February 14, 2008
Morphology of Benioff zones: Morphology of Benioff zones: Mexican arcMexican arc
Xyoli Pérez-Campos
Seismic Profiling of Seismic Profiling of subduction zones – subduction zones –
Lithosphere above Benioff Lithosphere above Benioff zonezone
Outline
• Introduction• Seismology techniques and their objectives
– Local seismicity: Benioff zone; overriding plate stresses
– Surface wave dispersion: continental crust– Ray tracing: continental and oceanic slab structure– Receiver functions: continental and oceanic crust
lithosphere – Global tomography: Global view of the Cocos plate– P-wave tomography: Cocos plate in depth– Attenuation: mantle wedge
Arcchemistry
Why Seismic Profiling?
?
Determine
Structure
Velocity
Attenuation
Anisotropy
Surfacestrain
Infer
Viscosity
Density
Flowdirection
Temperature
Melting /Dehydration
Seismic profiling can provide
Pardo and Suárez (1995)
Relation of Cocos plate subduction with volcanic activity
Oblique arc: Trans-Mexican Volcanic Belt (TMVB)
• Diference in subduction angles
• Flat subduction under south-central Mexico
Observations:
MASE:MesoAmerican
Subduction Experiment
100 broadband seismic stations
Objective: Dynamic model of the subduction system under south-central Mexico
Local Seismicity in South-Central Mexico
The Wadati-Benioff zone does not extend past a depth of 60 km and disappears before it reaches the TMVB.
Convergence rates vary from northwest to southeast between 4.4 cm/yr to 5.2 cm/yr (DeMets et al., 1994), with a convergence direction almost perpendicular to the trench.
The seismic activity is related to stresses generated by the subduction of the oceanic Cocos plate under the North American continent.
Pacheco and Singh (2008)
Identified mechanisms: 1. Shallow-angle thrust events along the plate interface.2. Down-dip tension within the subducted plate.3. Down-dip compression within the subducted plate4. Others not related to those previous ones, mainly strike-
slip or normal fault striking oblique to the trench.
Local seismicity
Shallow-angle thrust events along the plate interface
Strike-slip or normal fault striking oblique to the trench
Pa
che
co a
nd
Sin
gh
(2
00
8)
Local seismicity
The down-dip compression type is restricted to locations near the coast, while the down-dip tension type is found both, along the coast and further inland, leaving a gap of seismicity.
Down-dip extension Down-dip compression
Pa
che
co a
nd
Sin
gh
(2
00
8)
Local seismicity
There is no continuity of the Wadati-Benioff zone if a small swath of 50 km is taken to generate the cross section. The sense of continuity comes about from the flattening of the subducted plate from West to East.
Pacheco and Singh (2008)
3
4
5
5 10 15 20 25 30 35Period (s)
Multiple FilterDziewonsky et al., 1969
Gro
up
Ve
loc
ity
(k
m/s
)
Time (s)
-104º -102º -100º -98º -96º
16º
18º
20º
22º
SAPE
MAXE
20/01/2006M=4.4,d=16km
TMVB
Mexico
Surface wave tomography
Use surface waves from regional recording
Objective:Crustal structure
Dispersion curves
Figure courtesy of Arturo Iglesias
The dispersion curves for an earthquake recorded at two different stations are different.
Dispersion curves
2
3
4
5
20 40 60 801002
3
4
5
20 40 60 801002
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4
5
20 40 60 801002
3
4
5
20 40 60 801002
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5
20 40 60 801002
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5
20 40 60 801002
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5
20 40 60 801002
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4
5
20 40 60 801002
3
4
5
20 40 60 80100
2
3
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5
20 40 60 801002
3
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5
20 40 60 801002
3
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5
20 40 60 801002
3
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5
20 40 60 801002
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5
20 40 60 801002
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5
20 40 60 801002
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5
20 40 60 801002
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5
20 40 60 801002
3
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5
20 40 60 80100
2
3
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5
20 40 60 801002
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5
20 40 60 801002
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5
20 40 60 801002
3
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5
20 40 60 801002
3
4
5
20 40 60 801002
3
4
5
20 40 60 80100
BQ
2
3
4
5
20 40 60 801002
3
4
5
20 40 60 801002
3
4
5
20 40 60 80100
2
3
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5
20 40 60 801002
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20 40 60 801002
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5
20 40 60 801002
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20 40 60 801002
3
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5
20 40 60 801002
3
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5
20 40 60 801002
3
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5
20 40 60 801002
3
4
5
20 40 60 801002
3
4
5
20 40 60 80100
2
3
4
5
20 40 60 801002
3
4
5
20 40 60 801002
3
4
5
20 40 60 801002
3
4
5
20 40 60 801002
3
4
5
20 40 60 801002
3
4
5
20 40 60 801002
3
4
5
20 40 60 801002
3
4
5
20 40 60 801002
3
4
5
20 40 60 80100
2) Preprocess (Rmean, Rtrend)
3) Dispersion Curves1) Event selection (Position, distance, depth)
Figures courtesy of Arturo Iglesias
-104°
-104°
-102°
-102°
-100°
-100°
-98°
-98°
-96°
-96°
16° 16°
18° 18°
20° 20°
22° 22°
Surface wave tomography
Paths event-station
4) Tomographic images for each period (continuous regionalization: Debayle
and Sambridge, 2004)
Figures courtesy of Arturo Iglesias
Construction of local dispersion curves
Tomographic image at particular period.Using various periods, one can construct a local dispersion curve
Figures courtesy of Arturo Iglesias
Surface wave tomography
The local dispersion curves can be inverted to obtain a local S-wave velocity model.
10
30
40
60
20
50
De
pth
(km
)
3.0 5.04.0b(km)
2
3
4
5
10 20 30 40 50
Gro
up
Ve
loci
ty (
km)
Period(s)-70
-50
-30
-10
100 200 300 400 500 600
3
4
5
0
1000
2000
3000
100 200 300 400 500 600
-102°-101°
-100°-99°
-98°
16°
17°
18°
19°
20°
21°
22°
h (m
)
U(km
/s)
Distance from trench (km)
dep
th (
km)
Moho
S-wavevelocity
S-wave velocityDispersion curveTopography
Velocity models at stations along the line can be used to construct a velocity profile.
Figures courtesy of Arturo Iglesias The crust thickens under the TMVB
Ray Tracing
Objective: Propose a velocity structure such that satisfies the observed arrival times.
Use earthquakes close to the line of receivers
Figures courtesy of Carlos Valdés-González
Possible to model the continental and oceanic lithosphere.
What is a Receiver Function (RF)?
• It is the transfer function of the inner structure below the seismic station
Shallowstructure
Ei (t )
InstrumentI (t )
P wave groupTeleseismic record
P
pP
sP
Figu
re fro
m h
ttp://e
qse
is.geosc.p
su.e
du
/~ca
mm
on
/HTM
L/Rftn
Docs/rftn
01
.htm
l
SourceS (t )
Given the distance, the arrival angle of the P wave is almost vertical. Therefore, the S energy is mostly concentrated in the horizontal plane.
By deconvolving the horizontal components with the vertical components is possible to obtain the transfer function of the shallow structure.
Characteristics of a RF
• Arrival times and amplitudes are sensitive to the local structure
Figure from http://eqseis.geosc.psu.edu/~cammon/HTML/RftnDocs/rftn01.html
Direct arrival
ConversionP-S
Multiples
Surface
S waves
Amplitude
Thickn
ess
Station(3 components)
Discontinuity d
Tim
e
P waves
Receiver Function
Polarity of the RF• The polarity is related with the change of impedances
Fig
ure
cou
rtesy
of
Fern
an
do G
reen
an
d L
izb
eth
Esp
ejo
Time
Velocity
Am
plitu
de
Receiver function profile
Acapulc
o Tem
po
al
Mexico
C
ity
TMVB
Depth
[km]
Altitude
[km]
Distance from the coast [km]
The Cocos plate underplates the continental crust and subducts horizontally for 250 km.
The continental crust is thicker under the TMVB and thinner toward the coasts.
Active volcanoes of the TMVB is between the 80 and 200 km isodepth curves of the Cocos plate
Global tomography
Gorbatov and Fukao (2005)
Global tomography shows the changes in dip of the slab subduction. Under the TMVB, the slab subducts abruptly. The TMVB is between the 100 and 200 km isodepth contours of the top of the slab.
GT represents the differences in velocities given a reference model
Slower material than the surrounding.
P-wave tomography
A teleseismic event is recorded at all stations along the line (bottom), its P-wave arrival is aligned (top right). The difference in arrival times (bottom right) is the parameter that helps us to describe the structure underneath.
Courtesy of Allen Husker
P-wave tomography
TMVB
After 275 km of underthrusting the North American plate, the oceanic slab dips steeply with and angle of ~75°.
It seems to stop at 500 km depth, by the northern end of the TMVB.
The active volcanoes lie between the 80 and 200 km iso-depth contours.Courtesy of Allen Husker
The slab is a slow feature within a faster background.
Attenuation
Attenuation can be used as a proxy for viscosity.A region of low resistivity roughly coincides with low Q (high attenuation) under the TMVB.Both might be explained by the presence of subduction-related fluids and partial melts.
Singh et al. (BSSA, 2007)
Resistivity from Jödicke et al. (2006)
1000/Q
Attenuation:Proxy for Viscosity
Q
Distance from the coast [km]
Distance from the coast [km]
Dep
th [
km
]
Courtesy of J. Chen
Low Q (high attenuation) underneath the TMVB
Up to date results
Flat subduction for 275 km from the trench
There is an extension stress regime in the
overriding plate
No room for mantle wedge
No seismicity present
within the slab
Consistent with rollbackModeling:
flat slab can be generated by shrinking low-viscosity zone.
Up to date results
Slab dips steeply (~75°) after horizontal segment
Active volcanoes between 100 and 200
km iso-depth contours of the top of
the slab
No seismicity present.
Consistent with slab tear
Slab stops at 500 km
depth, at 400 km inland
Up to date results
Attenuation in the wedge is a factor of 2 higher than the
surrounding mantle.
Low Q region is focused under the
TMVB
Coincides with low resistivity
zone
Consistent with presence of fluids or melts