Most continental transform ruptures start on a minor branch fault, then propagate unilaterally: Implications for the physics of slip
Peter Bird, & Ross S. SteinSubmitted September 8, 2024, SCEC Contribution #13787, 2024 SCEC Annual Meeting Poster #130
We propose a new model for typical large-earthquake ruptures on continental transforms, based on two recent discoveries:
(1) A review of the seismological literature [Stein & Bird, 2024, SRL] shows that the 5 Mw≥7.8 continental transform earthquakes since 2000 all nucleated on minor branch faults.
(2) We studied the 100 largest (Mw≥6.49) continental transform earthquakes, 1977-2022, to determine whether their ruptures were bilateral or unilateral [Bird & Stein, 2024, SRL]. High migration velocities (from epicenter to epicentroid) and asymmetric early aftershocks are indicators of unilateral rupture. We also found some ambiguous cases where these indicators disagreed, 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.
Key Words
rupture, unilateral, friction, dynamic, stress drop,
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.
Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)
