Average properties of Southern California earthquake ground motions envelopes… G. Cua, T. Heaton Caltech

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

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?

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

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

 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 log 10 (distance in km) Magnitude Database

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

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

ROCK S-wave SOIL S-wave  Scaling for small magnitudes-

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

 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 AccelerationVelocity (filtered) Displacement

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

S-wave acceleration (ROCK) P-wave acceleration (ROCK)  scaling for small magnitudes,

 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 Vertical P and S wave Horizontal P and S wave Something curious …

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

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

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

Acceleration Amplification Relative to Average Rock Station

Velocity Amplification Relative to Average Rock Station

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

Rock versus Soil  CDMG map of Preliminary Surface Geologic Material (Wald) and SCEC Phase III Report Velocity Calculator NEHRP Class Maximum Shear Wave Velocity in Upper 30 m (m/s) A (Hard Rock)1400 B (Soft Rock)724 BC686 C464 CD (Alluvium)372 D301 DE298 E (Mud) CISN ROCK Stations 120 CISN SOIL Stations Our Binary Classification

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

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

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

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

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