Download Average properties of Southern California earthquake ground

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 Swaves
 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?
Ground motion envelope: our definition
Full acceleration time history
envelope definition– max.absolute value over 1 second window
Modeling ground motion envelopes
model for observed envelope
 P,S-wave envelopes – rise
time, duration, constant
amplitude, 2 decay parameters
 Noise - constant
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
log10(distance in km)
Functional form for M, R-dependence
of ground motion amplitudes
C(M)
C(M) term “turns on”
amplitude saturation for
M>5
Magnitude
* Modified from Campbell (1981)
Main Points
 saturation of rock vs soil sites
 attenuation characteristics of P vs Swaves
 importance of station corrections
ROCK
S-wave
 Scaling for small magnitudes-
SOIL
S-wave
Saturation of rms horizontal
acceleration S-wave (rock vs soil)
Acceleration
 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
Velocity
(filtered)
Displacement
Main Points
 saturation of rock vs soil sites
 attenuation characteristics of P
vs S-waves
 importance of station corrections
P-wave acceleration (ROCK)
 scaling for small magnitudes,
S-wave acceleration (ROCK)
Something curious …
Horizontal P and S wave
Vertical P and S wave
 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
Average Rock and Soil envelopes as functions of M, R
ACCELERATION
Main Points
 saturation of rock vs soil sites
 attenuation characteristics of P vs Swaves
 importance of station corrections
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 
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

Our Binary Classification
30 CISN ROCK Stations
120 CISN SOIL Stations
NEHRP
Class
Maximum Shear
Wave Velocity in
Upper 30 m (m/s)
A (Hard Rock)
1400
B (Soft Rock)
724
BC
686
C
464
CD (Alluvium)
372
D
301
DE
298
E (Mud)
163
Average Rock and Soil envelopes as functions of M, R
VELOCITY
Average Rock and Soil envelopes as functions of M, R
(filtered) DISPLACEMENT
S-wave
magnitude-dependence
anaelastic attenuation
saturation
geometric attenuation
constant
a
b
c1
c2
d
e
ROCK
0.7789
-2.56E-03
1.4775
1.1054
-1.352
-0.645
S-wave
SOIL
0.8354
-2.32E-03
1.768
1.03
-1.563
-0.338
P-wave acceleration (ROCK)
a
b
c1
c2
d
e
P-wave
0.718
-3.27E-03
1.6
1.045
-1.195
-1.064
S-wave acceleration (ROCK)
S-wave
0.779
-2.56E-03
1.478
1.105
-1.105
-0.645
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