Dr. Judith Chester

Presentation: Structural-Petrologic Characterization of the San Andreas Fault Zone from SAFOD

The presentation will outline the drilling and sampling activities at the EarthScope San Andreas Fault Observatory at Depth (SAFOD), and illustrate how we are using observations from the drillhole to interpret: 1) the development of fault zone architecture, 2) the occurrence of seismic and aseismic deformation, 3) the absolute strength of the fault, and 4) the energy budget of earthquakes. Earthquakes are one of our greatest natural hazards, and understanding the fundamental processes of earthquake faulting is one of the most challenging problems in the Earth sciences. Understanding the physics of earthquake rupture requires the study of faulting over a wide range of spatial and temporal scales. The macroscopic characteristics of earthquake rupture nucleation, propagation, and arrest depend in part on processes operating at the mesoscopic and microscopic scales. Geologic observations of active and exhumed faults, rock deformation experiments, geophysical observations of seismic sources, and theoretical models have contributed to our current understanding of these processes. Research has been impeded, however, by our inability to observe active fault zones directly, and to sample earthquake source regions at depth. The overall scientific objective of SAFOD is to drill, sample and monitor the San Andreas Fault at seismogenic depths to understand the physical and chemical processes of deformation and earthquake generation within an active, plate-bounding fault. Collection of rock samples from the active slipping zones of the San Andreas Fault at depth is a major achievement and provides a wonderful opportunity to test hypotheses and constrain earthquake faulting models. Rock samples taken from SAFOD include a complete suite of cuttings from the borehole, tens of small, sidewall core samples from within the fault zone proper, and six continuous, large-diameter spot cores from different locations across the San Andreas fault zone, two of which cross the actively slipping segments identified by geologic and down-hole geophysical observations. As part of our EarthScope research, we are mapping the mesoscale structure and lithology of the cores, and performing optical and electron microscopy, XRF, XRD, and stable isotope analyses to characterize the deformation mechanisms, mineral reactions, and fluid-rock reactions that are important to seismic and aseismic slip in the San Andreas fault zone.

Dr. Matthew Fouch

Presentation: EarthScope explores the depths of western North America

EarthScope’s USArray and other geophysical studies reveal the structure of the crust and mantle of the western United States. Several big-picture questions for the region, including the role of the mantle in crustal deformation, the structure of the lithosphere, and the coupling/decoupling of lithosphere and sublithospheric mantle, will be discussed in the presentation. Case studies focusing on the convergent margin of the Pacific Northwest, the Columbia Plateau and High Lava Plains, Basin and Range Province, and Colorado Plateau will be shown to highlight recent work investigating the structure and deformation of the western edge of the North American continent.

Dr. Tim Melbourne

Presentation: EarthScope imaging of Cascadia megathrust slow-slip events

Recent increases in GPS and seismic instrumentation, including the EarthScope Plate Boundary Observatory (PBO) and USArray networks, reveal new observations about episodic tremor and slip (ETS) in the Cascadia Subduction Zone. Together these data suggest that Cascadia, and presumably other subduction zone ETS events, are comprised largely of a form of microseismic clustering unique to subduction zones and in fact have little aseismic creep associated with them. Thirty-six isolated Cascadia ETS events have been observed since 1997, including two in 2007. ETS events occur throughout the Cascadia forearc, from northern California to southwestern British Columbia. Events located in well-instrumented regions can be tracked as they migrate laterally from north to south along the plate boundary, but increasing station density has resolved many smaller transients that could not previously be identified. There is a well-resolved 14-month average recurrence interval for ETS events in northern Washington and southwestern British Columbia based on five events since 1997. Along central Vancouver Island, a host of smaller events distinct from the 14-month recurrence interval occur in a nonperiodic fashion. Sporadic smaller events also appear throughout the subduction zone to the south, including within the region known for the 14-month periodicity. In southern Washington, some of the largest transient displacements are observed, but lack any obvious periodicity in their recurrence. Observed recurrence intervals are 18 months along central Oregon and 11 months in northern California. Analysis of GPS data for the 14 largest ETS events suggests equivalent moment magnitudes ranging from 6.3 (smallest resolvable with GPS) to 6.8; equivalent slip is typically 2-3 cm. In marked contrast to other subduction zones, the largest spatial extent of the events resolved to date is just under 500 km along strike, and maximum duration is seven weeks. Averaged over many ETS events, the upper limit of transient slip in the vicinity of Seattle lies just west of the heavily urbanized Puget Sound region, suggesting that the lower limit of megathrust seismic rupture may extend much closer to this area than previously thought. From 2005 to 2007, a systematic quantification of seismic tremor observed both during and outside of ETS events has demonstrated a linear relationship between tremor duration and inverted GPS equivalent moment. Moreover, no GPS transients during this time period are observed in the absence of tremor. This suggests that the GPS-measured transient deformation may be the integrated near-field offsets resulting from many small, discrete seismic slip events (meaning aseismic, transient creep is not needed to explain the GPS signal). This hypothesis is supported from inferences about tremor source characteristics drawn from borehole seismic spectral features and tremor locations determined from surface arrays. The observed quantity, timing and location of tremor thus largely explain the ETS signal as observed on surface and borehole strain- and tiltmeter arrays.

Dr. Suzan van der Lee

Presentation: EarthScope and the Cenozoic history of the western US

Like surface rocks have recorded geologic history, important clues on geologic history can also be found in the upper mantle. Seismic tomography uses seismic waves generated by earthquakes and recorded at arrays of seismic stations to image the structure of the upper mantle. A new, dense, moving array of seismometers in the United States offers unprecedented resolving power for such structures. This array is called USArray and is part of the national Earthscope program. In this talk I will review some of what we currently know about the structure of the North-American upper mantle. I will address the correlation between uppermost mantle structure and geology, in particular the contrast between the rigid, cool North-American Craton and the easily deformable, hot upper mantle beneath the western United States. The bottom half of the upper mantle here appears to contain subducted fragments from the former Farallon Plate, which could help constrain Cenozoic history. Finally I will speculate about what upper-mantle structure might be telling us about the geologic future of the United States.

Dr. George Zandt

Presentation: Insights on Lithospheric Foundering from the Sierra Nevada EarthScope Project (SNEP)

Interdisciplinary studies in eastern California have documented ongoing removal (delamination) of the dense residual root from beneath the southern Sierra Nevada Batholith. Many questions remain concerning the timing, spatial extent, mechanism, and consequences of the lithospheric foundering process. The Sierra Nevada EarthScope Project (SNEP) is investigating these questions with a two-phase (two-year) seismic deployment of 46 broadband Flex-Array stations embedded in the existing stations of the USArray transportable network. During the two phases, approximately 80 sites have been occupied with ~25 km spacing from the northern edge of Kings Canyon north to Honey Lake and from the edge of the Central Valley into the Great Basin. The presentation focuses on the most recent seismic results that image the 3D lithospheric structure beneath the central and northern Sierra Nevada. Crustal thickness, reflectivity (layering), Moho “brightness”, and mantle wavespeeds vary dramatically across and along the length of the Sierra Nevada, but these changes are not simply correlated with present-day topography or surface geology. The talk explores the hypothesis that the removal process progresses intermittently northward beneath the region, changing the crust and upper mantle, and partially controlling the volcanic and uplift history of the range. Data from the recently-completed Phase 2 deployment, to nearly the northern limit of the exposed Sierra Nevada Batholith, will constrain the northern extent of the affected region and investigate the character of the batholith where root removal may not have yet occurred or is in an embryonic stage. (Note: This work represents collaboration between G. Zandt of the University of Arizona, H. Gilbert of Purdue University, T. Owens of the University of South Carolina, and C. Jones of the University of Colorado, among others. The interpretations in this abstract are those of G. Zandt and don’t necessarily reflect the views of all collaborators).