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

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5

20 40 60 801002

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BQ

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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

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mm

on

/HTM

L/Rftn

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01

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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