Swiss Earthquake Hazard

Introduction

Earthquakes are a serious threat to society and human beings. In order to implement measures to mitigate the consequences of earthquakes, the seismic hazard needs to be well assessed and known. Seismic hazard is defined as the probable level of ground shaking associated with the recurrence of earthquakes in a given time period. The assessment of seismic hazard is the first step in the evaluation of seismic risk, obtained by combining the seismic hazard with vulnerability factors (type, value and age of buildings, as well as infrastructure, population density and land use). Seismic hazard is assessed by combining the history of past earthquakes with the knowledge of the present seismotectonic setting and the local properties of the waves generated by earthquakes.

We present the results of a new generation of probabilistic seismic hazard assessment for Switzerland. This study replaces the previous intensity based generation of hazard maps from 1976. It is the first to systematically consider alleatory and epistic uncertainties and compute spectral hazard.

Based on a revised moment magnitude earthquake catalog for the period 1300 - 2002, sets of recurrence parameters (a, b-value and Mmax) are estimated for two models of seismotectonic sources zones, using two different completeness reporting models. To estimate alternative a- and b-value sets and their relative weighting, we introduce a novel approach based on the Bayesian information Criterion.

From these input parameters, we simulate synthetic earthquake catalogs of 1 Million year duration, reflecting the difference in depth distribution between the alpine Foreland and the Alps. Using a new predictive spectral ground motion attenuation and scaling model (Bay et al., 2004), we estimate expected ground accelerations at 0.5 - 12 Hz for all of Switzerland and return periods of 100, 475, 1000, 2500 and 10'000 years. Hazard maps for the frequency range and the different return periods are found under Swiss Hazard Maps.

Switzerland is characterized by moderate seismic activity. The epicenter map shows the instrumentally recorded earthquakes of the period 1975 to 2002 with magnitudes >= 2.5, as well as all earthquakes since the 13th century known to have caused damage (intensity >= VI). In this period approximately 10’000 events have been felt by the population, 12 are known to have caused severe damage. Moderate seismicity in populated areas combined with high vulnerability entails moderate hazard but high risk.

The hazard map for 5Hz and a return period of 475 years can be obtained for free from the Swiss Seismological Service. The frequency of 5Hz corresponds to the portion of the spectrum where the local soil conditions are likely to enhance seismic motion. This is also the resonance frequency of 2 to 5 story buildings representing the largest contribution to the building stock of Switzerland. In the map, blue to green colors indicate areas of expected moderate shaking and damage, while red to orange highlight areas expecting more severe shaking and damage in the next 475 years. The zones of higher hazard in Switzerland are the Wallis, the Basel region, Graubünden and the Alpine front. This hazard map is the basic input for the zonation of the Swiss territory and the definition of the design spectra included in the new national seismic building code SIA 261, which is valid since 2004.

More detail about the data, methodology and results is found in the report ‘Seismic Hazard Assessment in Switzerland 2004’ (see link at the right).

Hazard maps for the frequency range and the different return periods are found under Swiss Hazard Maps. We also show the hazard at various sites in Switzerland as hazard curves and uniform hazard spectra.