Seismic Hazard Mathilde B Srensen and Jens Havskov

Seismic Hazard Mathilde B. Sørensen and Jens Havskov Dept. of Earth Science, University of Bergen

Seismic Hazard Assessment An Introduction

What is seismic hazard? and seismic risk?

Basic definitions Seismic hazard Describes the probability of experiencing a given level of earthquake shaking at a given location within a given time frame Seismic risk Describes the probability of economical or human losses due to earthquake shaking Risk = Hazard · Vulnerability · Value

Earthquakes don’t kill, buildings do!

China, 12. mai 2008

What controls the damage? • Amplitude of the seismic waves • Magnitude • Distance • Local geology • Frequency content of ground shaking • Building quality • Indirect damages (secondary effects) • Fire • Landslide/rock fall • Tsunami • Liquefaction

Effects of local geology

Frequency content

Building quality May 2008 Sichuan, China earthquake

Building quality

Indirect damages - liquefaction

Indirect damages - tsunami

Indirect damages - tsunami Meulaboh, NW Sumatra

Indirect damages - landslides Before After quake After 1 st flood After 2 nd flood

Indirect damage: Fire Japan

Prevention • Earthquakes cannot be avoided, but disasters can be prevented • Hazard and risk mapping • Building codes and retrofitting • City planning • Early warning

Hazard mapping Based on statistical analysis of the seismicity in an area

Hazard mapping Based on scenarios expected for the future

Building codes Regulate the loads a building must withstand Based on hazard maps Depend on the usage of the building

City planning Buildings sinking due to bad foundation at a delta complex (Izmir, Tyrkey)

Early warning systems

Mexico early warning Warning time for coastal events is 60 -100 s

Early warning systems

Applications of hazard and risk • Can assist in defining building design specifications or determining if a site is appropriate for a partiular application • Can be accounted for in city planing or used as basis for risk mitigation • Is used by the insurance industry to define insurance level and costs • Allows society to make risk based decisions based on quantitative cost/benefit analyses for multiple hazards • Deagregation of the results can help identifying critical scenarios which can be associated with a specific hazard level

Presentation of seismic hazard Hazard curves giving annual probability vs. ground motion level Petersen et al. , 2007 Hazard maps showing maximum expected ground motion for a given time frame and probability level Petersen et al. , 2007

Main steps in seismic hazard assessment Data required: 1) Tectonics, geology, earthquake catalog 2) Earthquake catalog 3) Ground motion prediction equation Reiter, 1990

Step 1: source zones • Defines zones within which the seismicity is assumed to be homogeneous • Can be defined as point, line, area or volume sources • Accurracy in zonation depends mainly on amount of available information • Input information: – Seismicity – Tectonic information – Geological information

2. Recurrence and Mmax For each source zone, the earthquake activity must be described - Recurrence in terms of the Gutenberg-Richter relation - Maximum magnitude to be produced by the source zone - Minimum magnitude to be considered (smaller events are not expected to cause damage) Linear Truncated Grünthal, 2003

3. Ground motion • For every source, an appropriate GMPE must be chosen • Ideally, relation derived from local data • In most cases, relation from similar tectonic regime Petersen et al. (2004)

4. Calculation of hazard • For all sources, the effect of all earthquakes of all sizes and at all locations, with different occurrence probabilities are integrated • Output: Probability of exceeding different GM levels at a site within a given time: Mean rate of occurrence of earthquakes with m 0≤m≤mmax in source i (from Gutenberg-Richter) Expected number of exceedances of z within a given time From GMPE PDFs of m and r

4. Calculation of hazard Petersen et al. (2007)

HHaiti earthquake catalog 1900 -february 2010 Frankel et al. , 2010 Franke l

Frankel et al. , 2010

Shear wave velocity Frankel et al. , 2010

Frankel et al. , 2010

Conclusion Calculation of seismic hazard requires: -Good knowledge of past seismicity -Study of local attenuation Seismic hazard calculation and seismic zonation maps are essential for planning purposes and construction of new buildings Many areas have no zonation maps despite a high seismic hazard