Preliminary Results from the CAUGHT Experiment: Investigation of the North Central Andes Subsurface
University of Arizona
University of North Carolina, Chapel Hill
Using Receiver Functions and Ambient Noise Tomography
1
2
1
-12˚
-74˚
-73˚
-72˚
-71˚
-70˚
-69˚
-68˚
-67˚
-66˚
-65˚
Shear Velocity (km/s)
2.0
2.5 Noise
3.0
3.5
4.0
Ambient
Results:
The cross-section below was done in two segments to take advantage of the station spacing along the
densest part of the array; the western segment at an azimuth of 58 degrees centered around the south-western
most point on the dense station line, and the eastern segment at an azimuth of 30 degrees centered around the
mid-point of the station line (see station map).
Peru
-13˚
-5
0
Peru
-15˚
WC
AP
5km
Bolivia
-13˚
AP
-15˚
3
-71˚
-70˚
-69˚
-68˚
-67˚
-66˚
-65˚
Ambient-Noise Tomography (ANT) results
are attained from the cross-correlation of
background noise between station pairs.
These cross-correlations are used to
estimate the Empirical Green’s Function
response of Raleigh waves traveling
between station pairs. Dispersion curves
between station pairs are then used to invert
for phase velocities at every prescribed grid
point (Barmin et. al, 2001). Where the phase
velocity resolution met acceptable criteria
we also inverted for absolute shear wave
velocity (Herrmann, 1987).
-19˚
depth (km)
Figure 5: Receiver function cross-section along dense line; Gaussian 2.5, SNR >= 2, 15 km bins.
In the above cross-section we observe a Moho arrival beneath most of the Altiplano, Eastern Cordillera and
Subandean provinces, ranging from 60 km underneath the Altiplano to ~35 km underneath the Subandean
zone. It appears that the transition from 60 km to ~50 km takes place gradually in the Eastern Cordillera, while
an abrupt shallowing of the Moho may occur under the Subandean zone.
radial
Figure 2: Basic receiver function conversions.
transverse
seconds
Figure 3: Example calculated receiver function prior to CCP stacking.
CB25 --- CB26
CB25 --- CB27
CB25 --- CB28
CB25 --- CB2A
CB25 --- CB2B
CB25 --- CB2C
CB25 --- CB2D
CB25 --- CB2E
CB25 --- CB2F
Seconds
Figure 4: Representative example of cross-correlated station pairs sorted by increasing interstation distance from top down.
Figure 14: Combined cross-section sections of receiver functions.
0
10
km 20
30
40
50
-70.5
-71˚
-70˚
-69˚
-68˚
-67˚
-66˚
-12˚
Bolivia
-13˚
-14˚
SA
-15˚
AP
WC
EC
3.
75
-16˚
A
3 3.25
-17˚
-17˚
B
Pacific
Ocean
-19˚
-19˚
-20˚
-20˚
-73˚
-66˚
-72˚
-17˚
-18˚
-16˚
-19˚
Chile
-71˚
-70˚
-69˚
-68˚
-67˚
Figures 6,7: 5 km (6) and 20km (7) slice shear velocity results. Thick black lines delineate the major morphotectonic provinces of the Central Andes from
west to east: Western Cordillera (active arc), Altiplano, Eastern Cordillera, Sub-Andes (active thrusting).
-20˚
-66˚
B
3
3.25
3
-70.0
3.25
3.5
3.75
3.5
-69.5
-69.0
3.75
-68.5
Preliminary results from the 5 km depth slice
show low velocity areas that correlate well with
the Altiplano and Subandean Zone and a high
velocity body beneath the Eastern Cordillera. In
the 20 km depth slice we observe a high velocity
body along the Eastern Cordillera that extends
SW into the area below Lake Titicaca.
Figure 15: Map-view image showing locations
for cross-section segments.
Figure 13: Combined cross-section sections of ambient-noise tomography.
CB25 --- CB29
A
-72˚
-67˚
20km
-13˚
-13˚
-18˚
-18˚
Chile
-68˚
20 km
0
10
20
km 30
40
50
-68.0 -70.0
3.5
3.2
5
-69.5
3.5
3.25
-69.0
3.5
-68.5
-68.0
-67.5
Figures 8,9: Cross-section results from a transect perpendicular to strike (8) and along the dense station line (9) with the best phase velocity resolution.
Combined Results and Interpretation:
PpPhs PpShs Pp Ps
-20˚
-73˚
5
B
Pacific
Ocean
3.25
5
-18˚
2.7
2.7
?
-69˚
Peru
-16˚
A
-17˚
Amplitude relative to direct P-arrival
Ambient Noise Methodology:
?
?
The depth of the Moho under the westernmost Altiplano and Western Cordillera is uncertain at this time.
High amplitude arrivals at ~40 km and ~75 km are potential candidates. During the last service trip we installed
an additional station to fill the gap near the Peru-Bolivia border. The additional data should lead to improved
images for this region.
relative amplitude
The raw waveforms are cut, checked for
appropriate signal-to-noise ratios and then
rotated into their radial and transverse
components. We then deconvolved the rotated
components using iterdecon to create the
receiver functions. Common conversion-point
(CCP) stacking was later utilized to take large
numbers of quality-controlled receiver functions
and collapse them around the points where
phase conversions occurred.
?
-20˚
Figure 1: CAUGHT station map with USGS slab contours overlain. A-A’-A’’ cross-section
line defines the two transects used for receiver functions.
Receiver functions utilize P-S conversions generated at sharp
impedance boundaries in the Earth, i.e. velocity transitions. A
comparison of the arrival times of the primary phase to the
converted phases yields the depth to the interface causing the
conversion, while the amplitude of the arrival is dependent on the
size of the velocity contrast. Generally the largest amplitude
arrivals in receiver functions are the primary arrival from the
direct P wave, and the high velocity contrast conversion arrival
associated with the Moho.
?
Absolute shear wave velocity (km/s)
-72˚
-70˚
4.5
3.5
-73˚
4.0
3
-74˚
-19˚
-71˚
EC
WC
-16˚
3.5
4.5
-15˚
-15˚
2.5
Chile
Pacific
Ocean
-19˚
PULSE station
CAUGHT station
PBP station
LPAZ station
Holocene Volcano
Arequipa, Peru
La Paz, Bolivia
-18˚
-72˚
SA
5
~ 7 cm/yr
-18˚
-20˚
A
-73˚
-66˚
-12˚
-12˚
-14˚
-14˚
SA
EC
-17˚
A’
-67˚
-14˚
-16˚
-17˚
-68˚
2.7
-16˚
-69˚
4.0
3. 5
0
0
-5
00
-70˚
3.0
3.5
75
-35
-3
0
A’’
-71˚
2.5
3.0
2.5
3.
-40
A’’
-72˚
5 km
Shear Velocity (km/s)
2.0
2.0
3
0
50
-2
-20
-10
-150
-15˚
A
-14˚
Bolivia
A’
-73˚
-12˚
4.5
Shear Velocity (km/s)
75
3.
00
-4
50
-14˚
Receiver Function Methodology:
For the shear wave inversion, our starting
model is 100 kilometers thick with
5-kilometer thick layers. The top 60
kilometers of the model are assigned a Vp
equal to 6.5 km/s and a Vs equal to 3.71
km/s and the remaining 40 kilometers are
assigned a Vp equal to 8.0 km/s and a Vs
equal to 4.57 km/s.
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Receiver Function Results:
The CAUGHT Experiment:
The Central Andean Uplift and
Geodynamics of High Topography
(CAUGHT) seismology component is a
deployment of 50 broadband seismometers
in northwestern Bolivia and southern Peru
to investigate the interplay between crustal
shortening, lithospheric removal and
surface uplift.
There is a total of 275 km of documented
upper crustal shortening in northwest
Bolivia (15° to 17° S) (McQuarrie et al, 2008).
Associated with such shortening is crustal
thickening and the potential for lithospheric
removal as the thickened lithospheric root
becomes unstable.
The instruments have been out for
slightly over a year, and ~6 months of data
have been retrieved and cataloged. With
that data receiver functions images were
created to constrain Moho depth
underneath the Central Andes, and ambient
noise tomography was used to estimate an
improved shear wave velocity model.
2
5
1
3.
1
3.5
1
3.7
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Jamie Ryan , Kevin M. Ward , Ryan Porter , Susan Beck , George Zandt , Lara Wagner , Estela Minaya and
1
El Observatorio San Calixto, La Paz, Bolivia
4 4
Hernando Tavera Instituto Geofísico del Perú, Lima, Peru
3
First order correlations can be
seen between the two
techniques in both cross
section segments. The western
segments show a distinct
velocity transition pattern from
slow to a fast velocity anomaly
in the mid-crust, followed by a
decrease in velocity above the
Moho. The eastern segments
show patterns of low- and
high-velocity transitions that
seem to indicate the presence of
a high-velocity body in the
shallow- to mid-crustal region.
Presented here are three cross-sections from
the Central Andes that portray different models
of lower crustal deformation (McQuarrie et., al
(2008 ), Baby et., al (1997), and Schmitz, M., and
J. Kley, (1997). As more data becomes available
from the CAUGHT array, we hope to combine
crustal thickness estimates from receiver
functions with ambient-noise tomography
results to improve our understanding of crustal
deformation in the Central Andes.
tion of Neogene shortening
and to estimate its contribution to crustal thickening.
Mechanism of crustal underplating by tectonically eroded
material is proposed to explain 70 km crustal thickness
below Altiplano. “Short”
Figures 10,11,12: Crosssection at 16-17°S (10),
McQuarrie et., al, 2008;
cross-section, 21°-22°S (11)
Baby et. al, 1997;
cross-section at ~21°S (12)
Schmitz, M., and J. Kley,
1997
21° S
References
Baby, P., Rochat, P., Masclé, G., and Hérail, G., 1997, Neogene shortening contribution to crustal thickening in the back arc of the Central Andes: Geology, v. 25, p. 883–886.
Barmin, M.P., Ritzwoller, M.H. and Levshin, A.L., 2001. A fast and reliable method for surface wave tomography. Pure and applied geophysics, 158(8): 1351-1375.
Herrmann, R.B., 1987. Computer Programs in Seismology. St. Louis University, St. Louis, Mo. International Seismological Centre, EHB Bulletin, http://www.isc.ac.uk, Internatl. Seis.
Cent., Thatcham, United Kingdom, 2009.
Ligorria, J. and Ammon, C. (1999) Iterative deconvolution and receiver-function estimation. Bulletin of the Seismological Society of America (October 1999), 89(5):1395-1400.
McQuarrie, N., Barnes, J.B. & Ehlers, T.A. (2008) Geometric, kinematic, and erosional history of the central Andean Plateau, Bolivia (15–17°S). Tectonics, TC3007, doi: DOI:
0.1029/2006TC00205.
Schmitz, M., and J. Kley, 1997, The geometry of the Central Andean backarc crust: joint interpretation of cross-section balancing and seismic refraction data:Journal of South
American Earth Sciences, v. 10, p. 99–110.
Acknowledgments
We’d like to acknowledge the following groups: NSF Continental Dynamics for funding the CAUGHT project (#0907880), our collaborators and the staff members of both El
Observatori San Calixto in Bolivia and the Instituto Geofísico del Perú in Peru, IRIS-PASSCAL for providing the equipment, technical support and field assistance, and also the
generous people and families in Bolivia and Peru who graciously allowed us the use of their property for this experiment. J. Ryan and K. Ward also received support from
Chevron and Conoco-Phillips.