Constraining fault constitutive behavior with slip and stress heterogeneity

We study how enforcing self-consistency in the statistical properties of the preshear and postshear stress on a fault can be used to constrain fault constitutive behavior beyond that required to produce a desired spatial and temporal evolution of slip in a single event. We explore features of rupture dynamics that (1) lead to slip heterogeneity in earthquake ruptures and (2) maintain these conditions following rupture, so that the stress field is compatible with the generation of aftershocks and facilitates heterogeneous slip in subsequent events. Our three-dimensional finite element simulations of magnitude 7 events on a vertical, planar strike-slip fault show that the conditions that lead to slip heterogeneity remain in place after large events when the dynamic stress drop (initial shear stress) and breakdown work (fracture energy) are spatially heterogeneous. In these models the breakdown work is on the order of MJ/m^2, which is comparable to the radiated energy. These conditions producing slip heterogeneity also tend to produce narrower slip pulses independent of a slip rate dependence in the fault constitutive model. An alternative mechanism for generating these confined slip pulses appears to be fault constitutive models that have a stronger rate dependence, which also makes them difficult to implement in numerical models. We hypothesize that self-consistent ruptures could also be produced by very narrow slip pulses propagating in a self-sustaining heterogeneous stress field with breakdown work comparable to fracture energy estimates of kJ/M^2

The large‐scale freshwater cycle of the Arctic

This paper synthesizes our understanding of the Arctic\u27s large‐scale freshwater cycle. It combines terrestrial and oceanic observations with insights gained from the ERA‐40 reanalysis and land surface and ice‐ocean models. Annual mean freshwater input to the Arctic Ocean is dominated by river discharge (38%), inflow through Bering Strait (30%), and net precipitation (24%). Total freshwater export from the Arctic Ocean to the North Atlantic is dominated by transports through the Canadian Arctic Archipelago (35%) and via Fram Strait as liquid (26%) and sea ice (25%). All terms are computed relative to a reference salinity of 34.8. Compared to earlier estimates, our budget features larger import of freshwater through Bering Strait and larger liquid phase export through Fram Strait. While there is no reason to expect a steady state, error analysis indicates that the difference between annual mean oceanic inflows and outflows (∼8% of the total inflow) is indistinguishable from zero. Freshwater in the Arctic Ocean has a mean residence time of about a decade. This is understood in that annual freshwater input, while large (∼8500 km3), is an order of magnitude smaller than oceanic freshwater storage of ∼84,000 km3. Freshwater in the atmosphere, as water vapor, has a residence time of about a week. Seasonality in Arctic Ocean freshwater storage is nevertheless highly uncertain, reflecting both sparse hydrographic data and insufficient information on sea ice volume. Uncertainties mask seasonal storage changes forced by freshwater fluxes. Of flux terms with sufficient data for analysis, Fram Strait ice outflow shows the largest interannual variability

The tegula tango: A coevolutionary dance of interacting, positively selected sperm and egg proteins

Reproductive proteins commonly show signs of rapid divergence driven by positive selection. The mechanisms driving these changes have remained ambiguous in part because interacting male and female proteins have rarely been examined. We isolate an egg protein the vitelline envelope receptor for lysin (VERL) from Tegula, a genus of free-spawning marine snails. Like VERL from abalone, Tegula VERL is a major component of the VE surrounding the egg, includes a conserved zona pellucida (ZP) domain at its C-terminus, and possesses a unique, negatively charged domain of about 150 amino acids implicated in interactions with the positively charged lysin. Unlike for abalone VERL, where this unique VERL domain occurs in a tandem array of 22 repeats, Tegula VERL has just one such domain. Interspecific comparisons show that both lysin and the VERL domain diverge via positive selection, whereas the ZP domain evolves neutrally. Rates of nonsynonymous substitution are correlated between lysin and the VERL domain, consistent with sexual antagonism, although lineage-specific effects, perhaps owing to different ecologies, may alter the relative evolutionary rates of sperm- and egg-borne proteins. © 2012 The Author(s). Evolution © 2012 The Society for the Study of Evolution