Scaling Relations for the Thermal Structure of Segmented Oceanic Transform Faults

Abstract:

Mid-ocean ridge-transform faults (RTFs) are a natural laboratory for studying strike-slip earthquake behavior due to their relatively simple geometry, well-constrained slip rates, and quasi-periodic seismic cycles. However, deficiencies in our understanding of the limited size of the largest RTF earthquakes are due, in part, to not considering the effect of short intra-transform spreading centers (ITSCs) on fault thermal structure. We use COMSOL Multiphysics to run a series of 3D finite element simulations of segmented RTFs with visco-plastic rheology. The models test a range of RTF segment lengths (L = 10-150 km), ITSC offset lengths (O = 1-30 km), and spreading rates (V = 2-14 cm/yr). The lithosphere and upper mantle are approximated as steady-state, incompressible flow. Coulomb failure incorporates brittle processes in the lithosphere, and a temperature-dependent flow law for dislocation creep of olivine activates ductile deformation in the mantle. ITSC offsets as small as 2 km affect the thermal structure underlying many segmented RTFs, reducing the area above the 600˚C isotherm, A₆₀₀, and thus the size of the largest expected earthquakes, Mc. We develop a scaling relation for the critical ITSC offset length, O_C, which significantly reduces the thermal affect of adjacent fault segments of length L₁ and L₂. O_C is defined as the ITSC offset that results in an area loss ratio of R = (A_unbroken – A_combined)/A_unbroken – A_decoupled) = 63%, where A_{unbroken =} C₆₀₀(L1+L2)^1.5V^-0.6 is A₆₀₀ for an RTF of length L₁ + L₂; A_decoupled = C₆₀₀(L₁^1.5+L₂^1.5)V^-0.6 is the combined A₆₀₀ of RTFs of lengths L₁ and L_2, respectively; and A_{combined =} A_unbroken exp(-O/ O_C) + A_decoupled (1-exp(-O/ O_C)). C₆₀₀ is a constant. We use O_C and kinematic fault parameters (L1, L2, O, and V) to develop a scaling relation for the approximate seismogenic area, A_seg, for each segment of a RTF system composed of two fault segments. Finally, we estimate the size of Mc on a fault segment based on A_seg. We show that small (<1 km) offsets in the fault trace observed between M­_W6 rupture patches on Gofar and Discovery transform faults, located at ~4S on the East Pacific Rise, are not sufficient to thermally decouple adjacent fault patches. Thus additional factors, possibly including changes in fault zone material properties, must limit the size of Mc on these faults.