Geophysical Journal International

SummaryElastic rock physics models are widely used to estimate the saturation of hydrate in isotropic sediments. However, for isotropic media, the influence of heterogeneously distributed hydrate on the P- and S-wave velocities remains unclear, leading to uncertainties in hydrate saturation estimates. To address this issue, in this work we proposed a double-solid-matrix model for predicting the velocities of sediments hosting heterogeneously distributed hydrates. A comparison of simulated velocities of our model and two rock physics schemes designed for homogeneous distributed hydrate (i.e. matrix-supporting and pore-floating models) show that, our model predicts higher S-wave velocity than matrix-supporting and pore-floating models, but yields similar P-wave velocity estimates as matrix-supporting model. We apply our model to two marine hydrate sites in the Cascadia margin: Site 1245 from Ocean Drilling Program Leg 204 and Site U1328 from International Ocean Drilling Program Expedition 311. Two locations yield similar results: velocity estimates from our model are much closer to downhole measurements than matrix-supporting and pore-floating models. Moreover, we estimate in situ hydrate saturation and clay concentration using our model, matrix-supporting model, and pore-floating model independently, and find that (i) hydrate saturations predicted by our model conform better with the saturations from chloride concentration and (ii) clay contents calculated by our model fit the best with results from smear slide analysis. This study demonstrates that our double-solid-matrix model can be an effective tool to understand the effect of heterogeneously distributed hydrates on velocities, as well as obtain accurate hydrate content in marine isotropic sediments.

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.