A69. Bird, P., Y. Y. Kagan, D. D. Jackson, F. P. Shoenberg, & M. J. Werner [2007] Earthquake production by subduction zones is not linear in relative plate velocity (abstract), Eos Trans. AGU, 88(52), Fall Meeting Supplement, Abstract S32B-02.

The ratio of {long-term-average seismic moment production per unit length of plate boundary} to {relative plate velocity} is determined by the “coupled thickness” of seismogenic lithosphere, and also by elastic moduli and geometric factors that are fairly well known.  It is generally assumed that coupled thickness is constant within a given class of plate boundary, such as Bird’s [2003, G³]: CCB Continental Convergent Boundary, CRB Continental Rift Boundary, CTF Continental Transform Fault, OCB Oceanic Convergent Boundary, OSR Oceanic Spreading Ridge, OTF Oceanic Transform Fault, or SUB Subduction zone.  However, Bird et al. [2002, Geodyn. Ser.] and Bird & Kagan [2004, BSSA] found two exceptions: OSR and OTF both have greater coupled thickness at low relative plate velocities.

We test for variation of coupled thickness with relative plate velocity in each of the 7 classes of plate boundary.  We use shallow (<70 km) earthquakes from the Harvard CMT catalog, 1982.01.01-2007.03.31, above magnitude M_W threshold of 5.51 or 5.66.  In order to reduce the influence of aftershock swarms, we estimate the probability of independence of each earthquake according to the likelihood stochastic declustering method of Kagan & Jackson [1991; GJI] and use this as a weight.  We use the algorithm of Bird & Kagan [2004, BSSA] to assign 95% of shallow earthquakes to plate boundary steps and plate boundary classes, rejecting all earthquakes that fall into one of the 13 orogens of Bird [2003, G³].  We order the plate-boundary steps outside orogens in each class by relative plate velocity according to the PB2002 model of Bird [2003].  Then, we plot cumulative earthquake count as a function of cumulative model tectonic moment (assuming constant coupled thickness and other parameters within each plate boundary class).  The null hypothesis is a linear relation; we use 2 measures (Kolmogorov/Smirnov, and Cramer/von-Mises) to quantify departures from this line.  We use 10,000 simulations of each class with random Poissonian seismicity in each plate boundary step (with expectations based on the tectonic model) to assess the significance of  the measures obtained.

Subduction zones have velocity-dependent coupled thickness: P < 0.001 for the null hypothesis.  Subduction zones with relative plate velocity <67 mm/a (which would comprise 35% of the model tectonic moment rate, in the null hypothesis) actually produce only 20% of the global subduction zone earthquakes (outside orogens), and thus have a coupled thickness about half that of faster subduction zones (if corner magnitude and spectral slope are constant).  This result contradicts the uniform coupling of subduction zones inferred by Kreemer et al. [2002, Geodyn. Ser.]; the difference may be due to their exclusion of several slow subduction zones including Aegean, Cascadia, New Zealand, Caribbean, and South Shetland.  Continental CCBs show a similarly strong relation (P < 0.001), with an increase in coupled thickness when velocity exceeds 25 mm/a.

OSRs show coupled thickness declining with velocity, as in previous studies.  OTFs and OCBs give complex results with significant variations (P < 0.01; P < 0.05) that are not easy to interpret.  For CRBs and CTFs we do not reject the null hypothesis of constant coupled thickness.