A. Nyblade

Seismic Anisotropy of the Upper Mantle Below the Western Rift, East Africa

Although the East African rift system formed in cratonic lithosphere above a large‐scale mantle upwelling, some sectors have voluminous magmatism, while others have isolated, small‐volume eruptive centers. We conduct teleseismic shear wave splitting analyses on data from 5 lake‐bottom seismometers and 67 land stations in the Tanganyika‐Rukwa‐Malawi rift zone, including the Rungwe Volcanic Province (RVP), and from 5 seismometers in the Kivu rift and Virunga Volcanic Province, to evaluate rift‐perpendicular strain, rift‐parallel melt intrusion, and regional flow models for seismic anisotropy patterns beneath the largely amagmatic Western rift. Observations from 684 SKS and 305 SKKS phases reveal consistent patterns. Within the Malawi rift south of the RVP, fast splitting directions are oriented northeast with average delays of ~1 s. Directions rotate to N‐S and NNW north of the volcanic province within the reactivated Mesozoic Rukwa and southern Tanganyika rifts. Delay times are largest (~1.25 s) within the Virunga Volcanic Province. Our work combined with earlier studies shows that SKS‐splitting is rift parallel within Western rift magmatic provinces, with a larger percentage of null measurements than in amagmatic areas. The spatial variations in direction and amount of splitting from our results and those of earlier Western rift studies suggest that mantle flow is deflected by the deeply rooted cratons. The resulting flow complexity, and likely stagnation beneath the Rungwe province, may explain the ca. 17 Myr of localized magmatism in the weakly stretched RVP, and it argues against interpretations of a uniform anisotropic layer caused by large‐scale asthenospheric flow or passive rifting.

Observed rapid bedrock uplift in Amundsen Sea Embayment promotes ice-sheet stability

A quick rebound for Antarctic crust Earths crust deforms under the load of glaciers and ice sheets. When these masses are removed, the crust rebounds at a time scale determined by the viscosity of the upper mantle. Using GPS, Barletta et al. found that the viscosity of the mantle under the West Antarctic Ice Sheet is much lower than expected. This means that as ice is lost, the crust rebounds much faster than previously expected. Although estimates of total ice loss have to be revised upward, the surprising finding indicates that the ice sheet may stabilize against catastrophic collapse. Science, this issue p. 1335 A new viscosity model requires a much lower viscosity under the Amundsen Sea Embayment, stabilizing the Antarctic Ice Sheet. The marine portion of the West Antarctic Ice Sheet (WAIS) in the Amundsen Sea Embayment (ASE) accounts for one-fourth of the cryospheric contribution to global sea-level rise and is vulnerable to catastrophic collapse. The bedrock response to ice mass loss, glacial isostatic adjustment (GIA), was thought to occur on a time scale of 10,000 years. We used new GPS measurements, which show a rapid (41 millimeters per year) uplift of the ASE, to estimate the viscosity of the mantle underneath. We found a much lower viscosity (4 × 1018 pascal-second) than global average, and this shortens the GIA response time scale to decades up to a century. Our finding requires an upward revision of ice mass loss from gravity data of 10% and increases the potential stability of the WAIS against catastrophic collapse.

Near‐Surface Environmentally Forced Changes in the Ross Ice Shelf Observed With Ambient Seismic Noise

Author(s): Chaput, J.; Aster, R. C; McGrath, D.; Baker, M.; Anthony, R. E; Gerstoft, P.; Bromirski, P.; Nyblade, A.; Stephen, R. A; Wiens, D. A; Das, S. B; Stevens, L. A

Tsunami excitation of the Ross Ice Shelf, Antarctica

The responses of the Ross Ice Shelf (RIS) to the September 16, 2015 8.3 Mw Chilean earthquake tsunami (>75 s period) and infragravity (IG) waves (50-300 s period) were recorded by a 34 element broadband seismic array deployed on the RIS for one year from November 2014. Tsunami and IG-generated signals travel from the RIS front as water-ice coupled flexural waves at gravity wave speeds (~70 m/s). Displacements across the RIS are affected by gravity wave incident direction, bathymetry under and north of RIS, and water and ice shelf thickness/properties. Horizontal displacements are about 5 times larger than vertical, producing extensional motions that may facilitate expansion of existing fractures. Excitation is continuously observed throughout the year, with horizontal displacements highest during the austral winter (>20 cm). Because flexural waves exhibit weak attenuation, significant flexural wave energy reaches the grounding zone. Flexural waves provide year-round excitation of the RIS that likely promo...