Coupling at subduction zones Background reading: Pacheco, Sykes, Scholz (1993) Hyndman, Yamano, Oleskevich (1997) Carl Tape November 9, 2007 Thanks to Dietmar Mueller for the unpublished updated seafloor age grids.

Seismic coupling is the extent to which the relative motion between the upper and lower plate at a subduction zone, along a specified down-dip length, is accommodated via slip during earthquakes. Fundamental Questions What defines a subduction zone? How is the relative motion between the upper plate and lower plate accommodated at each subduction zone? Is the motion confined to the interplate interface? If yes, then what fraction slips seismically (coseismic, post-seismic) or aseismically (“slow earthquakes”, sliding behavior). What observations can we use? Convergence velocities from plate models (GPS!). Seismic catalogs. How long is the catalog relative to the expected largest earthquake cycle? How complete is the catalog? Does the catalog distinguish the mechanism of each event? Because we want to exclude all non-interplate events from the analysis. Paleoseismic studies. Geodetic observations of interseismic strain accumulation.

Fundamental Questions For a given segment of a subduction zone, what is the maximum (moment magnitude) recorded earthquake? For a given segment of a subduction zone, what stage of what seismic cycle is it in? (How can seismicity and GPS help answer this?)

Procedure of Pacheco et al. (1993): Select subduction zones and catalog. Determine interplate events. Compute slip rate due to earthquakes and compare with slip rate due to plate model.

Procedure of Hyndman et al. (1997): Compute thermal model for subduction zones based on: (1) age of subducting plate at trench; (2) convergence rate; (3) sediment thickness on subducting plate; (4) dip of interplate interface From the thermal model, infer the downdip locking depth.

Hyndman et al. (1997)

Ruff and Kanamori (1980)

Ruff and Kanamori (1980)

Linear regression model : Mw-max(A, Vc) Ruff and Kanamori (1980)

Computing the convergence velocity, the trench-normal convergence velocity, and the obliquity. Should we only consider the major plates (NUVEL)?

Southernmost Andaman subduction zone Dips angle on thrust events = 27 +/- 4 Harvard CMT = 8 Engdahl et al. (2007)

What can a 30-year record of seismicity tell us about coupling? --> Harvard CMT catalog: Jan 1976 to April 2007 Questions to keep in mind when examining the seismicity pattern at each subduction zone Is there a pattern of thrust events defining the interplate interface? What is the convergence rate (and what data is it derived from)? What is the maximum event known to have occurred? What is its magnitude and when did it occur? Is there geodetic evidence of strain accumulation? What event the strain accumulation related to?

Geodetic (not seismic!) coupling at Sumatra, derived from coral uplift data and GPS interseismic strain accumulation Chlieh et al. (2008)

Chlieh et al. (2008)

Chlieh et al. (2008)

EXTRA SLIDES

Eastern Japan subduction zone Miura et al. (2005)

Southern Kurile subduction zone Nakanishi et al. (2004)

Accretionary margin Erosive margin Clift and Vannucchi (2004)

Accretionary plate margins Clift and Vannucchi (2004)

Non-accretionary and erosive plate margins Clift and Vannucchi (2004)