Tomographic imaging of the effects of Peruvian flat slab subduction on the Nazca slab and surrounding mantle under central and southern Peru
Alissa Scire (ascire@email.arizona.edu), George Zandt , Susan Beck , Brandon Bishop , C. Berk Biryol , Lara Wagner , Maureen Long , Estela Minaya , and Hernando Tavera
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1
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3
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5
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Department of Geosciences, University of Arizona, Tucson, AZ, United States, Department of Geological Sciences, University of North Carolina - Chapel Hill, Chapel Hill, NC, United States, Carnegie Institution of Washington, Washington, D.C., United States,
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Department of Geology and Geophysics, Yale University, New Haven, CT, United States, El Observatorio San Calixto, La Paz, Bolivia, El Instituto Geofisico del Peru, Lima, Peru
PERU
B
-66˚
A’
BRAZIL
-64˚
-8˚
-10˚
-10˚
-12˚
-12˚
BOLIVIA
Ri
dg
e
a
zc
Na
-18˚
-80˚
-78˚
-76˚
-74˚
-72˚
-70˚
PULSE stations
CAUGHT stations
Permanent stations
-68˚
-66˚
-64˚
PBO stations
PERUSE stations
Holocene volcano
Direct S
Number of
Events
Number of
Rays
150
85
14,124
144
6,889
6,014
Elevation (meters)
0.6
0.6
0.4
0.4
0.2
0.2
0
0
-0.2
-0.2
-0.4
-0.4
-0.6
-0.6
-0.8
-0.8
0
100
200
300
400
500
600
700
800
900
-1
-10
-8
-6
-4
-2
0
2
4
6
8
10
II. Aligned and weighted seismograms and cross-correlation plots
0.6
0.01
0.4
0.2
0.005
0
-0.2
0
-0.4
-0.005
-0.01
0
100
200
300
400
500
600
700
800
900
RESOLUTION
-0.8
-10˚
100
200
300
400
500
600
700
>> Cross-correlation and weighting
The MCCC method for
picking arrivals was
III. Beam window
0.12
described by
VanDecar and Crosson 0.1
0.08
(1990) and modified
0.06
by Pavlis and Vernon
0.04
(2010). Raw data
0.02
0
(above) is crosscorrelated and traces -0.02
-0.04
are aligned using a
-0.06
multi-channel
-0.08
0
100 200 300 400
cross-correlation
algorithm. Traces are weighted and then
stacked in the beam window. First arrival
is picked in the beam window (right).
Theoretical, infinitesimally thin
ray path
Fresnel Zone
130 km
-14˚
-14˚
-16˚
-16˚
-18˚
-18˚
-20˚
-20˚
-80˚ -78˚ -76˚ -74˚ -72˚ -70˚ -68˚ -66˚ -64˚
-8˚
280 km
800
900
Lower sensitivity
in the center
Higher sensitivity
towards the
periphery
-8˚
-10˚
-12˚
-14˚
-14˚
-16˚
-16˚
-18˚
-18˚
-20˚
-20˚
-10˚
700
Vp
5%
-14˚
-14˚
-14˚
-16˚
-16˚
-16˚
-18˚
-18˚
-18˚
-20˚
-20˚
-20˚
200 km
-10˚
-10˚
-10˚
-12˚
-12˚
-12˚
-14˚
-14˚
-14˚
-16˚
-16˚
-16˚
-18˚
-18˚
-18˚
-20˚
-20˚
-20˚
280 km
505 km
-10˚
-12˚
-12˚
-14˚
-14˚
-16˚
-16˚
-18˚
-18˚
-20˚
-20˚
-80˚ -78˚ -76˚ -74˚ -72˚ -70˚ -68˚ -66˚ -64˚
-8˚
280 km
-10˚
-10˚
-12˚
-12˚
-12˚
-14˚
-14˚
-14˚
-16˚
-16˚
-16˚
-18˚
-18˚
-18˚
-20˚
-20˚
-20˚
365 km
-8˚
365 km
-8˚
-10˚
-10˚
-10˚
-12˚
-12˚
-12˚
-14˚
-14˚
-14˚
-16˚
-16˚
-16˚
-18˚
-18˚
-18˚
-20˚
-20˚
-20˚
-80˚ -78˚ -76˚ -74˚ -72˚ -70˚ -68˚ -66˚ -64˚
-9%
-5%
0
Vp
9%
5%
A ridge-parallel tear is proposed N of the Nazca ridge to
explain an offset in the slab anomaly. S of the ridge, the slab
appears to be continuous. The prominent sub-slab low
velocity is interpreted as either the result of limited partial
melting due to sub-slab flow or asthenospheric mantle at
shallow depths due to the existence of thinned oceanic
lithosphere under the Nazca Ridge (See Poster T23A-4629).
0
100
100
200
200
300
400
300
400
500
500
600
600
-75˚ -74˚ -73˚ -72˚ -71˚ -70˚ -69˚ -68˚
B
B’
6
4
2
0
-2
-4
-6
-8
B
0
100
100
200
200
300
400
300
400
500
500
600
600
˚ -72˚ -71˚ -70˚ -69˚ -68˚ -67˚ -66
˚
-76˚ -75˚ -74˚ -73
B’
6
4
2
0
-2
-4
-6
-8
0
˚ -72˚ -71˚ -70˚ -69˚ -68˚ -67˚ -66
˚
-76˚ -75˚ -74˚ -73
Earthquakes (yellow dots) from EHB catalog (Engdahl et al., 1998). Black contours are from
Slab1.0 (Hayes et al., 2012). Ridge outline from Hampel (2002). Orange dots are underside
wide-angle reflection points from James & Snoke (1990) which constrain the top of the slab.
100
SA
200
EC
0
100
200
300
400
500
600
-20
-18
-16
-14
-12
-10
-8
-66
-64
-68
-74
-78
-80
E
WC
AP
-72
-70
-76
N
A
C
AZ
DG
I
R
300
400
100
500
Sub-slab
low velocity
anomaly
600
Very steep (~70°)
slab dip
-20
200
300
Tear?
400
-18
500
-16
600
-14
-12
-80˚ -78˚ -76˚ -74˚ -72˚ -70˚ -68˚ -66˚ -64˚
Vs
>> The effect of the Nazca Ridge
6
4
2
0
-2
-4
-6
-8
-75˚ -74˚ -73˚ -72˚ -71˚ -70˚ -69˚ -68˚
-8˚
-10˚
A’
N
-10
-8
-64
-66
-68
-70
-72
-74
-76
-78
-80
CONCLUSIONS
280 km
-80˚ -78˚ -76˚ -74˚ -72˚ -70˚ -68˚ -66˚ -64˚
-8˚
200 km
-8˚
A
130 km
-80˚ -78˚ -76˚ -74˚ -72˚ -70˚ -68˚ -66˚ -64˚
-12˚
-8˚
600
-10˚
-12˚
-80˚ -78˚ -76˚ -74˚ -72˚ -70˚ -68˚ -66˚ -64˚
500
-8˚
-12˚
-10˚
Sensitivity Kernels
-5%
Our synthetic tests use a “checkerboard” defined by
alternating fast (blue) and slow (red) velocity
anomalies. About 60% of the input anomaly
amplitude is recovered by the P-wave inversion. The
checkerboard tests show good lateral resolution
throughout much of the model space; lateral
smearing increases and amplitude recovery
decreases towards the edges of the model space.
Input
Recovered P
-8˚
0
-12˚
-8˚
<< Checkerboard tests
-0.6
-1
-12˚
400
350
0.8
-12˚
400
350
1
0.015
The radius of the Fresnel zone where a medium is sampled
is proportional to the velocity and the depth, and inversely
proportional to the frequency of a seismic wave (below).
Sampling within the Fresnel zone itself is variable, with
higher sensitivity
towards the periphery
(bottom). This study
uses a finite frequency
tomography inversion
in MATLAB (Schmandt
and Humphreys, 2010)
-10˚
-8˚
6
4
2
0
-2
-4
-6
-8
-8˚
-10˚
300
250
0.8
130 km
200
0.8
>> Fresnel zones and tomography
-8˚
-10˚
300
250
1
130 km
200
1
-80˚ -78˚ -76˚ -74˚ -72˚ -70˚ -68˚ -66˚ -64˚
-8˚
FINITE FREQUENCY TOMOGRAPHY
I. Raw seismograms and cross-correlation plots
-80˚ -78˚ -76˚ -74˚ -72˚ -70˚ -68˚ -66˚ -64˚
-8˚
-8˚
Map of station locations with slab contours from the USGS Slab 1.0 model (Hayes et. al., 2012). Projection of the subducted Nazca
Ridge based on the conjugate feature, the Tuamotu Plateau from Hampel (2002)
Cross section lines in red correspond to sections shown to the right.
MULTI-CHANNEL CROSS-CORRELATION (MCCC)
Vs
150
-20˚
-20˚
PKIKP
Vp
150
B’
Direct P
50
4000
0
30 0
2000
00
10
00
-1 0
50
-3 0
00
-4 0
50
-6 0
00
0
-8
00
0
-18˚
Our tomographic images show that the Peruvian flat slab extends
further inland than previously proposed along the projection of
the Nazca Ridge. Once the slab re-steepens inboard of the flat slab
region, the Nazca slab dips very steeply (~70°) from about 150 km
depth to 410 km depth.
0
A
Direct P- and S-wave arrivals are picked for
earthquakes located between 30 and 90 degrees
from the array (red & blue circles) while PKIKP arrivals
are picked for earthquakes between 155 and 180
degrees from the array (green circles). The location of
the study area is indicated by the black square.
10
0
10
-16˚
>> The location of the Nazca slab
0
40 45 5 55
0 00 0
0
50
-16˚
-14˚
<< Earthquake locations
10
0
35
0
30
0
25
200
150
-14˚
The modern central Peruvian Andes are
dominated by a laterally extensive region
of flat slab subduction. The Peruvian flat
slab extends for ~1500 km along the
strike of the Andes, correlating with the
subduction of the Nazca Ridge in the
south and the theorized Inca Plateau in
the north. We have used data from the
CAUGHT and PULSE experiments for
finite frequency teleseismic P- and
S-wave tomography to image the Nazca
slab in the upper mantle below 95 km
depth under central Peru between 10°S
and 18°S as well as the surrounding
mantle.
Vs
Abstract: T23A-4627
Depth (km)
-68˚
A’
Elevation (km)
-70˚
A
Depth (km)
-72˚
Vp
Elevation (km)
-74˚
Elevation (km)
-76˚
Depth (km)
-78˚
RESULTS
Depth (km)
-80˚
-8˚
EARTHQUAKE DATA
Elevation (km)
INTRODUCTION
3
Depth (km)
2
Depth (km)
1
Depth (km)
1
505 km
-80˚ -78˚ -76˚ -74˚ -72˚ -70˚ -68˚ -66˚ -64˚
Our images of the Nazca slab suggest that steepening of the slab inboard of the subducting Nazca Ridge locally occurs ~100 km further inland than was indicated in previous studies. When the slab
steepens inboard of the flat slab region, it does so at a very steep (~70°) angle. The transition from the Peruvian flat slab to the more normally dipping slab south of 16°S below Bolivia is characterized
by an abrupt bending of the slab anomaly in the mantle in response to the shift from flat to normal subduction. A potential tear in the slab is inferred from an observed offset in the slab anomaly north
of the Nazca Ridge extending subparallel to the ridge axis between 130 and 300 km depth. A high amplitude (-5-6%) slow S-wave velocity anomaly is observed below the projection of the Nazca
Ridge. This anomaly appears to be laterally confined to the mantle directly below projection of the Nazca Ridge but descends to ~300 km depth in the mantle. This sub-slab slow anomaly may
correspond to vertical mantle flow induced by movement of material through the inferred tear in the slab north of the Nazca Ridge or alternately may represent a long-lived feature of the sub-slab
mantle possibly associated with thinning of the lithosphere under the Nazca Ridge by the Easter-Salas hot spot.
ACKNOWLEDGEMENTS
Support for this research was provided by the National Science Foundation as part of the CAUGHT (NSF Award EAR-0907880) and PULSE (NSF Award EAR-0943991) projects. The seismic instruments for the CAUGHT and PULSE arrays were
provided by the Incorporated Research Institutions for Seismology (IRIS) through the PASSCAL Instrument Center at New Mexico Tech, Yale University, and the University of North Carolina, Chapel Hill. Data collected is or will be available
through the IRIS Data Management Center. We would also like to thank everyone who helped with the field work for the CAUGHT and PULSE networks in Bolivia and Peru, particularly our collaborators at OSC in Bolivia and IGP in Peru. Thank
you to Paul Davis and Rob Clayton for access to the data for the PeruSE stations included in this study. The authors would like to thank the GEOFON Program at GFZ Potsdam as a source of additional waveform data. The authors thank
Brandon Schmandt at the University of New Mexico and Gene Humphreys at the University of Oregon as the original authors of the teleseismic tomography code used in this study. Also, thanks to the GSAT group at the University of Arizona
for their support and help.
REFERENCES
Engdahl, E.R., R.D. van der Hilst, and R. Buland (1998), Global teleseismic earthquake relocation with improved travel times and procedures for depth determination, Bull. Seism. Soc. Am., 88, 722-743.
Hampel, A. (2002), The migration history of the Nazca Ridge along the Peruvian active margin: a re-evaluation, Earth Planet. Sci. Lett., 203, 665-679.
Hayes, G. P., D. J. Wald, and R. L. Johnson (2012), Slab 1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res. 117, B01032, doi:10.1029/2011JB008524.
James, D.E. and J.A. Snoke (1990), Seismic evidence for continuity of the deep slab beneath central and eastern Peru, J. Geophys. Res., 85, 4989-5001.
Pavlis, G., and F. Vernon (2010), Array processing of teleseismic body waves with the USArray, Computers & Geosciences, 36, 910-920.
Schmandt, B., and E. Humphreys (2010), Seismic heterogeneity and small-scale convection in the southern California upper mantle, Geochem. Geophys. Geosyst., 11, Q05004, doi:10.1029/2010GC003042.
VanDecar, J.C., and R.S. Crosson (1990), Determination of teleseismic relative phase arrival times using multi-channel cross-correlation and least squares, Bull. Seism. Soc. Am., 80 (1), 150-169.