Geophysical Journal International

SummaryThe seismic wavefield is fully described by translation, rotation and strain. Until recently, the seismological community has not been able to measure rotation directly with portable sensors with satisfactory resolution. Portable blueSeis-3A (Exail) sensors allow measuring 3 components of rotational motions. Co-located with conventional seismometers, one can locally observe six degrees of freedom (6 DoF) of ground motion. To test the performance of the rotational sensors, an active source experiment was carried out in Fürstenfeldbruck, Germany, in November 2019. Five explosions with different yields and distances from our sensors, were fired. In a first stage, eight rotational sensors were deployed inside a bunker next to a seismometer. In a second stage, the rotational sensors were installed in two clusters of 4 sensors each. We compare the back azimuths derived using two different methods: (i) a method using horizontal rotational components and (ii) a standard polarization analysis using only 3C translational data. Back azimuths calculated using rotational data for 5 explosions have an average 10.2○ deviation from the theoretical back azimuths. Estimates using 3C translational data for the first stage of the experiment have a 1.8○ deviation from the theoretical back azimuth. For the second stage we found a 29.4○ deviation using the seismometer from stage 1. We conclude that within our distance range from 50 m to 1070 m, all rotational sensors provide reliably back azimuths of explosive sources when using only horizontal rotational components. For future applications of rotational sensors in other environments this is promising as back azimuths can be derived reliably.

SummaryA new global mascon solution using GRACE and GRACE Follow-On data is co-estimated with Satellite Laser Ranging (SLR) measurements to seven major geodetic satellites. This combined solution is compared with an otherwise similar GRACE-only solution to determine improvements in the estimate. We find similar performance between both solutions in the recovered mass change, but significant improvements in the associated errors in the combination solution. Errors in recovered basin mass change are 10-20% better for the combination solution across all basin sizes, with the greatest improvements in high latitude ice sheets. These results lead to our recommendation that all GRACE Level 3 mascon and spherical harmonic user-oriented gridded solutions should include SLR information during the solution inversion. As an ancillary contribution, we also provide validation of the choice of truncated spherical harmonics used in determining GRACE Technical Note 14, the current recommended mechanism for including SLR information with GRACE solutions in post-processing.

SummaryThe 1929 Grand Banks earthquake (Mw 7.1) generated a large landslide that broke submarine cables near the source area. A large tsunami was generated by the large landslide that killed 28 people on the southern coast of the Burin Peninsula, Canada. The tsunami was also observed at a tide gauge in Halifax, Canada. In this study, the initial landslide mass distribution was estimated by fitting the first pulse of the tsunami waveform observed at Halifax and the timings of the submarine cable break with those computed values. A deep-sea landslide tsunami computation method was developed by modifying a previously developed Tsunami Squares method with a different friction term and an effect of deep water. The results showed that the tsunami waveform and timing of the cable break were well explained by the tsunami computed from the initial landslide mass volume of about 450 km3 located along the continental slope near the source area. The method developed to compute deep-sea landslide tsunamis should be useful for studying other types of landslide tsunamis.

SummaryWe conduct a comprehensive comparison of Ice Mass Balance (IMB) estimates for Greenland derived from satellite observations of ice Surface Elevation Changes (SEC), gravity and GNSS observations. Our analysis integrates data from the ICESat and CryoSat-2 satellite altimetry missions, augmented by optical stereo-imagery for peripheral glaciers, and GRACE satellite gravimetry mission, spanning the 2003-2008 and 2011-2015 periods. We also consider three firn densification models (FDM) and five Glacial Isostatic Adjustment (GIA) models for correcting the datasets for these effects when necessary. Our results reveal significant differences among FDM corrections applied to SEC observations, with particularly large variations in IMB estimates reaching up to 90 Gt/yr. To address this, we develop an innovative method for estimating equivalent firn corrections to the ice elevation observations, based on a least-squares fit of filtered ice SEC observations to GRACE mass-change estimates. This approach is both simple and independent from climate models assumptions and shows minimal sensitivity to GIA model differences. Using this method, we estimate IMBs for Greenland at -217.6 ± 15.7 Gt/yr for 2003-2008 and -253.2 ± 18.8 Gt/yr for 2011-2015. Importantly, these values indicate an acceleration of the thinning rate, not consistently captured by the IMB estimates inferred from the ice SEC observations corrected by FDMs. Finally, we compute elastic ground deformation induced by ice mass change during 2011-2015, using the four proposed mass-variation distributions and compare the predicted vertical velocities with GNSS observations in Greenland, accounting for all GIA models. While all models are consistent with most of the GNSS-derived uplift rates, they cannot fully explain the observed vertical velocities, especially in the South-East Greenland, which confirms the need to refine our understanding of GIA contributions in this region.

SummaryIn this study, we adopt a horizontally layered model consisting of air, seawater and undersea porous rock and develop an analytically-based method to calculate the seismic and EM fields generated by undersea earthquakes. We conduct numerical simulations to investigate the characteristics of the EM response at the receivers located at the seafloor, in the seawater near the sea surface and in the air, respectively. The results show that two kinds of EM signals can be identified in the EM records at these receivers, namely, the early EM wave (seismic-to-EM conversion at the seafloor interface) arriving before the seismic waves and the coseismic EM fields with apparent speeds of the seismic waves. The EM signals observed at the seafloor are mostly stronger than those observed in the seawater and air near the sea surface. The method is applied to simulating the EM response to the 2022 Mw 7.3 earthquake that took place in the sea near Fukushima, Japan. At a receiver with 76 km epicentral distance at the seafloor, the predicted coseismic electric and magnetic signals reach 2 μV/m and 2 nT, respectively, which are within the detectability of the current EM equipment. This suggest a possibility to monitor the EM disturbances associated with undersea earthquakes and use them to serve the earthquake early warning, helping to mitigate the societal impact of large earthquakes.

SummaryLow-frequency laboratory measurements provide direct access to the elastic properties of samples within the seismic frequency band, offering calibration data for seismic survey analysis. Additionally, µCT imaging can quantify actual saturations and provides insights into phase distributions at the pore scale. To conduct laboratory triaxial measurements at seismic frequencies while simultaneously imaging the rock interior, we developed an X-ray transparent low-frequency apparatus. Our apparatus determines rock mechanical properties at seismic frequencies (0.5–150 Hz) and strain amplitudes (10−7–10−5), measuring Young’s modulus, Poisson’s ratio, and attenuation. In addition P- and S-wave velocities at ultrasonic frequencies are measured. We conducted imbibition-drainage experiments to assess the effect of saturation and patch size on seismic and ultrasonic elastic properties in sandstone. Additional tests with liquid and gaseous CO2 reveal the impact of partial CO2-gas saturation. The imbibition-drainage experiment demonstrated that P-wave velocity at ultrasonic frequencies was elevated during drainage and reduced during imbibition. Drainage caused patchy saturation, while imbibition resulted in uniform saturation. This implies that ultrasonic measurements, with wavelengths comparable to the pore fluid patch size, are likely influenced by scattering. In contrast, low-frequency measurements, where the wavelength surpasses the patch size, capture effective medium properties and therefore are not affected by scattering effects. The results of the CO2 test suggest that low-frequency measurements can detect even low gas saturations (4% gaseous CO2). In contrast, ultrasonic velocity measurements primarily reflect the response of the fully saturated sample at low gas saturations and do not indicate a reduction in velocity. Identifying fluid-solid interactions and estimating saturation via µCT imaging is crucial, especially with minimal gas presence. Our combined approach allows precise determination of elastic properties at seismic frequencies and shows the importance of low-frequency over ultrasonic measurements.

SummaryIntraplate earthquakes in stable continental regions exhibit diverse characteristics in terms of timing, spatial distribution, and magnitude. They are often unexpected, and their underlying physical mechanisms are not well understood. This complexity is particularly apparent in Norway, where seismicity is mostly localised on the continental margin and coastal areas. Various studies have attempted to explain the causes of seismicity in Norway by invoking different sources of stress, ranging from regional stress due to ridge push to local effects such as topography or deglaciation. In this study, we revisit these questions by investigating the distribution of seismicity in southwestern Norway using an enhanced earthquake catalogue. To achieve this, we revised the Norwegian National Seismic Network seismic catalogue from 2000 to 2023 and built a new catalogue using machine-learning-based techniques on data from a temporary seismic deployment in the region. Thanks to the increased station density during this deployment, we were also able to calculate new fault plane solutions that consistently showed a WNW-ESE direction for the most compressive axis. Furthermore, we demonstrate that seismicity in southwestern Norway, while diffuse, tends to be localised around the major crustal shear zones of the region, such as the Bergen Arc Shear Zone and the Hardangerfjord Shear Zone.

SummaryHaematite-bearing red beds are widespread across the Earth and play a pivotal role in palaeomagnetic studies. However, chemical remanent magnetisation (CRM) typically associated with authigenic haematite is not fully understood, which precludes more accurate interpretations of natural remanent magnetisation (NRM) in red beds. Here, we use electron microscopy, rock magnetism, and palaeomagnetism to investigate authigenic haematite in Early Triassic red beds in North China. Our findings reveal that the biotite-hosted haematite grains with grain sizes of several to tens of microns carry a significant portion of the NRM in these sedimentary rocks. We propose that these authigenic haematite particles primarily form during the early stages of diagenesis process. This authigenic haematite's growth is controlled by the crystal structure of the host biotite. Furthermore, this authigenic haematite displays high coercivity (> 100 mT) and high unblocking temperature (> 650 ° C), comparable to that of typical detrital haematite (30–1000 mT, > 650 ° C), which is usually the primary carrier of detrital remanent magnetisation (DRM) in such red beds. This study highlights the significance of combining mineralogical analysis with rock magnetism and palaeomagnetism to differentiate between CRM and DRM and thereby identify the primary NRM component within red beds. We hypothesize that the abundant iron supplied by biotite promotes the growth of authigenic haematite. This study illustrates the need to use caution when studying sedimentary NRM, particularly in rocks from source areas containing acidic igneous and metamorphic rocks (e.g. granite, diorite, and biotite gneiss) that contain a large proportion of iron-bearing minerals, such as the biotite observed in this study.