Spectral Characteristics of the 1960 Tsunami at Crescent City, CA

Spectral characteristics of sea level fluctuations during the May 1960 Chilean Earthquake tsunami are investigated using digitized strip chart recordings from two docks within Crescent City Harbor. Peaks in sea level spectra at the two docks near 10-3 Hz and near 2.1 x10-3 Hz correspond to the two lowest frequency harbor modes, occurring above the frequency band most strongly excited by the tsunami. Tidal modulation of harbor spectral structure at very short periods is observed. Theoretical estimates of shelf edge wave resonant modes fall within the frequency band strongly excited by the tsunami, in contrast to modeled edge waves from a seismic event near Cape Mendocino that show no evidence of the reflection necessary for a strong shelf resonance. This suggests that heightened susceptibility of sea level (but not necessarily currents) at Crescent City to tsunami is not due primarily to either harbor or shelf resonances

Tidal and thermal stresses drive seismicity along a major Ross Ice Shelf rift

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters, 46(12), (2019): 6644-6652, doi:10.1029/2019GL082842.Understanding deformation in ice shelves is necessary to evaluate the response of ice shelves to thinning. We study microseismicity associated with ice shelf deformation using nine broadband seismographs deployed near a rift on the Ross Ice Shelf. From December 2014 to November 2016, we detect 5,948 icequakes generated by rift deformation. Locations were determined for 2,515 events using a least squares grid‐search and double‐difference algorithms. Ocean swell, infragravity waves, and a significant tsunami arrival do not affect seismicity. Instead, seismicity correlates with tidal phase on diurnal time scales and inversely correlates with air temperature on multiday and seasonal time scales. Spatial variability in tidal elevation tilts the ice shelf, and seismicity is concentrated while the shelf slopes downward toward the ice front. During especially cold periods, thermal stress and embrittlement enhance fracture along the rift. We propose that thermal stress and tidally driven gravitational stress produce rift seismicity with peak activity in the winter.NSF grants PLR‐1142518, 1141916, and 1142126 supported S. D. Olinger and D. A. Wiens, R. C. Aster, and A. A. Nyblade respectively. NSF grant PLR‐1246151 supported P. D. Bromirski, P. Gerstoft, and Z. Chen. NSF grant OPP‐1744856 and CAL‐DPR‐C1670002 also supported P. D. Bromirski. NSF grant PLR‐1246416 supported R. A. Stephen. The Incorporated Research Institutions for Seismology (IRIS) and the PASSCAL Instrument Center at New Mexico Tech provided seismic instruments and deployment support. The RIS seismic data (network code XH) are archived at the IRIS Data Management Center (http://ds.iris.edu/ds/nodes/dmc/). S. D. Olinger catalogued and located icequakes, analyzed seismicity and environmental forcing, and drafted the manuscript. D. A. Wiens and B. P. Lipovsky provided significant contributions to the analysis and interpretation of results and to the manuscript text. D. A. Wiens, R. C. Aster, A. A. Nyblade, R. A. Stephen, P. Gerstoft, and P. D. Bromirski collaborated to design and obtain funding for the deployment. D. A. Wiens, R. C. Aster, R. A. Stephen, P. Gerstoft, P. D. Bromirski, and Z. Chen deployed and serviced seismographs in Antarctica. All authors provided valuable feedback, comments, and edits to the manuscript text. Special thanks to Patrick Shore for guidance throughout the research process.2019-11-2

Near-surface environmentally forced changes in the Ross Ice Shelf observed with ambient seismic noise

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 11,187-11,196, doi:10.1029/2018GL079665.Continuous seismic observations across the Ross Ice Shelf reveal ubiquitous ambient resonances at frequencies >5 Hz. These firn‐trapped surface wave signals arise through wind and snow bedform interactions coupled with very low velocity structures. Progressive and long‐term spectral changes are associated with surface snow redistribution by wind and with a January 2016 regional melt event. Modeling demonstrates high spectral sensitivity to near‐surface (top several meters) elastic parameters. We propose that spectral peak changes arise from surface snow redistribution in wind events and to velocity drops reflecting snow lattice weakening near 0°C for the melt event. Percolation‐related refrozen layers and layer thinning may also contribute to long‐term spectral changes after the melt event. Single‐station observations are inverted for elastic structure for multiple stations across the ice shelf. High‐frequency ambient noise seismology presents opportunities for continuous assessment of near‐surface ice shelf or other firn environments.NSF Office of Polar Programs Grant Number: PLR-11425182019-04-1