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ChimeIn Link for review paper(s) discussion

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ChimeIn Link for review paper(s) discussion

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How much viscosity variation is left to be explained by physical property variations?

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How much viscosity variation is left to be explained by physical property variations?

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If the viscosity can vary by ~2 orders of magnitude based on timescale, as suggested by some studies, how much is left for physical property variation (temperature, etc)? From some studies, it seems that there would hardly be any such variation due to that, as they explain almost all variations through timescale. What implications does this have for the attempt to scale viscosity from seismic velocity, and why/how to regions like the deforming western North America and stable eastern North America become different?

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To the experimental community: What aspects of rheology would you most like the geodynamics and geodesy communities to test?

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Question for Shawn from Jim Gaherty online

Any thoughts on the effects of focusing on attenuation estimates? Maybe important to consider in regions where q is high and vp is low (ie zone 1)?

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How can we make progress joining the dots between deformation mechanisms  observed in the lab (e.g. grain boundary sliding, back stress), properties observed from seismology (e.g. grain size), and how these are included in geodynamic models? Particularly when thinking about how to represent this complexity in (often) quite basic models.

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The word "effective" has been used a lot this week. What's "effective" to you? A specific rock that dominates deformation dynamics? Some average of different rock types? Another model of an effective medium? Specifying the meaning of "effective" can help with cross-disciplinary constraints on a given parameter.

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1. Based on what we know right now, on what timescales is transient theology important (1 year? 10 years? 100 years?)? And how does it depend on the depth range that a particular observed process is sensitive to?

2. This week we heard about progress from different experimental labs. In some cases there seem to be conflicting results (e.g., whether adding opx to olivine affects Q), and in other cases, it would be interesting and helpful to better understand whether different experimental results are or are not consistent with each other (e.g., the Yamauchi and Faul experiments).

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Please post questions here (by 5:30 pm Thursday) that you'd like the panel to discuss!

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Thanks for the excellent talk! Visually, it looked like the agreement in the lower mantle between high Vs and high attenuation was stronger than the association of low Vs and low attenuation in the LLSVPs. It is clear that the LLSVPs are not characterized by high attenuation, but it seemed like they are some combination of low and average attenuation. Am I over-interpreting the lateral variations in the lower mantle, or is it true that the strongest relationship is the high Vs - high attenuation association.

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Hi! Just checking that you are happy with me also saying something about LLSVP density and what that says about viscosity when fitting the geoid?

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Experimentalists: what do you and don't you agree on?

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Experimentalists: what do you and don't you agree on?

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We have seen some really wonderful talks today. As a non-specialist, it seems to me that the speakers concluded radically different things about some fundamental issues. Would it be possible to get votes on specific assertions?

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Are solidus, melting temperature, and eutectic temperature the same? I feel the usage of these terms at this workshop is somehow different from what Google/Wikipedia tells me.

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You attribute the greater dissipation for San Carlos vs solgel to the presence of melt. San Carlos shows higher dissipation even at 950C. Is there melt present even at low T of 950?

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Thanks for the great talks about all the new progress. But at the end, I wonder whether your models agree better or worse with each other?

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Your new results are very different, so what does this mean for seismologists who have been using your models to interpret their attenuation observations? Can we still use them?

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What’s going on physically, at grain scale to explain the oxygen fugacity dependence of attenuation?

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1. Do you see a relationship with homologous temperature like in the borneol experiments?

2. Have you considered extrapolating to upper mantle conditions by Maxwell freq normalization rather than just extrapolating in grain size? Does it change the conclusions?

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All Q&A questions will eventually be moved here. you can still comment/interact with them though!

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One the stress is reduced and from that time onward, does the integral of stress over the sample stay constant? It seemed that it didnt.. and I guess I am trying to understand where this ``irreversibility'' comes from?

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Thanks for that beautiful talk! I was wondering about pressure effects, after you talked about the effect of temperature on the dislocation loop morphology, and mentioned an effective stress (\sigma - \sigma_b ? ) (an analogy to poroelasticity?). Do you have a feeling for how such effects on morhpology map into a macroscopic, effective activation volume of the back stress model, and backstress-driven transient creep? What might the (uncertainty in the) pressure effect be in that beautiful plot of transient viscosity as a function of depth in response to a stress pulse. What is competition between pressure and temperature in the transient, when thinking about the morphology of dislocation interactions ?

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1. Related to the real Earth example you showed on the last slide, can you think of a location or scenario on Earth where you might be able to test the predictions.

2. How much does the timescale over which the applied stress is applied matter, e.g., instantaneously applied versus gradually applied?

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I was amazed to see that (if I understood you) there are near-GPa internal stresses within grains that oscillate in sign over distances of < 50 microns. How/Does this impact diffusion mechanisms within the same grains? How is this complicated and extreme stress pattern accounted for in simple deformation laws.

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This lateral change in the southern extent of the ice sheet at the LGM coincides with the western boundary of the Superior Craton. Is that just a coincidence?

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A question for GIA modelers and geodesists

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A question for GIA modelers and geodesists

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I would argue even the best global tomography models miss a lot of short-wavelength heterogeneity in the shallow upper mantle. What is the shortest wavelength or spatial length scale that is important to understand the surface response to changes in loading?

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...on the state of attenuation knowledge now. The paper can summarize how we study attenuation from various methods, and differences in the methodologies (assumptions, limitations, etc.), as well as expected attenuation values in different geologic/tectonic settings. This can be an introductory type paper for the broader community, while also demonstrating where further developments are needed.

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Most of our reference velocity models/dispersion curves/etc. are based on data from land seismometers. Could we increase the local accuracy of velocity models using the growing ocean bottom seismometer data set (direct seismic observations), hopefully increasing the accuracy of parameters deduced using these observations?

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I am posting this for Roland: could you speculate on the role of magma/mushy crust in Ethiopia vs a relative lack of this magmatically-weakened crust in interior Saudi Arabia? I would think that the effect of this crustal lateral heterogeneity might affect the inferences of asthenosphere heterogeneity.

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When you link seismic properties to rheology, what grain size and solidus do you use?

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A related sub-topic from Zach's project on frequency-dependent response vs lateral heterogeneity: In magmatic systems, we should consider the possibility that the effective lateral rheologic heterogeneity itself is time-dependent due to the transport and redistribution of magma/fluids (e.g. intrusion, degassing). Unlike post-seismic relaxation, where the impulsive stress perturbation and transient response are clearer, here the perturbations may themselves be protracted. How do we disentangle timescales of loading and the frequency-dependent transient (poroviscoelastic? plastic?) responses from the timescales at which the lateral heterogeneity itself evolves -- feedbacks lead to non-linear

coupling?

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For complex viscoelastic models that have one or more kelvin elements, my impression is that the starting value of the strain of the kelvin elements is very important. That is, there is a history term that needs to be tracked. How does that history effect get accounted for in these large-scale models?

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Is the lack of lower crystal involvement at Hector Mine a feature of the lower crust in general? Or something specific to this region?

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propose a group project or activity! vote or comment on existing proposoals.

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Group topic - how do we discriminate frequency dependence from spatial heterogeneity?

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Group topic - how do we discriminate frequency dependence from spatial heterogeneity?

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Different methods, especially seismic, have some form of 1:1 relationship between spatial aperture of sensitivity and frequency. Different observation frequencies thus sense different volumes of the planet, potentially with different average thermodynamic states. But Earth materials might/do also have frequency dependence to their transient deformation responses. How do we disentangle these effects?

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Click the "Post" button to ask a question during a talk. Posts are sorted by date/time with most recent first. after some time we'll move posts over to the "archived" column to keep the Q&A column reasonable length.