1. Institute of Geological & Nuclear Sciences Limited, P.O. Box 30368, Lower Hutt, New Zealand Ph: +64-4-5701444 Russell Robinson & Rafael Benites Synthetic Seismicity of Multiple Interacting Faults and its use for Modelling Strong Ground Motion

2. New Zealand tectonic and bathymetric setting Image from NIWA National Institute of Water and Atmospheric Research Ltd K e r m a d e c T r e n c h H i k u r a n g i T r o u g h A l p i n e F a u l t

3. Wellington region topography

4. Major faults of the Wellington region

6. Earthquake Commission (EQC) A small fraction of fire insurance premiums is used for earthquake insurance A small fraction of fire insurance premiums is used for earthquake insurance They asked GNS: What is the probability of two (or more ) large earthquakes in the Wellington region within a few years of one another?What is the probability of two (or more ) large earthquakes in the Wellington region within a few years of one another? What sort of shaking should we expect from a large earthquake on the Wellington Fault?What sort of shaking should we expect from a large earthquake on the Wellington Fault?

7. Synthetic Seismicity: Computer model of a network of interacting faults and a driving mechanism. Generates long catalogues of seismicity so that questions can be answered by statistical analysis. Computationally efficient but reasonably realistic. Fault properties are tuned to reproduce known slip rates/directions and other fault properties.

8. Features: Coulomb Failure Criterion. Static/dynamic friction law, modified to include healing. Okada’s (1992) dislocation routines for calculating induced stresses. Stress propagation is at the shear wave velocity.

9. Features: Induced changes in pore pressure are included. Mimics dynamic rupture effects to some degree. All interaction terms are kept in RAM. The program has been “parallelized” to run on a Beowulf PC cluster.

10. Fault Interactions

11. Stress history of a single cell

12. Model faults

15. Wellington Fault Fault Length: 75 km Fault Width:20 km Fault Dip:90 o Cell Size: 1 x 1 km Coefficient of Friction: Asperity regions: Random between 0.65 and 0.95 Non-Asperity:Random between 0.40 and 0.70 Stress Drop: 25% Static/Dynamic Strength: 0.85 Healing Time: 3.0 s Dynamic Enhancement Factor: 1.2 Pore Pressure: Initially ~ hydrostatic; varies with time Stress Propagation Velocity: 3.0 km/s

16. Typical ‘Characteristic’ Event Moment: 1.41 x 10 20 N-m; Mw 7.40 ModelSommerville (1999) Rupture Area 1500 km 2 2810 km 2 Average Slip 2.35 m1.96 m Area of Asperities 345 km 2 630 km 2 Area of Largest Asperity 272 km 2 458 km 2 Radius of Largest Asperity~9 km 2 13 km Num. of Asperities 2 + 1 very small2.6 Area Covered by Asperities23%22% Average Asperity Slip 1.672.01 Contrast Corner Spatial Wavenumber, Along Strike 0.01 km -1 0.01 km -1 Along Dip 0.01 km -1 0.02 km -1 Slip Duration 3.0 s2.55 s Rupture Duration~30 s-

17. Final slip distribution

18. Rupturing ‘snapshots’ for a characteristic Wellington Fault event

21. METHOD Discrete wave number Generalised reflection/transmission coefficients (Bouchon 1979, Kennet 1973, Chin and Aki 1991) In the plane k-z k kyky kxkx

22. in which t n is the time shift corresponding to the time step n, X P and X S are the directivity correction factors for P and S waves, respectively, applied to each subfault m, and defined by: with  r = average rupture velocity, L = length of the subfault m, and  the angle between the point source corresponding to the subfault m and the station. The components of the wavefield contribution of each subfault in the k-z plane are rotated to the geographical coordinates.

23. The complete wavefield in the source layer L is computed from: for P-SV waves; and for SH waves, where: The propagation through the layers is performed by applying the generalized reflection/transmission coefficients.

24. Ground displacement at x=5 km, y=70 km

25. Velocity and acceleration

26. Institute of Geological & Nuclear Sciences Limited, P.O. Box 30368, Lower Hutt, New Zealand Ph: +64-4-5701444 Russell Robinson & Rafael Benites www.gns.cri.nz