Seismic anisotropy of the lithosphere and asthenosphere was investigated with a dense broadband seismic transect nearly orthogonal to the central San Andreas fault (SAF). A contrast in SK(K)S splitting was found across the SAF, with a clockwise rotation of the fast orientation ~26° closer to the strike of the SAF and greater delay times for stations located within 35 km to the east. Dense seismograph spacing requires heterogeneous anisotropy east of the SAF in the uppermost mantle or crust. Based on existing station coverage, such a contrast in splitting orientations across the SAF may be unusual along strike and its location coincides with the high‐velocity Isabella anomaly in the upper mantle. If the Isabella anomaly is a fossil slab fragment translating with the Pacific plate, the anomalous splitting east of the SAF could indicate a zone of margin‐parallel shear beneath the western edge of North America

Clayton, Robert W.

Jiang, Chengxin

Schmandt, Brandon

Caltech Authors - Main

 
 
1 
 
 
Geophysical Research Letters 
Supporting Information for 
An anisotropic contrast in the lithosphere across the central San Andreas fault 
Chengxin Jiang1, Brandon Schmandt1, Robert W. Clayton2 
1 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131. 
2 Seismological Laboratory, California Institute of Technology, Caltech 252-21, Pasadena, CA 91125. 
  
Contents of this file  
 
Figures S1 to S8 
Tables S1 to S2 
Introduction  
This supporting file contains the following figures and table:  
Figure S1. An example of selected non-null splitting measurement for a SKS arrival at station 
CC17.  
Figure S2. Quality control criteria for each parameter used for the selection of good null and 
non-null measurements in this study. 
Figure S3. Compilations of splitting results from using a different culling threshold of defining 
the null measurements. 
Figure S4. Comparison of splitting parameters and associated uncertainties from using a 
different culling threshold of SNR with the same criteria of 10° angular difference between fast 
direction and event back-azimuth to define null measurements. 
Figure S5: Comparison of splitting parameters and associated uncertainties from using a 
different culling threshold of SNR with the same criteria of 15° angular difference between fast 
direction and event back-azimuth to define null measurements. 
Figure S6: Rose diagrams showing the fast direction of each non-null station.  
Figure S7: Comparison of the splitting fast direction at SAFOD sites with those from our study.  
Figure S8: Synthetic splitting results for modelling observed azimuth-dependent splitting 
behaviors for stations located within 30 km on both sides of the SAF. 
Table S1: Definitions of the parameter used for the quality control in section 2. 
Table S2: Summary of the stacked SKS/SKKS splitting measurements along CCSE array as well 
as for 7 long-term regional stations. 
 
 
 
2 
 
 
Figure S1. An example of selected non-null splitting measurement for a SKS arrival at station 
CC17. (a) The raw radial (solid black line) and transverse (dashed red line) waveform 
components. The grey corridor shows the selected time window for subsequent splitting 
analysis. (b) Diagnostic plot resulted from the grid-search based splitting analysis with the x-
axis showing the delay time and the y axis showing the fast direction. The grey shaded region 
represents the estimates of 95% confidence, and the red cross denotes the optimal splitting 
parameters with exact values marked on the top of the figure. (c) Corrected waveforms for the 
fast (solid black line) and slow direction (dashed red line). (d) Corrected Q (solid black line) and 
T (dashed red line) waveforms. 
  
0 20 40 60 80
Delay time (sec)
-1.5
-1
-0.5
0
0.5
1
Am
pli
tu
de
104 2015-05-12 07:05:19
selected
Q
T
CC17 (-50.98,  2.2)
0 1 2 3 4
Delay time (sec)
-90
-60
-30
0
30
60
90
Fa
st 
dir
ec
tio
n 
(d
eg
)
0 10 20 30
Delay time (sec)
-1
-0.5
0
0.5
Am
pli
tu
de
corFast
corSlow
0 10 20 30
Delay time (sec)
-10000
-5000
0
5000
Am
pli
tu
de
corQ
corT
(a) (b)
(c) (d)
Figure S1
 
 
3 
 
 
Figure S2. Quality control criteria for each parameter (defined in Table S1) used for the 
selection of good null and non-null measurements in this study, including SNR (a), angular 
difference (b), T-to-Q amplitude ratio of raw waveforms (c), and T-to-Q amplitude ratio for 
corrected waveforms (d). The red lines with arrows denote the range of each parameter to 
define good non-null and null measurements. 
  
0
50
100
150
200
Co
un
ts
0 2 4 6 8
SNR
(a)
Figure 2
0
50
100
150
200
0 10 20 30 40 50 60
|phi−baz|
(b)
0
50
100
150
200
Co
un
ts
0.0 0.2 0.4 0.6 0.8 1.0
initial T/Q
(c)
0
50
100
150
200
0.0 0.2 0.4 0.6 0.8 1.0
T/Q after correction
(d)
selected waveforms selected non-nulls
se
lec
te
d 
nu
lls
se
lec
te
d 
no
n-
nu
lls
an
d 
nu
lls
 
 
4 
 
 
Figure S3. Compilations of splitting results from using a different culling threshold, which 
requires non-null measurements to have angular differences between fast direction and 
backazimuth direction larger than 15° (compared to an original culling threshold of 10°). (a) 
The averaged splitting parameter for the CCSE array resulted from the new data culling; (b) 
number of measurements at each CCSE stations; (c) fraction of null measurements for each 
station; (d) the splitting measurements for the best SKS event; (e) the splitting measurements 
for the best SKKS event. Note the results here are similar to those in Figure 2 and Figure 3a. 
Abbreviations: SAF-San Andreas Fault, GV-Great Valley. 
 
  
SAF
SAF SAF
−121˚ −120˚ −119˚
2s
(a) Averaged splitting 
36.0˚
36.5˚
(d) Best event for SKS phase (e) Best event for SKKS phase
(b) Number of meaurements (c) Fraction of Null measurements
36.0˚
36.5˚
36.0˚
36.5˚
−121˚ −120˚ −119˚
2s
−121˚ −120˚ −119˚
2s
−121˚ −120˚ −119˚−121˚ −120˚ −119˚
SAF SAF
Figure S3
GV
GV GV
GVGV
 
 
5 
 
 
 
Figure S4. Comparison of single-station average splitting results from using a different culling 
threshold of SNR with the same criteria of 10° when defining null measurements in the quality 
control. The left column is from the culling with a SNR of 2, and the right column is with a SNR 
of 2.5. The first panel shows the geographic distribution of splitting parameters across the 
CCSE array; the mid panel shows the delay time variations along with the 1-sigma uncertainty 
from the splitting analysis; the lower panel shows the fast direction variations with the 1-sigma 
uncertainty. Note that the uncertainties are calculated based on the good non-null 
measurements at each station. 
  
−121˚ −120˚ −119˚
36˚00'
36˚30'
2s
60
90
120
150
ph
i (
de
g)
−121 −120 −119
0
1
2
3
4
dt
 (
s)
−121 −120 −119
−121˚ −120˚ −119˚
2s
60
90
120
150
−121 −120 −119
0
1
2
3
4
−121 −120 −119
SAF
SAF
SAF
SAF
SAF
SAF
(a)
(b)
(c)
(d)
(e)
(f)
 
 
6 
 
 
 
Figure S5. Similar comparison as Fig. S4 of single-station average splitting results from using a 
different culling threshold of SNR but with the same criteria of 15° when defining null 
measurements in the quality control. The left column is from the culling with a SNR of 2, and 
the right column is with a SNR of 2.5.  
 
  
−121˚ −120˚ −119˚
36˚00'
36˚30'
2s
60
90
120
150
ph
i (
de
g)
−121 −120 −119
0
1
2
3
4
dt
 (
s)
−121 −120 −119
−121˚ −120˚ −119˚
2s
60
90
120
150
−121 −120 −119
0
1
2
3
4
−121 −120 −119
SAF
SAF
SAF
SAF
SAF
SAF
(a)
(b)
(c)
(d)
(e)
(f)
 
 
7 
 
 
Figure S6. Fast polarization directions of good non-null measurements for each non-null 
station (with a selection criteria same to that for Fig. 3b) shown as red vectors on a unit circle. 
The station name and the number of measurements are labeled beneath each diagram. Note 
that the stations from the two colored circles in Fig. 3a of the main text are illustrated with a 
colored short bar on the top left of the diagram.    
CC04; 13 CC05; 4 CC06; 7 CC07; 8 CC08; 8
CC09; 5 CC10; 8 CC13; 3 CC14; 6 CC15; 9
CC16; 6 CC17; 9 CC18; 7 CC19; 5 CC25; 3
CC26; 3 CC27; 5 CC28; 6 CC30; 4 CC31; 6
CC32; 6 CC33; 7 CC34; 6 CC35; 11 CC36; 7
CC37; 11 CC38; 11 CC64; 9
Figure S6
270
270
270
270
270
270
90
90
90
90
90
0 0 0 0 0
 
 
8 
 
 
Figure S7. (a) Comparison of the splitting fast direction at SAFOD sites (red column) from 
Becker et al., (2012) with those from ~35 km west (grey column) and east (blue column) of the 
SAF from this study. (b) A zoomed-in plot of the splitting measurements at SAFOD sites. 
  
−121.0˚ −120.5˚ −120.0˚
35.8˚
36.0˚
36.2˚
2s
SAF
(b)
0
10
20
30
40
50
60
pe
rc
en
ta
ge
 (%
)
0 30 60 90 120 150 180
fast direction (deg)
(a)
 
 
9 
 
 
 
Figure S8. Synthetic splitting results for modelling observed azimuth-dependent splitting 
behaviors for stations located within 30 km on both sides of the SAF. The forward calculation is 
conducted using a simple two anisotropic layers with a detailed explanation of model 
parameters in section 4.2 of the text. The black dots indicate the splitting parameter averaged 
for all stations within each 20° back-azimuth bin (modulo 180°), and the error bars represent 
the two standard deviations of all measurements within each bin. Note that the dot at 70° 
back-azimuth has no error bar due to the lack of good events within that back-azimuth range.      
     
 
  
0
50
100
150
200
Fa
st 
dir
ec
tio
n 
(d
eg
)
0 50 100 150
Backazimuth (deg)
West of SAF
(a)
0
1
2
3
De
lay
 tim
e 
(s
)
0 50 100 150
Backazimuth (deg)
(b)
0
50
100
150
200
Fa
st 
dir
ec
tio
n 
(d
eg
)
0 50 100 150
Backazimuth (deg)
East of SAF
(c)
0
1
2
3
De
lay
 tim
e 
(s
)
0 50 100 150
Backazimuth (deg)
(d)
Figure S4
 
 
10 
 
Variable	 Definition	
SNR	of	waveforms	 The	sum	of	RMS	of	the	noise	window	
(~70	s	ahead	of	the	signal	window)	for	
the	raw	Q	and	T	components	over	the	
sum	of	RMS	of	the	signal	window	for	the	
raw	Q	and	T	components	
T/Q	of	the	initial	waveform	 The	maximum	amplitude	of	the	signal	
window	on	raw	T	component	over	the	
maximum	amplitude	of	the	same	
window	on	raw	Q	component	
T/Q	of	the	corrected	waveforms	 The	maximum	amplitude	of	the	signal	
window	on	corrected	T	component	over	
the	maximum	amplitude	of	the	same	
window	on	corrected	Q	component		
Table S1. Definitions of the parameter used for quality control in section 2.   
  
 
 
11 
 
station	 longitude	 latitude	 phi	(deg)	 1-sigma	 dt	(s)	 1-sigma	
CC04	 -121.25	 36.016	 118.31	 22.39	 1.6	 0.65	
CC05	 -121.172	 36.025	 98.09	 57.12	 1.8	 0.98	
CC06	 -121.115	 35.984	 94.04	 25.35	 1.6	 0.57	
CC07	 -121.054	 35.975	 106.18	 28.74	 1.2	 0.59	
CC08	 -120.961	 35.992	 90	 10.59	 2.6	 0.82	
CC09	 -120.889	 36.025	 98.09	 11.35	 1.6	 0.78	
CC10	 -120.824	 36.046	 92.02	 28.52	 2	 0.85	
CC13	 -120.618	 36.121	 122.36	 36.57	 2	 1.03	
CC14	 -120.539	 36.131	 118.31	 15.96	 2.4	 0.44	
CC15	 -120.443	 36.099	 118.31	 12.29	 2.2	 0.97	
CC16	 -120.371	 36.127	 138.54	 41.14	 3.2	 1.03	
CC17	 -120.343	 36.194	 124.38	 26.56	 1.6	 0.82	
CC18	 -120.238	 36.177	 114.27	 29.12	 1.4	 0.91	
CC19	 -120.165	 36.135	 90	 26.07	 3	 0.86	
CC20	 -120.105	 36.21	 0	 0	 0	 0	
CC21	 -119.995	 36.227	 0	 0	 0	 0	
CC23	 -119.855	 36.249	 0	 0	 0	 0	
CC25	 -119.72	 36.258	 126.4	 36.7	 2.2	 0.72	
CC26	 -119.637	 36.279	 79.89	 15.31	 2.2	 0.83	
CC27	 -119.566	 36.286	 96.07	 36.21	 1	 0.58	
CC28	 -119.489	 36.298	 142.58	 33.54	 2.4	 1.16	
CC30	 -119.342	 36.31	 90	 14.43	 1.4	 1.28	
CC31	 -119.274	 36.318	 90	 2.02	 2.2	 0.7	
CC32	 -119.193	 36.333	 71.8	 25.77	 2.2	 0.75	
CC33	 -119.122	 36.344	 100.11	 13.8	 1.2	 0.67	
CC34	 -119.058	 36.36	 0	 0	 0	 0	
CC35	 -119.006	 36.376	 73.82	 33.8	 1	 0.96	
CC36	 -118.962	 36.389	 94.04	 36.75	 1.4	 0.85	
CC37	 -118.9	 36.388	 102.13	 30.3	 1.2	 0.61	
CC38	 -118.841	 36.382	 90	 27.62	 2.2	 0.91	
CWC	 -118.08	 36.44	 102.13	 28.76	 1.4	 0.53	
HAST	 -121.551	 36.389	 106.18	 29.08	 2	 0.48	
HELL	 -119.023	 36.68	 77.87	 24.61	 1.2	 0.57	
ISA	 -118.474	 35.663	 71.8	 23.01	 1.8	 0.57	
RAMR	 -120.87	 35.636	 114.27	 11.72	 1.2	 0.72	
SMM	 -119.996	 35.314	 112.25	 34.54	 1	 0.64	
TIN	 -118.23	 37.054	 67.75	 15.39	 1.8	 0.44	
 
 
 
12 
 
Table S2. Summary of the stacked SKS/SKKS splitting measurements along CCSE array as well 
as for 7 long-term regional stations used to test the consistency of our analysis with prior 
results. Note that the stations with 0 value of fast direction and delay time are regarded as null 
stations (defined in section 3).  
 


An anisotropic contrast in the lithosphere across the central San Andreas fault

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An anisotropic contrast in the lithosphere across the central San Andreas fault

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