[1] Slow slip and tectonic tremor in subduction zones take place at depths where there is abundant evidence for distributed shear over broad zones ($10-10 3 m) composed of rocks with marked differences in mechanical properties. Here we model quasi-dynamic rupture along faults composed of material mixtures characterized by different rateand-state-dependent frictional properties to determine the parameter regime capable of producing slow slip in an idealized subduction zone setting. Keeping other parameters fixed, the relative proportions of velocity-weakening (VW) and velocity-strengthening (VS) materials control the sliding character (stable, slow, or dynamic) along the fault. The stability boundary between slow and dynamic is accurately described by linear analysis of a double spring-slider system with VW and VS blocks. Our results place bounds on the volume fractions of VW material present in heterogeneous geological assemblages that host slow slip and tremor in subduction zones.

Catastrophic landslides cause billions of dollars in damages and claim thousands of lives annually, whereas slow-moving landslides with negligible inertia dominate sediment transport on many weathered hillslopes. Surprisingly, both failure modes are displayed by nearby landslides (and individual landslides in different years) subjected to almost identical environmental conditions. Such observations have motivated the search for mechanisms that can cause slow-moving landslides to transition via runaway acceleration to catastrophic failure. A similarly diverse range of sliding behavior, including earthquakes and slow-slip events, occurs along tectonic faults. Our understanding of these phenomena has benefitted from mechanical treatments that rely upon key ingredients that are notably absent from previous landslide descriptions. Here, we describe landslide motion using a rate-and state-dependent frictional model that incorporates a nonlocal stress balance to account for the elastic response to gradients in slip. Our idealized, one-dimensional model reproduces both the displacement patterns observed in slowmoving landslides and the acceleration toward failure exhibited by catastrophic events. Catastrophic failure occurs only when the slip surface is characterized by rate-weakening friction and its lateral dimensions exceed a critical nucleation length h * that is shorter for higher effective stresses. However, landslides that are extensive enough to fall within this regime can nevertheless slide slowly for months or years before catastrophic failure. Our results suggest that the diversity of slip behavior observed during landslides can be described with a single model adapted from standard fault mechanics treatments.landslides | slope failure | rate and state friction | pore-water pressure | effective stress L aboratory experiments (1, 2) and numerical models (3,4) suggest that for slow-moving landslides that persist over periods of years to centuries (5) the shear strength that resists motion increases with slip rate-a characteristic referred to as rate strengthening-whereas the opposite is true for landslides that exhibit runaway acceleration and catastrophic failure. Two primary mechanisms are invoked frequently to describe the former rate-strengthening behavior. The first characterizes landslide materials as viscoplastic (i.e., Bingham-plastic), so that an increase in velocity corresponds with an increase in strain rate and viscous resistance. These models are able to reproduce field-based measurements of velocity for seasonally active slow-moving landslides (3). However, this constitutive behavior contradicts measurements that suggest landslide displacement is dominated by frictional sliding along basal and lateral faults (6); furthermore, such rheological models are unable to capture the transition from slow sliding to catastrophic failure. The second modeling approach applies Coulomb friction and invokes shear-zone dilatancy to regulate porewater pressure changes so that sliding increases strength because of ...

The Hikurangi subduction zone hosts shallow slow‐slip events, possibly extending to the seafloor. The mechanisms allowing for this behavior are poorly understood but are likely a function of the frictional properties of the downgoing seafloor sediments. We conducted friction experiments at a large range of effective stresses, temperatures, and velocities on incoming sediment to the Hikurangi subduction zone to explore the possible connection of frictional properties to slow‐slip events. These experiments were conducted on multiple apparatuses, allowing us to access a wider range of deformation conditions than is available on any one machine. We find that the material frictionally weakens and becomes less velocity strengthening with increasing effective stress, whereas temperature has only a small effect on both friction and frictional stability. When driven at the plate convergence rate, the sediment exhibits velocity‐weakening behavior. These results imply that the frictional properties of the sediment package subducting at Hikurangi could promote slow‐slip events at the pressures, temperatures, and strain rates expected along the plate boundary thrust up to 10‐km depth without requiring elevated pore fluid pressures. The transition to velocity‐strengthening behavior at faster slip rates could provide a mechanism for limiting unstable slip to slow‐sliding velocities, rather than accommodating deformation through ordinary earthquakes.

Episodic tremor and slip (ETS) along the subduction interface takes place where there is abundant evidence for elevated, near-lithostatic pore pressures, at sufficiently great depths (30-45 km) that chemical dehydration reactions must act as their dominant source. We simulate fluid and heat flow while tracking the location of a vertically oriented, one-dimensional column of material as it subducts through the slow slip and tremor zone. The material in the column is transformed through a pressure-dependent and temperature-dependent dehydration reaction that we describe with a generalized nonlinear kinetic rate law. Column deformation is largely dominated by viscous creep, with a closure rate that depends linearly on porosity. This behavior causes the dehydration reaction to generate traveling porosity waves that transport increased fluid pressures within the slow slip region. To explore the possibility that the observed periodicity of slow slip and tremor in subduction zones can be explained by the migration of such porosity waves, we derive a dispersion relation that accurately describes our numerical results. We also obtain an expression for how the thickness of the dehydrating layer is expected to vary as a function of the parameters in the reaction rate law. Although the amplitudes of pore pressure perturbations rival those that are produced by known external forcings (e.g., tides or passing surface waves), our analysis suggests that given reasonable estimates of rock viscosity, permeabilities in the range 6:5310 215 to 5310 210 m 2 are required for porosity wave trains to form at periods comparable to those of slow slip and tremor.

We present a MATLAB graphical user interface (GUI) software package for analyzing rate and state friction experiments. Called RSFit3000, the software allows users to easily determine frictional parameters by fitting velocity-step and slide-hold-slide events using the aging- and slip-law forms for state variable evolution. RSFit3000 includes features for removing strain hardening or weakening trends from the data, and provides options for using two state variables, applying a weighting function, and treating stiffness as a fitting parameter. Completed fits are conveniently saved in MATLAB structure arrays that contain fitted parameter values with their error intervals, and all information required to reproduce a given fit. The GUI makes the program simple to use, as all fitting tasks are completed via interaction with the interface. Here we describe how to use the software, and illustrate its flexibility and utility by analyzing two sets of synthetic friction data, as well as some previously published experimental data. Although descriptions of rate and state friction fitting routines have been published in the past, RSFit3000 marks the first time a software package for analyzing friction experiments has been described in the literature.