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