Dr. Kaj Johnson

Presentation: Potential for Earthquakes On and Off Major Faults in California

The quantity and quality of GPS measurements of surface motions in the Western U. S. has been significantly enhanced through the Earthscope Program of the National Science Foundation. A fundamental question we wish to answer with these observations is: “where is stress accumulating in the crust, and will it be released in future large earthquakes?” Answering this question is not straightforward because it requires an understanding of how observed strain accumulation at Earth’s surface relates to accumulation of stress on faults at depth. Furthermore, the current GPS surface velocity field provides only a time snapshot of surface deformation rates that vary over decadal to millennial time scales due to time-variable crustal and mantle deformation processes. Therefore, inferring potential for earthquakes on faults requires models that implement what we know about the behavior of faults in the lower crust and deformation processes in the mantle. In this talk I will show how we are using deformation models and GPS data from California to: 1) obtain estimates of long-term slip rates on major faults; 2) infer where faults are locked and accumulating stress and where faults are creeping and not accumulating stress; and 3) estimate the potential size and frequency of large earthquakes on and off major faults.

Dr. Meghan S. Miller

Presentation: From Structure to Dynamics: New Insights into Continental Dynamics from the EarthScope USArray Seismic Observatory

The EarthScope USArray of seismic instruments consists of the Transportable Array (TA), the Flexible Array (FA) and a Reference Network. The TA component has occupied over 800 sites in the Western United States during the past 6 years, providing the seismology community with ~20 terabytes of data. These seismic data are being used to image the lithospheric and mantle structure by techniques such as receiver functions and tomographic inversions, which are then integrated with geological observations and results of geochemical analyses in order to better understand tectonic processes and the overall geologic evolution of the region. The large study area and wealth of data allow for investigation into the three-dimensional structure of a variety of tectonic features, including the San Andreas plate boundary, Mendocino Triple Junction, Cascadia Subduction Zone, Sierra Nevada, Snake River Plain, Basin and Range Province, and Colorado Plateau. All of these features have been influenced by Farallon plate subduction and its aftermath.

Dr. Gary L. Pavlis

Presentation: Three-dimensional Imaging with the USArray: New Constraints on the Geometry of the Juan de Fuca/Farallon Slab

The presentation illustrates new results from the application of a fully three-dimensional, direct imaging method applied to data recorded by the EarthScope Transportable Array (TA). Input data are receiver-function estimates from the EarthScope Automated Receiver-function Survey (EARS). The image volume is an estimate of P- to S-wave conversion scattering amplitude for the entire Cordillera to a depth of 700 km. The waveforms from EARS were first stacked to produce composite events from a set of source regions defined by a radial grid with a center in the western US. The composite event waveforms were imaged with a three-dimensional, pre-stack migration method using a plane-wave decomposition and an inverse generalized Radon transform algorithm to yield estimates of radial and transverse component scattering potential. The migrated, 3-D image was stacked to produce a single, three-dimensional image volume. The Moho, 410 km, and 660 km discontinuities are clearly imaged throughout the study volume. The focus of the presentation is a set of east-dipping conversion horizons seen throughout the Cordillera above the 410 km discontinuity. A direct comparison with published tomography models of comparable scale, produced from the TA data, suggests that this horizon can be interpreted as the top of the Juan de Fuca/Farallon slab. This hypothesis is tested by fitting a surface through picks made on this horizon, with constraints imposed on the depth of the slab by the positions of volcanoes and the surface trace of the trench. The up-dip projection of the slab in the San Andreas region is modeled as linking to the base of the crust at the current location of the transform plate boundary. The resulting surface is well matched with no kinks or mismatched dips, suggesting strongly that we are imaging a structure linked to the subducting slab. Correlation with tomography models shows that this surface is remarkably consistent with the top of high velocity bodies interpreted in previous papers as marking the subducting lithosphere. Using this assumption a refinement of this surface is constructed with a set of two generalized coordinates : (1) flow lines based on the current Juan de Fuca-North America pole of rotation; and (2) time since subduction. A wireframe formed from a surface defined by these coordinates provides insights into the large-scale geodynamics of North America.

Dr. Harold J. Tobin

Presentation: From SAFOD to Subduction: An Inside Look at Plate Boundary Fault Zone Properties and Processes

This talk presents an overview of the EarthScope San Andreas Fault Observatory at Depth (SAFOD) and other drilling studies, weaving together an integrated view of the structure and conditions inside mature fault zones. The large faults that mark the main boundary zones between tectonic plates in the crust--and create the largest earthquakes--have an internal anatomy distinct from other crustal faults. As tens to hundreds of kilometers of slip accumulate on these faults during thousands of large earthquakes, they develop their own internal structure, composition and physical properties. These internal properties govern the nature of locking and fault slip--and ultimately the size and occurrence of earthquakes--by controlling fault strength, friction, pore fluid pressure development, and geochemical changes due to fluids, heat, and pressure. Because active fault zones lie deep beneath Earth’s surface, a concerted effort has taken place in the past decade to drill into them. Prominent examples include SAFOD in California, as well as drillholes in Taiwan, the Nankai Trough off Japan, and elsewhere. These projects are providing data from the geology and petrophysics of core samples, well logs, and seismological imaging that, for the first time, are detecting and mapping fault properties. Concurrent seismological studies reveal that a previously undetected, wide range of types of slip and new forms of earthquakes occur along plate-boundary faults. In the light of new data on fault materials and stress state, the new drillhole information challenges us to understand and explain the how and why of the newly-discovered slip and earthquake characteristics.

Dr. Steven G. Wesnousky

Presentation: The Walker Lane and Basin and Range Fault Systems of Western North America: Styles and Rates of Deformation, Fault Mechanics, and Insights to the Structural Evolution of a Major Transform Plate Boundary

The Great Basin physiographic province of the western United States encompasses an area reaching ~800 km in width between the Sierra Nevada to the west and the Wasatch Mountains to the east. Within reside the Walker Lane and Basin and Range fault systems, which together accommodate upwards of 20-25% of the total 5 cm/yr of ongoing Pacific-North American transform plate motion. The Walker Lane is manifest as an approximately 50 km zone of disrupted topography and discontinuous, northwest-trending, strike-slip and normal faults along the eastern flank of the Sierra Nevada. The Basin and Range extends eastward from there and is marked by relatively regularly-spaced, north-northeasterly striking normal faults. The Center for Neotectonic Studies at the University of Nevada-Reno has been conducting Quaternary mapping and paleoseismic trenching studies of active faults across the region to elucidate the sense, rate, style and pattern of fault deformation and the recurrence characteristics of earthquakes that occurred during the Late Pleistocene. In this talk I combine these observations with published geodetic measurements of ongoing elastic strain accumulation and geological documentation of longer-term, cumulative fault offsets. These studies address the relationship of strain accumulation to strain release over different time scales and put forth the idea that the Walker Lane fault system is analogous to an earlier stage in the structural development of the San Andreas--when the system was transtensional and before sufficient slip accumulated to yield the now throughgoing, San Andreas fault.