Geophysical Journal International

SummaryThe Epidemic-Type Aftershock Sequence (ETAS) model is the state-of-the-art approach for modeling short-term earthquake clustering and is preferable for short-term aftershock forecasting. However, due to the large variability of different earthquake sequences, the model parameters must be adjusted to the local seismicity for accurate forecasting. Such an adjustment based on the first aftershocks is hampered by the incompleteness of earthquake catalogs after a mainshock, which can be explained by a blind period of the seismic networks after each earthquake, during which smaller events with lower magnitudes cannot be detected. Assuming a constant blind time, direct relationships based only on this additional parameter can be established between the actual seismicity rate and magnitude distributions and those that can be detected. The ETAS-Incomplete (ETASI) model uses these relationships to estimate the true ETAS parameters and the catalog incompleteness jointly. In this study, we apply the ETASI model to the SE Türkiye earthquake sequence, consisting of a doublet of M7.7 and M7.6 earthquakes that occurred within less than half a day of each other on February 6, 2023. We show that the ETASI model can explain the catalog incompleteness and fits the observed earthquake numbers and magnitudes well. A pseudo-prospective forecasting experiment shows that the daily number of detectable m ≥ 2 can be well predicted based on minimal and incomplete information from early aftershocks. However, the maximum magnitude (Mmax ) of the next day’s aftershocks would have been overestimated due to the highly variable b value within the sequence. Instead, using the regional b value estimated for 2000-2022 would have well predicted the observed Mmax  values.

SummaryAtmospheric and oceanic pressure perturbations deform the ground surface and the seafloor, respectively. This mechanical deformation, where the fluid perturbations propagate as plane waves, occurs not only on Earth but also on other planets/bodies with atmospheres, such as Mars, Titan, and Venus. Studying this type of deformation improves our understanding of the mechanical interaction between the fluid layer (atmosphere/ocean) and the underlying solid planet/body, and aids investigation of subsurface structures. In this study, we utilize eigenfunction theory to unify existing theories for modelling this deformation and to comprehensively demonstrate possible scenarios of this deformation in homogeneous and 1D elastic media, including static loading, air-coupled Rayleigh waves, and leaky-mode surface waves. Our computations quantitatively reveal that the deformation amplitude generally decays with depth and that reducing seismic noise due to Martian atmosphere requires deploying seismometers at least 1 m beneath Martian surface. We also apply our theory to illustrate how this deformation and the corresponding air-to-solid energy conversion vary on different planetary bodies. Finally, we discuss how medium anelasticity and other factors affect this deformation.

SummarySatellite observations of the geomagnetic field contain signals generated in Earth’s interior by electrical currents in the core and by magnetized rocks in the lithosphere. At short wavelengths the lithospheric signal dominates, obscuring the signal from the core. Here we present details of a method to co-estimate separate models for the core and lithospheric fields, which are allowed to overlap in spherical harmonic degree, that makes use of prior information to aid the separation. Using a maximum entropy method we estimate probabilistic models for the time-dependent core field and the static lithospheric field that satisfy constraints provided by satellite observations while being consistent with prior knowledge of the spatial covariance and expected magnitude of each field at its source surface. For the core field, we find that between spherical harmonic degree 13 and 22 power adds coherently to the established structures, and present a synthetic test that illustrates the aspects of the small scale core field that can reliably be retrieved. For the large scale lithospheric field we also find encouraging results, with the strongest signatures below spherical harmonic degree 13 occurring at locations of known prominent lithospheric field anomalies in north-Eastern Europe, Australia and eastern North America. Although the amplitudes of the small scale core field and large scale lithospheric field are underestimated we find no evidence that obvious artefacts are introduced. Compared with conventional maps of the core-mantle boundary field our results suggest more localized normal flux concentrations close to the tangent cylinder, and that low latitude flux concentrations occur in pairs of opposite polarity. Future improvements in the recovery of the small scale core field and large scale lithospheric field will depend on whether more detailed prior information can be reliably extracted from core dynamo and lithospheric magnetisation simulations.

SummaryA theory for modeling the evolution of elastic moduli of grain packs under increasing pressure is combined with a method that accounts for the presence of fine grained particles to develop a new conceptual framework for computing the seismic velocities of compacting sediments. The resulting formulation is then used to constuct a seismic velocity model for California’s Central Valley. Specifically, a set of 44 sonic logs from the San Joaquin Valley are combined with soil textural data to derive the three-dimensional velocity variations in the province. An iterative quasi-Newton minimization algorithm that allows for bounded variables provided estimates of the 9 free parameters in the model. The estimates low- and high-pressure exponents that resulted from the fit to the sonic log velocities are close to 1/2 and 1/3, respectively, values that are observed in laboratory experiments. Our results imply that the grain surfaces are sufficiently rough that there is little or no slip between grains. Thus the deformation may be modeled using a strain energy function or free energy potential. The estimated Central Valley velocity model contains a 27% increase in velocity from the surface to a depth of 700 meters. Lateral variations of around 4% occur within the layers of the model, a consequence of the textural heterogeneity within the subsurface.

SummaryWe define the degenerate orthorhombic anisotropy models which have two symmetric singularity lines with constant phase velocity for S1 and S2 waves. Depending on the singularity line trajectory, we consider two types of degenerate models (VTI- and HTI-type). In addition to this singularity line, there is always one isolated singularity point in one of non-essential symmetry planes. The degenerate orthorhombic model has 7 independent parameters and can be parameterized by different combinations of the stiffness coefficients. Exploiting the fact that the second-order derivatives matrix computed from the Christoffel polynomial is degenerate, we also compute the group velocity image of this singularity line.

SummaryAlong the Aleutian megathrust, the Shumagin seismic gap (162°W–158.5°W) had not hosted a large megathrust earthquake during the observational period 1946-2020. Geodetic evidence suggests a prominent trench-parallel transition from strong to weak kinematic coupling in this segment, indicating varying frictional properties of the megathrust. In July and October 2020, the occurrence of two large but dissimilar earthquakes in this seismic gap (the Mw 7.8 Simeonof Island thrust event on 22 July 2020 followed by the Mw 7.6 Sand Point intra-slab strike-slip event on 19 October 2020) presented a unique opportunity to examine the interaction between stresses on the megathrust and within the downgoing slab. We use geodetic and geophysical evidence to derive a more accurate kinematic coupling model of the megathrust in this area and show that the MW 7.6 Sand Point earthquake within the down-going Pacific slab likely occurred as a result of both trench-perpendicular interseismic shear stress caused by variable kinematic coupling of the megathrust and co-seismic stress changes resulting from the Simeonof Island event. Furthermore, we show that the location of the strike-slip event coincides with along strike change in the megathrust gravitational anomaly and flexural bending of the downgoing plate, suggesting a long-term interaction between megathrust frictional properties and the structure of the plate interface and downgoing slab.

SummaryThis paper proposes the use of the parametric bootstrap as a simple way of obtaining reliable confidence intervals for the parameters in the space-time epidemic-type aftershock sequence (ETAS) model. Using an earthquake data catalogue spanning almost 20 years of seismic activity in the Pacific Ocean off the shores of Vancouver Island (British Columbia, Canada), the authors show through simulation that the confidence intervals based on asymptotic maximum likelihood theory can sometimes be misleading. In contrast, confidence intervals based on empirical quantiles from bootstrap samples have reliable nominal coverage, provided that edge effects are properly taken into account, both in the bootstrap procedure and in simulation studies. An R package called ETASbootstrap, developed by the authors, facilitates the use of this resampling procedure.