The inhabitants of Basel had been warned on this particular October 18th. Between lunch and afternoon snack at about 4 pm a first tremor shook the town with its 7,000 inhabitants. When the bells for dinner rang at around 5 pm the earth shook four times. By this time at the latest, many people fled the town, saving their lives. The main quake did not arrive until 10 pm, when it destroyed or badly damaged many buildings. Even in Bern, 71 kilometres away, a church vault collapsed. The amount of property damage was enormous; the number of deaths was comparatively low. Between 100 and 300 people are estimated to have lost their lives.
Nowadays an earthquake of this magnitude would have much more serious consequences. About half a million people live in the Switzerland-France-Germany border triangle. The chemical and pharmaceutical industries, nuclear power plants and countless tanks containing petrol, diesel and kerosene harbour additional risks. According to one estimate 1,500 to 2,000 people could lose their lives; in Switzerland alone, losses up to 60 million Euro are conceivable.
The Basel earthquake of 1356 is not only the strongest quake documented in Central Europe north of the Alps by written records. It also emphatically proves that the countries located far away from the large earthquake zones in the world around the Mediterranean or the Pacific should also prepare themselves against such devastating catastrophes. However, to take such precautions one must first know which regions are threatened by earthquakes in the first place and what damage is to be expected. Earthquake hazard maps of exactly this type are being created by Gottfried Grünthal from the Helmholtz Centre in Potsdam – German Research Centre for Geosciences – GFZ and his colleagues in GFZ Section 2.6 “Seismic Hazard and Stress Field”.
The euro of the earthquake researchers
“First of all we prepare an earthquake catalogue which extends as far back in time as possible”, explains Gottfried Grünthal. Reports from archives on past earthquakes are analysed: how and where was the event perceived by humans, what damage was suffered by buildings? The answers to these questions show the researchers where the epicentre of the earthquake was located and its intensity. Gottfried Grünthal has played a significant part in developing the methods for assessing such intensity values which have been in uniform use in Europe since 1998. In this way the Helmholtz researcher has initiated a uniform “currency” valid in all regions of Europe and further afield, in other words a kind of euro for earthquake research. From these intensity values the “moment magnitude” can be very reliably calculated, which today is the uniform measure of the strength of earthquakes in catalogues. “A hazard map must be based on uniform fundamentals”, Gottfried Grünthal explains this step.
Such catalogues immediately show the researchers how many earthquakes of a certain magnitude have occurred in any one region in recent centuries and at roughly which average rates they could occur again. The archive data are however very different; in certain regions they are even completely missing for specific periods. For example, in the time of Turkish rule the archives were burned in many places and information about the time before this was destroyed. In addition to this, most records in Central Europe go back a thousand years at the most. A strong earthquake which took place 2500 years ago and which can be repeated after thousands of years should naturally also be taken into consideration.
A look into the earth
“For this reason, paleoseismological data are also included in earthquake hazard calculations”, reports Gottfried Grünthal. The researchers search for layers in the subsurface which from a geologist’s point of view have been deposited in recent times and have abruptly shifted up against one another. If they find several such typical earthquake traces along a several kilometre-long line, they can also determine the moment magnitude of such quakes. The GFZ researchers then have further values which reach much further back into the past than the historical records. In the Lower Rhine Embayment between Cologne, Belgium and the Netherlands, several earthquakes are proved to have occurred in the last 50,000 to 100,000 years, reaching a moment magnitude of up to 6.7 and thus comparable to the strength of the Basel earthquake. They released about 30 times more energy than the strongest quake in this region in historical times, with a magnitude of 5.7.
After collecting the data the researchers analyse the subsurface as far as possible: what are the geological conditions, which stresses occur? The computers are then fed with a wealth of data, particularly including all data concerning the earthquakes in the area under investigation. The calculations must of course also take into account the effects of events at large distances. For this reason, an area must be investigated which extends roughly 200 kilometres beyond the region actually being analysed. Finally, hazard curves are obtained for a grid whose individual points in the region are for example ten kilometres apart from each other. The researchers then compile this data to create hazard maps which show the whole of Europe, German-speaking countries or specific countries.
One can then see regions at particular risk such as the Rhine Valley from Basel to the Netherlands or the Swabian Alb, but also inconspicuous regions such as the North German Plain with its specific vibration values. In this way authorities and civil engineers obtain clues about the vibrations for which they should design the buildings. “However, we make more exact information concerning the soil acceleration spectra to be expected for any location in Germany available in the Internet”, Gottfried Grünthal continues. With this information, engineers can then work on measures for earthquake-resistant building. And these can be very simple. “The simpler the layout, the better a building can withstand an earthquake”, Gottfried Grünthal explains one of the possibilities. Square or rectangular layouts are especially stable. And because the simplest and thus safest form is a circle, the containment shells for installations at particular risk, such as the inside of a nuclear power plant, are of circular construction.
The GFZ researchers are meanwhile applying their collective know-how to related areas. For example, they have been working on maps on which the danger of a tsunami for the coasts of the Mediterranean can be seen. “On July 9, 1956, a tsunami occurred there which ran aground with 20-metre waves on the Aegean island of Amorgos and cost a large number of lives”, Gottfried Grünthal underlines the importance of such maps. A further project is investigating how earthquakes can be triggered by technical projects and how strong they might be. The researchers are paying particular attention to the industrial production of geothermal energy at large depths. Do these interventions change the stress field in the Earth’s interior in such a way that earthquakes are triggered, and how can their strength be limited? Answers to questions such as these are urgently required in times in which the energy industry is to become much more sustainable than before.