Average properties of Southern California earthquake ground motions envelopes …

1. Average properties of Southern California earthquake ground motionsenvelopes… G. Cua, T. Heaton Caltech

2. Main Points • saturation of rock vs soil sites • attenuation characteristics of P vs S-waves • importance of station corrections

3. Motivation: Seismic Early Warning Q1: Given available data, what is most probable magnitude and location estimate? Q2: Given a magnitude and location estimate, what are the expected ground motions?

4. Ground motion envelope: our definition Full acceleration time history envelope definition– max.absolute value over 1 second window

5. Modeling ground motion envelopes • P,S-wave envelopes – rise time, duration, constant amplitude, 2 decay parameters • Noise - constant model for observed envelope

6. Database • 70 events • 2 < M <=7.3 • 0 < R < 200 km • 9 channels • Z, EW, NS • acc, vel, disp • binary classfication • rock - NEHRP BC, above • soil - NEHRP C, below Magnitude log10(distance in km)

7. C(M) term “turns on” amplitude saturation for M > 5 Functional form for M, R-dependence of ground motion amplitudes C(M) Magnitude * Modified from Campbell (1981)

8. Main Points • saturation of rock vs soil sites • attenuation characteristics of P vs S-waves • importance of station corrections

9. ROCK S-wave SOIL S-wave • Scaling for small magnitudes-

10. Saturation of rms horizontal acceleration S-wave (rock vs soil)

11. Acceleration Velocity • Saturation is most pronounced in • acceleration in close to large events • Also present to some degree in velocity • and displacement • Rock and soil approach similar amplitude levels in close to large events • Displacements are high-passed filtered (filtered) Displacement

12. Main Points • saturation of rock vs soil sites • attenuation characteristics of P vs S-waves • importance of station corrections

13. S-wave acceleration (ROCK) P-wave acceleration (ROCK) • scaling for small magnitudes,

14. Horizontal P and S wave Vertical P and S wave Something curious … • horizontal P-wave amplitudes saturate more than horizontal S-wave • difference between P- and S-waves is more pronounced in horizontal than vertical • uniquely decomposing P and S wave at close distances is problematic, particularly on horizontal

15. Average Rock and Soil envelopes as functions of M, R ACCELERATION

16. Main Points • saturation of rock vs soil sites • attenuation characteristics of P vs S-waves • importance of station corrections

17. How much do station corrections improve standard deviation? rock + soil=0.315 rock only=0.308 rock w/ station corr=0.243 ~21% reduction in 

18. Acceleration Amplification Relative to Average Rock Station

19. Velocity Amplification Relative to Average Rock Station

20. Conclusions • Saturation of rock and soil sites • Soil sites saturate ground motions more than rock • Stronger saturation at higher frequencies • Difference between rock and soil sites decreases with increasing ground motion amplitude • P-waves appear to have higher degree of saturation than S-waves ? • Station-specific data contributes to ~20% variance reduction

21. Rock versus Soil • CDMG map of Preliminary Surface Geologic Material (Wald) and SCEC Phase III Report Velocity Calculator Our Binary Classification 30 CISN ROCK Stations 120 CISN SOIL Stations

22. Average Rock and Soil envelopes as functions of M, R VELOCITY

23. Average Rock and Soil envelopes as functions of M, R (filtered) DISPLACEMENT

24. S-wave S-wave magnitude-dependence anaelastic attenuation saturation geometric attenuation constant

25. S-wave acceleration (ROCK) P-wave acceleration (ROCK)

26. Station Corrections • Average residual at a given station relative to expected ground motion amplitude given by attenuation relationship • Defined for stations with 2 or more available records • Consistent with generally known station behavior • PAS, PFO are typically used as hard rock reference sites • SVD anomalous due to proximity to San Andreas • Some “average” rock stations are: DGR, JCS, HEC, MWC, AGA, EDW