Webinars - Detail

GAGE/SAGE Plenary Session: Behavior at and coupling across key Earth interfaces
Heather Ford (UC-Riverside) and Diego Melgar (University of Oregon)


Plenary Abstract: This session is aimed at understanding the structure and processes associated with interfaces and boundaries within the Earth system. Examples include the boundary between the solid Earth and its fluid envelopes, the boundary between the solid mantle and the liquid outer core, the land/ocean interface at continental margins, and Earth's permeable near-surface boundary layer known as the critical zone. It will include contributions that highlight new data sets and observation strategies that target these interfaces, those that reveal new understanding of the processes operating at them, and studies that explore the ways that different regions of the Earth interact and how different components of the Earth system are coupled across interfaces.
Dr. Ford's Abstract: The lithosphere-asthenosphere boundary is a rheological boundary that connects the Earth’s rigid lithosphere to the underlying convecting mantle asthenosphere. Better understanding the range of physical and rheological properties of the lithosphere-asthenosphere boundary has important implications for plate tectonics and mantle dynamics, yet considerable uncertainty in our understanding of this boundary remains. The boundary is commonly defined as the depth at which heat transfer changes from conductive (lithosphere) to convective (asthenosphere). However, other factors including chemical composition, water content, grain size and melt are thought to play a role. One important characteristic of the lithosphere-asthenosphere boundary (among others) is the well-established correlation between lithospheric age and boundary depth, with older lithosphere typically corresponding to a deeper lithosphere-asthenosphere boundary. Where well-resolved deviations from this correlation exist, tectonic and/or dynamic processes may be invoked in order to explain these differences.

In this webinar we will begin by providing a framework for imaging and analyzing the characteristics of the lithosphere-asthenosphere boundary. We provide this framework by first presenting a Sp receiver function case study in which pronounced lateral variations in seismic properties at the lithosphere-asthenosphere boundary are observed to be coincident with the surface expression of the Pacific-North American plate boundary in California. We then report new evidence of a well-resolved step in lithospheric thickness coincident with the surface expression of the Taconic/Gander boundary in eastern North America. Here, our work suggests that the present-day lithosphere-asthenosphere boundary is inherited from the collision of the Laurentia and Appalachian terranes, and that subsequent tectonism and/or thermal evolution have not overprinted this structure. Such a conclusion runs counter to our current understanding of the thermal evolution of the lithosphere-asthenosphere system, as described in the first paragraph of this abstract, and future discussion and research is needed.

Dr. Melgar's Abstract: It is widely accepted that the degree to which a fault is coupled, or locked, should influence how hazardous it is perceived to be. As a result, over the last decades, observations from space geodesy, and in particular from Global Navigation Satellite Systems (GNSS) have been used to measure interseismic velocities and infer locking on faults. For subduction megathrusts, where large portions of the fault are offshore, there can be significant ambiguity in whether this portion of the fault is locked or not. This is due to the resolution of onshore measurements decaying rapidly with distance. Seafloor geodesy through the GPS-A technique continues to gain traction and GPS-A sites are being deployed at many subduction zones worldwide. As the networks slowly grow this promises to increase resolution and sharpen the picture of where and to what degree megathrusts are locked offshore. However, once the locking is known, how should we use the information in a quantitative sense to inform hazards assessments? In this talk we will show a new approach which uses locking models as pre-conditions to efficiently generate thousands of stochastic rupture scenarios which can in turn be utilized to simulate tsunamis or strong shaking at locations of interest. As a demonstration of the methodology we will focus on the Cascadia subduction zone and show how assuming different locking models leads to very different probabilistic tsunami hazard assessments. We will also show the comparison to when no locking model is used to precondition the simulated ruptures and conclude that knowledge of coupling at the fault is a first order control in tsunami hazard assessment.


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