Spring 2021
Fridays at 12:00 PM
April 2, 2021
Speaker: Léo Martire, JPL
Title: Characterisation of Infrasound in the Coupled Solid-Atmosphere System: Numerical Modelling, Terrestrial and Planetary Applications
Abstract: This presentation focuses on the mechanical coupling between a planet’s solid body and its atmosphere. We study natural and anthropogenic geophysical events under the scope of seismic waves and infrasound. These phenomena are keys to uncover the atmospheric structure of Earth, the interior of Venus, and Mars as a whole. Acoustic and seismic waveforms contain valuable information, about both the source event and the propagation medium. Our work is two-fold. Firstly, we develop a numerical simulation software for the coupled ground-atmosphere system. We rely on the linearised Navier-Stokes equations to model the atmosphere, and on visco-elastodynamics to model the sub-surface. We employ a discontinuous spectral finite elements method, allowing the simulation of full waveforms. The implementation is validated using two techniques: analytical and manufactured solutions. Our software can model all types of air-ground couplings, and accurately accounts for acoustic and seismic wave propagation. Complex topographies can be used, as well as range-dependant atmospheric models. As a result, it is particularly well suited to study most geophysical phenomena in planetary atmospheres. Example events include seismic waves, microbaroms, underground and overground explosions, or gravity waves. Secondly, we study numerous application cases related to the aforementioned planetary science objectives. With the exploration of Venus’ interior in mind, we conduct terrestrial experiments to study seismically-induced infrasound, and involve balloon-borne instruments. We show that it is possible to infer the properties and structure of the sub-surface from these infrasonic waves. These instrumented balloons also render the localisation of ground events possible, which is crucial both for planetary exploration and for the airborne monitoring of the Earth. Finally, we demonstrate that the Martian atmosphere features infrasound, establishing for the first time the existence of infrasound on another planet. This is achieved thanks to InSight’s seismometer SEIS, able to measure the faint ground motion caused by passing airwaves.
April 16, 2021
Speaker: Clara Maurel, MIT
Title: Paleomagnetic evidence for partial differentiation of planetesimals and long-lived dynamo activity
Abstract: Most meteorites are remnant pieces of planetesimals, the first 1- to 1000-km planetary bodies to form in the solar system. Meteorites are divided into two principal groupings: chondrites (unmelted accretional aggregates) and achondrites (products of planetary melting). This division has commonly been interpreted as evidence that planetesimals either never melted or otherwise melted throughout their entire interiors—a view that is challenged by the idea that some planetesimals were only partially differentiated, with both chondritic and achondritic constituent materials. Understanding the phenomenon of partial differentiation can place important constraints on the timing and mechanism of planetesimal formation and evolution. However, little remains known about the natures and structures of these objects, partly because none have so far been identified in the asteroid population. The IIE iron meteorites contain both achondritic and chondritic silicate inclusions of common origin and are proposed to come from a partially-differentiated body. Here, I will talk about paleomagnetic measurements we conducted on three IIE irons to search for evidence of a molten metallic core in order to constrain the internal structure of the IIE parent planetesimal. We find that the meteorites experienced a magnetic field most likely powered by a core dynamo, implying that metallic core, melted silicates and chondritic material coexisted on the IIE parent body. Combining these measurements with geochronometry data, we constrain the timing of the IIE dynamo, which points towards a structure with a substantial metallic core overlain by achondritic and chondritic silicates. This result further challenges the topology of existing meteorite classification schemes, and provides some constraints on planetary accretion and differentiation mechanisms allowing for the coexistence of melted and unmelted material on the same body.
April 23, 2021
Speaker: Evan Bjonnes, Brown University
Title: Understanding the Ice Shells of Ganymede and Callisto through Their Impact Crater Records
Abstract: Both Ganymede and Callisto, Jupiter’s two largest moons, host numerous impact craters and basins on their ice shells. Although it is not yet possible to directly probe them, the wide ranges of impact crater sizes provide a possible avenue through which we can begin to understand the internal structures of these ice shells surrounding these moons. Here I will present findings from iSALE models looking at the development of complex craters and multiring basins and the implications for rheologic structure within Ganymede and Callisto’s ice shells. Icy complex craters show an inflection point in their depth-diameter measurements, which we show to be due to a transition from colder, conductive ice at the surface to warmer, convective ice in the subsurface. The depth of this transition indicates that the upper conductive portion of the ice shell has a conductive thermal gradient of approximately 10 K/km. We also set out to test if the onset of multiring basin morphologies are sensitive to the total thickness of the ice shell. Preliminary results show that thinner ice shells and higher thermal conditions—both conductive thermal gradient and convective ice temperature—facilitate multiring basin formation.
April 30, 2021
Speaker: Ilya Zaliapin, University of Nevada, Reno
Title: Localization of background seismicity: Estimation and application to tracking preparation of large earthquakes
Abstract: Progressive localization of deformation is a basic mechanical process that produces simultaneously reduced strength and increasing strain in a deforming rock volume. The localization framework describes the progressive evolution of deformation from distributed failures in a rock volume to localized shear zones, culminating in generation of primary slip zones and large earthquakes [2]. This framework considers the development of conditions during deformation in a volume (rather than on pre-existing faults or sets of faults) that allow an appropriate “trigger” to produce large ruptures. Recent observations provide new promising perspectives related to observable localization signals preceding large earthquakes. This talk discusses estimation of localization using earthquake catalogs and its applications to tracking preparation processes of large earthquakes [1]. The estimation methodology is based on comparing “spikiness” of spatial measures using the Relative Operating Characteristic (ROC) approach. The comparison is done both between space distributions of observed background events within distinct time windows (relative localization), and between an observed space distribution and a uniform measure with the same support (absolute localization). We also discuss a nearest-neighbor methodology for earthquake delcustering, which is an essential component of our (and many other) analyses of earthquake catalogs [3]. The results reveal generation of earthquake-induced rock damage on a decadal timescale around eventual rupture zones, and progressive localization of background seismicity on a 2-3 yr timescale before several M > 7 earthquakes in southern and Baja California, and M7.9 events in Alaska. This is followed by coalescence of earthquakes into growing clusters that precede the mainshocks. Corresponding analysis around the 2004 M6 Parkfield earthquake in the creeping section of the San Andreas fault shows opposite tendencies to those associated with the large seismogenic faults. The results are consistent with observations from laboratory experiments and physics-based models with heterogeneous materials not dominated by a pre-existing failure zone. This is a joint work with Yehuda Ben-Zion, USC. [1] Ben-Zion, Y., & Zaliapin, I. (2020) Localization and coalescence of seismicity before large earthquakes. Geophysical Journal International, 223(1), 561–583. [2] Kato, A., & Y. Ben-Zion (2021) The generation of large earthquakes. Nature Reviews Earth & Environment, 2, 26–39, https://doi.org/10.1038/s43017-020-00108-w. [3] Zaliapin, I. and Y. Ben-Zion (2020) Earthquake declustering using the nearest-neighbor approach in space-time-magnitude domain, J. Geophys. Res., 125, e2018JB017120, doi: 10.1029/2018JB017120.
May 7, 2021
Speaker: Stefania Soldini, University of Liverpool
Title: The Effect of "Mascons" Interior Mass Distribution onto the Dynamic Environment Around Asteroids
Abstract: A sphere cluster (SPH-Mas) based gravity model allows a semi-analytic expression of the linearised equations around irregular-shaped celestial bodies Equilibrium Points (EPs) and an easy method for searching families of periodic orbits around them. The SPH-Mas model can retrieve the same dynamical objects of the shape model when the spheres share a uniform density distribution. The dynamics are solved for a rotating asteroid-fixed frame of angular velocity equivalent to the asteroid spin axis. The SPH-Mas model has the advantage to define the same particles mesh distribution for both astrophysical and astrodynamics tools. To the core of this study, we aim to gain a general insight on the dynamics around Didymos and other minor celestial bodies in term of stable and unstable orbits for studying the dynamics of ejecta particles.
May 14, 2021
Speaker: Michelle Thompson, Purdue University
Title:
May 21, 2021
Speaker: Martijn van den Ende, University of Central Arkansas
Title: What’s that wiggle? The challenges (and solutions) of using fibre-optic cables as seismological antennas
May 28, 2021
Speaker: Wenyuan Fan, UC San Diego
Title:
June 4, 2021
Speaker: Alexis Ault, Utah State University
Title: Hematite fault rock microtextures and thermochronometry inform earthquake processes
Abstract: Hematite is a ubiquitous secondary phase in fault zones. Iron is the 4th most abundant element in Earth’s crust and hematite grows in a range of shallow fault rocks due to the redox potential of diverse Fe-bearing minerals. Hematite in fault rocks exhibits grain morphologies and nano- to microscale textures that preserve evidence for different fault slip rates and deformation conditions. Hematite is also amenable to (U-Th)/He thermochronometry, a powerful tool that, when combined with textural observations, informs the timing and temperatures (and thus rates) of fault slip. In this talk, I share examples of how we fuse textural observations and thermochronometry data patterns from natural and experimental fault surfaces to inform earthquake mechanics on thin slip surfaces.
