State of stress before and after the 1994 Northridge Earthquake

The state of tectonic stress in the epicentral area of the 17 January 1994, Northridge earthquake (Mw 6.7) is investigated by applying a stress inversion method to P-wave polarity data from earthquakes in Northridge from July 1981 to January 1994 and from the Northridge aftershocks during January 1994 to December 1995. A 3-D crustal model is used to trace the rays taking off from the hypocenter, which reduced the effects of large structural heterogeneities on the determination of the stress tensor. We found significant temporal changes of stress orientations induced by the Northridge earthquake. The principal pressure (P) axis is oriented N32°E from 1981 to June 1992, and N30°E from 28 June 1992 to 16 January 1994, suggesting that the stress field in Northridge was not affected by the 1992 Landers earthquake. During two weeks following the Northridge mainshock, the P-axis is oriented N13°E, which is a significant (17°) change from that before the earthquake (N30°E). Between February 1994 and August 1995 the P-axis orientation changes from N18°E to N26°E, and finally ends up at N34° by the end of 1995, which is close to that before the Northridge earthquake. These results suggest that the stresses rotated coseismically, then rotated more slowly back to their original orientation. The aftershocks caused by the mainshock changed the stress distribution in the crust, which showed up as a regional stress change. The stress recovery appears to have completed within two years after the mainshock, which is very short compared to the time scale of the earthquake cycle

Broadband Seismic Observation at Dome-F and Contribution to Global Network

第4回極域科学シンポジウム特別セッション：[S] 南極研究の将来展望―第Ⅸ期6か年計画策定に向けて11月13日（水） 国立極地研究所 ２階大会議

Nucleation and seismic tremor associated with the glacial earthquakes of Whillans Ice Stream, Antarctica

The ability to monitor transient motion along faults is critical to improving our ability to understand many natural phenomena such as landslides and earthquakes. Here, we usedata from a GPS and seismometer network that were deployed to monitor the regularly repeating glacial earthquakes of Whillans Ice Stream, West Antarctica to show that a unique pattern of precursory slip precedes complete rupture along the bed of the ice stream. Additionally, we show that rupture can be independently tracked by increased levels of microseismic activity, including harmonic tremor, that are coincident with the onset of slip at any location, thus providing a remote means of monitoring stress and rupture propagation during the glacial earthquakes

Seasonal and spatial variations in the ocean-coupled ambient wavefield of the Ross Ice Shelf

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Baker, M. G., Aster, R. C., Anthony, R. E., Chaput, J., Wiens, D. A., Nyblade, A., Bromirski, P. D., Gerstoft, P., & Stephen, R. A. Seasonal and spatial variations in the ocean-coupled ambient wavefield of the Ross Ice Shelf. Journal of Glaciology, 65(254), (2019): 912-925, doi:10.1017/jog.2019.64.The Ross Ice Shelf (RIS) is host to a broadband, multimode seismic wavefield that is excited in response to atmospheric, oceanic and solid Earth source processes. A 34-station broadband seismographic network installed on the RIS from late 2014 through early 2017 produced continuous vibrational observations of Earth's largest ice shelf at both floating and grounded locations. We characterize temporal and spatial variations in broadband ambient wavefield power, with a focus on period bands associated with primary (10–20 s) and secondary (5–10 s) microseism signals, and an oceanic source process near the ice front (0.4–4.0 s). Horizontal component signals on floating stations overwhelmingly reflect oceanic excitations year-round due to near-complete isolation from solid Earth shear waves. The spectrum at all periods is shown to be strongly modulated by the concentration of sea ice near the ice shelf front. Contiguous and extensive sea ice damps ocean wave coupling sufficiently so that wintertime background levels can approach or surpass those of land-sited stations in Antarctica.This research was supported by NSF grants PLR-1142518, 1141916, 1142126, 1246151 and 1246416. JC was additionally supported by Yates funds in the Colorado State University Department of Mathematics. PDB also received support from the California Department of Parks and Recreation, Division of Boating and Waterways under contract 11-106-107. We thank Reinhard Flick and Patrick Shore for their support during field work, Tom Bolmer in locating stations and preparing maps, and the US Antarctic Program for logistical support. The seismic instruments were provided by the Incorporated Research Institutions for Seismology (IRIS) through the PASSCAL Instrument Center at New Mexico Tech. Data collected are available through the IRIS Data Management Center under RIS and DRIS network code XH. The PSD-PDFs presented in this study were processed with the IRIS Noise Tool Kit (Bahavar and others, 2013). The facilities of the IRIS Consortium are supported by the National Science Foundation under Cooperative Agreement EAR-1261681 and the DOE National Nuclear Security Administration. The authors appreciate the support of the University of Wisconsin-Madison Automatic Weather Station Program for the data set, data display and information; funded under NSF grant number ANT-1543305. The Ross Ice Shelf profiles were generated using the Antarctic Mapping Tools (Greene and others, 2017). Regional maps were generated with the Generic Mapping Tools (Wessel and Smith, 1998). Topography and bathymetry data for all maps in this study were sourced from the National Geophysical Data Center ETOPO1 Global Relief Model (doi:10.7289/V5C8276M). We thank two anonymous reviewers for suggestions on the scope and organization of this paper

Complex and diverse rupture processes of the 2018 Mw 8.2 and Mw 7.9 Tonga-Fiji deep earthquakes

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(5), (2019):2434-2448, doi:10.1029/2018GL080997.Deep earthquakes exhibit strong variabilities in their rupture and aftershock characteristics, yet their physical failure mechanisms remain elusive. The 2018 Mw 8.2 and Mw 7.9 Tonga‐Fiji deep earthquakes, the two largest ever recorded in this subduction zone, occurred within days of each other. We investigate these events by performing waveform analysis, teleseismic P wave backprojection, and aftershock relocation. Our results show that the Mw 8.2 earthquake ruptured fast (4.1 km/s) and excited frequency‐dependent seismic radiation, whereas the Mw 7.9 earthquake ruptured slowly (2.5 km/s). Both events lasted ∼35 s. The Mw 8.2 earthquake initiated in the highly seismogenic, cold core of the slab and likely ruptured into the surrounding warmer materials, whereas the Mw 7.9 earthquake likely ruptured through a dissipative process in a previously aseismic region. The contrasts in earthquake kinematics and aftershock productivity argue for a combination of at least two primary mechanisms enabling rupture in the region.We thank the Editor Gavin Hayes and two anonymous reviewers for their helpful comments that improved the quality of the manuscript. The seismic data were provided by Data Management Center (DMC) of the Incorporated Research Institutions for Seismology (IRIS). The facilities of IRIS Data Services, and specifically the IRIS Data Management Center, were used for access to waveforms, related metadata, and/or derived products used in this study. IRIS Data Services are funded through the Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation under Cooperative Agreement EAR‐1261681. W. F. acknowledges supports from the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the Weston Howland Postdoctoral Scholarship. S. S. W. and D. T. are supported by the MSU Geological Sciences Endowment.2019-08-2

A frozen collision belt beneath ice: an overview of seismic studies around the Gamburtsev Subglacial Mountains, East Antarctica

The Gamburtsev Antarctic Mountains Seismic Experiment (GAMSEIS, 2007–2010) was jointly conducted by the United States, China, and Japan during and after the International Polar Year 2007–2008. Broadband seismic stations were deployed across the ice-covered Gamburtsev Subglacial Mountains (GSM) and other previously unexplored areas in the interior of East Antarctica. Using GAMSEIS data, published results not only have revealed the deep structure of Antarctica, but also improved our understanding of the tectonic evolution of Antarctica and the supercontinent Gondwana, and of the relationship between geothermal heat flux and glaciers. This contribution draws together the major findings from recent studies, and also offers further investigation into the relationship between tectonic history and the East Antarctic Ice Sheet. The elevation of the GSM is largely supported by thickened crust, with Moho depths of ~60 km near the crest of the range. The GSM are underlain by thick (>200 km) and cold continental lithosphere that likely formed after collision of two ancient crustal blocks during the pan-African orogeny. Heat flux results obtained from seismic inversion support a model of ice sheet basal melting that depends more strongly on bedrock topography than on geothermal heat flux, while ice surface and ice thickness are inversely correlated with heat flux

Implications of sea ice on Southern Ocean microseisms detected by a seismic array in West Antarctica

The proximity of Southern Ocean storms coupled with seasonal variation in sea ice make Antarctica ideal for the study of microseism sources. We explore frequency-dependent beamforming results using a short-duration, 60 km aperture, broad-band seismic array located on the Whillans Ice Stream, West Antarctica. Locations of single-frequency microseism (13–16 s period) generation are in regions where the continental shelf is ice-free, consistent with previous studies, and show Rayleigh wave sources remaining at consistent backazimuths throughout the duration of the array. Beamforming analysis of daily noise correlations shows that long-period double-frequency microseisms (9–11 s) consist predominantly of Rayleigh waves excited by storms in the Southern Ocean. Modelling of source locations based on wave–wave interaction provides a good fit to our data at these periods. We show that short-period double-frequency microseisms (5–7 s) in Antarctica consist of crustal phase Lg and body waves. Lg arrivals propagate through regions of continental crust and our data show that the Lg energy is generated when storm systems interact with the sea-ice-free continental shelf during austral summers. Ultra-short-period (0.3–2 s) microseismic body waves back project to regions that correlate with oceanic storm systems in both the Southern and Northern Hemispheres

Tidal pacing, skipped slips and the slowdown of Whillans Ice Stream, Antarctica

We summarize new observations of the deceleration and stick–slip motion of Whillans Ice Stream (WIS), Antarctica. We refine the location of the large sticky spots that resist motion between slip events, the locations of which are controlled by the patterns of subglacial water flow. Our examination of the long-term velocity time series for the ice stream reveals that the decadal-scale deceleration is not occurring at a steady rate, but varies at the sub-decadal timescale. This unsteady deceleration modulates the temporal evolution of a broad (~50 km across) surface-elevation bulge forming at the junction between the relatively narrow upstream portion of the ice stream and broad ice plain that constitutes the downstream end of WIS. Comparison of observations from April 2003 and November 2010 reveals significant changes in the tidally modulated stick–slip cycle that regulates motion on the ice plain. We observe that the timing of slip events has become less regular in response to decreased flow speed in the upstream portions of the ice stream. The decreased regularity of slip events has reduced the release of stored elastic strain during slip events, increasing the rate of deceleration

Glacial Earthquakes and Precursory Seismicity Associated With Thwaites Glacier Calving

We observe two (~MS 3) long‐period (10–30 s) seismic events that originate from the terminus of Thwaites Glacier, Antarctica. Serendipitous acquisition of satellite images confirm that the seismic events were glacial earthquakes generated during the capsizing of icebergs. The glacial earthquakes were preceded by 6 days of discrete high‐frequency seismic events that can be observed at distances exceeding 250 km. The high‐frequency seismicity displays an increasing rate of occurrence, culminating in several hours of sustained tremor coeval with the long‐period events. A series of satellite images collected during this precursory time period show that the high‐frequency events and tremor are the result of accelerating growth of ancillary fractures prior to the culminating calving event. This study indicates that seismic data have the potential to elucidate the processes by which Thwaites Glacier discharges into the ocean, thus improving our ability to constrain future sea level rise

Ocean-excited plate waves in the Ross and Pine Island Glacier ice shelves

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Glaciology 64 (2018): 730-744, doi:10.1017/jog.2018.66.Ice shelves play an important role in buttressing land ice from reaching the sea, thus restraining the rate of grounded ice loss. Long-period gravity-wave impacts excite vibrations in ice shelves that can expand pre-existing fractures and trigger iceberg calving. To investigate the spatial amplitude variability and propagation characteristics of these vibrations, a 34-station broadband seismic array was deployed on the Ross Ice Shelf (RIS) from November 2014 to November 2016. Two types of ice-shelf plate waves were identified with beamforming: flexural-gravity waves and extensional Lamb waves. Below 20 mHz, flexural-gravity waves dominate coherent signals across the array and propagate landward from the ice front at close to shallow-water gravity-wave speeds (~70 m s−1). In the 20–100 mHz band, extensional Lamb waves dominate and propagate at phase speeds ~3 km s−1. Flexural-gravity and extensional Lamb waves were also observed by a 5-station broadband seismic array deployed on the Pine Island Glacier (PIG) ice shelf from January 2012 to December 2013, with flexural wave energy, also detected at the PIG in the 20–100 mHz band. Considering the ubiquitous presence of storm activity in the Southern Ocean and the similar observations at both the RIS and the PIG ice shelves, it is likely that most, if not all, West Antarctic ice shelves are subjected to similar gravity-wave excitation.Bromirski, Gerstoft, Chen and Diez were supported by NSF grant PLR 1246151. Stephen was supported by NSF grant PLR-1246416. Wiens, Aster and Nyblade were supported under NSF grants PLR-1142518, 1141916 and 1142126, respectively