Geophysical Journal International

AbstractSurface waves such as Rayleigh, Love and Scholte waves can exhibit dispersion, i.e., variations in phase velocity with wavelength as a function of frequency. This property enables the inversion of 1D models of seismic velocity and density in the subsurface. Conventional deterministic and stochastic inversion schemes are widely applied to surface wave data but face two main challenges. The first is the identification of dispersion curves for fundamental and higher modes on wavefield-transformed images, which is often done manually. The second is the quantification of uncertainty, which can be computationally expensive in stochastic approaches or limited to data-propagated uncertainty in deterministic inversions. Our objectives are to (1) eliminate the need for manual or automatic dispersion curve picking, and (2) directly infer ensembles of 1D velocity models - and their associated uncertainties - from the full velocity spectrum, i.e., the complete dispersion image containing all modes. To this end, we employ Bayesian Evidential Learning, a predictive framework that reproduces experimental data from prior information while allowing prior falsification. In our application, ensembles of prior Earth models are sampled to predict 1D subsurface structures in terms of seismic velocity and, where applicable, attenuation from near-surface seismic wave data. This approach bypasses traditional inversion schemes and provides a computationally efficient tool for uncertainty quantification.

SummaryRepeating earthquakes are believed to result from recurring ruptures of a single asperity, driven by surrounding aseismic creep. However, their occurrence and behavior in intraplate regions remain poorly understood. This study investigates the repeating earthquakes in the Gyeryongsan region of the Korean Peninsula, a tectonically stable intraplate region, following the 2008 Mw 3.56 earthquake. We augmented the earthquake catalogue from 2007 to 2022 using template matching and identified one repeating earthquake family comprising ten events with irregular recurrence intervals. The repeating earthquakes, with a median magnitude of Mw 1.22, occurred within the rupture area of the Mw 3.56 mainshock, beginning in late 2010 and subsequently recurring intermittently between 2011 and 2019. Stress drops of nearby earthquakes increased gradually from 0.3-0.9 MPa to 8.6 MPa over a decade, indicating a fault strength recovery period substantially longer than that typically observed at plate boundaries. We interpret that the earthquakes occurred within a damaged fault zone, reflecting extremely low loading rates in the intraplate region. Our study provides insights into earthquake behaviour within intraplate damaged fault zones and documents a rare case of a repeating earthquake family that persisted over ∼12 years.

SummaryThe Palaeomagnetic Intensity (PINT) database documents variations in the full-vector of the ancient geomagnetic field that can be used to provide insights into the operation and evolution of the geodynamo. In this study, we report an update of PINT and the evolving behaviour of the palaeomagnetic field since 17 Ma. The update is the addition of 206 recently published site-mean data with ages between 0.06 and 2610 Ma that have been assessed using the palaeointensity quality criteria (QPI). Using this database, we analysed, for the first time, the distribution of values of the palaeosecular variation index (PSVi) in intervals drawn from the past 17 million years. Our results indicate that this index was enhanced prior to 5 Ma reflecting both lower average virtual dipole moments and higher angular deviations of the virtual geomagnetic pole (VGP) from the geographic pole. The present Brunhes chron is highlighted as being associated with especially high measurements of dipole moment which we hypothesise may be related to its already long duration relative to most other chrons of the last 17 Myr.

SummaryThe icy parts of the Earth, known as the cryosphere, are an integral part of the climate system. Comprehensively understanding the cryosphere requires dense observations, not only of its surface, but also of its internal structure and dynamics. Seismic methods play a central role in this endeavour. Fibre-optic sensing is emerging as a valuable complement and alternative to well-established inertial seismometers. Offering metre-scale channel spacing, interrogation distances of up to ∼100 km, and a bandwidth from mHz to kHz, it has enabled new seismological applications, for instance, under water, in cities and on volcanoes. Cryosphere research particularly benefits from fibre-optic sensing because long cables can be deployed with relative ease in icy environments where dense arrays of seismometers are difficult to install, including glaciers, ice sheets and deep boreholes. Intended to facilitate future fibre-optic seismology research in the cryosphere, this Expository Review combines a classical publication review with theoretical background, a practical field guide, a cryospheric signal gallery, and open-access data examples for hands-on training. Following a summary of recent findings about firn and ice structure, glacial seismicity, hydrology and avalanche dynamics, we derive the ideal instrument response of a distributed fibre-optic deformation sensor. To approach this ideal in field experiments, we propose numerous practical dos and don’ts concerning the choice and handling of fibre-optic cables, required equipment, splicing in the field at low temperatures, cable layout and trenching, and the deployment and coupling of cables in boreholes. A cryospheric signal gallery provides examples of data from a wide range of sources, such as explosions, land and air traffic, electricity generators, basal stick-slip icequakes, surface crevassing, englacial icequake cascades, floating ice shelf resonance, surface water flow and snow avalanches. Many of these data are enclosed as an open-access training resource, together with code for reading, visualisation and simple analyses. This review concludes with a discussion of grand open challenges in our understanding of cryosphere structure and dynamics, and how further advances in fibre-optic sensing may help to overcome them.