Eitan Shelef

Elevated shear zone loading rate during an earthquake cluster in eastern California

We compare geodetic velocity to geologic fault slip rates to show that tectonic loading was doubled across the eastern California shear zone (ECSZ) during a cluster of major earthquake activity. New slip rates are presented for six dextral faults that compose the ECSZ in the central Mojave Desert. These rates were determined from displaced alluvial fans dated with cosmogenic 10 Be and from a displaced lava flow dated with 40 Ar/ 39 Ar. We find that the sum geologic Mojave ECSZ slip rate, ≤6.2 ± 1.9 mm/yr, is only half the present-day geodetically measured velocity of 12 ± 2 mm/yr. These rates account for cumulative fault slip and geodetic observations that span the 60-km-wide shear zone; therefore this difference cannot be attributed to postseismic relaxation. Redistribution of tectonic loading over the earthquake cycle at a regional scale suggests that earthquake clustering may be enhanced via feedback with weakening of ductile shear zones.

Symmetry, randomness, and process in the structure of branched channel networks

The branched structure of channel networks has a primary impact on the spatial distribution of elevation, water, and life across Earths surface from the hillslope to the continental scale and is also observed on other planets. However, the link between this dendritic multiscale structure and the erosional processes that sculpt it has remained elusive for more than six decades. In fact, many topologic measures fail to distinguish natural networks from those generated by random walks. Here we show that a fundamental multiscale topologic symmetry is ingrained into the structure of these networks and reflects the equal elevation drop spanned by flows that split at the drainage divide and meet again downslope. We demonstrate that this symmetry distinguishes random-walk networks from natural ones, captures the temporal evolution of these networks, and divulges information about the processes that shape them.

Recent topographic evolution and erosion of the deglaciated Washington Cascades inferred from a stochastic landscape evolution model

In this study, we model postglacial surface processes and examine the evolution of the topography and denudation rates within the deglaciated Washington Cascades to understand the controls on and time scales of landscape response to changes in the surface process regime after deglaciation. The postglacial adjustment of this landscape is modeled using a geomorphic-transport-law-based numerical model that includes processes of river incision, hillslope diffusion, and stochastic landslides. The surface lowering due to landslides is parameterized using a physically based slope stability model coupled to a stochastic model of the generation of landslides. The model parameters of river incision and stochastic landslides are calibrated based on the rates and distribution of thousand-year-time scale denudation rates measured from cosmogenic 10Be isotopes. The probability distributions of those model parameters calculated based on a Bayesian inversion scheme show comparable ranges from previous studies in similar rock types and climatic conditions. The magnitude of landslide denudation rates is determined by failure density (similar to landslide frequency), whereas precipitation and slopes affect the spatial variation in landslide denudation rates. Simulation results show that postglacial denudation rates decay over time and take longer than 100 kyr to reach time-invariant rates. Over time, the landslides in the model consume the steep slopes characteristic of deglaciated landscapes. This response time scale is on the order of or longer than glacial/interglacial cycles, suggesting that frequent climatic perturbations during the Quaternary may produce a significant and prolonged impact on denudation and topography.