10 episodes

Open dialogue about important issues in earthquake science
presented by Center scientists, visitors, and invitees.

USGS Earthquake Science Center Seminar‪s‬ USGS

    • Natural Sciences
    • 4.8 • 5 Ratings

Open dialogue about important issues in earthquake science
presented by Center scientists, visitors, and invitees.

    • video
    Susan Hough

    Susan Hough

    Recent current events have shined a harsh light on the nature and consequences of continuing systemic inequalities in society, inspiring the scientific community to take an overdue look in the mirror. While some progress has been made in recent decades, improving the gender diversity in geosciences, very little progress has been made improving racial diversity in a field that has historically been the purview of white, male scientists. As steps are now taken to improve diversity of the geoscience community, it is important to note that systemic socioeconomic biases potentially impact not only our community, but also geoscience itself. In this talk I focus on earthquake observations contributed by human observers, which provide an invaluable source of information to investigate both historical and modern earthquakes. Commonly, the observers whose eyewitness accounts are available to scientists come from a self-selected minority of those who experience a given earthquake. As such these may not be representative of the overall population that experienced shaking from the event. Eyewitness accounts can contribute to modern science only if they are recorded and archived in an accessible repository. I discuss the extent to which geopolitics and socioeconomic disparities can limit the number of earthquake observers whose observations can contribute to science. I first revisit a late-19th century earthquake in the central U.S. in 1882 that provides an illustrative example of an event that has been poorly characterized due to a reliance on English-language archival materials. For modern earthquakes, I consider data collected for recent earthquakes in California and India via the online "Did You Feel It?" system. In California, online data-collection systems appear to be increasingly effective in gathering eyewitness accounts from a broad range of socioeconomic groups. In India, however, a recent study by Hough and Martin (SRL, 2021) shows that responses to the "Did You Feel It?" system reveal a strong bias towards responses from urban areas as opposed to rural settlements, as well a strong bias with literacy rate. The dissimilarity of these results from modern earthquakes in the U.S. and India provides a caution that, in some parts of the world, contributed felt reports can still potentially provide a grossly unrepresentative view of earthquake effects, especially if online data collection systems are not designed to be inclusive. This limitation can in turn potentially shape our understanding of an earthquake's impact and the characterization of seismic hazard. A lesser degree of bias is found in DYFI data for California, suggesting there is room for improved engagement with underserved communities.

    • 1 hr
    • video
    Leah Langer

    Leah Langer

    When an earthquake occurs, slip models of the event may be produced by inverting geodetic data for slip on a finite fault. This process generally requires coseismic Green's functions, which must be calculated in advance. The vast majority of such studies use Green's functions that ignore any 3D structure that is present in the region. The planet's largest earthquakes, magnitude 8-9 subduction zone ruptures, occur underneath regions with the largest topographic gradients ranging from deep ocean trenches up to onshore mountain ranges. These regions also include wide variations in material properties. Inland earthquakes can also occur in regions with significant heterogeneity, such as mountain ranges or sedimentary basins, which have the potential to affect the deformation field.

    Here, we investigate the effects of 3D structure on forward models of coseismic deformation and on earthquake static slip inversions. Using a newly-developed software package, SPECFEM-X, we show that the presence of topography alters the shape of predicted surface deformation patterns for the 2010 Maule and 2015 Gorkha earthquakes. We then compute coseismic Green's functions for these earthquakes in domains with and without topography, and perform Bayesian inversions using geodetic data. In both cases, we find that the use of Green's functions with topography yields a different distribution of slip. We then turn our attention to 3D elastic structure, and use SPECFEM-X to examine the impact of sedimentary basins on forward models of coseismic deformation. Our analysis of the 1984 Morgan Hill earthquake, which occurred near the Evergreen Basin, and of a hypothetical earthquake on the Sanchiao Fault near the Taipei Basin show that sedimentary basins generally affect the magnitude rather than the shape of surface deformation patterns. These findings suggest that the effects of 3D elastic structure may be qualitatively different from those of topography, and that 3D structure should be accounted for when estimating static slip in regions with significant heterogeneity.

    • 1 hr
    • video
    Andrew Michael

    Andrew Michael

    During the 2018 eruption of Kilauea, the central part of the caldera collapsed by up to 500 meters and caused M~5 earthquakes, almost daily from the end of May until the beginning of August. These earthquakes damaged the USGS Hawaiian Volcano Observatory and future events present a hazard to other structures around the caldera. But how can we assess the hazard of these highly clustered earthquakes, which do not follow a Poisson process? After all, in his seminal paper on Probabilistic Seismic Hazard Assessment (PSHA), Allin Cornell wrote, "the assumption that the occurrences of earthquakes follow the behavior of the Poisson process model can be removed only at a great penalty" (BSSA, 1968). I will discuss a pair of papers by Andrea Llenos and myself that address this issue. In the first, we model the occurrence of these earthquakes by combining a distribution for the occurrence of the underlying process of caldera collapses with a distribution for the earthquake response to those collapses. This approach could also be used to model the hazard due to aftershocks, which can be considered the earthquake response to the underlying process of mainshocks. While simulations can be used to compute seismic hazard including behavior such as aftershocks (e.g. Field et al., BSSA, 2017), the computational requirements may be daunting for many applications. A clear goal of Cornell was an analytic method to allow users to explore the impact of a variety of parameters. Therefore, our second paper introduces an analytic approach for PSHA that allows for arbitrary distributions of earthquake occurrence without an appreciable computational penalty. We apply this method to the caldera collapse earthquakes and explore general implications of non-Poissonian behavior for PSHA.

    • 1 hr
    • video
    Francois Renard

    Francois Renard

    If most of continental earthquakes occur in the first 15-20 km of the upper crust, lower crust earthquakes exist at depths of 50-60 km in the roots of mountain chains. Such earthquakes are observed, for example, below the Himalayas. Three mechanisms may explain how elastic strain energy is released in the form of earthquakes in the lower crust where rocks are often inferred to be ductile. The first mechanism involves local stress concentrations related to displacements along shear zones. The second mechanism involves the triggering of lower crust earthquakes by strain pulses produced by upper crust earthquakes. The third mechanism involves hydration reaction that may cause embrittlement of the rock. In Norway, outcrops of lower crust have recorded earthquake activity that occurred more than 400 million years ago during the Caledonian orogeny and that allow to test these three mechanisms. Fossil lower crust earthquake are observed because coseismic frictional heating produced numerous pseudotachylytes and fault offsets of up to two meters. Field data at the outcrop scale and images of fault samples in 2D (electron microscopy) and 3D (synchrotron X-ray microtomography) reveal the anatomy of the fault zone. Seismic slip and associated melting are preceded by fracturing, asymmetric fragmentation, and comminution of the wall rock caused by a dynamically propagating rupture. Data also show that earthquake activity preceded or was coeval with shear zone development and that fluids infiltrated into the earthquake damage zones. These observations indicate that, under mountain chains, the lower continental crust can be initially strong and not as ductile as usually proposed. The damage produced by earthquakes may explain how the rock rheology may evolve from brittle (e.g. granulites) to ductile (e.g. eclogites) through fluid infiltration.

    • 1 hr
    • video
    Eleanour Snow

    Eleanour Snow

    This seminar will focus on the programs in the Office of Science, Quality, and Integrity (OSQI) that provide education, training, and opportunities for emerging and early career scientists. This includes internships and outreach opportunities in the Youth and Education in Science (YES) office, as well as programs that engage diverse and early career scientists. It will highlight resources and best practices for USGS personnel who wish to inclusively engage youth.

    Eleanour Snow is the USGS Manager of Youth and Education programs, and has been working in the areas of Justice, Equity, Diversity, and Inclusion for decades. She is currently the co-lead of the Federal Interagency Working Group for Inclusion in STEM, and leader of the DOI Interagency Youth Team.

    • 1 hr
    • video
    Drake Singleton

    Drake Singleton

    South-central Alaska is a region of high seismicity that is frequently impacted by intraslab earthquakes. Intraslab earthquakes, which occur at mid-crustal depths, do not produce the characteristic surficial faulting, land-level change, or tsunami deposits typically associated with megathrust earthquakes, but instead generate a more subtle signal in the geologic record. The emerging field of lacustrine and fjord paleoseismology utilizes relatively small and well-defined basins with unique depositional characteristics (e.g., varve formation), in combination with earthquake-generated turbidity deposits as paleoseismic proxies, to construct earthquake histories that are sensitive enough to record intraslab events. The 2018 Anchorage Earthquake resulted in high-intensity shaking across the upper Cook Inlet and highlighted the region's vulnerability to intraslab earthquakes. Recent work on Eklutna Lake by Van Daele et al. (2020) has confirmed the presence of earthquake-generated turbidites as a result of the 2018 earthquake across the lake's two sub-basins. The results of their study provided some of first inputs necessary to begin calibrating south-central Alaska's natural seismograph. However, important questions remain, including the minimum MMI necessary to trigger turbidity currents, which depositional environments are most susceptible to failure by seismic triggers, and the potential of relative turbidite thickness as an indicator of epicentral direction. I'll present preliminary results from fieldwork carried out this summer 2020 in south-central Alaska that focused on the collection of short-barrel gravity cores, MCS Sparker profiles, and high-resolution Chirp data. The goal of this project will be to characterize the sedimentary response of south-central Alaska's lakes and fjords to the 2018 Anchorage Earthquake. As a first step, short-barrel gravity cores are used to investigate the spatial extent of turbidity currents triggered by the 2018 Anchorage Earthquake, and if possible, identify a correlating acoustic signature.

    • 1 hr

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