JGR–Solid Earth

We determine high-resolution tomographic models of isotropic P-wave velocity (Vp) and tilting-axis anisotropy of the Alaska subduction zone using a large number of local and teleseismic data recorded at many portable and permanent network stations in and around Alaska. We find a flat high-Vp slab in the mantle transition zone (410–670 km depths) beneath western Alaska, which is connected with the subducting Pacific slab at 0–410 km depths, suggesting that a big mantle wedge has formed under western Alaska. Our tilting-axis anisotropy model reveals complex mantle flows in the asthenosphere. Corner flow in the mantle wedge above the subducting Pacific slab and toroidal flow in the big mantle wedge are revealed, which may cause the Cenozoic intraplate volcanoes in western Alaska and the Bering Sea. In central Alaska, the mantle wedge beneath the Denali volcanic gap is characterized by high-Vp and subhorizontal fast velocity directions normal to the volcanic arc, which may reflect a remnant of the subducted Yakutat slab. In SE Alaska, the shallow subduction of the Wrangell slab is visible above 150 km depth, and hot mantle upwelling through the Wrangell-Yakutat slab gap may contribute to the Wrangell volcanic field.

Two-phase flow, a system where Stokes flow and Darcy flow are coupled, is of great importance in the Earth's interior, such as in subduction zones, mid-ocean ridges, and hotspots. However, it remains challenging to solve the two-phase equations accurately in the zero-porosity limit, for example, when melt is fully frozen below solidus temperature. Here we propose a new three-field formulation of the two-phase system, with solid velocity (v s ), total pressure (P t ), and fluid pressure (P f ) as unknowns, and present a robust finite-element implementation, which can be used to solve problems in which domains of both zero porosity and non-zero porosity are present. The reformulated equations include regularization to avoid singularities and exactly recover to the standard single-phase incompressible Stokes problem at zero porosity. We verify the correctness of our implementation using the method of manufactured solutions and analytic solutions and demonstrate that we can obtain the expected convergence rates in both space and time. Example experiments, such as self-compaction, falling block, and mid-ocean ridge spreading show that this formulation can robustly resolve zero- and non-zero-porosity domains simultaneously, and can be used for a large range of applications in various geodynamic settings.

The magnitude-frequency distribution (MFD) describes the relative proportion of earthquake magnitudes and provides vital information for seismic hazard assessment. The b-value, derived from the MFD, is commonly used to estimate the probability that a future earthquake will exceed a specified magnitude threshold. Improved MFD and b-value estimates are of great importance in the central and eastern United States where high volumes of fluid injection have contributed to a significant rise in seismicity over the last decade. In this study, we recalculate the magnitudes of 8,775 events for the 2011 Prague, Oklahoma sequence using a relative magnitude approach that depends only on waveform data to calculate magnitudes. We also compare the distribution of successive magnitude differences to the MFD and show that a combination of the magnitude difference distribution (MDFD) and relative magnitudes yields a reliable estimate of b-value. Using the MDFD and relative magnitudes, we examine the temporal and spatial variations in the b-value and show that b-value ranges between ∼0.6 and 0.85 during the aftershock sequence for at least 5 months after the M 5.7 mainshock, though areas surrounding the northeast part of the sequence experience higher b-values (0.7–0.85) than the southwestern part of the Meeker-Prague fault where b-value is the lowest (0.6–0.7). We also identify a cluster of off-fault events with the highest b-values in the catalog (0.85). These new estimates of MFD and b-value will contribute to understanding of the relations between induced and tectonic earthquake sequences and promote discussion regarding the use of b-value in induced seismic hazard estimation.

The Conrad Rise (CR), located midway between Antarctica and the Southwest Indian Ridge (SWIR), remains one of the least explored submarine large igneous provinces (LIPs) in the Indian Ocean to date. Relying on only seafloor paleomagnetic records, early studies hypothesized that the formation of the CR occurred during the Late Cretaceous. Here, we present new geochemical and geochronological data, including Sr‒Nd‒Pb‒Hf isotopes and 40Ar/39Ar data. Our results indicate that the uppermost part of the CR (Ob and Lena seamounts) unexpectedly formed later than previously predicted, at approximately 40 Ma in an intraplate setting. Another small seamount north of the Ob seamount formed later, at 8.5 Ma. The isotopic composition of lava from the small seamount north of the Ob seamount overlaps with that commonly defined by the Indian plume component. Overall, the isotopic variations defined by the volcanic suite from the CR could be accounted for by a three-component mixing model involving the common component, lower continental crust, and depleted mantle endmembers. The newly obtained 40Ar/39Ar ages imply that the CR volcanism might have been triggered by major regional plate reorganizations during the middle to late Eocene and the late Miocene, inducing the release of a small upwelling rising from the African large low-velocity province.

The Cretaceous Normal Superchron (CNS, 84–121 Ma) is a singular period of the geodynamo's history, identified by a prolonged absence of polarity reversals. To better characterize the paleosecular variation (PSV) of the geomagnetic field at the end of this interval, we sampled seven continuous sequences of lava flows from the Okhotsk-Chukotka Volcanic Belt, emplaced 84–89 Ma in the vicinity of the Kupol ore deposit (NE Russia). From a collection of 1,024 paleomagnetic cores out of 82 investigated lava flows, we successfully determined the paleodirections of 78 lava flows, which led to 57 directional groups after removing the serial correlations. The resulting paleomagnetic pole is located at 170.0°E, 76.8°N (A 95 = 5.2°, N = 57), in good agreement with previous estimates for north-eastern Eurasia. Aiming at quantifying PSV at a reconstructed paleolatitude (λ) of ∼80°N, we obtained a virtual geomagnetic pole (VGP) scatter Sb=21.5°|19.3°24.0°(N=57) ${{S}_{\mathrm{b}}=21.5{}^{\circ}\vert }_{19.3{}^{\circ}}^{24.0{}^{\circ}}\,(N=57)$, the value of which was corrected for within-site dispersion and is little dependent on the choice of the selection criteria. Compared to previous paleodirectional data sets characterizing PSV at various paleolatitudes during the CNS, our S b estimate confirms a relative latitudinal increase S b(λ = 90°)/S b(λ = 0°) on the order of 2–2.5. Focusing on PSV at high paleolatitude within the 70°–90° range, we show that S b was ∼15% lower at the end of the CNS than during the past 10 Myr, confirming that the singular polarity regime of the geodynamo observed during the CNS is likely accompanied with reduced PSV.

Submarine volcano monitoring is vital for assessing volcanic hazards but challenging in remote and inaccessible environments. In the vicinity of Kita-Ioto Island, south of Japan, unusual M ∼ 5 non-double-couple volcanic earthquakes exhibited quasi-regular recurrence near a submarine caldera. Following the earthquakes in 2008 and 2015, a distant ocean bottom pressure sensor recorded distinct tsunami signals. In this study, we aim to find a source model of the tsunami-generating earthquake and quantify the pre-seismic magma overpressure within the caldera's magma reservoir. Based on the earthquake's characteristic focal mechanism and efficient tsunami generation, we hypothesize that submarine trapdoor faulting occurred due to highly pressurized magma. To investigate this hypothesis, we establish mechanical earthquake models that link pre-seismic magma overpressure to the size of the resulting trapdoor faulting, by considering stress interaction between a ring-fault system and a reservoir of the caldera. The trapdoor faulting with large fault slip due to magma-induced shear stress in the submarine caldera reproduces well the observed tsunami waveform. Due to limited data, uncertainties in the fault geometry persist, leading to variations of magma overpressure estimation: the pre-seismic magma overpressure ranging approximately from 5 to 20 MPa, and the co-seismic pressure drop ratio from 10% to 40%. Although better constraints on the fault geometry are required for robust magma pressure quantification, this study shows that magmatic systems beneath calderas are influenced significantly by intra-caldera fault systems and that tsunamigenic trapdoor faulting provides rare opportunities to obtain quantitative insights into remote submarine volcanism hidden under the ocean.

No abstract is available for this article.