Engineers use response spectra to work out the design requirements for structures on various substrates (which are divided into subsoil categories for the purpose of the standards). Based on these response spectra, they identify the measures that need to be implemented to ensure that a structure built on the subsoil at a given location complies with the standards. The adjustments made to the response spectra in the revised standard are based on an in-depth analysis of earthquake recordings at many seismic stations together with geophysical measurements determining the local properties of the subsoil at the station locations. The standard takes account of the fact that loosely deposited sediments amplify the seismic waves. As a result, structures at such locations must be designed to withstand stronger ground movements.

In Switzerland, cantons are responsible for checking compliance with construction standards. In some cantons, building legislation explicitly requires compliance with the applicable SIA standards or lays down earthquake-specific requirements as part of the planning permission process. In others, implementation is not explicitly regulated by law and is not monitored either. According to the Swiss Federal Office for the Environment, the additional costs involved in making a structure earthquake-resistant represent no more than 1% of the total, provided that this is factored into the design at an early stage.

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Earthquake-Resistant Construction

Seismic Hazard in Switzerland

SIA 261 (in German, French and Italian)

Time and again, Switzerland is faced with natural events that have the potential to endanger human life and infrastructure. In light of this, the federal agencies responsible for monitoring natural hazards have joined forces to ensure that the public is adequately informed, warned and protected in such critical situations. They publish warnings about storms, heavy precipitation and snowfall, slippery roads, ground frost, heat waves, thunderstorms, forest fires, floods and avalanches, and issue alerts regarding current earthquakes.

The agencies jointly operate the Natural Hazards Portal with the aim of reaching as many people as possible when a natural event occurs and providing them with standardised information. Furthermore, they publish all warnings and alerts about natural hazards on this website and also issue these warnings and alerts through the MeteoSwiss app. As a result, reliable and consolidated information on current natural hazards situations and events is available to the public.

Following a revamp, a new-look Natural Hazards Portal is now being released. The updated portal features adjustments to how hazards are shown and illustrations depicting recommended behaviours. For example, rivers and lakes of national interest are now colour-coded based on their warning status, making this easier to see, while forest-fire risk is now shown along cantonal borders, with pictograms on key recommended behaviours making this information more user-friendly.

The federal agencies responsible for monitoring natural hazards firmly believe that these adjustments will help the Swiss public to locate the relevant information more easily and understand it more readily, both before and during natural hazards events.

The Federal Office for the Environment (FOEN), the Federal Office of Meteorology and Climatology (MeteoSwiss), the WSL Institute for Snow and Avalanche Research (SLF), the Swiss Seismological Service (SED) at ETH Zurich and the Federal Office for Civil Protection (FOCP) are officially mandated by the federal government to provide information and issue warnings to the public, the media and the authorities about impending natural hazards. In critical situations, the natural hazard agencies work particularly closely together, coordinating their forecasts and warnings and compiling joint press releases and bulletins about natural hazards.

You can find further information at www.natural-hazards.ch.

This text was adapted, with permission, from a MeteoSwiss blog entry (German).

By analysing month-to-years long datasets from over 300 seismic stations around the world, the study, led by Dr Thomas Lecocq from the Royal Observatory of Belgium, was able to show how the seismic noise decreased in many countries and regions since the start of the lockdown measures. The researchers could visualise the resulting "wave of quiet” moving through China, then to Italy, and around the rest of the world. The seismic lockdown sees the total effect of physical and social distancing measures, reduced economic and industrial activity and drops in tourism and travel. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record.

Traditionally, seismology focuses on measuring and analysing seismic waves arising from earthquakes. High-frequency vibrations (“buzz”) of human activity, however, contaminate the seismic records from natural sources. Walking around, driving cars, and getting the train, but also heavy industry and construction work generate unique seismic signals in the subsurface that influence the recordings of natural phenomena. The strongest seismic noise reductions from the COVID-19 lockdown were found in urban areas, but the study also observed signs of the lockdown on sensors installed hundreds of meters below the surface and in more remote areas, such as in sub-Saharan Africa. Furthermore, the researchers found a strong correlation between seismic noise reduction and human mobility datasets.

In Switzerland, in particular the stations belonging to the Swiss Strong Motion Network (SSMNet) did show similar effects. Many of these measuring stations are located in urban areas and a significant reduction in seismic noise was recorded in Lugano, Martigny, Zurich, Basel and Geneva, among others. Since the lockdown was declared in mid-March, levels of seismic noise in these towns and cities have been nearly as low on working days as they were during weekends before the lockdown began. Meanwhile, the seismic noise is back to almost normal levels in Switzerland and in most countries analysed in the study.

Will the 2020 seismic noise quiet period allow new types of signals to be detected? The study has shown the first evidence that previously concealed earthquake signals, especially during the daytime, appeared much clearer on seismic sensors in urban areas during the lockdown. In Switzerland, the overall effect is a decrease in the detection threshold for earthquakes by about 0.1 - 0.2 magnitude units across the country, for many urban areas this decrease is even greater at 0.3 magnitude units or more.

SED seismologist Dr Frédérick Massin, one of the co-authors of the study, says that the researchers hope that their work will spawn further research on the seismic lockdown. Finding previously hidden signals from earthquakes and volcanoes will be one key goal. Due to the growing urbanisation and increasing populations, it becomes more important than ever to characterise the anthropogenic noise humans cause so that seismologists can better listen to the Earth, especially in cities, and monitor the ground movements beneath our feet.

More information: https://science.sciencemag.org/lookup/doi/10.1126/science.abd2438