Information for Prospective Students
My research group is currently focused on the processes that control the evolution of fault systems. We directly document fault growth in the laboratory with physical experiments and use numerical models to both investigate fault propagation within simple systems and simulate complex active faulting in southern California.
Active faulting in Southern California
My students and I have developed mechanical models of the active faults in Southern California that include all sorts of three-dimensional complexity along the restraining bend at the San Bernardino Mountains and other regions. With the models, we examine how the fault geometry both hinders and facilitates slip along faults. We work with paleoseismologists, sedimentologists and geomorphologists to compare the slip rates, deposition and uplift patterns from the models with the geologic record. Our models can also simulate interseismic deformation so that we can compare the surface displacements of the model to the GPS station velocities and the stress states of the model to focal mechanisms. We also work with dynamic rupture modelers who use stress conditions from our steady-state models as input for inital conditions in their earthquake simulations.
Asessing Fault Evolution using Work Minimization
Faults grow in order to minimize work of the system. My students and I are exploring this hypothesis with a variety of analytical, analog and numerical approaches. Analog experiments using either a sandbox or claybox provide a wonderful opportunity to observe fault growth under known conditions. We are also collaborating with sandbox modelers who record how the applied force changes during fault system evolution. In the physical modeling lab at UMass, we run claybox experiments and analyze the work budget by simulating deformation within numerical models.We use two digital cameras to document the deformation and from this we can extract the horizontal and vertical deformation throughout the experiment. Within the numerical models that simulate the experiments, we calculate the Wfric (frictional heating), Wgrav (work of uplift against gravity), Wint (internal mechanical work), Wseis (seismic radiated energy), Wprop (work of fault propagation) and the Wtot (tectonic work). The tradeoff among these work terms as faults evolve reveals insights into the mechanisms of fault growth and abandonment.
We've been learning a lot about fault growth through the numerical models and analog experiments but we have a lot more work to do. Future projects are likely to involve more claybox modeling. Our recent investigations into clay rheology show that wet kaolin is an excellent analog for Earth materials. The ease of fault reactivation in clay makes it a very important comparison to sandbox models. All projects involve some degre of mechanical modeling as this is a strength of the geomechanics group. The UMass Geomechanics YouTube channgel has animations of the experiments as well as explanatory videos.
We have been investigating the growth of faults by work optimization, which postulates that faults grow in order to minimize work on the entire system. To do this. we use the new code GROW, written by current student, Jess McBeck. GROW searches for the optimal fault orientation and automatically propagates faults in the optimal direction. We've applied GROW to releasing stepovers, lab experiments of fault propagation and coalescence of microcracks. Over the next few years, I look forward to applying this tool to a variety of settings.
Field work opportunities.
While none of our current projects centers around field data collection, all students in the geomechanics group have opportunities to work with colleagues at other universities. Many of these opportunities include field investigations. UMass Geomechanics gradaute students have assisted with trenching across the San Andreas fault, worked with geodetic experts at JPL, mapped Martian and Venutian features with planetary geologists, gathered geochronology samples with tectonic geomorphologists, worked with earthquake dynamic rupture modelers, helped collect paleomag data, studied in Spain with Pyrenean experts and run experiments in Iowa, Paris and Potsdam Germany.
