Transcript Sismos: Lo que la Tierra intenta decirnos

Seismic Profiling of
subduction zones –
Lithosphere above Benioff
zone
Morphology of Benioff zones:
Mexican arc
Xyoli Pérez-Campos
February 14, 2008
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
Why Seismic Profiling?
Seismic profiling
can provide
Structure
Velocity
Attenuation
Anisotropy
Surface
strain
Arc
chemistry
Viscosity
Determine
Infer
Density
Flow
direction
Temperature
Melting /
Dehydration
Observations:
Oblique arc: TransMexican Volcanic
Belt (TMVB)
• Diference in
subduction angles
• Flat subduction under
south-central Mexico
Relation of Cocos plate
subduction with volcanic activity
Pardo and Suárez (1995)
100 broadband seismic stations
MASE:
MesoAmerican
Subduction
Experiment
Objective: Dynamic
model of the
subduction system
under south-central
Mexico
Local Seismicity in South-Central Mexico
The seismic activity is related to stresses
generated by the subduction of the
oceanic Cocos plate under the North
American continent.
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 Wadati-Benioff zone does not extend
past a depth of 60 km and disappears
before it reaches the TMVB.
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 plate
4. Others not related to those previous ones, mainly strikeslip or normal fault striking oblique to the trench.
Pacheco and Singh (2008)
Local seismicity
Strike-slip or normal fault
striking oblique to the trench
Pacheco and Singh (2008)
Shallow-angle thrust events
along the plate interface
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 compression
Pacheco and Singh (2008)
Down-dip extension
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)
Surface wave tomography
Objective:
Crustal structure
22º
The dispersion curves for
an earthquake recorded at
two different stations are
different.
M e x ic o
20º
Use surface waves
from regional recording
SAPE
T M VB
M AXE
18º
P ac
fic
O ce
an
16º
2 0 /0 1 /2 0 0 6
M = 4 .4 ,d= 1 6 k m
-1 0 4 º
-1 0 2 º
-1 0 0 º
-9 8 º
-9 6 º
Dispersion curves
M ult iple F ilte r
D zie w o ns ky e t al., 19 6 9
Group Velocity (km/s)
5
4
3
5
10
15
20
25
30
P e r io d (s )
T im e ( s )
Figure courtesy of Arturo Iglesias
35
Dispersion curves
1) Event selection (Position, distance, depth)
2) Preprocess (Rmean, Rtrend)
3) Dispersion Curves
5
5
5
4
4
4
3
3
3
2
2
20 40 60 80 100
2
20 40 60 80100
20 40 60 80 100
5
5
5
4
4
4
3
3
3
2
2
20 40 60 80 100
2
20 40 60 80100
20 40 60 80 100
5
5
5
4
4
4
3
3
3
BQ
2
2
20 40 60 80 100
20 40 60 80 100
5
5
5
4
4
4
3
3
3
2
2
20 40 60 80 100
2
20 40 60 80100
20 40 60 80 100
5
5
5
4
4
4
3
3
3
2
Figures courtesy of Arturo Iglesias
2
20 40 60 80100
2
20 40 60 80 100
2
20 40 60 80100
20 40 60 80 100
Surface wave tomography
4) Tomographic images for each period
(continuous regionalization: Debayle
and Sambridge, 2004)
-104°
-102°
-100°
-98°
-96°
22°
22°
20°
20°
18°
18°
16°
16°
Paths event-station
-104°
-102°
-100°
Figures courtesy of Arturo Iglesias
-98°
-96°
Construction of local dispersion curves
Tomographic image at particular period.
Figures courtesy of Arturo Iglesias
Using various periods, one can
construct a local dispersion
curve
22°
21°
19°
18°
17°
16° 02°
-1
1
-10
The local dispersion curves can be
inverted to obtain a local S-wave
velocity model.
20°
Surface wave tomography
°
-10
0°
-99°
Velocity models at stations along the
line can be used to construct a velocity
profile.
-98°
Topography
5
S-wave velocity
3000
h (m)
Dispersion curve
2000
1000
0
20
4
100
3
S-wave
velocity
30
40
4
60
2
10
20
30
40
50
Period(s)
3 .0
4 .0
b ( km )
5 .0
3
U(km/s)
50
300
400
500
600
-30
Moho
-50
-70
100
Figures courtesy of Arturo Iglesias
200
-10
5
depth (km)
Depth (km)
Group Velocity (km)
10
200
The crust thickens under the TMVB
300
400
500
Distance from trench (km)
600
Ray Tracing
Use earthquakes close
to the line of receivers
Possible to model the continental
and oceanic lithosphere.
Objective: Propose a velocity structure such that satisfies the observed arrival times.
Figures courtesy of Carlos Valdés-González
What is a Receiver Function (RF)?
Instrument
I (t )
Source
S (t )
P
sP
Shallow
structure
Ei (t )
P wave group
pP
Teleseismic record
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.
Figure from http://eqseis.geosc.psu.edu/~cammon/HTML/RftnDocs/rftn01.html
• It is the transfer function of the inner
structure below the seismic station
Characteristics of a RF
• Arrival times and amplitudes are sensitive
to the local structure
Amplitude
Station
(3 components)
Surface
P waves
Discontinuity d
Thickness
S waves
Direct arrival
Conversion
P-S
Time
Multiples
Receiver Function
Figure from http://eqseis.geosc.psu.edu/~cammon/HTML/RftnDocs/rftn01.html
Polarity of the RF
Velocity
Time
Figure courtesy of Fernando Green and Lizbeth Espejo
Amplitude
• The polarity is related with the change of impedances
Receiver function profile
Active volcanoes of the TMVB is
between the 80 and 200 km isodepth
curves of the Cocos plate
Tempoal
Mexico City
Acapulco
Altitude [km]
The Cocos plate underplates the
continental crust and subducts
horizontally for 250 km.
TMVB
The continental crust
is thicker under the
TMVB and thinner
toward the coasts.
Depth [km]
Distance from the coast [km]
Global tomography
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.
Gorbatov and Fukao (2005)
GT represents the
differences in velocities
given a reference model
Slower material
than the
surrounding.
P-wave tomography
Courtesy of Allen Husker
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.
P-wave tomography
The slab is a slow feature within a faster background.
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 isodepth contours.
Courtesy of Allen Husker
TMVB
Attenuation
Singh et al. (BSSA, 2007)
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.
Resistivity from Jödicke et al. (2006)
Q
Attenuation:
Proxy for Viscosity
1000/Q
Low Q (high attenuation) underneath the TMVB
Courtesy of J. Chen
Depth [km]
Distance from the coast [km]
Distance from the coast [km]
Up to date results
Flat subduction for 275 km
from the trench
No room for
mantle wedge
Modeling:
flat slab can be
generated by
shrinking lowviscosity zone.
There is an
extension stress
regime in the
overriding plate
No seismicity
present
within the
slab
Consistent with
rollback
Up to date results
Slab dips steeply
(~75°) after
horizontal segment
No seismicity
present.
Active volcanoes
between 100 and 200
km iso-depth contours
of the top of the slab
Slab stops at
500 km
depth, at 400
km inland
Consistent with slab tear
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