Dr. Lucy Flesch

Presentation: Integration of Plate Boundary Observatory and USArray Data to Quantify the Forces Driving Deformation in the Western United States

Understanding the forces and factors responsible for deforming the continental lithosphere remains a fundamental question in geophysics. Over the past 15 years there has been an explosion of GPS measurements in the western United States, which has been accelerated by the EarthScope Plate Boundary Observatory. This effort has identified measureable surface motions away from the plate boundary as far east as Montana and New Mexico. These data allow for the quantification of the long-term surface motions and strain rates over the North American plate boundary zone and provide constraints needed to address the fundamental question: "What are the forces driving the observed deformation?" Geodynamic models of continental-scale deformation quantify the vertically-averaged deviatoric stress field arising from internal buoyancy forces, the accommodation of relative plate motions, and the level of tractions acting at the base of the lithosphere. But such models are highly dependent on the assumed model parameters. Crustal structure determinations, using EarthScope data from the USArray Transportable Array as well as previously deployed seismic stations in the western United States, are therefore important contributions to the construction of dynamic models that illustrate the role of previous tectonic events in driving present-day surface motions.

Dr. James P. Evans

Presentation: Earthquake Petrology: The Signatures of Seismic Slip and Fluid Flow in Faults

Slip along faults is the principal mechanism by which strain is accommodated in the brittle crust. In the past 20 years numerous workers have made great advances in determining the nature of fluid-rock interactions in fault zones, and exploring the relationship between seisimc slip and faulting. The later issue is hampered by the presence of few direct seismic-slip indicators in rocks, but by drawing analogies between results from drilling and field studies of rocks exhumed from as deep as 15 km, we can constrain the mechanisms of slip and fluid-rock interactions during faulting. This talk reviews results of our current research that reveals the complex nature of some fault systems, including portions of the San Andreas fault where it cuts accretionary complex lithologies, to deeply exhumed faulted granitoids in which the relationships between pseudotachylytes, narrow slip surfaces, and highly polished surfaces that reveal surprising complexity at the micron scale. The presentation also relates these observations to geochemical and petrologic studies of faults and shear zones, and derives relationships between the energetics of seismic slip and the energy required to drive reactions within faults.

Dr. Bridget Smith-Konter

Presentation: Locked, Loaded, and Looming? Exploring Earthquake Cycle Stress History of Seismically-active Faults of the San Andreas Fault System

Prompted by the lingering absence of a major earthquake over the past 300 years along the southern San Andreas fault, EarthScope is leading a large-scale community effort to understand the nature of earthquake hazards along the Pacific-North American plate boundary. Several key questions motivate this work: How does plate boundary stress evolve spatially and temporally throughout the earthquake cycle? What can past and present-day stress accumulation scenarios tell us about the current state of seismic hazard? GPS data from EarthScope’s Plate Boundary Observatory (PBO) provide a large-scale perspective of present-day crustal motions. But a three-dimensional, synoptic picture of stress evolution spanning the full earthquake cycle requires both sophisticated computational models and improved paleoseismic chronologies. One of the fundamental issues in evaluating stress rates from crustal motion models is a lack of knowledge of the variable depths to which the San Andreas fault system is locked and accumulating stress. This talk presents an overview of geodetic and paleoseismic observations of the San Andreas fault system. It relates how the resulting data are synthesized into earthquake stress evolution models to probe the depths, stress accumulation rates, and stress relationships of past earthquakes along seismically-acitive faults in California.

Dr. Terry Plank

Presentation: Extending a Continent: Magmatism and Lithosphere Dynamics across the Basin and Range Province of the western United States

Why is there volcanism in the Basin and Range Province? Cinder cones have popped up in various regions from California to Utah, bringing mantle melts and xenoliths to the surface. Some of these volcanic vents are clearly related to recent faulting and Basin and Range extension, while others are not. Some recent work points to small-scale convective phenomena in the mantle, such as lithospheric drips, edge-driven convection, or flow around subducting slabs, which may also drive melting. What is the role of prior subduction and hydration of the lithosphere in generating magmas? Such questions are fundamental to how the continent evolves in regions of active extension, and yet there are no consensus answers. EarthScope provides a new platform to constrain the dynamics of mantle melting and evolution of the continental lithosphere. Specifically, seismic velocity structure can be integrated with petrological information to constrain the melting regions in the mantle, and the temperatures and water contents involved. We are finding large variations in the temperature of the shallow mantle beneath the Western US, which points to the potential importance of lithospheric downwellings and asthenospheric upwellings as drivers of magmatism and consequences of continental extension.

Dr. William L. Ellsworth

Presentation: Earthquakes from the Top to the Bottom of the Magnitude Scale: Insights into Earthquake Physics from EarthScope

Earthquakes are among the most terrifying and deadly of all natural phenomena that we face on a recurring basis. In this century alone, earthquakes have been responsible for over 700,000 deaths worldwide. They strike without warning and occur throughout the world as a primary agent of tectonic activity and occasionally as unintended consequences of human activity. Why are earthquakes so difficult to predict and how well do our current models characterize the hazard they pose? One of many challenges faced by earthquake researchers is the enormous range of scales spanned by the phenomenon, from grain boundary fractures to 1,000 km-long ruptures of plate boundaries. Another challenge is the remoteness of the nucleation process, with rupture invariably beginning kilometers deep in the Earth. This talk presents an overview of the underpinnings of earthquake science and discusses important results being obtained from a new generation of scientific tools for studying earthquakes. These tools include: the first scientific drill hole into the earthquake nucleation zone of a major fault at EarthScope's San Andreas Fault Observatory at Depth (SAFOD); other EarthScope observatories, namely the Plate Boundary Observatory (PBO) network of geodetic instruments and the USArray of seismometers; similar geodetic and seismic networks around the world; and advanced computational approaches to the modeling and interpretation of earthquake processes.