Geophysical Journal International

SummaryThe Hainan volcanic field (HNVF) is one of China’s most active Holocene volcanic areas. Due to a lack of comprehensive geophysical research, questions persist regarding the deep magma system of the HNVF. For example, it is unclear whether the intense seismic activity in its eastern part may be a precursor to renewed volcanic activity. We present new three-dimensional electrical conductivity images, derived from magnetotelluric data, that provide a new understanding of the deep magma system in the HNVF. Our results reveal the presence of multiple sets of low-resistivity structures in both shallow and deep regions. Although once associated with past volcanic activity, a widespread shallow low-resistivity layer on the northwest side of the HNVF is not currently indicative of shallow magma chambers. Instead, a deeper large-volume low-resistivity structure in the western part of the HNVF may represent the current crustal magmatic plumbing system. Our analysis suggests that the intense seismic activity in the east of HNVF lacks corresponding low-resistivity structures, which indicates that there is no direct correlation between seismicity and movement of magma. Recent volcanic eruptions are primarily concentrated near the Changliu-Xiangou fault, which may indicate that the migration of magma has utilized crustal weak zones.

SummaryUnderstanding the effects of pore pressure changes on soil stability is important in geohazards and geotechnical studies. In situ measurements of PP are difficult at large scales. Geophysical methods can offer an indirect approach in understanding the effects of PP in soils. In this laboratory study, we investigate the complex conductivity (CC) signatures of soils undergoing increasing pore pressure inside a rigid cylinder. We experimented on different synthetic soil mixtures (with various clay percentages) as well as a natural soil sample collected from central Oklahoma, United States. We measured the CC response of the soil as we increased the pore pressure in small increments starting from atmospheric pressure up to 200 kPa. Our results show that the CC method is sensitive to changes in pore pressure values with imaginary conductivity magnitude increasing with increasing pore pressure for the samples containing clay minerals. The pure sand soil sample showed a less pronounced yet similar trend to clayey mixtures. The natural soil sample and samples with montmorillonite showed a direct relationship between imaginary conductivity and PP while real conductivity and PP showed an inverse relationship. In the samples without montmorillonite, we observed no changes in the characteristic relaxation time (τpeak) indicating no pore geometry changes in these samples. However, the samples with montmorillonite showed a direct linear relationship between PP and τpeak. Our findings indicate that under our controlled conditions, the CC measurements are sensitive to PP changes in clayey natural and synthetic soils, and although further research in the field with site-specific calibration, a wider spectrum of natural soil types and larger PP increments, is needed to validate our results; this is a starting point to evaluate the possible sensitivities of CC measurements to PP changes in earth materials.

SummaryThe Neuwied Basin within the East Eifel Volcanic Field (EEVF) is characterized by increased microseismicity, long hypothesized to be linked to the subsurface Ochtendung Fault Zone (OFZ). However, the source of this unrest remained elusive due to limited hypocenter resolution. Here, we present an extended local earthquake catalogue, compiled from a year-long Large-N deployment and a machine learning-based detection and location approach, including over 1,000 microearthquakes recorded between September 2022 and August 2023. This high-resolution dataset reveals new seismicity clusters, repeated waveforms, and distinct temporal bursts of activity, suggesting fluid-induced earthquake triggering. Probabilistic moment tensor inversion for 192 high-quality events (Mw 0.6-2.7) resolves predominantly strike-slip faulting along the OFZ, with localised clusters of normal faulting nearby, potentially associated with a previously unknown border fault of the NWB. Notably, we observe systematic rotations in P-axis orientations along the OFZ, which we interpret as localized stress perturbations induced by an overpressured reservoir beneath the Laacher See volcano - the youngest explosive eruption centre in the EEVF. These patterns, coupled with elevated magmatic CO2 emissions in the region and high waveform similarity, suggest that active magmatic and transcrustal fluid processes are influencing the stress regimes and driving the seismicity in the NWB. Our high-resolution seismicity and moment tensor catalogue offers new insights into the interplay between tectonics and fluid-driven processes beneath the youngest volcanoes in the EEVF.

SummaryThis study presents the first comparative measurements of seismic attenuation between Mauna Loa and Kīlauea volcanos on Hawai’i Island. The focus is on key physical variables found within Kīlauea, and extending our knowledge of these from Kīlauea to Mauna Loa. The measurements of attenuation, elastic/anelastic moduli (µ rigidity and K bulk), T temperature, P pressure, basalt activation energy, are uniformly applied to these adjacent volcanos (34 km separation) for comparative analyses. While numerous seismic attenuation studies have been conducted at Kīlauea, Mauna Loa has remained unexamined in this context until now. We extend previous methodologies to measure both shear (Qµ) and bulk (QK) attenuation over propagation paths from both volcanic calderas to the Aloha Cabled Observatory (ACO), located 442-464 km away at 4728 m depth. Utilizing earthquake displacement source spectra from shallow (near sea level) events beneath both calderas, we derive frequency-dependent effective Q values across the 2-35 Hz frequency band. Our analytical approach employs the t* formulation (ratio of travel time to Q) to separate attenuation along path segments, allowing direct comparison between the two volcanic systems. Results reveal that Mauna Loa exhibits substantially higher attenuation (lower Q values) than Kīlauea for both bulk and shear waves. At 10 Hz, Qµ is approximately four times higher for Kīlauea (∼400) than Mauna Loa (∼115), while QK displays even greater contrast with Kīlauea (∼425) exceeding Mauna Loa (∼25) by a factor of 17. Both volcanoes demonstrate QK < Qµ across most frequencies, emphasizing the significance of bulk losses in volcanic environments. This contradicts traditional assumptions held, that bulk attenuation is negligible in Earth. The pronounced difference in attenuation between these adjacent volcanoes, which share the same hot spot origin, cannot be explained solely by temperature-pressure dependent activation energy models. While we calculated expected Q variations using established basalt activation energies (59-68 kJ/mole), the observed differences exceed predictions by an order of magnitude. This suggests additional mechanisms are at work, likely involving partial melting processes. Our findings indicate that the internal structure of Mauna Loa may contain a greater proportion of partial melt or different melt geometry than Kīlauea, significantly affecting seismic wave propagation. At higher frequencies (17-33 Hz), both volcanoes show evidence of comparable scattering effects. This research provides new insights into the internal composition and dynamics of Hawaiian volcanoes, demonstrating that despite their proximity and shared magmatic source, Mauna Loa and Kīlauea possess distinctly different attenuation characteristics that reflect fundamental differences in their internal structure and melt distribution. These findings enhance our understanding of volcanic processes and contribute to improved interpretation of seismic data in volcanic environments.

SummaryCentral America’s tectonic complexity arises from the interaction of multiple plates and diverse plate boundaries, resulting in high seismic activity and intricate subduction processes. Three-dimensional seismic velocity models can provide critical constraints on subduction processes and associated earthquake hazard models. Although regional tomographic studies have offered insights into seismic activity and lithospheric processes in Central America, there have been few studies that image the entire region in a consistent manner, likely due to the geological complexity and numerical challenges. In this study, we develop a new high-resolution three-dimensional (3-D) compressional (P)-wave velocity model to investigate the subduction dynamics of the region. We apply the teletomoDD method, which uses both local and global body-wave arrivals to resolve velocity structures. We use the International Seismological Center’s EHB catalog to extract data from 6,026 regional earthquakes from 1965 to 2019, recorded by seismic stations both inside and outside of our study area. Our model is further constrained by incorporating about 30,000 global events recorded by the seismic stations within our study area. We perform both checkerboard and restoration tests to assess the resolution of the model and find that the main features are resolved robustly regardless of the initial models. The model shows a coherent high-velocity anomalies along the Middle America Trench at 50 km depth, suggesting cold, dense subducting slabs. It also captures notable variations in slab geometry, including a slab window in the southern Cocos region starting at ∼75 km depth. Low-velocity anomalies beneath major volcanic systems such as the Central American Volcanic Arc and the Trans-Mexican Volcanic Belt point to slab dehydration, fluid migration, and partial melting processes, whereas the discontinuous distribution of volcanism in Mexico and Central America appears to be influenced by the subduction of the Cocos Plate. Additionally, we identify high-velocity anomalies near the Siqueiros and Clipperton Transform Faults on the East Pacific Rise, possibly caused by mafic magmatic cumulates. The high-velocity anomaly near Swan Islands Transform Fault may reflect locally increased density inferred from previous gravity studies. Our new velocity model offers a consistent seismic structural foundation for further investigations into seismogenic processes, slab geodynamics, petrology, and rheology in Central America.

SummaryRecently, semi-airborne transient electromagnetic (TEM) systems have gained attention in geophysical investigations due to their ability for fast mapping and minimal ground access requirement. These systems consist of a ground-based transmitter source and an inductive receiver coil, carried by an uncrewed aerial vehicle. This study investigates how transmitter source selection in field-based semi-airborne TEM systems affects model parameter uncertainty, using synthetic subsurface models. The simulated dB/dt responses highlight distinct signal characteristics between the galvanic-based system (herein galvanic source) and the inductive-based system (herein inductive source), with differences observed across varying subsurface conditions. An analysis of four synthetic 3-layer models highlights that the inductive-based system resolves shallow conductors better at short offsets, whereas the galvanic-based system is better at resolving parameters for deeper targets at large offsets. Both systems, however, face challenges in accurately resolving resistive targets embedded between conductors, highlighting the need for strategic selection of the transmitter source. The galvanic-based system consistently achieves a higher signal-to-noise ratio (SNR), particularly at large offsets, making it better suited for deep exploration. In contrast, the inductive-based system exhibits lower SNR, higher noise susceptibility, and sign-changing dB/dt responses at increasing offsets adding complexity to data processing and interpretation. Despite these limitations, inductive-based systems enable earlier time measurements with signal magnitudes at short offsets comparable to galvanic-based, due to shorter current turn-off times. In this analysis we have used two system setups utilizing inductive and galvanic sources that reflect commonly used systems, but obviously, assumptions regarding transmitter characteristics such as type, size, waveform, and current amplitude, will influence the results when examining details more closely.

SummaryAccurate quantification of natural gas hydrate is essential for resource potential and climate impact assessment. Archie’s empirical equations are commonly used to quantify hydrates from electrical resistivity measurements. One dominant Archie equation parameters, i.e. saturation parameter (n), is generally assumed to be constant for different hydrate saturation range for a given reservoir. However, n actually varies with hydrate saturation and morphology, and the exact relationship between n and hydrate saturation or morphology still remains poorly understood, leading to great uncertainties in resistivity-derived saturations. Here we investigate the effect of hydrate saturation and dominant hydrate morphologies on n using well logs from four sites in both fine and coarse-grained sediments: two sites with fluid-displacing hydrate (site W11 from the third Guangzhou Marine Geologic Survey; and Mallik 5L-38 well in the Mackenzie Delta) and two sites with fracture-filling hydrate (site 10 from Indian National Gas Hydrate Program Expedition 01; and site W08 from the second Guangzhou Marine Geologic Survey). We calculated n value using Archie’s law with hydrate saturation determined from velocity. Our results demonstrate a clear negative relationship between hydrate content and n value. Moreover, n estimates from two fracture-filling sites show greater variability compared to fluid-displacing sites. At a fracture-filling hydrate site, site 10, various trends between n and hydrate saturation are possibly caused by the distint gas compositions of hydrate. Our results demonstrate that significant effects of hydrate morphology and saturation on n that are site specific, and can be used to enhance the accuracy of gas hydrate quantification.

SummaryFull-waveform inversion (FWI) is a method that utilizes seismic data to invert the physical parameters of subsurface media by minimizing the difference between simulated and observed waveforms. Due to its ill-posed nature, FWI is susceptible to getting trapped in local minima. Consequently, various research efforts have attempted to combine neural networks with FWI to stabilize the inversion process. This study presents a bidirectional physics-constrained full waveform inversion (BP-FWI) framework that leverages transfer learning by pre-training on simple initial models and utilizing the results. Additionally, it employs FWI gradients to co-optimize both the neural network and the adaptive residual learning module under bidirectional physics constraints. By eliminating the reliance on a large amount of manually constructed synthetic datasets, the proposed training strategy addresses the challenge of data dependency. Furthermore, through the joint optimization strategy guided by bidirectional constraints, the neural network is able to focus on integrating physically-informed prior knowledge into global stratigraphic representations, while the adaptive residual learning module specializes in learning residual mappings from the network’s output, thereby capturing subtle inter-layer velocity variations in local geological structures. Evaluating the method on two benchmark models under various conditions, including absent low-frequency data, noise interference, non-uniform receiver configurations, and differing initial models, along with corresponding ablation experiments, consistently demonstrates the superiority of the proposed approach.