Search for: All records

Quantitative estimates of magma storage are fundamental to evaluating volcanic dynamics and hazards. Yet our understanding of subvolcanic magmatic plumbing systems and their variability remains limited. There is ongoing debate regarding the ephemerality of shallow magma storage and its volume relative to eruptive output, and so whether an upper-crustal magma body could be a sign of imminent eruption. Here we present seismic imaging of subvolcanic magmatic systems along the Cascade Range arc from systematically modelling the three-dimensional scattered wavefield of teleseismic body waves. This reveals compelling evidence of low-seismic-velocity bodies indicative of partial melt between 5 and 15 km depth beneath most Cascade Range volcanoes. The magma reservoirs beneath these volcanoes vary in depth, size and complexity, but upper-crustal magma bodies are widespread, irrespective of the eruptive flux or time since the last eruption of the associated volcano. This indicates that large volumes of melts can persist at shallow depth throughout eruption cycles beneath large volcanoes. 

Abstract The solid inner core, suspended within the liquid outer core and anchored by gravity, has been inferred to rotate relative to the surface of Earth or change over years to decades based on changes in seismograms from repeating earthquakes and explosions^1,2. It has a rich inner structure^3–6and influences the pattern of outer core convection and therefore Earth’s magnetic field. Here we compile 143 distinct pairs of repeating earthquakes, many within 16 multiplets, built from 121 earthquakes between 1991 and 2023 in the South Sandwich Islands. We analyse their inner-core-penetrating PKIKP waves recorded on the medium-aperture arrays in northern North America. We document that many multiplets exhibit waveforms that change and then revert at later times to match earlier events. The matching waveforms reveal times at which the inner core re-occupies the same position, relative to the mantle, as it did at some time in the past. The pattern of matches, together with previous studies, demonstrates that the inner core gradually super-rotated from 2003 to 2008, and then from 2008 to 2023 sub-rotated two to three times more slowly back through the same path. These matches enable precise and unambiguous tracking of inner core progression and regression. The resolved different rates of forward and backward motion suggest that new models will be necessary for the dynamics between the inner core, outer core and mantle. 

Abstract Seismic wave amplitudes have tremendous sensitivity to subduction structure; however, they are affected by attenuation, scattering and focusing, and have therefore been sparsely used compared with traveltimes. We measure and model teleseismic body wave amplitudes recorded at a dense broadband array in the Washington Cascades. These data show anomalous amplitude variations with complex azimuthal dependence at frequencies as low as 0.05 Hz, accompanied by significant multipathing. We demonstrate using spectral‐element numerical simulations that focusing of the teleseismic wavefield by the Juan de Fuca slab is responsible for some of the amplitude anomalies. The focusing effects can contaminate the apparent differential attenuation measurements and produce at least 20% of the inferred attenuation signal. Our results indicate that the amplitudes are sensitive to the subducting slab geometry and subduction structure, and can be used to refine seismic images. Ubiquitous and consistent amplitude anomalies are observed along the arc, suggesting that the Juan de Fuca slab may be continuous from Canada to northern California.