Browsing by Author "Schmerr, Nicholas"
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Item Constraints on Seismic Anisotropy in the Mantle Transition Zone from Long-Period SS Precursors(2019) Huang, Quancheng; Schmerr, Nicholas; Waszek, Lauren; Beghein, Caroline; Schmerr, NicholasThe mantle transition zone (MTZ) of Earth is demarcated by solid-to-solid phase changes of the mineral olivine that produce seismic discontinuities at 410 and 660 km depths. Mineral physics experiments predict that wadsleyite can have strong single-crystal anisotropy at the pressure and temperature conditions of the MTZ. Thus, significant seismic anisotropy is possible in the upper MTZ where lattice preferred orientation (LPO) of wadsleyite is produced by mantle flow. Here, we use a body wave method, SS precursors, to study the topography change and seismic anisotropy near the MTZ discontinuities. We stack the data to explore the azimuthal dependence of travel times and amplitudes of SS precursors, and constrain the azimuthal anisotropy in the MTZ. Beneath the central Pacific, we find evidence for ~4% anisotropy with a SE fast direction in the upper mantle, and no significant anisotropy in the MTZ. In subduction zones, we observe ~4% anisotropy with a trench-parallel fast direction in the upper mantle, and ~3% anisotropy with a trench-perpendicular fast direction in the MTZ. The transition of fast directions indicates that the LPO of wadsleyite induced by MTZ flow is organized separately from the flow in the upper mantle. Global azimuthal stacking reveals ~1% azimuthal anisotropy in the upper mantle, but negligible anisotropy (< 1%) in the MTZ. Finally, we correct for the upper mantle and MTZ anisotropy structures to obtain a new MTZ topography model. The anisotropy correction produces ±3 km difference, and therefore has minor overall effects on global MTZ topography.Item The Deployment of the Seismometer to Investigate Ice and Ocean Structure (SIIOS) in Northwest Greenland: An experiment to study icy ocean world seismic deployments(2020) Marusiak, Angela; Schmerr, Nicholas; DellaGiustina, Daniella; Avenson, Brad; Bailey, S Hop; Bray, Veronica; Brodbeck, Juliette; Carr, Chris; Dahl, Peter; Habib, Namrah; Pettit, Erin; Wagner, Natalie; Weber, ReneeIn anticipation of future spacecraft missions to icy ocean worlds, the Seismometer to Investigate Ice and Ocean Structure (SIIOS) was funded by NASA to prepare for seismologic investigations of these worlds. During the summer of 2018, the SIIOS team deployed a seismic experiment on the Greenland Ice Sheet situated approximately 80 km north of Qaanaaq, Greenland. The deployment included one Trillium 120 s Posthole (TPH) broadband seismometer, thirteen Silicon Audio flight-candidate seismometers, five Sercel L28 4.5 Hz geophones, and one HTI 60-min hydrophone. Seismometers were buried 1 m deep in the firn in a cross-shaped array centered on a co-located TPH, hydrophone, and Silicon Audio instrument. One part of the array consisted of Silicon Audio and Sercel Geophones situated 1 m from the center of the array in the ordinal directions. A second set of four Silicon Audio instruments situated 1 km from the center of the array, in the cardinal directions. A mock-lander spacecraft was placed at the array center and instrumented with four Silicon Audio seismometers. We performed an active-source experiment and a passive-listening experiment that lasted for approximately 12 days. The active-source experiment consisted of 9-12 sledgehammer strikes to an aluminum plate at ten separate locations up to 100 m from the array center. The passive experiment recorded the ice-sheet ambient background noise, as well as local and regional events. Both datasets will be used to quantify differences in spacecraft instrumentation deployment strategies and for evaluating science capabilities for single-station and small-aperture seismic arrays in future geophysical missions. Our initial results indicate that the flight-candidate seismometer performs comparably to the TPH at frequencies above 0.1 Hz and that instruments coupled to the mock-lander perform comparably to ground-based instrumentation in the frequency band of 0.01-10 Hz. For future icy ocean world missions, a deck-coupled seismometer would perform similarly to a ground-based deployment.Item Field Mapping and Modeling of Terrestrial Lava Tube Magnetic Anomalies as an Analog for Lunar Lava Tube Exploration and Prospecting(Wiley, 2022-05-19) Bell, Ernest Jr.; Schmerr, Nicholas; Young, Kelsey; Esmaeili, Sanaz; Garry, W. Brent; Jazayeri, Sajad; Kruse, Sarah; Richardson, Jacob; Whelley, PatrickLava tubes are a commonplace feature on the terrestrial planets, and knowledge of tube size and location informs lava flow processes. Future exploration of lava tubes on the Moon can provide access to geologic environments that likely remain unaltered from their emplacement billions of years ago. Lunar lava tubes may also provide astronauts protection from thermal extremes, meteoroid impacts, and radiation. High-resolution magnetic identification and characterization of lava tubes can be used to help inform future scientific investigations of lava tubes for human exploration and utilization. We demonstrate how magnetometry is useful for determining the geometry and extent of lava tubes on the Earth and, by proxy, the Moon, by relating the magnetic anomalies produced by lava tubes to their location and geomorphology. Using a proton-precession total field magnetometer, we surveyed an area of more than 100,000 m2, with cross-tube linear traverses spaced at 3–5 m, perpendicular to an approximately 1,000 m length of the Modoc Crater lava tube complex, within the Lava Beds National Monument (California, USA). The observed magnetic anomalies of the sections known as Incline, Skull, and Ship Caves are compared against synthetic predictions, and the sensitivity of the magnetic anomalies to the tube geometry used to derive a basic relationship between the two. We use our model of terrestrial lava tube magnetic anomalies and adjust for the lunar magnetic environment to predict the signature of anomalies resulting from tubes on the Moon.Item LiDAR and Magnetic Data for: Field Mapping and Modeling of Terrestrial Lava Tube Magnetic Anomalies as an Analog for Lunar Lava Tube Exploration and Prospecting(2022) Bell, Ernest; Schmerr, Nicholas; Richardson, Jacob; Whelley, Patrick; Young, Kelsey; Garry, BrentArchived are Magnetic and LiDAR data collected at Lava Beds National Monument by the NASA funded TUBEX project (Dr. Kelsey Young – PI) in 2017 and 2018. The archived data are for a paper submitted to a JGR Planets special issue on Exploring Planetary Caves as Windows into Subsurface Geology, Habitability, and Astrology with the title: Field Mapping and Modeling of Terrestrial Lava Tube Magnetic Anomalies as an Analog for Lunar Lava Tube Exploration and Prospecting. We use observed data and detailed magnetic models to understand the practicality of mapping magnetic anomalies to determine lava tube locations and internal geometries within a lava flow.Item Mesozoic subduction shaped lower mantle structures beneath the East Pacific Rise(2024) Wang, Jingchuan; Lekic, Vedran; Schmerr, Nicholas; Gu, Yu Jeffrey; Guo, Yi; Lin, Rongzhi; Lekic, Vedran; Schmerr, NicholasThe morphology of the Large Low Shear Velocity Provinces (LLSVPs) has been a subject of debate for decades. Large-scale features of the Pacific LLSVP, as revealed by cluster analysis of global tomographic models, suggest three distinct portions. Notably, the East Pacific Anomaly and the Superswell Anomaly are characterized by a ~20 deg wide gap. The cause of the structural gap remains unclear, and there has been no direct evidence for a subduction episode beneath the region. In this study, we take advantage of an up-to-date SS precursor data set that samples the Nazca Plate and investigate the high-resolution seismic structure at mantle transition zone (MTZ) depths. We find that much of the southern East Pacific Rise is underlain by a thin MTZ due to the depressed 410 by up to 15 km, which suggests along-ridge temperature variations extending into the MTZ. East of the East Pacific Rise, the MTZ is characterized by anomalous thickening and fast seismic velocities from seismic tomography, consistent with the presence of cold subducted slab material intersecting the MTZ. Furthermore, recent global tomographic models reveal a slab-like structure throughout the MTZ and lower mantle, which is also evidenced by tomographic vote maps, albeit with less visibility. The observations reconcile with Mesozoic intraoceanic subduction beneath the present-day Nazca Plate, which is predicted by an earlier plate reconstruction model of proto-Pacific Ocean. The subduction initiated ~250 Myr ago and ceased before 120 Myr ago. The implications of this discovery are that the shape of the eastern portion of the Pacific LLSVP was separated by downwelling associated with this ancient subducted slab.Item Northwest Greenland Active Source Seismic Experiment(2021) Schmerr, Nicholas; Maguire, Ross; Pettit, Erin; Riverman, Kiya; Gardner, Christyna; DellaGiustina, Daniella; Avenson, Brad; Wagner, Natalie; Marusiak, Angela; Habib, Namrah; Broadbeck, Juliette; Bray, Veronica; Bailey, Samuel; Carr, Christina; Dahl, Peter; Weber, ReneeIn summer of 2018, the Seismometer to Investigate Ice and Ocean Structure (SIIOS) team conducted a geophysical field investigation on the Greenland ice sheet in northwestern Greenland at a location where a previous airborne radar survey by Palmer et al. (2013) had detected the signatures of a subglacial lake. The field site is located approximately 50 km north of the town of Qaanaaq. This site was chosen for the SIIOS project as it provides an opportunity for studying how a lander station could be used to detect subsurface water at an icy-ocean world. The purpose of the investigation was to confirm the presence of the subglacial lake and to measure its physical properties such as seismic impedance, as well as to estimate its depth and volume. One component of the investigation consisted of an active source seismic survey that was used to create a reflection image of the lake, as well as to measure the ice-bottom reflection coefficient. The survey was conducted along a roughly northeast oriented traverse, which started above the subglacial lake and crossed the lake’s eastern boundary.Item The Seismic Signatures of Recently Formed Impact Craters on Mars(Journal of Geophysical Research Planets, 2019-09) Schmerr, Nicholas; Banks, Maria; Daubar, IngridWe investigated the seismic signatures of recent impact crater clusters on Mars that would be recorded by the InSight Seismic Experiment for Interior Structure (SEIS) seismometers. We used a database of 77 measured and dated impact sites, with craters with effective diameters between 2.1 and 33.8 m, along with inferred impact angle, bolide trajectory, and varying target material properties to empirically scale for the momentum, expected seismic source function, and radiation pattern of impacts. The impact source is simulated in a local 3-D finite difference wave propagation code and coupled to teleseismic distances by scaling the spectra of 1-D global synthetic seismograms. We use the SEIS noise floors to estimate seismic detectability of impact(s) across azimuth and distance. Our experiments reveal that impact clusters have a higher peak corner frequency resulting from energy contributed by smaller craters to the power spectrum. We also find that the time separation between individual impacts in a cluster is small (< 10-15 milliseconds) and a require a seismometer closely situated to the source (< 10 km) and a high sampling rate (> 100 Hz) to resolve individual impacts within the cluster. Two of the clusters in our database (> 20 m effective diameter) would have been detectable by InSight, with the assumptions that the martian background noise and background attenuation are both low. Joint detection of surface changes from newly formed crater(s) in images and by SEIS will provide precise source locations that are crucial for constraining the internal structure of Mars.