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February 3, 2017
3:00pm in PLS 1140
Tom Pratt from USGS-Reston
The influence of eastern U.S. Atlantic Coastal Plain strata on earthquake ground motions, and damage in Washington, DC, during the 2011 Mineral, Virginia, Earthquake

Abstract: During the 2011 Mw5.8 Mineral, VA earthquake, many buildings in Washington, DC, including national landmarks like the Washington National Cathedral, the Smithsonian “Castle,” and the Washington Monument, sustained damage despite being 130 km from the epicenter. The surprisingly large amount of damage from weak ground motions raises questions of how much the southeast-thickening sedimentary strata of the Atlantic Coastal Plain (ACP) strata beneath the city amplify and trap seismic energy. Partially consolidated ACP marine sedimentary strata overlie crystalline or indurated sedimentary rocks throughout coastal regions of the eastern U.S., extending more than 200 km inland from the coast. The strata taper landward from as much as 1 km near the coast to pinching out in the Washington, DC area. Shallow sedimentary strata are known to amplify earthquake ground motions due to low seismic impedance and strong reverberations. Between November 2014 and August 2015, we used 27 seismometers to measure ground motions across Washington, DC, using four sites on crystalline rocks as reference sites. We also used data from continental-scale seismic experiments that span the ACP to examine the influence of the broader ACP strata on earthquake ground motions. Recordings of teleseisms and regional earthquakes provided data with sufficiently high signal-to-noise for computing amplitude ratios relative to the bedrock sites. Amplifications of 10 or greater are found in the Washington, DC area due to the ACP strata, with the peak amplifications occurring near the estimated resonant frequencies of buildings throughout the city. Amplitudes decrease as the strata thicken, but even coastal sites on 600 m of ACP strata show amplification factors as great as 5. We use the frequency of the resonance peaks to invert for an average velocity function within the ACP strata. This work indicates that amplification of short-period ground motions by thin ACP strata contributed to the damage in Washington, DC, during the 2011 earthquake, and documents longer-period amplifications that could affect larger structures beneath coastal regions of the eastern U.S. during earthquakes.

February 10, 2017
3:00pm in PLS 1140
Yingwei Fei from Geophysical Laboratory
Recovery of an oxidized majorite inclusion from Earth's deep mantle

Abstract: Minerals recovered from the deep mantle provide a rare glimpse into deep Earth processes. In this study I describe the first discovery of ferric iron-rich majoritic garnet found as inclusions in a garnet host within an eclogite xenolith from the Trans-North China Orogen formed at the end of oceanic subduction during the Paleoproterozoic (~1.85 Ga). The ferric iron-rich majoritic garnet inclusions show a deep origin, formed at least at a depth of 380 km. The host garnet and majorite inclusion record two distinctive depths in the asthenosphere that has important implications for the ancient supercontinent assembly and breakup. The preservation of the ferric iron-rich majorite, particularly in a garnet host, provides a rare opportunity to understand the mantle chemistry and dynamic process.

February 17, 2017
3:00pm in PLS 1140
Brent Grocholski from AAAS / Science Magazine
Scientific Publishing from the Inside Out

Abstract: Effective scientific communication requires both good writing and understanding the publication process. We will discuss a wide range of topics from the perspective of a Science editor that touch on all aspects of the writing, peer review, and publication process. This includes insights into what make for a successful submission to high profile journals like Science.

Additional Information: Facebook Live video of this presentation is available here.

February 24, 2017
3:00pm in PLS 1140
Kevin Lewis from Johns Hopkins University
Exploration of Gale Crater Mars with the Curiosity Mars Rover

Abstract: The Curiosity Mars rover has been exploring its landing site at Gale crater since 2012. Over this time it has begun to climb the lower slopes of Mount Sharp, a 5 kilometer high mound of sedimentary rock located within the crater. In this talk, we will combine orbital and rover-based geological and geophysical tools to understand the formation of Mount Sharp, with potential implications for other crater-hosted mounds found commonly in the Martian equatorial region. The ultimate goal of this work, and one of the key objectives of the Curiosity mission, is to understand the climate information recorded in the strata of Mount sharp exposed along the rover traverse.

Additional Information: Facebook Live video of this presentation is available here.

March 17, 2017
3:00pm in PLS 1140
Luciana Astiz from NSF
USArray Data quality: A look at seismicity across the lower 48 and preliminary results on Earth Tide observations
March 31, 2017
3:00pm in PLS 1140
Daniel Viete from Johns Hopkins University
Metamorphism can record individual earthquake events in the subduction setting: evidence from the Franciscan Complex, California

Abstract: Rhythmic major-element zoning has been documented in garnets from high pressure/low temperature (HP/LT) lenses within a number of worldwide subduction mélanges (e.g. California, Chinese Tianshan, Cuba, Greek Cyclades, Guatemala, Japan, Venezuela). These features reflect some fundamental process(es) in the subduction setting. In this talk, conditions of rhythmic zoning acquirement in HP/LT garnets of the Franciscan Complex, California are investigated by use of thermodynamic modeling of phase equilibria, and Raman and synchrotron Fourier transform infrared (FTIR) microspectroscopy.

Hornblende, omphacite and zoisite in the Franciscan rocks are also complexly zoned in major elements. Modeling of phase equilibria shows that modal contours for garnet, amphibole and zoisite are gently dipping in the P–T region that corresponds to the peak-metamorphic mineral assemblage. Metamorphic assemblage diagrams suggest that hydration/dehydration reactions involving garnet <—> zoisite (which also involve amphibole exchange or omphacite for glaucophane) are incredibly sensitive to changes in P (e.g. 5–10 vol.% absolute gain/loss of garnet for ΔP = 250 MPa). Major-element zoning in the Franciscan minerals may record repeated growth–partial dissolution cycles in response to P fluctuations in the subduction setting.

Quartz-in-garnet Raman barometry reveals varying P—on the order of 100–350 MPa, over radial distances of 10s of µm—in association with the major-element zoning in the Franciscan garnets. Results from synchrotron FTIR microspectroscopy demonstrate association between zone overgrowths and OH in garnet (a proxy for crystallization pressure in pyrope garnet). The microspectroscopy results confirm changes in P attended development of the rhythmic garnet zoning.

Steep compositional gradients defining the rhythmic major-element zoning limit time scales at peak T (and garnet growth–dissolution) conditions to < 1 Myr, requiring that individual growth–partial dissolution cycles were extremely brief. Overpressure on the order of 100s of MPa can develop by tectonic loading of the crust and is relieved with earthquake rupture. Seismic cycles represent ephemeral phenomena capable of accounting for development of rhythmic major-element zoning in HP/LT garnet, during subduction, as a result of fluctuations in P (and garnet stability). Metamorphic rocks may carry detailed records of the catastrophism that punctuates longer-term tectonometamorphic processes.

April 7, 2017
3:00pm in PLS 1140
Johnny Zhang from Scripps Institution of Oceanography
Fe-Ni-S-C Liquid in the Earth's Mantle

Abstract: Fe-Ni-S-C phases are accessory phases in the Earth’s mantle, but carry important geochemical and geophysical implications due to the contrasting physical and chemical properties between metallic and silicate phases. In the shallow mantle (<200 km), the metallic phase occurs as monosulfide solid solution (mss) or melt with near-monosulfide stoichiometries. To constrain the sulfide melt stability field and its Fe-Ni exchange with mantle silicate minerals, we performed experiments at comparable conditions (P, T, fO2) to Earth’s shallow mantle. In the deeper part of the upper mantle (200-410 km), the mantle become reduced, corresponding to an increase of metal activities in sulfide melt. To contain the composition of Fe-Ni-S melt and its storage of deep carbon, we performed experiments and thermodynamic calculations to show the evolution of Fe-Ni-S-C compositions and mantle silicates at deep upper mantle conditions. Based on the experimental and modeling exercise, further discussion will be made on the recent Fe-Ni-S-C liquid from deep diamonds (Smith et al. 2016). In the deepest part of Earth’ mantle (<2900 km), we propose that small quantities of Fe-Ni-S-C liquid is the cause for the two large low shear velocity provinces (LLSVPs). These Fe-Ni-S-C liquid is likely trapped during the crystallization of a dense basal magma ocean and therefore a potential carrier of primordial geochemical signature.

April 14, 2017
3:00pm in PLS 1140
Sabine Stanley from Johns Hopkins University
The Ancient Lunar Dynamo: How to Resolve the Intensity and Duration Conundrums

Abstract: Modern paleomagnetic analysis of Apollo lunar samples indicate that the Moon possessed a strong surface magnetic field with intensities in the range 10-100 microTeslas between at least 4.25 and 3.26 Ga. The paleofield intensity subsequently decreased to below ~4 microTeslas. A core dynamo seems to be the only plausible explanation for this long-lived field. However, any model of the lunar dynamo should be able to explain both the early high-field epoch as well as the rapid decline to a lower field epoch and the subsequent cessation of the dynamo. Typical dynamo power sources such as thermal or thermochemical convection appear able to reproduce field strengths in the later low field epoch, but do not appear to be able to explain the earlier high field epoch. Mechanical driving has therefore been suggested as an alternative source for the strong fields. Here we propose a modification of this mechanical dynamo in which we consider the role of a solid inner core that is gravitationally locked to a precessing mantle. We use numerical dynamo simulations and scaling laws to demonstrate that this scenario results in novel magnetic instabilities that can explain the intensity and longevity of the lunar dynamo during the high field epoch.

April 21, 2017
3:00pm in PLS 1140
Stephen Macko from University of Virginia
Deciphering the Diets of Modern and Ancient Humans using Stable Isotopes

Abstract: Fundamental to the understanding of human history is the ability to make interpretations based on artifacts and other remains which are used to gather information about an ancient population. Sequestered in the organic matrices of these remains can be information, for example, concerning incidence of disease, genetic defects and diet. Stable isotopes have long been used to interpret diet and trophic interactions in modern ecosystems. In fact, using stable isotopes to quantify the influence of corn on the modern American diet was accomplished this way in the recent Independent Film on agriculture sustainability, “King Corn”. In this talk, it is also suggested that the isotopic compositions of a commonly overlooked material, human hair, may represent an ideal tool to be used in addressing human diets of ancient civilizations.

Hair can be well-preserved and is amenable to isotopic analysis for distinguishing sources of nutrition. Based on this observation, we have isotopically characterized hair from both modern and ancient individuals. There is a wide diversity in carbon, nitrogen and sulfur isotope values owing, at least partially, to the levels of seafood, corn-fed beef and other grains in diet. Using these isotope tracers, new information regarding historical figures (George Washington, 1799 AD) to perhaps the most ancient of mummies, the Chinchorro of Chile (possibly more than 7000 BP) as well as the Moche of Peru (1500 BP) and the best preserved mummy, the Neolithic Ice Man of the Oetztaler Alps (5200 BP), have been deciphered. It appears that the often-overlooked hair in archaeological sites may represent a significant new approach for understanding ancient human communities and their environments.

April 28, 2017
3:00pm in PLS 1140
Lindy Elkins-Tanton from Arizona State University
The Psyche mission and early planetary formation

Abstract: In the first few million years of our solar system, planetesimals rapidly grew to tens to hundreds of kilometers in radius. At this size, heat from the decay of short-lived radioactive 26Al was trapped, melting their interiors. The molten interiors quickly differentiated: denser material settled to their centers, leaving lighter silicates to cool into thick mantles surrounding metal cores. Dense iron and nickel alloys were now imprisoned forever deep inside these orbiting planetoids.

The inner solar system was still a high-energy, crowded place; over the next few tens of millions of years, many planetesimals crossed paths catastrophically. Colliding worlds merged into even larger planets. In other cases, “hit and run” impacts caused one protoplanet to lose almost all its mantle material, leaving mostly core metal that re-formed into (16) Psyche. Psyche is the only core that humankind can see, and it will be the first metal world we have visited. The Psyche mission has been selected as the fourteenth in the NASA Discovery program. In this talk I will introduce what is known and what is hypothesized about Psyche, and talk how we have planned and won the opportunity to fly this mission.

The coordinator for the Colloquium Series is Dr. Nicholas Schmerr. You can contact him at