush into Puget Sound and build up into a wavefront many meters high before inundating the city. “The quake would be similar to the Tohoku­Oki tremor in Japan in March 2011“, says Igel. Given that the cities of Seattle and Vancouver lie in the danger zone, and that more people live there than in the area worst hit in Japan, such a scenario would cause “an awful catastrophe”.

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Prof. Dr. Heiner IgelBorn in 1963, Heiner Igel is Professor of Seismol­ogy and Geophysics at LMU. He began his studies at the Technical University in Karlsruhe, did his doctoral work at the Institut de Physique du Globe de Paris (IPGP), and obtained his doctoral degree at the University of Paris in 1993. He then spent five years at Cambridge University, and was awar­ded a Heisenberg Fellowship by the German Re­search Foundation (DFG) in 1998. In 1999 he took up his present position at LMU Munich.