1. February 14, 2008 Morphology of Benioff zones: Mexican arc Xyoli Pérez-Campos Seismic Profiling of subduction zones – Lithosphere above Benioff zone

2. 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

3. Arc chemistry Why Seismic Profiling? ? Determine StructureVelocity Attenuation Anisotropy Surface strain Infer ViscosityDensity Flow direction Temperature Melting / Dehydration Seismic profiling can provide

4. 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:

5. MASE: MesoAmerican Subduction Experiment 100 broadband seismic stations Objective: Dynamic model of the subduction system under south-central Mexico

6. 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 plate 4.Others not related to those previous ones, mainly strike- slip or normal fault striking oblique to the trench.

7. Local seismicity Shallow-angle thrust events along the plate interface Strike-slip or normal fault striking oblique to the trench Pacheco and Singh (2008)

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 Pacheco and Singh (2008)

9. 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)

10. 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.

11. Dispersion curves 2) Preprocess (Rmean, Rtrend) 3) Dispersion Curves 1) Event selection (Position, distance, depth) Figures courtesy of Arturo Iglesias

12. Surface wave tomography Paths event-station 4)Tomographic images for each period (continuous regionalization: Debayle and Sambridge, 2004) Figures courtesy of Arturo Iglesias

13. 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

14. Surface wave tomography The local dispersion curves can be inverted to obtain a local S-wave velocity model. Moho S-wave velocity S-wave velocityDispersion curve Topography 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

15. 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.

16. What is a Receiver Function (RF)? It is the transfer function of the inner structure below the seismic station Shallow structure E i (t ) Instrument I (t ) P wave group Teleseismic record P pP sP Figure from http://eqseis.geosc.psu.edu/~cammon/HTML/RftnDocs/rftn01.html Source S (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.

17. 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 Conversion P-S Multiples Surface S waves Amplitude Thickness Station (3 components) Discontinuity d Time P waves Receiver Function

18. Polarity of the RF The polarity is related with the change of impedances Figure courtesy of Fernando Green and Lizbeth Espejo Time Velocity Amplitude

19. Receiver function profile Acapulco Tempoal Mexico City 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

20. 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.

21. 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

22. 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.

23. 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)

24. 1000/Q Attenuation: Proxy for Viscosity Q Distance from the coast [km] Depth [km] Courtesy of J. Chen Low Q (high attenuation) underneath the TMVB

25. 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 rollback Modeling: flat slab can be generated by shrinking low- viscosity zone.

26. 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

27. 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