Fault networks at plate boundaries are the fundamental systems that enable the differential motion of tectonic plates and are home to the majority of the world's earthquake activity. Whether at the convergent India-Asia collision zone or strike-slip dominated Pacific-North America boundary the differential motion of major tectonic plates causes slip on some of the most recognizable linear features on the Earth's surface such as the Altyn Tagh and San Andreas faults. While individual faulting processes are understood in remarkable theoretical detail we know far less about how fault systems interact to build broad boundaries and mountain belts. Our work spans earthquake science and geodynamics to understand the nature of the most active parts of the Earth's crust exploiting both space based geodetic and geologic observations. Answering these questions inevitably provides insights on earthquake hazard in some of the worlds most populated regions. We are currently studying three major thematic areas:

Geodetic constraints on plate boundaries deformation, earthquake hazard, and fault system evolution

Large aperture GPS networks have provided revolutionary views of the Earth's surface in motion and provide the fundamental observations for understanding the nature of continental deformation. We have developed linear block modeling theory to interpret these measurements in active tectonic environments from Tibet to Japan and southern California. Block theory allows for the simultaneous imaging of fault slip rates, micro-plate rotations, fault coupling, and off-fault deformation decomposing complex GPS velocity fields into their constituent tectonic elements. These methods provide high-resolution images of fault systems in motion allowing us to answer questions about the nature of continental deformation: How do fault systems coordinate motion to partition slip across plate boundary zones? How complex are fault systems and how can we understand fault interactions within them? What processes are most important for controlling the long-term evolution of fault networks? Where are large earthquakes most likely to occur in the future?

• Geodetic constraints on plate boundaries deformation, earthquake hazard, and fault system evolution

Understanding the earthquake cycle and lithospheric rheology

Several lines of evidence suggest that there are subtle geodetic signals that can be interpreted to gain information about the range of earthquake cycle behavior and rheology of the lithosphere. Moderately sized (Mw<7.0) silent earthquakes slipping rates too slow to cause significant shaking and have been widely observed in the Cascadia and Nankai subduction zones. We have shown the predicted signature of great (Mw>7.5) silent earthquakes may be quite different from that of the moderately sized events and that the pervasive observation of partially coupled fault interfaces may in fact be the signature of great silent earthquakes occur over hundreds of years. To understand the behavior of deeper lithosphere we are using both geologic and geodetic observations to develop models that explain all phases of earthquake cycle deformation using extended linear rheologies. The remarkable correlation between geologic and geodetic slip rate estimates leads of a view of the earthquake cycle as far more steady than classical Maxwell models predict, requiring high-resolution fault system models to image these subtle, yet important, behaviors.

Understanding the earthquake

Mountain building in the context of climate change

Mountain belts grow and evolve in response to tectonic and erosional forcing. Erosion rates may vary due to climate change and mountains may gain or lose mass as they adjust their slopes towards a critical taper. We have developed a simple two-dimensional coupled orogen-slab model that allows for the prediction of orogen size and plate motion in response to both tectonic and erosional forcing. As applied to the Andes of western South America our models show that Miocene aridifcation may have led to reduced erosion, increased orogen growth, greater frictional resistance to subduction, and, ultimately, to a ~50% reduction in the convergence rate between the Nazca and South American plates.

Mountain building in the context of climate change