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Geophysics and Tectonics Seminar - spring-2024

Chasing Darwin’s Shadow: Geophysics and Evolution in the Galapagos

April 3, 2024
noon - 1 p.m.
Geology 1707

Presented By:

  • Mark Richards - University of Washington
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In the autumn of 1835, young geologist Charles Darwin spent a month exploring the Galapagos Islands, coming to the realization that these new and ephemeral volcanic habitats had given rise to new species of otherwise familiar creatures, and leading him to conclude that biological evolution was a fact, with natural selection as the primary mechanism. The active Galapagos mantle plume (“hotspot”) and mid-ocean ridge system is a spectacular showplace for plume-ridge interaction, with geophysical and geochemical signatures that elucidate upper mantle dynamics and evolution, and a geological history of continuously emerging and subsiding island habitats that have provided stepping stones for at least 20 million years of evolutionary divergence for mainland-derived species such as iguanas, tortoises, and finches. This talk will discuss how several different lines of geophysical investigations are revealing new secrets about the geological evolution of the Galapagos plume-ridge system, how these discoveries are helping us understand volatile (H2O) fluxes from the deep mantle, and new horizons for how to formally combine increasingly rich genetic constraints on evolutionary biology with the geological history of ancient island habitats to obtain a more integrated understanding of geo-biological evolution in ocean island systems.

How high was sea level in the Holocene?

April 4, 2024
noon - 1 p.m.
3853 Slichter Hall

Presented By:

  • Roger Creel - Woods Hole Oceanographic Institution
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I will present work that aims to constrain sea-level change over the last glacial cycle by merging relative sea-level observations and glacial isostatic adjustment models via Bayesian statistical frameworks. I will first reconstruct Norwegian sea level over the last 16,000 years. I will then infer global mean sea level during the Holocene (11.7 - 0 thousand years ago), which is the last time global temperatures may have exceeded early Industrial (1850 CE) values. I will show that the available evidence is consistent with global mean sea level that exceeded early industrial levels in the mid-Holocene. I will also present the first quantitative estimates of Holocene mountain glacier volume and sea level change due to ocean thermal expansion.

Spatiotemporal Imaging of the Earth’s Near Surface with Fiber-Optic Sensors

April 10, 2024
noon - 1 p.m.
3853 Slichter Hall

Presented By:

  • Yan Yang - Caltech
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Understanding the Earth's subsurface is essential for mitigating natural hazards and ensuring environmental sustainability. Seismological methods, which measure seismic velocity (v) and its relative temporal changes (dv/v), provide a 4D (space-time) view of the subsurface. However, high-resolution imaging and monitoring of the near surface—the top tens of meters subject to rapid spatiotemporal changes—remains challenging. This is primarily due to the prohibitive costs of dense seismic arrays necessary for capturing high-frequency signals that can probe shallow depths. Distributed Acoustic Sensing (DAS) offers an affordable solution by converting telecommunication fiber-optic cables into ultra-dense seismic arrays, enhancing our ability to study environmental phenomena with details as fine as tens of meters. When combined with seismic ambient noise interferometry, DAS enables time-lapse imaging of the near surface. In the talk, I will present an exemplary work in Ridgecrest, California, demonstrating a comprehensive spatiotemporal imaging approach for the near surface. The results provide new insights into high-resolution urban seismic hazard mapping and vadose zone soil moisture monitoring during California's drought periods.

Bridging seismology and oceanography with seafloor fiber-optic sensing

April 11, 2024
noon - 1 p.m.
3853 Slichter Hall

Presented By:

  • Ethan Williams - University of Washington
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Observational geophysics conventionally relies on point sensors, from locating earthquakes and imaging subsurface structure with seismometers to forecasting coastal wave heights and detecting tsunamis with buoys. The emerging field of fiber-optic sensing offers a fundamentally different paradigm: distributed instead of point sensing, leveraging pre-existing telecommunications infrastructure to construct inherently multi-scale images of Earth’s dynamic processes over distances >100 km using a single instrument. In two topical vignettes, I will summarize some of my recent and ongoing research utilizing distributed acoustic sensing (DAS) on seafloor cables at the intersection of seismology and oceanography. (1) Offshore site characterization and ground motion. From quantitative estimation of tsunamigenic landslide hazard to interpretation of the turbidite record as a paleo-seismometer, accurate models of shallow sediment shear-wave velocity structure are required to anticipate the distribution of offshore ground motion in great earthquakes. Yet, because the water column is opaque to shear waves, this structure often remains poorly constrained, especially in the top 100s of meters. Interferometry of ambient seismic Scholte waves recorded with DAS reveals a typical power-law shear wave velocity profile immediately below the seafloor, with Vs30 as low as 150 m/s at locations ranging from the North Sea to the Oregon margin. This value is nearly an order of magnitude lower than used in Cascadia M9 simulations and in previous studies of turbidite triggering, suggesting that offshore site amplification has been systematically underestimated. With physics-based modeling of ambient seismic noise amplitude, I will show site amplification varies by nearly an order of magnitude over sub-kilometer distances and is strongly correlated with topography. Finally, I will summarize an ongoing experiment offshore southern Alaska combining seafloor compliance under ocean surface gravity wave loading with recordings of over 3000 local earthquakes to infer the nonlinear response of shallow sediment to strong ground motion. (2) Long-period DAS: from ocean mixing to seafloor geodesy. Where fiber-optic cables are unburied and exposed at the seafloor, DAS is also sensitive to temperature fluctuations from internal wave and tide dynamics in the bottom boundary layer, a region of enhanced ocean mixing but scarce observations. I will highlight two novel DAS datasets and discuss ongoing work to quantify rates of turbulent kinetic energy dissipation. DAS data recorded on a power cable across the Strait of Gibraltar show temperature transients up to 4 K associated with passing large-amplitude internal waves propagating on the near-surface thermocline. On the slope of Gran Canaria, an island off the coast of west Africa, temperature variability of about 2 K at 1-km depth decreasing to 0.2 K at 2.5-km depth reveals the bore-like propagation of the nonlinear internal tide at locations where the slope is near critical. The latter dataset also includes a long-wavelength signal proportional to the barotropic tidal pressure, including the lunar fortnightly variation, likely due to elastic strain from ocean-tidal loading. The demonstrated sensitivity of order 1 microstrain at 14-day period suggests that contemporary DAS already possesses sufficient long-period stability for (some) seafloor geodetic applications.

New Earth and Planetary Science Discoveries Enabled by the Optical Fiber Sensing Revolution

April 17, 2024
noon - 1 p.m.
3853 Slichter Hall

Presented By:

  • Bradley Lipovsky - University of Washington
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Optical fiber sensors constitute the biggest revolution in geophysical and environmental sensor technology since digitization. Although traditional sensors have been refined through decades of incremental progress, optical fiber sensors provide an entirely new lens with which to study fundamental processes. These sensors are particularly advantageous for systems that require high spatial and temporal resolution (i.e., on the order of 1-10m spatial scale and 100 s to 100 kHz sampling rate). As director of the UW Fiber Lab, Dr. Bradley Paul Lipovsky has deployed these technologies in Antarctica, Greenland, Alaska, New Zealand, and at a dozen sites in Europe and the lower United States. The main focus of this research has been on studying Earth's cryosphere, submarine, urban, and otherwise difficult-to-instrument environments. This talk will focus specifically on use cases where basic knowledge has been gained regarding the calving front of large, ocean-terminating glaciers in Greenland, paleoclimatic history of the Antarctic ice sheet, monitoring of clean energy systems, and earthquake detection and ground motion hazard characterization. The presentation will conclude with a forward looking discussion regarding the rapid pace of development of basic optical physics and engineering, and the prospects for future growth at the intersection of optical fiber sensors and the Earth, Planetary, and Space Sciences

Geophysics of the changing hydro-cryosphere: From California’s aquifers to Greenland’s supraglacial lakes

April 18, 2024
noon - 1 p.m.
3853 Slichter Hall

Presented By:

  • Stacy Larochelle - Columbia University
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Human activity and climate change are rapidly transforming Earth's hydrological and glaciological systems, posing critical challenges for global water security, coastal communities, and ecosystem resilience. In recent years, geophysics has become pivotal in both tracking hydro-cryospheric changes and unveiling the physics behind them. In this talk, I will discuss insight gained from applying a comprehensive geophysical approach to two critical and rapidly evolving systems: The Sacramento Valley aquifer system in California and supraglacial lakes on the Greenland Ice Sheet. Combining GNSS, InSAR, and GRACE/-FO satellite observations with in situ groundwater level measurements and lithologic logs in the Sacramento Valley reveals that the aquifer system has transitioned from a primarily reversible to irreversible deformation regime over the 2021-22 drought, indicating severe permanent compaction and loss of aquifer storage capacity that pose a serious threat to California’s water resources and infrastructure. In Greenland, we harness observations from on-ice GNSS stations, satellite imagery, ice-penetrating radar, seismometers, and water pressure instruments to decipher how meltwater lakes that form at the surface of the ice sheet interact through englacial stress to rapidly funnel surface meltwater to the ice-sheet bed and modulate ice basal sliding. Together, these case studies highlight the importance of multi-technique geophysical surveys in understanding the Earth’s changing hydro-cryosphere and enabling space-based monitoring at the global scale.