Mid-lithosphere discontinuities are seismic interfaces likely located within the lithospheric mantle of stable cratons, which typically represent velocities decreasing with depth. The origins of these interfaces are poorly understood due to the difficulties in both characterizing them seismically and reconciling the observations with thermal-chemical models of cratons. Metasomatism of the cratonic lithosphere has been reported by numerous geochemical and petrological studies worldwide, yet its seismic signature remains elusive. Here, we identify two distinct mid-lithosphere discontinuities at ~89 and ~115 km depth beneath the eastern Wyoming craton and the southwestern Superior craton by analyzing seismic data recorded by two longstanding stations. Our waveform modeling shows that the shallow and deep interfaces represent isotropic velocity drops of 2–9% and 3–10%, respectively, depending on the contributions from changes in radial anisotropy and density. By building a thermal-chemical model including the regional xenolith thermobarometry constraints and the experimental phase-equilibrium data of mantle metasomatism, we show that the shallow interface probably represents the metasomatic front, below which hydrous minerals such as amphibole and phlogopite are present, whereas the deep interface may be caused by the onset of carbonated partial melting. The hydrous minerals and melts are products of mantle metasomatism, with CO2-H2O-rich siliceous melt as a probable metasomatic reagent. Our results suggest that mantle metasomatism is probably an important cause of mid-lithosphere discontinuities worldwide, especially near craton boundaries, where the mantle lithosphere may be intensely metasomatized by fluids and melts released by subducting slabs.

Lithospheric discontinuities, including the lithosphere-asthenosphere boundary (LAB) and the enigmatic mid-lithospheric discontinuities (MLDs), hold important clues about the structure and evolution of tectonic plates. However, P- and S-receiver-function techniques (PRF and SRF), two traditional techniques to image Earth’s deep discontinuities, have some shortcomings in imaging lithosphere discontinuities. Here, we propose a new method using reflections generated by teleseismic S waves (hereafter S reflections) to image lithospheric discontinuities, which is less affected by multiple phases than PRFs and has better depth resolution than SRFs. We apply this method to data collected by the Transportable Array and other regional seismic networks and obtain new high-resolution images of the lithosphere below the contiguous US. Beneath the tectonically active Western US, we observe a negative polarity reflector (NPR) in the depth range of 60–110 km, with greatly varying amplitude and depth, which correlates with active tectonic processes. We interpret this feature as the lithosphere-asthenosphere boundary below the Western US. Beneath the tectonically stable Central and Eastern US, we observe two NPRs in the depth ranges of 60–100 km and 100–150 km, whose amplitude and depth also vary significantly, and which appear to correlate with past tectonic processes. We interpret these features as mid-lithospheric discontinuities below the Central and Eastern US. Our results show reasonable agreement with results from PRFs, which have similar depth resolution, suggesting the possibility of joint inversion of S reflections and PRFs to constrain the properties of lithospheric discontinuities.

Seismic noise has been widely used to image Earth's structure in the past decades as a powerful supplement to earthquake signals. Although the seismic noise field contains both surface-wave and body-wave components, most previous studies have focused on surface waves due to their large amplitudes. Here, we use array analyses to identify body-wave noise traveling as PKP waves. We find that by cross-correlating the array-stacked horizontal- and vertical-component data in the time windows containing the PKP noise signals, we extract a phase likely representing PKS-PKP, the differential phase between PKS and PKP. This phase can potentially be used for shear-wave-splitting analysis. Our results also suggest that the sources of body-wave noise are extremely heterogeneous in both space and time, which should be accounted for in future studies

Ocean transform faults often generate characteristic earthquakes that repeatedly rupture the same fault patches. The westernmost Gofar transform fault quasi-periodically hosts ~M6 earthquakes every ~5 years, and microseismicity suggests that the fault is segmented into five distinct zones, including a rupture barrier zone that may have modulated the rupture of adjacent M6 earthquakes. However, the relationship between the systematic slip behavior of the Gofar fault and the fault material properties is still poorly known. Specifically, the role of pore fluids in regulating the slip of the Gofar fault is unclear. Here, we develop a new method using differential arrival times between nearby earthquakes to estimate the in-situ Vp/Vs ratio of the fault-zone materials. We apply this technique to the dataset collected by an ocean-bottom-seismometer network deployed around the Gofar fault in 2008, which recorded abundant microearthquakes, and find a moderate Vp/Vs ratio of 1.75–1.80 in the rupture barrier zone and a low Vp/Vs ratio of 1.61–1.69 in the down-dip edge of the 2008 M6 rupture zone. This lateral variation in Vp/Vs ratio may be caused by both pore fluids and chemical alteration. We also find a 5–10% increase in Vp/Vs ratio in the barrier zone during the nine months before the mainshock. This increase may have been caused by fluid migrations or slip transients in the barrier zone.