Structure and deformation of the uppermost mantle beneath...
1
Method Fig. 1 Several earthquake source regions lie on-axis with the RISTRA line in both directions. Events with Mw > 6.0 and within 20 degrees of the line are shown. Fig. 2 Mw 6.0 event in Kodiak, Alaska on 2000/11/06. Rayleigh waveforms vary considerably across the RISTRA array suggesting complex structure. Phase velocities are calculated for a moving window of vertical component seismograms. Blue box indicates the bin of stations stacked in figure 3. Delta, deg. time, min. UT53 UT52 UT51 AZ49 AZ48 AZ47 AZ46 NM43 NM42 NM40 NM39 NM38 NM37 NM33 NM32 NM31 NM30 NM27 NM26 NM24 NM23 NM22 NM20 NM17 NM15 NM10 NM08 TX06 TX05 TX03 TX01 2.5 km/s 3 km/s 4 km/s 5 km/s 8 km/s 35 36 37 38 39 40 41 42 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 Northwest Southeast Fig. 3 For each event, wavefields are slant-stacked over a range of slownesses in the fourier domain following the approach of McMechan & Yedlin (1981) and Herrmann & Ammon (2002). The modulus of the stack for four events are shown in A-D. Contours span .5 to 1.0 of the maximum value. Some events stack poorly due to missing data or noise such as in B. (Periods greater than 40 seconds are calculated separately using an identical procedure. ) 6 8 10 20 30 40 3 4 5 Period (s) Event: 2000.11.06 6 8 10 20 30 40 3 4 5 Period (s) Event: 2001.02.17 6 8 10 20 30 40 3 4 5 Period (s) Event: 2000.05.12 6 8 10 20 30 40 3 3.5 4 4.5 5 Period (s) Phase Velocity (km/s) Event: 1999.10.13 A B D C Fig. 4 Since not all events yield high quality period- velocity maps for a given set of stations, we perform a second stack over many events. Stacking multiple events compensates for spectral holes, multipathing and contamination from higher modes. Bootstrap error estimates (2σ shown) provide a measure of reliability. The maximum value at each period is considered the true phase velocity. 6 8 10 20 30 40 3 3.5 4 4.5 5 Period (s) Phase Velocity (km/s) Stack of 29 events Fig. 5 Average phase velocities across entire array compared to other studies. The intermediate velocities observed here reflect the mixture of terrains across the Ristra line, including the Colorado Plateau, Rio Grande Rift and Great Basin. Fig. 6 Phase velocity as a function of period and distance along the array. At each station, phase velocity is determined using a bin of neighboring stations (see fig. 2-4). A smaller bin is reliable for short periods, yielding higher resolution. Measurements with std. error > 0.15 km/s are masked out. Periods of 8-180 seconds are reliable across most of the array at the resolution shown above. Discussion Teleseismic S-wave tomography (Fig. 7) provides good lateral resolution of features in the uppermost mantle. However, velocities are only relative and the vertical resolution suffers due to steep ray paths in the shallow Earth. Surface wave tomography compliments this image by providing absolute velocities and increasing resolution in the upper 200 km. We are currently inverting phase velocity for shear wave structure. The constraints on Moho depth, determined from receiver functions, remove one of the largest sources of inversion non-uniqueness. As expected, high velocities are found for the cold lithosphere beneath the Great Plains. In contrast, we find low mantle velocities beneath the Colorado Plateau. Decreased density, implied by this velocity low, may support the existing high elevation over the plateau. 10 20 40 70 100 150 250 2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 Phase velocity (km/s) Period (sec) Average (and 2σerror) across Ristra array Canadian Shield (Brune & Dorman, 1963) Basin & Range (Priestly & Brune, 1978) Rio Grande Rift (Sinno et al., 1985) 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 -400 -300 -200 -100 0 100 200 300 400 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 Phase velocity, km/s TX01 TX02 TX03 TX04 TX05 TX06 NM07 NM08 NM09 NM10 NM11 NM12 NM13 NM14 NM15 NM16 NM17 NM18 NM19 NM20 NM21 NM22 NM23 NM24 NM25 NM26 NM27 Distance along array, km NM28 NM29 NM30 NM31 NM32 NM33 NM34 NM35 NM36 NM37 NM38 NM39 NM40 NM41 NM42 NM43 NM44 AZ45 AZ46 AZ47 AZ48 AZ49 AZ50 UT51 UT52 UT53 UT54 Period, s Colorado Plateau Great Plains Rio Grande Rift A A B C D stack bin width Jemez Lineament Rayleigh wave phase velocities Fig. 7 Shear wave tomography from teleseismic events analyzed by the U.T. Austin group (Grand and Gao). Top 200 km are broadly consistent with the phase velocity mapping: ~2% slow in the upper mantle beneath the Rift and Jemez Lineament; Higher Vs beneath the Great Plains and Colorado Plateau. Observations Several features are evident in the raw phase velocities: The Delaware and San Juan basins are seen in periods < 15 seconds. (label A) Periods sensitive to crustal structure (< ~30 seconds) are slow under the rift and the Jemez lineament. (label B) Rift and Jemez lineament mantle velocities are up to 0.3 km/s slow for periods < 100 seconds, coinciding with the gravity low (see other poster) and S-wave tomography low in Fig. 7. (label C) Periods > 100 seconds are ~0.3 km/s faster beneath the Great Plains but show little difference between the Rift and Colorado Plateau. (label D) Velocity Perturbation, %Vs 3 2 1 0 -1 -2 -3 -250 250 0 100 200 300 Depth (km) ut54 az50 az45 nm40 nm35 nm30 nm25 nm20 nm15 nm10 tx05 tx01 0 -500 500 Distance (km) Method The RISTRA array provided 23 earthquakes for the measurement of shear wave splitting. SKS and SKKS phases are used to measure seismic polarization anistropy beneath the array. When measuring the splitting, generally the second eigenvalue of the covariance matrix is minimized. In the case of SKS, a radially polarized phase, we minimize the energy in the transverse component. The error analysis was performed using an inverse F test to construct a confidence region for each set of shear wave splitting observations (Silver and Chan 1991). Observations Discussion It is commonly thought that shear wave splitting is due to strain-induced lattice preferred orientation (LPO) of anisotropic minerals in the upper mantle, in particular olivine. Development of the LPO may be due to the deformation of the lithosphere, strain associated with shear of the plate over the asthenosphere, or some combination of both processes. Most stations along the RISTRA array (NM14 to UT51) show a consistently oriented NE-SW fast direction (~40 degrees) with an average lag time ~1.2 sec. The anisotropy beneath these stations is best explained by the differential horizontal motion between the North American lithosphere and underlying asthenospheric mantle. Deviation of the fast direction from the absolute plate motion occurs in areas beneath the extreme northern and southern stations. The N-S fast direction beneath western Texas is similar to that observed beneath Las Cruces and El Paso in the southern rift. This contrast in fast direction indicates a fundamentally different dynamic in the mantle beneath the northern and southern Rio Grande Rift. Fig. 8 A well-split SKS phase. Contour plot shows the residual energy as a function of both fast direction and lag time. The first contour around the minimum represents the 95% confidence interval. Fig. 9a and 9b Shear wave splitting results. a) Red bars indicate fast directions from previous determinations (Sandvol 1995). Black bars indicate fast directions determined from this study. b) Fast direction versus station plot. Red line is the 5 point running average, absolute plate motion is indicated by a blue line and the black line indicates the averaged F, ~40 degrees. 30û 35û 0s .5s 1s 1.5s UT54 AZ45 NM34 NM23 NM12 TX02 -10 0 10 20 30 40 50 60 70 80 90 Absolute Plate Motion Fast Direction(Φ) 1 σ Absolute Plate Motion 2.5cm/yr NW SE Colorado Plateau Jemez Lineament Rio Grande Rift Great Plains a) b) Average Φ El Paso Las Cruces 0 0.5 1 1.5 2 2.5 20 40 60 80 100 120 140 160 180 ERROR PLOT : MINIMUM TRANSVERSE ENERGY Delta T (s) Phi (degrees) STN=AZ48 Φ=25±7 δt = 0.65±0.1 PHASE=SKS Event: 2000/08/15 04:30:0.8 h=357 km BAZ=234 DIST=95 RADIAL & TRANSVERSE Time (s) T R FAST & SLOW S F SHIFTED FAST & SLOW S F CORRECTED RADIAL & TRANSVERSE R 10 12 14 16 18 T 8 R T F S F S R T Shear wave splitting References Brune, J. & J. Dorman. Seismic waves and Earth structure in the Canadian shield. BSSA 53, 167-210 (1963) Herrmann, R. B. & C. J. Ammon. Computer programs in seismology, Version 3.15 (2002) McMechan, G. A., & M. J. Yedlin. Analysis of dispersive waves by wavefield transformation. Geophys. 46, 869-874 (1981) Priestly, K. & J. Brune. Surface waves and the structure of the Great Basin of Nevada and Western Utah. JGR 83, 2265-2272 (1978). Sandvol E. Mapping mantle azimuthal Seismic anistropy. Ph.D. Thesis, New Mexico State University (1995) Silver, P.G. and W. Chan. Shear wave splitting and mantle deformation. JGR 96,16429-16454 (1991) Sinno, Y. A. & G. R. Keller. A Rayleigh wave dispersion study between El Paso, TX and Albuquerque, NM. JGR 91, 6168-6174 (1986) Shear Wave Splitting -110û -105û Structure and deformation of the uppermost mantle beneath the RISTRA array Michael West, Rengin Gok, and the RISTRA Team* Contacts: [email protected] & [email protected] * The RISTRA Team is: Ni, J., Gok, R., West, M., Reynolds, S., New Mexico State University, Dept. of Physics, Las Cruces, NM 88003 Grand, S., Gao, W., University Of Texas, Austin, Department of Geological Sciences, Austin, TX 78712 Aster, R., Wilson, D. , Schlue, J., New Mexico Institute of Mining and Technology, Socorro, NM 87801 Baldridge, S., Los Alamos National Laboratory, EES-1, Los Alamos, NM 87545 Semken, S., Dine College, Division of Natural Sciences, Shiprock, NM 87420
Transcript of Structure and deformation of the uppermost mantle beneath...
Method
Fig. 1 Several earthquake source regions lie on-axis with the RISTRA line in both directions. Events with Mw > 6.0 and within 20 degrees of the line are shown.
Fig. 2 Mw 6.0 event in Kodiak, Alaska on 2000/11/06. Rayleigh waveforms vary considerably across the RISTRA array suggesting complex structure. Phase velocities are calculated for a moving window of vertical component seismograms. Blue box indicates the bin of stations stacked in figure 3.
Delta, deg.
tim
e, m
in.
UT53
UT52
UT51
AZ
49 A
Z48
AZ
47 A
Z46
NM
43 N
M42
NM
40
NM
39 N
M38
NM
37
NM
33 N
M32
NM
31 N
M30
NM
27 N
M26
NM
24 N
M23
NM
22
NM
20
NM
17
NM
15
NM
10
NM
08
TX06
TX05
TX03
TX01
2.5 km/s
3 km/s
4 km/s
5 km/s
8 km/s
35 36 37 38 39 40 41 42
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
Northwest Southeast
Fig. 3 For each event, wavefields are slant-stacked over a range of slownesses in the fourier domain following the approach of McMechan & Yedlin (1981) and Herrmann & Ammon (2002). The modulus of the stack for four events are shown in A-D. Contours span .5 to 1.0 of the maximum value. Some events stack poorly due to missing data or noise such as in B. (Periods greater than 40 seconds are calculated separately using an identical procedure. )
6 8 10 20 30 40
3
3.5
4
4.5
5
Period (s)
Pha
se V
eloc
ity (
km/s
)
Event: 2000.11.06
6 8 10 20 30 40
3
3.5
4
4.5
5
Period (s)
Pha
se V
eloc
ity (
km/s
)
Event: 2001.02.17
6 8 10 20 30 40
3
3.5
4
4.5
5
Period (s)
Pha
se V
eloc
ity (
km/s
)
Event: 2000.05.12
6 8 10 20 30 40
3
3.5
4
4.5
5
Period (s)
Pha
se V
eloc
ity (
km/s
)
Event: 1999.10.13
A B DC
Fig. 4 Since not all events yield high quality period-velocity maps for a given set of stations, we perform a second stack over many events. Stacking multiple events compensates for spectral holes, multipathing and contamination from higher modes. Bootstrap error estimates (2σ shown) provide a measure of reliability. The maximum value at each period is
considered the true phase velocity.
6 8 10 20 30 40
3
3.5
4
4.5
5
Period (s)
Pha
se V
eloc
ity (
km/s
)
Stack of 29 events
Fig. 5 Average phase velocities across entire array compared to other studies. The intermediate velocities observed here reflect the mixture of terrains across the Ristra line, including the Colorado Plateau, Rio Grande Rift and Great Basin.
Fig. 6 Phase velocity as a function of period and distance along the array. At each station, phase velocity is determined using a bin of neighboring stations (see fig. 2-4). A smaller bin is reliable for short periods, yielding higher resolution. Measurements with std. error > 0.15 km/s are masked out. Periods of 8-180 seconds are reliable across most of the array at the resolution shown above.
DiscussionTeleseismic S-wave tomography (Fig. 7) provides good lateral resolution of features in the uppermost mantle. However, velocities are only relative and the vertical resolution suffers due to steep ray paths in the shallow Earth. Surface wave tomography compliments this image by providing absolute velocities and increasing resolution in the upper 200 km. We are currently inverting phase velocity for shear wave structure. The constraints on Moho depth, determined from receiver functions, remove one of the largest sources of inversion non-uniqueness. As expected, high velocities are found for the cold lithosphere beneath the Great Plains. In contrast, we find low mantle velocities beneath the Colorado Plateau. Decreased density, implied by this velocity low, may support the existing high elevation over the plateau.
10 20 40 70 100 150 2502.8
3
3.2
3.4
3.6
3.8
4
4.2
4.4
4.6
Pha
se v
eloc
ity (
km/s
)
Period (sec)
Average (and 2σ error) across Ristra arrayCanadian Shield (Brune & Dorman, 1963)Basin & Range (Priestly & Brune, 1978)Rio Grande Rift (Sinno et al., 1985)
3.4
3.5
3.6
3.7
3.8
3.9
4
4.1
4.2
4.3
4.4
4.5
4.6
4.7-400 -300 -200 -100 0 100 200 300 400
102030405060708090
100110120130140150160170180
Pha
se v
eloc
ity, k
m/s
TX
01T
X02
TX
03T
X04
TX
05T
X06
NM
07N
M08
NM
09
NM
10N
M11
NM
12N
M13
NM
14N
M15
NM
16N
M17
NM
18N
M19
NM
20
NM
21N
M22
NM
23N
M24
NM
25N
M26
NM
27
Distance along array, km
NM
28N
M29
NM
30N
M31
NM
32
NM
33
NM
34N
M35
NM
36
NM
37
NM
38N
M39
NM
40N
M41
NM
42N
M43
NM
44A
Z45
AZ
46A
Z47
AZ
48A
Z49
AZ
50U
T51
UT
52
UT
53U
T54
Per
iod,
s
Colorado Plateau Great PlainsRio Grande Rift
A AB
C
D
stack bin width
Jemez Lineament
Rayleigh wave phase velocities
Fig. 7 Shear wave tomography from teleseismic events analyzed by the U.T. Austin group (Grand and Gao). Top 200 km are broadly consistent with the phase velocity mapping: ~2% slow in the upper mantle beneath the Rift and Jemez Lineament; Higher Vs beneath the Great Plains and Colorado Plateau.
ObservationsSeveral features are evident in the raw phase velocities:
� The Delaware and San Juan basins are seen in periods < 15 seconds. (label A)
� Periods sensitive to crustal structure (< ~30 seconds) are slow under the rift and the Jemez lineament. (label B)
�Rift and Jemez lineament mantle velocities are up to 0.3 km/s slow for periods < 100 seconds, coinciding with the gravity low (see other poster) and S-wave tomography low in Fig. 7. (label C)
� Periods > 100 seconds are ~0.3 km/s faster beneath the Great Plains but show little difference between the Rift and Colorado Plateau. (label D)
Velocity Perturbation, %Vs 3 2 1 0 -1 -2 -3
-250 250
0
100
200
300
Dep
th (k
m)
ut54 az50 az45 nm40 nm35 nm30 nm25 nm20 nm15 nm10 tx05 tx01
0-500 500
Distance (km)
Method
The RISTRA array provided 23 earthquakes for the measurement of shear wave splitting.
SKS and SKKS phases are used to measure seismic polarization anistropy beneath the array. When measuring the splitting, generally the second eigenvalue of the covariance matrix is minimized. In the case of SKS, a radially polarized phase, we minimize the energy in the transverse component.
The error analysis was performed using an inverse F test to construct a confidence region for each set of shear wave splitting observations (Silver and Chan 1991).
Observations
DiscussionIt is commonly thought that shear wave splitting is due to strain-induced lattice preferred orientation (LPO) of anisotropic minerals in the upper mantle, in particular olivine. Development of the LPO may be due to the deformation of the lithosphere, strain associated with shear of the plate over the asthenosphere, or some combination of both processes.
Most stations along the RISTRA array (NM14 to UT51) show a consistently oriented NE-SW fast direction (~40 degrees) with an average lag time ~1.2 sec. The anisotropy beneath these stations is best explained by the differential horizontal motion between the North American lithosphere and underlying asthenospheric mantle.
Deviation of the fast direction from the absolute plate motion occurs in areas beneath the extreme northern and southern stations. The N-S fast direction beneath western Texas is similar to that observed beneath Las Cruces and El Paso in the southern rift. This contrast in fast direction indicates a fundamentally different dynamic in the mantle beneath the northern and southern Rio Grande Rift.
Fig. 8 A well-split SKS phase. Contour plot shows the residual energy as a function of both fast direction and lag time. The first contour around the minimum represents the 95% confidence interval.
Fig. 9a and 9b Shear wave splitting results.a) Red bars indicate fast directions from previous determinations (Sandvol 1995). Black bars indicate fast directions determined from this study.
b) Fast direction versus station plot. Red line is the 5 point running average, absolute plate motion is indicated by a blue line and the black line indicates the averaged F, ~40 degrees.
30û
35û
0s .5s 1s 1.5s
UT54 AZ45 NM34 NM23 NM12 TX02 -10
0
10
20
30
40
50
60
70
80
90
Absolute Plate Motion
Fast
Dir
ecti
on
(Φ)
1 σ
Absolute Plate
Motion
2.5cm/yr
NW SE
Colorado Plateau
Jem
ez L
inea
men
t
Rio Grande Rift
Great Plains
a)
b)
Average Φ
El Paso
Las Cruces
0 0.5 1 1.5 2 2.5
20
40
60
80
100
120
140
160
180ERROR PLOT : MINIMUM TRANSVERSE ENERGY
Delta T (s)
Ph
i (d
eg
rees)
STN=AZ48 Φ=25±7 δt = 0.65±0.1 PHASE=SKS
Event: 2000/08/15 04:30:0.8 h=357 km BAZ=234 DIST=95
RADIAL & TRANSVERSE
Time (s)
T
R
FAST & SLOW
S
F
SHIFTED FAST & SLOW
S
F
CORRECTED RADIAL & TRANSVERSE
R
10 12 14 16 18 T8
RT
FS
FS
RT
Shear wave splitting
ReferencesBrune, J. & J. Dorman. Seismic waves and Earth structure in the Canadian shield. BSSA 53, 167-210 (1963)
Herrmann, R. B. & C. J. Ammon. Computer programs in seismology, Version 3.15 (2002)
McMechan, G. A., & M. J. Yedlin. Analysis of dispersive waves by wavefield transformation. Geophys. 46, 869-874 (1981)
Priestly, K. & J. Brune. Surface waves and the structure of the Great Basin of Nevada and Western Utah. JGR 83, 2265-2272 (1978).
Sandvol E. Mapping mantle azimuthal Seismic anistropy. Ph.D. Thesis, New Mexico State University (1995)
Silver, P.G. and W. Chan. Shear wave splitting and mantle deformation. JGR 96,16429-16454 (1991)
Sinno, Y. A. & G. R. Keller. A Rayleigh wave dispersion study between El Paso, TX and Albuquerque, NM. JGR 91, 6168-6174 (1986)
Shear Wave Splitting
-110û -105û
Structure and deformation of the uppermost mantle beneaththe RISTRA array Michael West, Rengin Gok, and the RISTRA Team*
Contacts: [email protected] & [email protected]* The RISTRA Team is:Ni, J., Gok, R., West, M., Reynolds, S., New Mexico State University, Dept. of Physics, Las Cruces, NM 88003Grand, S., Gao, W., University Of Texas, Austin, Department of Geological Sciences, Austin, TX 78712Aster, R., Wilson, D. , Schlue, J., New Mexico Institute of Mining and Technology, Socorro, NM 87801Baldridge, S., Los Alamos National Laboratory, EES-1, Los Alamos, NM 87545Semken, S., Dine College, Division of Natural Sciences, Shiprock, NM 87420
