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X - 38 AUDET ET AL.: SLAB MORPHOLOGY AND ETS IN CASCADIA
230˚
230˚
232˚
232˚
234˚
234˚
236˚
236˚
238˚
238˚
240˚
240˚
40˚ 40˚
42˚ 42˚
44˚ 44˚
46˚ 46˚
48˚ 48˚
50˚ 50˚
BC
WA
OR
CA
Juan de Fuca
Pacific
Explorer
Gorda
LINE 1LINE 2
LINE 3
LINE 4
POLARIS NVIPOLARIS SWBCCNSNCassidy et al. 1998GSCCASC93TAPNSNCNSN
Cascade volcanoes
Figure 1. Map of the Cascadia subduction zone with broadband seismic stations located
within the forearc that were used in this study. Black triangles indicate the location of
active Cascade volcanoes.
D R A F T August 25, 2008, 5:50pm D R A F T
AUDET ET AL.: SLAB MORPHOLOGY AND ETS IN CASCADIA X - 39
SurfaceStation
Pp sPs s P P s
xx
xx
Figure 2. Ray diagram showing P -to-S converted phases and free-surface multiples
from an incident P wave interacting with a dipping interface. Solid lines are rays travelling
as P waves, dashed lines are rays travelling as S waves. For a dipping, low-velocity zone
this set of wave interactions is generated twice at the top and bottom layer discontinuities,
with opposite polarities.
D R A F T August 25, 2008, 5:50pm D R A F T
X - 40 AUDET ET AL.: SLAB MORPHOLOGY AND ETS IN CASCADIA
0
5
10
15
20
25Tim
e [
sec]
a !0.3
!0.2
!0.1
0.0
0.1
0.2
0.3
0
5
10
15
20
25Tim
e [
sec]
b !0.3
!0.2
!0.1
0.0
0.1
0.2
0.3
060
120180240300360
BA
Z [
de
g]
c
0.04
0.05
0.06
0.07
0.08
px
[s/k
m]
d
0 5 10 15 20 25
Pxs Ppxs Psxs
-0.4
-0.2
0
0.2
0.4
e b b b ttt
Figure 3. Radial- (a) and transverse- (b) component receiver functions at station
PGC sorted by back-azimuth (c) of incident wavefield. Amplitudes are relative to P -
component. (d) shows the distribution in horizontal slowness. (e) illustrates arrivals of
converted phases (PxS, PPxS, PSxS) from the top [t] and bottom [b] of the LVZ within
the radial-component (a) receiver functions. Red and blue colors correspond to velocity
increase and decrease with depth, respectively.
D R A F T August 25, 2008, 5:50pm D R A F T
AUDET ET AL.: SLAB MORPHOLOGY AND ETS IN CASCADIA X - 41
0
5
10
15
20
25
30
Tim
e [
se
c]
LINE1
0
5
10
15
20
25
30
Tim
e [
se
c]
LINE3
0
5
10
15
20
25
30
Tim
e [
se
c]
LINE3
0
5
10
15
20
25
30
Tim
e [
se
c]
LINE4
0
5
10
15
20
25
30
Tim
e [
se
c]
LINE5
Figure 4. Radial-component receiver functions for all data sorted by station posi-
tion along each line and, for each station, by back-azimuth of incident wavefield. Note
that LINE5 is a pseudo-linear profile based on all data south of 49! that were organized
according to increasing depth of the LVZ beneath each station.
D R A F T August 25, 2008, 5:50pm D R A F T
X - 44 AUDET ET AL.: SLAB MORPHOLOGY AND ETS IN CASCADIA
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2Vp/Vs
Ps (SV) Ps (SH) Pps (SV)
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
Vp/Vs
15 20 25 30 35 40 45 50 55
Depth
Pps (SH)
15 20 25 30 35 40 45 50 55
Depth
Pss (SV)
15 20 25 30 35 40 45 50 55
Depth
Weighted average
Figure 7. Example of phase stacking results for station PGC (Fig. 3). In this realization
the dip that produces optimal results is 14!. The maximum and minimum of the weighted
average of each individual phase determines the depth to the top and bottom of the LVZ,
and VP/VS of overlying structure.
D R A F T August 25, 2008, 5:50pm D R A F T
AUDET ET AL.: SLAB MORPHOLOGY AND ETS IN CASCADIA X - 45
01020304050607080D
ep
th [
km
]LINE 1
1.4
1.6
1.8
2.0
2.2
Vp
/Vs
0 20 40 60 80 100 120 140 160
Distance along line [km]
LINE 2
60 80 100 120 140 160 180 200
Distance from trench [km]
01020304050607080D
ep
th [
km
]
LINE 3
1.4
1.6
1.8
2.0
2.2
Vp
/Vs
120 140 160 180 200 220 240 260
Distance from trench [km]
LINE 4
80 100 120 140 160 180 200 220
Distance from trench [km]
Figure 8. Phase stacking results for individual stations along LINE1-4. Note that
VP/VS for the vertical column overlying the bottom of the LVZ (in red) is consistently
higher than VP /VS of the column overlying the top of LVZ (in blue), with a few outliers.
D R A F T August 25, 2008, 5:50pm D R A F T
X - 46 AUDET ET AL.: SLAB MORPHOLOGY AND ETS IN CASCADIA
230˚
230˚
232˚
232˚
234˚
234˚
236˚
236˚
238˚
238˚
240˚
240˚
40˚ 40˚
42˚ 42˚
44˚ 44˚
46˚ 46˚
48˚ 48˚
50˚ 50˚
BC
WA
OR
CA
Juan de Fuca
Pacific
Explorer
Gorda
Figure 9. Depths contours of the top of LVZ (shallow slab model) and the top of
the plate interface (deep slab model) along the Cascadia margin. Earthquake epicenters
from the GSC and USGS catalogues for M>2 are shown as green dots; tremor epicenters
appear as black dots.
D R A F T August 25, 2008, 5:50pm D R A F T
AUDET ET AL.: SLAB MORPHOLOGY AND ETS IN CASCADIA X - 47
0
10
20
30
40
50
60
70
80
Depth
[km
]
LINE 1
0
10
20
30
40
50
60
70
80
Depth
[km
]
LINE 2
0
10
20
30
40
50
60
70
80
Depth
[km
]
LINE 3
0
10
20
30
40
50
60
70
80
Depth
[km
]
0 20 40 60 80 100 120 140 160 180 200 220 240 260
Distance from trench [km]
LINE 4
Figure 10. Comparison of the depth to top of slab for the shallow (dark blue curves)
and deep (light blue curves) slab models along the four lines. Superposed on the plots are
the earthquake (green dots) and tremor (black dots) hypocenters.
D R A F T August 25, 2008, 5:50pm D R A F T
X - 48 AUDET ET AL.: SLAB MORPHOLOGY AND ETS IN CASCADIA
0
5
10
15
20
25
30
Perc
enta
ge o
f tr
em
or
Vancouver Island
0
5
10
15
20
25
30
Perc
enta
ge o
f tr
em
or
15 20 25 30 35 40 45 50
Slab depth [km]
Washington
15 20 25 30 35 40 45 50
Slab depth [km]
Oregon and N. California
15 20 25 30 35 40 45 50
Slab depth [km]
Figure 11. Histograms of tremor epicenter distribution within 2.5 km depth contours
based on the shallow (top) and deep (bottom) slab models. Histograms for the shallow
slab model reveal Gaussian distributions peaking at ∼35 km. Histograms for the deep slab
model are more flat and do not show tremor clustering along specific slab-depth contours.
D R A F T August 25, 2008, 5:50pm D R A F T