From the brittle–ductile transition to earthquake-cycle metrics: a parameter-separated test of moment scaling and interseismic loading

The brittle–ductile transition (BDT) provides a first-order mechanical constrainton the portion of the crust capable of sustaining brittle faulting, but it does not by itself20 determine rupture magnitude or earthquake timing. Here I test a three-stage frameworkthat links a thermomechanical–Timoshenko estimate of the mechanically admissibleseismogenic thickness to rupture moment and interseismic loading. The centraldesign rule is parameter separation: stress drop is calculated once from independentlyconstrained slip and rupture width and is then reused in both the moment and loading25 calculations. For three data-constrained fault systems—Parkfield, the Main MarmaraFault, and the central Alpine Fault—the moment branch reproduces published characteristicmagnitudes to within approximately 0.02–0.50 magnitude units. The baselineelastic loading formulation underestimates observed paleoseismic recurrence intervalsby factors of about 3–5. An analytical reduction shows that, under the adopted pa30rameterization, this baseline recurrence ratio is independent of BDT thickness, rupturewidth, and shear modulus; changing BDT depth alone therefore cannot repair the discrepancy.I introduce an extended formulation containing a rupture-geometry coefficient,effective interseismic coupling, and a nucleation-threshold factor. Retrospectivetests on eight fault segments show improved agreement for most segments, but this35 result is interpreted as a consistency test rather than a predictive validation becausethe external provenance and spatial representativeness of the coupling and nucleationparameters remain decisive. The framework is therefore best presented as a physicallybased screening and hypothesis-testing tool for characteristic moment and interseismicloading, not as a deterministic earthquake-timing model. A pseudo-prospective test40 with all inputs fixed before the target event is the required next falsification step.