Geophysical Journal International

Niezabitowska, D K; Szaniawski, R

doi: 10.1093/gji/ggad129pmid: N/A

SUMMARYThe Holy Cross Mountains (HCM) in Poland, is an isolated natural outcrop of Palaeozoic rocks located within the Trans-European Suture Zone, a tectonic collage of continental terranes adjacent to the Tornquist margin of the Baltica. This uniqueness made the HCM a target for palaeogeographic research. Based on the facies differences, the HCM had been divided into two major units, the southern (the Kielce Unit) and northern (the Łysogóry Unit) part (SHCM and NHCM, respectively). Their position in relation to each other and the Baltica continent during Silurian times is still a matter of discussion, whether both parts of the HCM were separated terranes located along the Baltica margin or they shared in common palaeogeographic history. Here, we present the results of comprehensive rock magnetic measurements applied as a tool to interpret palaeoenvironmental conditions during deposition and burial and therefore allow discussion about the terranes’ relative position. To recognize the magnetic mineral composition and texture of studied Silurian graptolitic shales several rock magnetic measurements were conducted including low-temperature Saturated Isothermal Remanent Magnetization, thermal demagnetization of three-component IRM and hysteresis measurements, as well as anisotropy of magnetic susceptibility (AMS). The sampled rocks come from both units of the HCM. In all analysed samples we found single domain (SD) stoichiometric magnetite of mostly diagenetic (i.e. post-depositional) origin and goethite resulting likely from weathering. In turn, detrital magnetite, even if observed in previously investigated Silurian rocks from the Baltica margin, was not identified in this study, what we attribute to dissolution during diagenesis in the deep-water environment. Solely in the NHCM, SD hematite and maghemite grains were observed, which we interpret as detrital in origin. These grains have been preserved in the suboxic environment of the NHCM sub-basin bottom waters due to their resistance to dissolution in marine waters. Considering the deposition conditions (oxygenation of the near-bottom zone) rather similar for both HCM parts, we associate the presence of aeolian hematite grains solely in the NHCM rocks with a more proximal position of the NHCM than the SHCM in relation to the Baltica continent during late Llandovery (Silurian). This conclusion agrees with some existing palaeogeographic models. In addition to petromagnetic studies focused on the analysis of ferromagnets, AMS measurements were also carried out. The results indicate that the magnetic susceptibility is mainly governed by paramagnetic minerals, mostly phyllosilicates with small ferromagnetic contributions. Oblate AMS ellipsoid and distinct bedding parallel foliation indicate prevailing sedimentary-compactional alignment. Observed magnetic lineation of tectonic origin resulting from weak strain is related presumably to Variscian deformations.

Pfaffenzeller, Nikolas; Pail, Roland

doi: 10.1093/gji/ggad132pmid: N/A

SUMMARYSatellite gravity missions so far are medium size satellites consisting of one or a pair of satellites flying in near-polar or sun-synchronous orbital planes. Due to the limited observation geometry and the related space–time sampling, high-frequency non-tidal mass variation signals from atmosphere and ocean cannot be observed and cause temporal aliasing. For current single-pair satellite gravimetry missions as GRACE and GRACE Follow-On (GRACE-FO) temporal aliasing is the limiting factor and represents the major error source in the gravity field time-series. Adding a second inclined satellite pair to a GRACE-like polar pair (Bender constellation) currently is the most promising solution to increase the spatio-temporal resolution and to significantly reduce the temporal aliasing error. This shall be implemented with the MAGIC mission in future. With the ongoing developments in miniaturization of satellites and gravity-relevant instruments (accelerometers and intersatellite ranging), in future constellations of multiple small satellite pairs may solve this problem even beyond the capabilities of a Bender constellation. Therefore, in this study the capabilities of such constellations flying in specific formations are investigated in order to enable a retrieval of the temporal gravity field on short time scales. We assess the performance of up to 18 satellite pairs. The satellite configurations cover satellite pairs in polar and inclined orbits flying in pair-wise or pearl-string formation with varying mean anomalies and right ascensions of the ascending node (RAAN). As future potential miniaturized instruments optomechanical accelerometers with similar performance as those flying on GRACE-FO are a candidate, while for the intersatellite ranging instrument still some technological development is required. Therefore, in this study a microwave ranging system equivalent to the GRACE and GRACE-FO instruments performance is taken as baseline assuming that such instruments can be miniaturized in future as well. In numerical closed-loop simulations, up to nine different satellite configurations with up to 18 satellite pairs are evaluated based on the retrieval of the non-tidal temporal gravity field on a monthly basis. From the simulation results it is concluded that the best-performing satellite constellation of 18 polar satellite pairs already is outperformed by a typical Bender-like constellation of 1 polar and 1 inclined pair. In general, we identify that increasing the number of satellite pairs leads to an improved gravity field retrieval, either at low spherical harmonic degree and order (d/o) by the shift in RAAN or at high d/o by the shift in mean anomaly. By a two-step simulation approach, co-estimating also (sub-)daily gravity fields for selected configurations with a large number of satellite pairs distributed equally over the globe, it is possible to retrieve stand-alone gravity fields at 24, 12 and 6 hr temporal resolution. Ultimately it is concluded that a network of miniaturized satellites with instrument performances similar to GRACE-FO and flying in a well-defined constellation has the potential to observe (sub-)daily mass variations and therefore could drastically reduce the problem of temporal aliasing due to high frequency mass variations in the Earth system.

Fletcher, Jon B; Erdem, Jemile E; Baker, Lawrence M

doi: 10.1093/gji/ggad143pmid: N/A

SUMMARYWe use records from 24 stations with epicentral distances under 50 km to invert for slip and rupture time of an M4.5 aftershock of the 2019 Ridgecrest, California, earthquake sequence. Slip was determined by inverting source time functions (STFs), which were obtained at each station by decomposing a main shock, in this case an M4.5 aftershock, into a series of subevents using an empirical Green's function (eGf) as a template. The technique is iterative in the time domain with a positivity constraint. The STFs are characterized by a single pulse with durations that were longer in the southeast direction (about 1.0 s) and shorter in the northwest direction (about 0.6 s) suggesting rupture directivity to the northwest. We inverted the STFs for slip using a tomographic algorithm from that uses a high-frequency approximation to ray theory for the forward problem. The tomographic algorithm solves for slip and rupture time on a grid of points, which provides a better strategy for elucidating directivity than a model that uses subfaults. We solve for the strike-slip intensity on a near-vertical plane striking northwest, similar to the M7.1. After nine iterations, the slip model reduces the chi-squared estimate of fit by 68 per cent. Most of the slip is to the northwest, with a narrow well-defined peak in slip about 0.3 km NW of the origin. The peak is fairly symmetrical and about 1.5 km across its base. The model shows significant directivity to the northwest as suggested by the azimuthal variation of the STF pulse widths. Rupture velocity increases from nucleation to the peak in slip, becomes supershear, and then slows to about 3.2 km s−1, but there is little slip beyond 0.8 s contour in the forward direction. It appears that the peak in slip contributed stress to accelerate the rupture to the northwest. Low-level, parabolic-shaped slip patterns are seen to the northwest, which could be where the rupture front is dying out. Similar, but fainter curves facing the southeast suggests some bilateral rupture but the rupture to the northwest was stronger. Static stress drop was calculated using a formula for an elastic half-space giving a maximum of 35 bars at the peak in slip, but smoothing controls the absolute value. The average over a presumed fault plane is about 1.4 bars, much less than the peak. Slip and stress drop are highly heterogeneous.

Voigt, Christian; Sulzbach, Roman; Timmen, Ludger; Dobslaw, Henryk; Weise, Adelheid; Deng, Zhiguo; Stolarczuk, Nico; Pflug, Hartmut; Peters, Heino; Fietz, Michael; Thomas, Maik; Förste, Christoph; Flechtner, Frank

doi: 10.1093/gji/ggad147pmid: N/A

SUMMARYThe superconducting gravimeter GWR iGrav 047 has been installed on the small offshore island of Heligoland in the North Sea approximately at sea level with the overall aim of high-accuracy determination of regional tidal and non-tidal ocean loading signals. For validation, a second gravimeter (gPhoneX 152) has been setup within a gravity gradiometer approach to observe temporal gravity variations in parallel on the upper land of Heligoland. This study covers the determination of regional ocean tide loading (OTL) parameters based on the two continuous gravimetric time-series after elimination of the height-dependent gravity component by empirical transfer functions between the local sea level from a nearby tide gauge and local attraction effects. After reduction of all gravity recordings to sea level, both gravimeters provide very similar height-independent OTL parameters for the eight major diurnal and semidiurnal waves with estimated amplitudes between 0.3 nm s−2 (Q1) and 11 nm s−2 (M2) and RMSE of 0.1–0.2 nm s−2 for 2 yr of iGrav 047 observations and a factor of 2 worse for 1.5 yr of gPhoneX 152 observations. The mean absolute OTL amplitude differences are 0.3 nm s−2 between iGrav 047 and gPhoneX 152, 0.4 nm s−2 between iGrav 047 and the ocean tide model FES2014b and 0.7 nm s−2 between gPhoneX 152 and FES2014b which is in good agreement with the uncertainty estimations. As by-product of this study, OTL vertical displacements are estimated from the height-independent OTL gravity results from iGrav 047 applying proportionality factors ${\rm d}h/{\rm d}g$ for the eight major waves. These height-to-gravity ratios and the corresponding phase shifts are derived from FES2014b. The OTL vertical displacements from iGrav 047 are estimated with amplitudes between 0.4 mm (Q1) and 5.1 mm (M2) and RMSE of 0.1–0.7 mm. These OTL amplitudes agree with FES2014b within 0.0 (M2) and 0.8 mm (K1) with a mean difference of 0.3 mm only. The OTL amplitudes from almost 5 yr of GNSS observations show deviations of up to 6 mm (M2) compared to vertical displacements from both iGrav 047 and FES2014b, which suggests systematic effects included in the estimation of OTL vertical displacements from GNSS. With the demonstrated accuracy, height-independent sensitivity in terms of gravity and vertical displacements along with the high temporal resolution and the even better performance with length of time-series, iGrav 047 delivers the best observational signal for OTL which is representative for a large part of the North Sea.

Wang, Yichuan; Bedle, Heather; Marfurt, Kurt J

doi: 10.1093/gji/ggad148pmid: N/A

SUMMARYGas hydrates that occur on many continental margins have received global attention. In reflection seismic imaging, the bottom-simulating reflector (BSR) is a common indicator of gas hydrates. However, it is difficult to identify gas hydrates and quantify their amounts through the BSR alone. For gas-hydrate characterization, it is therefore useful to measure seismic stratigraphic and attenuation attributes. Short-scale patterns of layering that contain information about the amount and mechanism of gas hydrates can be identified through stratigraphic and attenuation attributes. We measure the complete time-variant spectra by using sparse strongest peaks, and the spectral differences at different times through attenuation parameters Q–1 and γ. The traditional Q–1 is associated with the attenuation of the frequency-dependent part of wavefield, and the γ characterizes the frequency-independent attenuation. The measurement approach is straightforward and requires no sophisticated inverse algorithm and is applied to surface seismic data acquired over the Hikurangi and Gondwana margins, eastern New Zealand. High-quality spectral and attenuation images are obtained. Spectral attributes correlate with BSRs and large positive Q–1 and negative γ-values are below and above the BSRs, which are interpreted as being related to free-gas and gas-hydrate accumulations. These results will aid the quantification of gas hydrates and the assessment of their roles as an energy resource, as a potential geological hazard, and in climate change and ocean warming.

Roselli, P; Improta, L; Kwiatek, G; Martínez-Garzón, P; Saccorotti, G; Lombardi, A M

doi: 10.1093/gji/ggad155pmid: N/A

SUMMARYWe present the results from a fully unconstrained moment tensor inversion of induced seismic events in a complex and high seismic hazard region (Val d'Agri basin, Southern Italy). The study area hosts two well-documented cases of induced microseismicity linked to (i) a wastewater injection well of a giant oilfield (the largest in onshore Europe), and (ii) severe seasonal level changes of an artificial lake. In order to gather information on the non-double-couple components of the source and to better understand the rupture mechanisms, we analyse seismic events recorded during daily injection tests in the disposal well. The computed moment tensors have significant non-double-couple components that correlate with the well-head injection pressure. The injection parameters strongly influence the rupture mechanism that can be interpreted as due to the opening/closing of a fracture network inside a fault zone of a pre-existing thrust fault. For the case of the reservoir-induced seismicity, no direct correlations are observed with the loading/unloading of the reservoir.

Datta, Arjun; Shekar, Bharath; Kumar, Pushp L

doi: 10.1093/gji/ggad158pmid: N/A

SUMMARYWe present a method for estimating seismic ambient noise sources by acoustic full waveform inversion (FWI) of interstation cross-correlations. The method is valid at local scales for laterally heterogeneous media, and ambient noise sources confined to the Earth’s surface. Synthetic tests performed using an actual field array geometry, are used to illustrate three unique aspects of our work. First: the method is able to recover noise sources of arbitrary spatial distribution, both within and outside the receiver array, with high fidelity. This holds true for complex velocity models and does not require a good initial guess for inversion, thereby addressing an outstanding issue in the existing research literature. Second: we analyse the extent of biases in source inversion that arise due to inaccurate velocity models. Our findings indicate that source inversion using simplified (e.g. homogeneous) velocity models may work reliably when lateral variations in velocity structure are limited to 5 or 10 per cent in magnitude, but is vitiated by strong variations of 20 per cent or higher, wherein the effect of strong scattering and/or phase distortions become significant. Finally, our technique is implemented without the adjoint method, which is usually inextricably linked to FWI. Inversions are performed using source kernels computed for each receiver pair, and this approach is computationally tractable for real-world problems with small aperture seismic arrays.

Moncinhatto, Thiago R; de Oliveira, Wellington P; Haag, Mauricio B; Hartmann, Gelvam A; Savian, Jairo F; Poletti, Wilbor; Brandt, Daniele; Sommer, Carlos A; Caselli, Alberto T; Trindade, Ricardo I F

doi: 10.1093/gji/ggad166pmid: N/A

SUMMARYPalaeosecular variation (PSV) determinations and studies of the geometry of the Earth’s main magnetic field provide important information about the field evolution, and to constrain numerical geodynamo models. Palaeomagnetic directional data from lava flows over the last few million years is of particular interest because the regional and global tectonic effects are minimal. However, the distribution of this type of palaeomagnetic data is uneven where the Southern Hemisphere is the destitute side. Therefore, the better knowledge of the geomagnetic field behaviour depends on the increased availability of high-quality data, especially in the Southern Hemisphere. A PSV and the time-averaged field (TAF) study was then performed in 0–5 Ma lava flows from the Caviahue–Copahue Volcanic Complex located in Northern Patagonia, Argentina (37°0′S, 71°10′W). The magnetic mineralogy of lava flows was investigated through thermomagnetic susceptibility curves, isothermal remanent magnetization (IRM) acquisition curves, hysteresis loops and first-order reversal curves (FORCs). Samples are essentially comprised of titanomagnetite with different Ti contents and magnetic domain structures typical of vortex state particles. A total of 50 volcanic sites were sampled, which provided 42 reliable palaeomagnetic site-mean directions after alternating field and thermal demagnetization. From these 42 sites, 36 are of normal and 6 are of reversed polarity. The mean direction from normal (reversed) sites is D = 356.2°, I = −50.1°, α95 = 4.0° and N = 36 (D = 176.5°, I = 59.5°, α95 = 14.1° and N = 6). Using only site-level data with the precision parameter k ≥ 100, we obtain 26 palaeomagnetic sites for PSV and TAF investigations in the study region. The filtered data set has a mean direction (D = 354.4°, I = −53.2° and α95 = 5.1°) close to the expected direction for a geocentric axial dipole (GAD) field (IGAD = −57.3°). The palaeopole (Plat = 84.4°, Plon = 229.1° and A95 = 5.7°) coincides with the Earth’s spin axis within the 95 per cent confidence interval. Virtual geomagnetic pole scatter ($S_{B} = {15.8^{18.9}_{11.8}}^{\circ }$) and the inclination anomaly ($\Delta I = 4.1_{-1.0}^{{9.2}^{\circ }}$) are both consistent at the 95 per cent confidence level with recent PSV and TAF models, respectively. Our results support the presence of small non-dipole field contributions (<3 per cent) superimposed on the GAD term, as reported by South American studies at mid southern latitudes.

Sato, Haruo; Emoto, Kentaro

doi: 10.1093/gji/ggad169pmid: N/A

SUMMARYFor the study of the random velocity fluctuation of the solid Earth medium, it is useful to measure the collapse of a seismic wavelet with increasing travel distance and the excitation of coda waves. Radiative transfer theory (RTT) is a powerful tool for synthesizing the propagation of a seismic wavelet in random media statistically characterized by the power spectral density function (PSDF) of the fractional velocity fluctuation. The Born scattering coefficient is a key building block of RTT. As the centre wavenumber of a wavelet increases, the phase shift across the correlation length increases and the Born approximation leads to an extremely large forward scattering exceeding the applicable range of the perturbation method. In such a case, the Eikonal approximation is able to explain the envelope broadening with increasing travel distance; however, it can not explain the coda excitation. To overcome the difficulty, we have proposed a hybrid Monte Carlo (MC) simulation for scalar waves. In the case of von Kármán-type random media, when the centre wavenumber is higher than the corner wavenumber, taking the centre wavenumber as a reference, we divide the PSDF into two spectral components. Applying the Born and Eikonal approximations to the high- and low-wavenumber spectral components, we statistically evaluate the wide-angle scattering and the narrow-angle ray bending, respectively. The proposed MC simulation serially using two kinds of scattering processes successfully synthesizes the time trace of the wave energy density from the onset to the late coda. The travel-distance fluctuation derived from the one-way propagation of the Eikonal approximation is also important. This paper extends this method for the propagation of a vector wavelet in random elastic media. We suppose that random fractional fluctuations of the P- and S-wave velocities and the mass density are linearly proportional to each other, which maintains the linear polarization of an Swave throughout the scattering process. Using the hybrid MC simulation with the spectrum division, we synthesize three-component energy density time traces for the anisotropic radiation from a moment tensor source, from which we derive three-component root mean square (RMS) velocity amplitude time traces at different azimuths. In parallel, we synthesize the propagation of a vector wavelet in many realized random elastic media by the finite-difference simulation, then we calculate three-component RMS velocity amplitude time traces. Using them as a benchmark, we confirm the validity of the proposed MC simulation for specific cases.

Biondini, E; Rhoades, D A; Gasperini, P

doi: 10.1093/gji/ggad123pmid: N/A

SUMMARYThe Every Earthquake a Precursor According to Scale (EEPAS) forecasting model is a space–time point-process model based on the precursory scale increase ($\psi $ ) phenomenon and associated predictive scaling relations. It has been previously applied to New Zealand, California and Japan earthquakes with target magnitude thresholds varying from about 5–7. In all previous application, computations were done using the computer code implemented in Fortran language by the model authors. In this work, we applied it to Italy using a suite of computing codes completely rewritten in Matlab. We first compared the two software codes to ensure the convergence and adequate coincidence between the estimated model parameters for a simple region capable of being analysed by both software codes. Then, using the rewritten codes, we optimized the parameters for a different and more complex polygon of analysis using the Homogenized Instrumental Seismic Catalogue data from 1990 to 2011. We then perform a pseudo-prospective forecasting experiment of Italian earthquakes from 2012 to 2021 with Mw ≥ 5.0 and compare the forecasting skill of EEPAS with those obtained by other time independent (Spatially Uniform Poisson, Spatially Variable Poisson and PPE: Proximity to Past Earthquakes) and time dependent [Epidemic Type Aftershock Sequence (ETAS)] forecasting models using the information gain per active cell. The preference goes to the ETAS model for short time intervals (3 months) and to the EEPAS model for longer time intervals (6 months to 10 yr).