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P-wave reflections from the 410- and 660-km mantle discontinuities are visible in stacks of ambient noise cross-correlation functions of USArray stations spanning the contiguous United States. The reflections are most visible on the vertical components at frequencies between 0.1 and 0.3 Hz during low-noise periods, which generally occur during the summer months in the Northern Hemisphere. Common reflection point stacking can be used to resolve apparent lateral differences in discontinuity structure across the continent and suggests the possible existence of sporadic reflectors at other depths. Visibility of the 660-km reflector is correlated with faster P-wave velocities at similar depth in a tomographic model for North America. However, the lack of clear agreement between these P-wave ambient noise features and prior mantle-transition-zone imaging studies using other methods suggests caution should be applied in their interpretation. Ambient noise sources from the southern oceans may not be distributed uniformly enough for cross-correlation stacks to provide unbiased estimates of the true station-to-station P-wave Green’s functions. However, the clear presence of 410- and 660-km reflections in the ambient noise data suggests that it should be possible to unravel the complexities associated with varying noise source locations to produce reliable P-wave reflection profiles, providing new insights into mantle structure under the contiguous United States. 

Abstract Seismograms from two borehole seismometers near the 2019 Ridgecrest, California, aftershock sequence do not return to pre-mainshock noise levels for over ten days after the M 7.1 Ridgecrest mainshock. The observed distribution of root mean square amplitudes in these records can be explained with the Reasenberg and Jones (1989) aftershock occurrence model, which implies a continuous seismic “hum” of overlapping aftershocks of M > −2 occurring at an average rate of 10 events per second after ten days, which prevents observing the background aseismic noise level at times between the body-wave arrivals from cataloged and other clearly observed events. Even after the borehole noise levels return at their quietest times to pre-mainshock conditions, the presence of overlapping low-magnitude earthquakes for 80 days is implied by waveform cross-correlation results provided using the matrix profile method. These results suggest a hidden frontier of tiny earthquakes that potentially can be measured and characterized even in the absence of detection and location of individual events. 

Abstract Mid‐ocean ridges generate basalt and harzburgite, which are introduced into the mantle through subduction as a mechanical mixture, contributing to both lateral and radial compositional heterogeneity. The possible accumulation of basalt in the mantle transition zone has been examined, but details of the mantle composition below the 660‐km discontinuity (hereafter d660) remain poorly constrained. In this study, we utilize the subtle waveform details ofS660S, the underside shear‐wave reflection off the d660, to interpret the seismic velocity, density, and compositional structure near, and particularly below, the d660. We identify a significant difference inS660Swaveform shape in subduction zones compared to other regions. The inversion results reveal globally enriched basalt at the d660, with a notably higher content in subduction zones, consistent with the smaller impedance jump andS660Speak amplitude. The basalt fraction decreases significantly to less than 10% near 800‐km depth, forming a global harzburgite‐enriched layer and resulting in a steep seismic velocity gradient just below the d660, in agreement with 1D global reference models. The striking compositional radial variations near the d660 verify geodynamic predictions and challenge the applicability of homogeneous radial compositional models in the mantle. These variations may also affect the viscosity profile and, consequently, the dynamics at the boundary between the upper and lower mantle. 

Abstract We apply the Matrix Profile algorithm to 100 days of continuous data starting 10 days before the 2019 M 6.4 and M 7.1 Ridgecrest earthquakes from borehole seismic station B921 near the Ridgecrest aftershock sequence. We identify many examples of reversely polarized waveforms, but focus on one particularly striking earthquake pair with strongly negatively correlated P and S waveforms at B921 and several other nearby stations. Waveform‐cross‐correlation‐based relocation of these events indicates they are at about 10 km depth and separated by only 115 m. Individual focal mechanisms are poorly resolved for these events because of the limited number of recording stations with unambiguous P polarities. However, relative P and S polarity and amplitude information can be used to constrain the likely difference in fault plane orientation between the two events to be 5–20°. We explore possible models to explain these observations, including low effective coefficients of fault friction and short‐wavelength stress heterogeneity caused by prior earthquakes. Although definitive conclusions are lacking, we favor local stress heterogeneity as being more consistent with other observations for the Ridgecrest region. 

Abstract We present a revision and update to the high‐precision relocated seismicity catalog presented by Matoza et al. (2021,https://doi.org/10.1029/2020ea001253) for the Island of Hawai'i from 1986 to 2018. The starting catalog of hypocenters (input data), on which the study by Matoza et al. (2021,https://doi.org/10.1029/2020ea001253) was based, contained an inconsistent depth datum for events before and after 00:00 UT, 29 December 2017. Here we present a recomputed version of the catalog using a consistent reference depth. We corrected the starting catalog to a common depth datum (all events now use the model depth reference datum) and re‐ran the entire workflow as described in the paper by Matoza et al. (2021,https://doi.org/10.1029/2020ea001253). This included pairing, cross‐correlating, and relocating all seismic events again based on the updated starting catalog. We consider 347,446 events representing 32 years of seismicity on and around the island from 1986 to 2018. We now successfully relocate 299,966 (86%) events using ∼2.53 billion differential times (PandS) from ∼194 million similar‐event pairs, derived from cross‐correlations between ∼887 million event pairs total, a significant increase from our original analysis. The resolution of fine‐scale seismicity features is improved and the median depth of shallow events (<5 km) under Kaluapele (Kīlauea summit caldera) in 2018 is shifted 926 m deeper as a result of the change. The interpretations and other major conclusions in the paper by Matoza et al. (2021,https://doi.org/10.1029/2020ea001253) are unchanged. 

Abstract 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 ∼87 and ∼117 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%–8% and 4%–9%, 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. 

Abstract Template matching has proven to be an effective method for seismic event detection, but is biased toward identifying events similar to previously known events, and thus is ineffective at discovering events with non‐matching waveforms (e.g., those dissimilar to existing catalog events). In principle, this limitation can be overcome by cross‐correlating every segment (possible template) of a seismogram with every other segment to identify all similar event pairs, but doing so has been previously considered computationally infeasible for long time series. Here we describe a method, called the ‘Matrix Profile’ (MP), a “correlate everything with everything” calculation that can be efficiently and scalably computed. The MP returns the maximum value of the correlation coefficient of every sub‐window of continuous data with every other sub‐window, as well as the best‐correlated sub‐window location. Here we show how MP methods can obtain valuable results when applied to months and years of continuous seismic data in both local and global case studies. We find that the MP can identify many new events in Parkfield, California seismicity that are not contained in existing event catalogs and that it can efficiently find clusters of similar earthquakes in global seismic data. Either used by itself, or as a starting point for subsequent template matching calculations, the MP is likely to provide a useful new tool for seismology research. 

Abstract Deep earthquakes at depths below 500 km are under prohibitive pressure and temperature conditions for brittle failure. Individual events show diverse rupture behaviors and a coherent mechanism to explain their rupture nucleation, propagation, and characteristics has yet to be established. We systematically resolve the rupture processes of 40 large deep earthquakes from 1990 to 2023 and compare the rupture details to their local metastable olivine wedge (MOW) structures informed from thermo‐mechanical simulations in seven subduction zones. Our results suggest that these events likely initiate from metastable olivine transformations within the cold slab core and rupture beyond the MOW due to sustained weakening from molten rock at the rupture tip. Over half of the earthquakes likely rupture beyond the MOW boundary and are controlled by both mechanisms. Rupturing outside the MOW boundary leads to greater moment release, increased geometric complexity, and a reduction in rupture length, causing greater stress drops. 

Abstract We identify 51 near-contemporaneous earthquake pairs along a 100 km segment of California’s San Andreas fault south of San Juan Bautista between 1981 and 2021 that are separated by 5–50 s in time and 5–50 km in space. The event pairs are found throughout the time period and generally involve events smaller than magnitude 2. For 42 of these pairs (82%), the later earthquake is northwest of the earlier event—an asymmetry that is hard to explain with standard earthquake triggering models and suggests an underlying physical connection between the events. We explore possible origins for these observations but are unable to identify a definitive explanation.