Tectonic faults produce a wide spectrum of slip modes, ranging from fast earthquakes to slow-slip events (SSEs). Whether slow-slip events and regular earthquakes result from a similar physics is debated.

In this project we present numerical simulations to show that slow-slip events can result from frictional sliding like seismic slip, with an additional mechanism of shear-induced dilatancy that prevents acceleration to fast slip due to the presence of fluids. The model succeeds in reproducing a realistic sequence of slow-slip events and provides an excellent match to the observations from the Cascadia subduction zone, including the earthquake-like cubicmoment-duration scaling.

In contrast to conventional and widely used assumptions of magnitude-invariant rupture velocities and stress drops, both simulated and natural SSEs have rupture velocities and stress dropsthat increase with event magnitudes. These findings support the same frictional origin for both earthquakes and SSEs while suggesting a new explanation for the observed SSEs scaling.


Scaling properties of slow-slip events

LUCA DAL-ZILIO

PROJECTS

Nowadays, the scientific dispute as to whether vertical or horizontal forces are the primary drivers of mountain building seems settled in favor of horizontal forces. For the central European Alps, however, this concept fails to explain first-order observations of a mountain belt. Recent stratigraphic, palaeo-altimetry and lithosphere structural evidence suggest that a rollback orogeny model is capable of explaining the construction of thick nappe successions and the large-scale evolution of the Swiss Alps.

In this project we investigate this hypothesis using a high-resolution, rheologically consistent, two-dimensional visco-elasto-plastic thermo-mechanical numerical model. We conduct a set of numerical experiments in which we systematically vary several major parameters responsible for the degree of rheological coupling between platesduring collision. The driving forces of orogeny are solely provided by the structure within the model, i.e., by the oceanic slab and the buoyancy of the crust.

Our model reproduces the self-consistent stages of oceanic subduction, continent-continent collision, spontaneous slab breakoff and subsequent evolution of the orogen. This leads to the coeval stackingof the crustal root in the lower plate and widening of the orogen. In particular, we discuss how the current crustal seismicity pattern implies the occurrence of extensional forces at work beneath the Molasse Basin and within the Alps. Our results thus support the emerging hypothesis that the remaining slab exerts a first-order control on the motions and deformations of the orogen.


Slab Rollback Orogeny model: bridging long-term tectonics and seismicity in the Alps

Modeling sequences of seismic and aseismic deformation in a nonlinear viscoelastic megathrust

A major goal in earthquake physics is to develop a constitutive framework for fault slip that captures the dependence of shear strength on sliding velocity, nonlinear rheology, and temperature. In this project, we present VELO2CYCLEs (Visco-ELastO 2-D Cycles of Earthquakes), a two-dimensional (2-D) thermo-mechanical computational framework for simulating earthquake sequences in a nonlinear viscoelastic compressible media.

The method is developed for a plane-strain problem and incorporates an invariant formulation of the classical rate- and state-dependent friction equations and an adaptive time-stepping that allows the time step to vary by many orders of magnitude during a simulation. The new method provides a unique computational framework to analyze earthquake sequences and connect forearc deformation with the dynamic properties of the megathrust, thus providing a physical link between observations of individual earthquakes and postseismic deformation with geological observations of long-term tectonic deformation.