François J. Saucier

Present‐day kinematics of Asia derived from geologic fault rates

Active fault geometry, available Quaternary rates on major faults, and the far-field plate motion are used to determine crustal kinematics in the collision zone between India and Asia. Using a finite element formalism to construct a spherical shell model of the Asian continent with embedded faults, we seek a velocity solution approaching the motion of rigid blocks by minimizing the elastic strain energy in the fault-bounded blocks. By doing so, we test the assumption that long-term deformation within continents is mostly localized into major faults. In the solution, fault motion accounts for more than 80% of the deformation, allowing us to describe our velocity model in terms of quasi-rigid block rotations on the sphere. South China is rotating clockwise about a pole located southwest of Borneo, implying an E-ESE velocity vector of ∼11 mm/yr for a point at Shanghai, in agreement with the velocity vector determined by very long baseline interferometry [Heki et al., 1995]. The eastward movement of south China is accommodated by oblique extension along the Red River fault at a rate of 10 ± 5 mm/yr in the south and by the combination of left-lateral strike-slip motion on the Qinling-Dabie Shan fault and the counterclockwise rotation of the Ordos and adjacent blocks in the north. The Tarim rotates clockwise with respect to Dzungaria about a pole located at 44°N, 92°E, consistent with increasing crustal shortening toward the west throughout the Tien Shan. Assuming incompressibility, a crustal volume budget over a domain encompassing the Tertiary mountain ranges in Asia shows that, over the last 10,000 years, 73 ± 4% of the north-south shortening between India and Asia has been absorbed by thickening of the lithosphere, and 27% has been accommodated by lateral extrusion of continental blocks. The present-day predominance of thickening in Asia results from the relatively slow eastward motion of south China, controlled by strike-slip faulting in the Qinling Shan and oblique extension in northeastern China.

Stress near geometrically complex strike‐slip faults: Application to the San Andreas Fault at Cajon Pass, southern California

Slip on an undulatory strike-slip fault induces predictable residual stresses in the adjacent crust. Elastic analytic and finite element models are developed to quantify these stresses for arbitrary fault geometries. Across fault-parallel planes, domains of reverse, normal, right-lateral and left-lateral residual stresses are induced near the bends in the fault. These residual stresses increase in magnitude from one slip event to the next, typically reaching failure level after several events. A two-dimensional analytic elastic plate model for slip on a sinusoidal fault provides general results on the scales and patterns of slip-induced stresses, and Fourier synthesis allows for the solution with arbitrary but small fault distortions. Maximum fault-normal residual stress is proportional to the square of the wavenumber of the sinusoidal trace and decays exponentially away from the fault, reduced to 2/e times the maximum value at a distance equal to 1/wavenumber. This fault-parallel residual shear stress oscillates between dextral and sinistral along the fault, with maximum magnitude adjacent to the maximum excursions in the fault trace. Fault-parallel residual shear stress is maximum at a distance from the fault of 1/wavenumber, where its magnitude is 1/e times the maximum normal stress on the fault. Two- and three-dimensional finite element analyses extend the analytic model; they account for depth-decaying fault perturbation amplitude and slip deficit and are valid for large fault undulations. Application of the model to the San Andreas fault in the Cajon Pass region produces the complex distribution of fault-parallel normal, reverse, right-lateral and left-lateral structures recognized near the main trace. Left-lateral stresses on planes subparallel to the San Andreas fault at the Cajon Pass well are predicted by this model.

Impact of a Two-Way Coupling between an Atmospheric and an Ocean-Ice Model over the Gulf of St. Lawrence

The purpose of this study is to present the impacts of a fully interactive coupling between an atmospheric and a sea ice model over the Gulf of St. Lawrence, Canada. The impacts are assessed in terms of the atmospheric and sea ice forecasts produced by the coupled numerical system. The ocean-ice model has been developed at the Maurice Lamontagne Institute, where it runs operationally at a horizontal resolution of 5 km and is driven (one-way coupling) by atmospheric model forecasts provided by the Meteorological Service of Canada (MSC). In this paper the importance of two-way coupling is assessed by comparing the one-way coupled version with a two-way coupled version in which the atmospheric model interacts with the sea ice model during the simulation. The impacts are examined for a case in which the sea ice conditions are changing rapidly. Two atmospheric model configurations have been studied. The first one has a horizontal grid spacing of 24 km, which is the operational configuration used at the Canadian Meteorological Centre. The second one is a high-resolution configuration with a 4-km horizontal grid spacing. A 48-h forecast has been validated using satellite images for the ice and the clouds, and also using the air temperature and precipitation observations. It is shown that the two-way coupled system improves the atmospheric forecast and has a direct impact on the sea ice forecast. It is also found that forecasts are improved with a fine resolution that better resolves the physical events, fluxes, and forcing. The coupling technique is also briefly described and discussed.

A 3‐D coupled ice‐ocean model applied to Hudson Bay, Canada: The seasonal cycle and time‐dependent climate response to atmospheric forcing and runoff

A coupled three-dimensional, time-dependent ice-ocean model is developed and applied in order to reproduce the basin-scale ice and mixed-layer physical properties of Hudson Bay and James Bay, Canada. Models for albedo, evaporation, storms, frazil ice production, and radiation are included. Observed monthly means of winds, temperature, precipitation, runoff, and cloudiness are used to force the model and obtain multiyear, steady state, and non-steady state solutions. The seasonal cycle in sea ice thickness, ice concentration, ocean temperature, and salinity is first reproduced. Then we consider a set of five experiments: (1) a strong westerly event from the North Atlantic Oscillation, (2) a year with anomalously high runoff, (3) regulated runoff from hydroelectric development, (4) high autumn winds, and (5) warm conditions. We find that preconditioning of the ocean for winter, controlled by the heat transfer to the atmosphere and freshwater input rates and also related to the mixed-layer depth attained before freezing, has a strong control over the following ice season. The results show that varying runoff has more of an effect on sea-ice production in southeastern Hudson Bay than do temperature changes associated with the North Atlantic Oscillation but that both have a small effect on the ice cover when compared to the observed interannual variability. Regulated runoff produces a positive sea-ice anomaly during the January-April period which is significant (greater than 10 cm or 10%) in the southeastern part of the bay but less than 1 cm (∼1%) on average. We conclude that ∼90% of the excess winter runoff remains liquid. No significant delay is computed for breakup dates (less than 3 days in southeastern Hudson Bay and less than 1 day overall). Other controls from the atmosphere are required to explain the natural interannual variability of the ice cover. Summer and autumn winds, and air temperature (which control heat loss and winter preconditioning), spring cloud cover (which controls heat gain), and snow cover (which controls the winter insulation) can explain relatively large changes in the system. Simple climate warming by 2°C produces large impacts in the ice-ocean system, reducing the winter ice volume by over 20%.

Shear instability in the St. Lawrence Estuary, Canada: A comparison of fine‐scale observations and estuarine circulation model results

A three-dimensional numerical model was used to predict the timing and the location of shear instabilities in the St. Lawrence Estuary. This model suggests that significant mixing occur during flood tides in the upper estuary. This mixing is associated with a strong bottom density current made of the cold Gulf of St. Lawrence intermediate waters flowing under the St. Lawrence mixed surface waters. Guided by these results, a field experiment was undertaken in summer 1997 to verify this and to document the conditions that favor the development of instabilities. The instabilities were found as predicted and documented from acoustic imaging, current profiler, and density measurements. The instabilities first develop in the form of wavelike disturbances before they break, like Kelvin-Helmholtz instabilities. The unstable waves have wavelength of ≈140–150 m and extend vertically between 10 and 25 m. The fine-scale observations of the semidiurnal evolution of the vertical structure of currents and density at the experimental site are compared with the numerical results. The model reproduces accurately the tidal variability of the currents but underestimates by a factor of 2 the amplitude of the density fluctuations. The general patterns of the shear squared S2 and the buoyancy frequency squared N2 are reasonably well reproduced by the model, but their intensities are ≈2 times smaller than the observations. This difference is attributed to the limited vertical resolution of the model at the pycnocline. However, the modeled Richardson numbers, Ri ≡ N2S−2, are reasonably well reproduced and appeared to be useful for the prediction of instabilities in such a complex environment.

Earthquake recurrence on the southern San Andreas modulated by fault‐normal stress

Earthquake recurrence data from the Pallett Creek and Wrightwood paleoseismic sites on the San Andreas fault appear to show temporal variations in repeat interval. These sites are located near Cajon Pass, southern California, where detailed mapping has revealed geomorphically and structurally expressed domains of alternating extension and contraction respectively associated with releasing and restraining bends of the San Andreas fault. We investigate the interaction between strike-slip faults and auxiliary reverse and normal faults as a physical mechanism capable of producing such variations. Under the assumption that fault strength is a function of fault-normal stress (e.g. Byerlees Law), failure of an auxiliary fault modifies the strength of the strike-slip fault, thereby modulating the recurrence interval for earthquakes. In our finite element model, auxiliary faults are driven by stress accumulation near restraining and releasing bends of a strike-slip fault. Earthquakes occur when fault strength is exceeded and are incorporated as a stress drop which is dependent on fault-normal stress. The model is driven by a velocity boundary condition over many earthquake cycles. Resulting synthetic strike-slip earthquake recurrence data display temporal variations similar to observed paleoseismic data within time windows surrounding auxiliary fault failures. Although observed recurrence data for the two paleoseismic sites are too short to be definitive about the temporal variations or the physical mechanism responsible for it, our simple model supports the idea that interaction between a strike-slip fault and auxiliary reverse or normal faults can modulate the recurrence interval of events on the strike-slip fault, possibly producing short term variations in earthquake recurrence interval.