Search for: All records

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 Modal analysis of freestanding rock formations is crucial for evaluating their vibrational response to external stimuli, aiding accurate assessment of associated geohazards. Whereas conventional seismometers can be used to measure the translational components of normal modes, recent advances in rotational seismometer technology now allow direct measurement of the rotational components. We deployed a portable, three-component rotational seismometer for a short-duration experiment on a 36 m high sandstone tower located near Moab, Utah, in addition to conducting modal analysis using conventional seismic data and numerical modeling. Spectral analysis of rotation rate data resolved the first three natural frequencies of the tower (2.1, 3.1, and 5.9 Hz), and polarization analysis revealed the orientations of the rotation axes. Modal rotations were the strongest for the first two eigenmodes, which are mutually perpendicular, full-height bending modes with horizontal axes of rotation. The third mode is torsional with rotation about a subvertical axis. Measured natural frequencies and the orientations of displacements and rotation axes match our numerical models closely for these first three modes. In situ measurements of modal rotations are valuable at remote field sites with limited access, and contribute to an improved understanding of modal deformation, material properties, and landform response to vibration stimuli. 

The 410‐ and 660‐km discontinuities define the top and bottom of the mantle transition zone (MTZ). The properties of these mineralogical phase transformation interfaces provide critical constraints on the dynamics, temperature, and composition of the MTZ. Triplicated body waves that bottom near these discontinuities carry rich information about them. To streamline the modeling of upper‐mantle triplications recorded at regional distances (13°–30°), we have developed a (Fast) Message Passing Interface (MPI)‐accelerated 1D (Tr)iplication Waveform (I)nversion (P)ackage (FastTrip). With triplication waveform data as input, FastTrip uses a global search method to output a set of acceptable 1D velocity models. Quantitative estimation of the model uncertainties can be further derived based on the range of acceptable models. FastTrip supports central processing unit (CPU) parallel acceleration (15,000 models within 2 hr with 100 CPUs) and is portable to other inversion problems that can be described by a relatively small number of model parameters.