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Hazard Maps for Switzerland - Introduction
| The Swiss Seismological Service (SED) at the ETH Zurich has completed a multi-year project to compute a new generation of probabilistic seismic hazard maps in the spring of 2003. This web page presents an overview of the results, the detailed description of the methodology can be found in (Sellami et al., 2004). We choose to apply the classical probabilistic seismic hazard assessment (PSHA) methodology based on a complete revision of the input parameters. Our new assessment, 25 years after the first study (Saeggesser and Mayer-Rosa, 1978), integrated results from several extensive research projects at ETH Zurich, which provided new input data, most notably:
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- The revised Swiss earthquake catalogue (ECOS 2002), which includes new historical information and instrumental data and is based on a homogeneous assessment of moment magnitude.
- The source models are based on new seismotectonic information (Kastrup, 2002; Deichmann et al., 2003) and on the updated seismicity distribution. For the regions around Switzerland we applied the existing source models used for GSHAP (Global Seismic Hazard Assessment Program).
- The new attenuation relation developed for Switzerland (Bay, 2002; Bay et al. 2003). It was derived for the magnitude range from 3.0 to 5.5 and distances from 10 to 300 km. The relation includes an extrapolation to the magnitude range above magnitude 5.5 by introducing a stress drop that is increasing with magnitude, up to a maximum of 30 bars. The attenuation model for the Alpine Foreland is valid for good quality bedrock with an estimated shear-wave velocity of about 1500m/s in the upper 30m.
The maps depict earthquake hazard by showing the earthquake ground motions expected for a selected return period. The ground motions being considered at a given location are those from all future possible earthquake magnitudes at all possible distances from that location. The method assumes a reasonable future
catalog of earthquakes, based upon historical earthquake locations and seismotectonic information. These national maps of earthquake shaking hazards provided the input for the building code SIA 261 used in Switzerland since the beginning of 2003.
The ground motion is given as spectral acceleration (SA) with 5% damping and peak ground acceleration (PGA). Spectral acceleration has been derived from observed ground motion data using as a base the best estimate for all the input parameters. Spectral acceleration values therefore
correspond to the best estimate and include no conservative choices. It is given for the Alpine Foreland rock. PGA values could not be derived directly from observed ground motion due to the frequency-band limited signals of the old seismic network. PGA had to be estimated from the spectral acceleration values, and therefore has a high level of uncertainty. PGA values are an estimate for good quality ground conditions (ground types A or B of SIA261). For the building code SIA261 a conservative estimate is preferred and PGA values are assigned to ground type A. For PGA values of other ground types, the bulding code SIA 261 specifies a simplified scaling factor. The choice of ground type B for calibrating the PGA values is considered to be a best estimate.
To capture the epistemic uncertainty we considered different alternative models. The final hazard maps were computed using our preferred model. The uncertainty range is relatively small for short return periods (100 to 500 years) and becomes rather large for longer return periods (2500 years). Beyond this
limit we consider the data insufficient for reasonable hazard estimation.
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