Group B, Poster #130, Fault and Rupture Mechanics (FARM)
Most continental transform ruptures start on a minor branch fault, then propagate unilaterally: Implications for the physics of slip
Poster Image:
Poster Presentation
2024 SCEC Annual Meeting, Poster #130, SCEC Contribution #13787 VIEW PDF
or where migration velocity was oblique to fault traces. The result was 55~70% unilateral ruptures and 30~45% bilateral.
Our synthesis: Major transform faults have large net slip, and this is associated with thick cataclasite. This makes them natural “superhighways” for Rice-type dynamic ruptures because: (a) the cataclasite layer is a low-velocity zone that focusses wave energy on the process zone; and (b) cataclasite is rich in pore water, which expands when heated by frictional work in the matrix, reducing effective normal stress and increasing Coulomb stress. Once a dynamic rupture is established, it can propagate a long way through crust with only moderate shear stresses, leaving shear stresses even lower after near-total stress drop. As tectonic motion rebuilds regional shear stress, the value needed to sustain another dynamic rupture is reached long before the value needed for quasi-static Byerlee-type initiation according to rate-and-state friction theory. Therefore, most large transform earthquakes actually nucleate on a minor branch fault with small net slip, little or no cataclasite, and high shear stress. The stress-drop on the initiating branch fault is governed by its existing state and is modest. The stress-drop on the connected major transform is limited to pre-existing shear stress and is also modest. One implication of this model is that major transform earthquakes may be slip-predictable but are not time-predictable. Another implication is for Earthquake Early Warning: modest (Mw 4~6) ruptures on branch faults adjacent to a transform may continue to grow as large (Mw 7~8) ruptures on the adjacent transform.
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Our synthesis: Major transform faults have large net slip, and this is associated with thick cataclasite. This makes them natural “superhighways” for Rice-type dynamic ruptures because: (a) the cataclasite layer is a low-velocity zone that focusses wave energy on the process zone; and (b) cataclasite is rich in pore water, which expands when heated by frictional work in the matrix, reducing effective normal stress and increasing Coulomb stress. Once a dynamic rupture is established, it can propagate a long way through crust with only moderate shear stresses, leaving shear stresses even lower after near-total stress drop. As tectonic motion rebuilds regional shear stress, the value needed to sustain another dynamic rupture is reached long before the value needed for quasi-static Byerlee-type initiation according to rate-and-state friction theory. Therefore, most large transform earthquakes actually nucleate on a minor branch fault with small net slip, little or no cataclasite, and high shear stress. The stress-drop on the initiating branch fault is governed by its existing state and is modest. The stress-drop on the connected major transform is limited to pre-existing shear stress and is also modest. One implication of this model is that major transform earthquakes may be slip-predictable but are not time-predictable. Another implication is for Earthquake Early Warning: modest (Mw 4~6) ruptures on branch faults adjacent to a transform may continue to grow as large (Mw 7~8) ruptures on the adjacent transform.
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Citation: Bird, P., & Stein, R. S. (2024, 09). Most continental transform ruptures start on a minor branch fault, then propagate unilaterally: Implications for the physics of slip. Poster Presentation at 2024 SCEC Annual Meeting.
Keywords: rupture, unilateral, friction, dynamic, stress drop
Relevant SCEC Themes:
Developing Rheologies and Bridging Multi-Scales
Improving Predictive Analyses of Seismicity
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