Attenuation and GMPE’s for earthquakes in Iceland

1. Attenuationand GMPE’s for earthquakes in Iceland Símon Ólafsson University of Iceland Earthquake Engineering Research Centre Austurvegur 2a - Selfoss UPStrat-MAFA Workshop, Selfoss 24-27 July 2012

2. Applying GMPs to Icelandic data • Not enough data exists for applying regression equations • Applying GMPEs from other regions does not give good results • GMPEs from other regions tend to underestimate close to fault and overestimate far from fault

3. Basic assumptions: Strike slip earthquake with near vertical fault plane Strong motion phase modelled Brune’s source model applied High frequency spectral decay modelled with an exponential term Geometrical spreading function applied Developing a GMPE

4. Recent Earthquakes in South Iceland • Mw6.0, 25 May 1987 • Mw 6.6, 17 June 2000 • Mw6.5, 21 June 2000 • Mw6.3, 28th May 2008

5. Modelling approach

6. Geometric spreading function d = epicentral distance h= depth parameter D1, D2 and D3are used to set the limits for the different zones of the spreading function Reference: Ólafsson, S. and Sigbjörnsson, R. (1999), “A theoretical attenuation model for earthquake-induced ground motion”, Journal of Earthquake Engineering 3, Imperial College Press, 287-315.

7. Effect of spreading function

8. Theoretical GMPE Parseval theorem: Theoretical attenuation model for rms acceleration: A relationship for peak acceleration is obtained by inserting: amax = parms

9. The function Y

10. Estimation of parameters

11. Discrete filter equations for GMPE

12. Earthquake 28 May 2008 - Parameters ParameterEstimate Units _____________________________________ Mo 3.4×1018 N m Mw 6.26 Fc0.24Hz r 6.4km k 0.053 s Ds 73.0 bar u 79.4 cm _____________________________________

13. GMPE for Iceland Mw4.8 – Mw6.6 ParameterEstimate Units Type ro2.8 g/cm3 Density b6.4km S-wave velocity k 0.04 s Spectral decay Ds 100 bar Stress drop r Fault radius D 2 4r Geom. att. param. n 2 Exponent h ≈ r Depth p 2.94 Peak factor

14. Magnitude MW6

15. Magnitude MW6.5

16. Magnitude MW6.5 – Linear scale

17. Magnitude MW6.5 Icelandic model fitted to NGA data

18. Magnitude MW6.5 Icelandic strong motion and formulas from Europe and America

19. Magnitude MW6.5 NGA data with formulas by Campbell, Ambraseys and Boore

20. Durationfunction Td = duration r = radius of dislocation b = S-wave velocity d =hypocentral distance c1, c2, c3 = model parameters sT= standard deviation of duration

21. Duration – 90% cumulative energy

22. Cumulativeenergy

23. Estimated parameters

24. GMPE (Mw6.5) with 50 – 90% cumul energy

25. Duration function for 50% - 90% cumulative energy

26. Macroseismic data and model

27. Articles regarding attenuation and GMPE for Iceland • Sigbjörnsson R, Ólafsson S (2004) Near-source decay of seismic waves in Iceland, article4462 in Proceeding of the 15WCEE. • Sigbjörnsson R, Ólafsson S, Snæbjörnsson JT (2007) Macroseismic effects related to strong ground motion: a study of the South Iceland earthquakes in June 2000. Bulletin of Earthquake Engineering 5, 591-608. • Sigbjörnsson R, Ólafsson S (2004) On the South Iceland earthquakes in June 2000: Strong-motion effects and damage. Bollettino di Geofisica Teorica ed Applicata, 45(3), 131-152.

28. Conclusions • The parameter n defines the rate of attenuation for the geometrical spreading function has been estimated to be very close to the theoretical rate of attenuation, n = 2, for ground motion in the near-field. Based on data from earthquakes Mw6.3 – Mw6.5. • The parameter D2 was estimated 5r. The depth parameter, h, was estimated as to be in the range 15.5 km to 12.2 km. • The standard deviation, s, is lowest for the estimation using the 90% duration. This result seem to favour the use of the 90% duration model for the GMPE.

29. End