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Stochastic tomography, made possible by dense deployments of seismic sensors, is used to identify phase changes in Earth's mantle that occur over depth intervals. This technique inverts spatial coherences of amplitudes and travel times of body waves to determine the depth and dependence of the spatial spectrum of seismic velocity. This spectrum is interpreted using the predicted thermodynamic stability of mineral composition and phase as a function of temperature and pressure, in which the metamorphic temperature derivative of seismic velocities is used as a proxy for the effects of heterogeneity induced in a region undergoing a phase change. Peaks in the temperature derivative of seismic velocity closely match those found from applying stochastic tomography to elements of Earthscope and Flex arrays. Within ±12 km, peaks in the fluctuation of P velocity at 425, 500, and 600 km depth beneath the western US agree with those predicted by a mechanical mixture of harzburgite and basalt, 180 K cooler than a 1600 K adiabat in the mantle transition zone. A broad peak at 250 km depth may be associated with chemical heterogeneity induced by dehydration of subducted oceanic sediments, and a peak at 775 km depth with a phase change in subducted basalt. Non‐detection of predicted phase changes less than 10 km in width is consistent with the resolution possible with the seismic arrays used in the inversion, including the sharp endothermic phase change near 660 km. These interpretations are consistent with the known history of plate subduction beneath North America. Plain Language Summary: The speed of seismic waves in a rock depends on the chemistry and arrangement of the atoms making up the minerals in the rock. These atoms are arranged in different ways, depending on temperature and pressure. Increasing pressure with depth in Earth squeezes the atoms of minerals into denser arrangements. An abrupt change with depth in the arrangement of mineral atoms, termed a phase change, can speed up, slow down, and scatter earthquake waves in different directions, changing their arrival times and amplitudes. This study describes a way in which the fluctuations in the arrival times and amplitudes of seismic waves, recorded by dense collections of seismometers, can be used to locate, detect, and image the depth and thickness of mineral phase changes in Earth's mantle. These phase changes are found to control much of the observed scattering of seismic waves in Earth's mantle from distant earthquakes and agree with the history of plate tectonics of western North America. Key Points: Stochastic tomography can capture higher order phase and chemical changes undetectable from reflection imagingThe heterogeneity spectrum of the upper mantle is marked by peaks associated with depth regions undergoing phase or compositional changeThe upper mantle beneath the western US is consistent with a history of subduction and slab stagnation in the transition zone [ABSTRACT FROM AUTHOR]

Shear attenuation provides insights into the physical and chemical state of the upper mantle. Yet, observations of attenuation are infrequent in the oceans, despite recent proliferation of arrays of ocean‐bottom seismometers (OBSs). Studies of attenuation in marine environments must overcome unique challenges associated with strong oceanographic noise at the seafloor and data loss during OBS recovery in addition to untangling the competing influences of elastic focusing, local site amplification, and anelastic attenuation on surface‐wave amplitudes. We apply Helmholtz tomography to OBS data to simultaneously resolve array‐averaged Rayleigh wave attenuation and maps of site amplification at periods of 20–150 s. The approach explicitly accounts for elastic focusing and defocusing due to lateral velocity heterogeneity using wavefield curvature. We validate the approach using realistic wavefield simulations at the NoMelt Experiment and Juan de Fuca (JdF) plate, which represent endmember open‐ocean and coastline‐adjacent environments, respectively. Focusing corrections are successfully recovered at both OBS arrays, including at periods <35 s at JdF where coastline effects result in strong multipathing. When applied to real data, our observations of Rayleigh wave attenuation at NoMelt and JdF revise previous estimates. At NoMelt, we observe a low attenuation lithospheric layer (Qμ ${Q}_{\mu }$ > 1,500) overlying a highly attenuating asthenospheric layer (Qμ ${Q}_{\mu }$ ∼ 50 to 70). At JdF, we find a broad peak in attenuation (Qμ ${Q}_{\mu }$ ∼ 50 to 60) centered at a depth of 100–130 km. We also report strong local site amplification at the JdF Ridge (>10% at 31 s period), which can be used to refine models of crust and shallow mantle structure. Plain Language Summary: Seismic tomography provides a tool for probing regions deep within the Earth that are otherwise inaccessible. The degree to which seismic waves lose energy as they travel (seismic attenuation) provides information about temperature and melt in Earth's interior. However, seismic attenuation is notoriously difficult to measure due to complicating effects on wave amplitudes from focusing and amplification of the waves as they travel through the heterogeneous Earth. Here we introduce a tool that utilizes both amplitude and travel‐time information observed across arrays of seismometers to account for these competing effects and accurately quantify seismic attenuation. We validate the approach using realistic simulations and apply it to real data sets at young (∼3 Myr) and older (∼70 Myr) locations in the Pacific Ocean. Our observations revise previous estimates of attenuation at the two locations, revealing high attenuation that extends deeper beneath the Juan de Fuca ridge than previously thought and high attenuation in the asthenosphere beneath the typical oceanic plate. These observations have important implications for our understanding of mantle temperature and melt content beneath the oceans. Key Points: Helmholtz tomography applied at ocean‐bottom seismometer arrays recovers Rayleigh wave attenuation and site amplification at 20–150 s periodSynthetic tests show that focusing/defocusing are successfully accounted for, including near the coastline where strong multipathing occursStrong site amplification observed at the Juan de Fuca ridge may be used to improve models of crust and shallow mantle [ABSTRACT FROM AUTHOR]

Two‐decades of onshore‐offshore, ocean‐bottom seismometer, and passive‐source data are integrated to obtain high‐resolution 3‐D constraints on the architecture of the Nankai subduction zone. Our model reveals large along‐strike variability in the width of the outer‐forearc, with the crustal backstop (VP ≥ 5 km/s) extending within 50 km of Nankai Trough offshore Tokai and Kii Peninsula, but is a factor of 2 further landward (90–115 km) offshore Cape Muroto and Kyushu. The crustal backstop broadly coincides with the down‐dip extent of shallow slow‐earthquakes and the up‐dip extent of co‐seismic slip and geodetic locking, suggesting a link with the shallow frictional transition. In the inner‐forearc, high seismic‐velocities (VP > 6.6 km/s) in Kii Peninsula reflect Miocene plutons, impacting the nucleation and segmentation of large megathrust earthquakes. The complex geometry of the subducting slab drives spatial variability in the location of the down‐dip frictional transition. Variability in the width of the outer‐forearc, the dip of the subducting slab, and the depth distribution of deep slow‐slip events combine to produce large changes in the width of the seismogenic zone. This is most pronounced across Bungo channel, where the seismogenic zone narrows from 90 to 120 km in Shikoku to <60 km offshore Kyushu. Pinching of the seismogenic zone may explain the smaller area, and thus magnitude, of earthquakes offshore Kyushu and the reduction in slip‐deficit rate. Our results suggest crustal‐scale architecture is a key driver of profound along‐strike transitions in the location of frictional transition zones, and the width and slip behavior of the intervening seismogenic zone at Nankai Trough. Plain Language Summary: Some subduction zones produce the largest earthquakes and tsunami on Earth, while others slip aseismically. To understand what factors impact subduction zone slip behavior, we have integrated dense geophysical datasets that have accumulated over >20 years in SW Japan to construct the first high‐resolution, 3‐D image of an entire subduction zone. This image reveals large variability in the offshore extent of dense, rigid crustal rocks and suggests the position of this crustal backstop may influence the shallow transition from aseismic to seismic slip along the Nankai megathrust. The landward extent of seismic slip is also spatially variable and appears to be predominantly controlled by the geometry of the subducting plate. Collectively, along trench variability in the position of the crustal backstop (up‐dip transition) and the geometry of the subducting slab (down‐dip transition) combine to produce large variability in the width of the seismogenic zone. Earthquake magnitude is proportional to rupture area and pinching of the seismogenic due to the convergence of shallow and deep frictional transitions may explain the smaller magnitude of earthquakes offshore Kyushu. Our results reinforce the importance of crustal scale architecture in modulating earthquake and tsunami hazard at subduction zones. Key Points: Two‐decades of geophysical data suggest the crustal backstop coincides with the up‐dip transition from slow to seismic slip at Nankai TroughSpatial variability in backstop position and subducting slab geometry produce large changes in width of the seismogenic zone along‐strikePinching of the seismogenic zone between Shikoku and Kyushu may explain reductions in earthquake magnitude and slip‐rate deficit [ABSTRACT FROM AUTHOR]

The complex tectonic setting of the central‐western Mediterranean has interested geoscientists for decades, but its geodynamic evolution remains a matter of debate. We rely on 807 seismometers from southern Europe and northern Africa to measure Rayleigh and Love phase velocities in the period range ∼5–200 s, based on teleseismic earthquakes and seismic ambient noise. By nonlinear joint inversion of the phase‐velocity maps, we obtain a 3‐D shear‐wave velocity (VS) model of the study area. At shallow depths, our model correlates with surface geology and reveals the presence of a sedimentary cover in the Liguro‐Provençal basin, as opposed to the Tyrrhenian basin where this is either very thin or absent. At ∼5‐km depth, high velocities below the Magnaghi, Vavilov, and Marsili seamounts point to an exhumed, scarcely serpentinized mantle. These are replaced by lower velocities at larger depths, likely connected to the presence of partial melt. At 50–60‐km depth, a very heterogeneous structure characterizes the Tyrrhenian basin, with low velocities pointing to the presence of fluids due to the lateral mantle inflow from the Ionian slab edges, and higher velocities associated with a relatively dry upper mantle. Such heterogeneity disappears at depths ≳75 km, replaced by more uniform velocities which are ∼2% lower than those found in the Liguro‐Provençal basin. We infer that, at the same depths, the Tyrrhenian basin is characterized by a larger concentration of fluids and possibly higher temperatures. Plain Language Summary: The Mediterranean area has long been considered by geoscientists as a source of inspiration for understanding geodynamic processes on a global scale, but its deep structure remains a matter of debate. We rely on seismic data from 807 receivers distributed across southern Europe and northern Africa to measure surface‐wave (Rayleigh and Love) phase velocities, based on both strong earthquakes and seismic ambient noise. We use these observables to obtain an improved shear‐velocity model of the central‐western Mediterranean, covering the Liguro‐Provençal and the Tyrrhenian basins. The shallow features of our model are consistent with known geological structures, and indicate a thick sedimentary cover in the Liguro‐Provençal basin which is absent beneath the Tyrrhenian Sea. At larger depths (∼50–60 km), we find a very heterogeneous structure in the Tyrrhenian basin, as opposed to the more uniform Liguro‐Provençal basin. The observation is consistent with a relatively high concentration of fluids in the south‐eastern part of the Tyrrhenian basin at the same depths, which can be ascribed to the subduction of the Ionian slab beneath Calabria. Our results are discussed in light of recent geodynamic models, with a focus on the role of slab dynamics in controlling the compositional character of back‐arc basins. Key Points: Rayleigh and Love phase‐velocity maps of the central‐western Mediterranean from seismic ambient noise and teleseismic earthquakesNew high‐resolution shear‐wave velocity model from joint inversion of Rayleigh and Love phase velocitiesVery heterogeneous structure in the Tyrrhenian basin at 50–60‐km depth associated with thermo‐chemical variations driven by slab dynamics [ABSTRACT FROM AUTHOR]

We use traveltimes from a temporary seismic deployment of 30 broadband seismometers and a national catalog of arrival times to construct a finite‐frequency teleseismic P‐wave tomographic model of the upper mantle beneath eastern Indonesia, where subduction of the Indo‐Australian plate beneath the Banda Arc transitions to arc‐continent collision. The change in tectonics is due to a change from oceanic to continental lithosphere in the lower plate as inferred from geological mapping and geophysical, geochemical, and geodetic measurements. At this inferred transition, we seismically image the subducted continent‐ocean boundary at upper mantle depths that links volcanism on Flores to amagmatic orogenesis on Timor. Our tomographic images reveal a relatively high‐velocity feature within the upper mantle, which we interpret as the subducted Indo‐Australian slab. The slab appears continuous yet deformed as a result of the change in buoyancy due to the composition of the incoming continental lithosphere. Accordingly, there is a difference in dip angle between the oceanic and continental sections of the slab albeit not a gap or discontinuity. We suggest the slab has deformed without tearing to accommodate structural and kinematic changes across the continent‐ocean boundary as the two sections of the slab diverge. These results suggest that deformation in tectonic collisions can be localized along a continent‐ocean boundary, even at depth. We propose that future slab tearing may develop where we observe slab deformation in our study region and that a similar process may take place in collisions generally. Key Points: New data set produces finite‐frequency P‐wave tomographic model with unparalleled resolution of young arc‐continent collision in IndonesiaThis model resolves a continuous slab that is deformed near the boundary between continental and oceanic crust but not tornThis implies deep continental subduction prior to slab tearing and the localization of deformation along compositional boundaries [ABSTRACT FROM AUTHOR]

We performed waveform inversion of the P waveforms recorded by our BroadBand Ocean Bottom Seismometers (BBOBSs) deployed in the Northwestern Pacific. Consequently, the depth profile of the P velocity of the oceanic upper mantle, which has not been well resolved by previous surface wave or receiver function analyses, was revealed. We considered the azimuthal anisotropy in the lithosphere, which significantly improved the variance reduction from 34% to 44%. The resulting P model exhibited higher and lower velocities in the lithosphere and asthenosphere, respectively. The velocity contrast was found to be second/third of that observed in the previous S models; however the obtained model appeared to have some trade‐off with the VS structures in the vicinity of the source. We compared our P model with the previous S model obtained using our BBOBSs and obtained the VP/VS model. The resulting VP/VS model has two notable features. First, the lithosphere is characterized by a rapid increase in the VP/VS values with depth, which implies chemical stratification. Second, the VP/VS values in the vicinity of the lithosphere‐asthenosphere boundary (LAB) are larger than the synthetic values of any major upper mantle mineral predicted by considering the anharmonic effects, which suggests the effects of anelasticity or melt. Key Points: We inverted P waveforms recorded by our BBOBSs to obtain the depth profile of the P velocity beneath the older Pacific plateCombining our P model with a previous S model revealed two notable features in Vp/VS structuresThe obtained Vp/Vs model suggests chemical stratification in the lithosphere and the effects of anelasticity or melt at the LAB [ABSTRACT FROM AUTHOR]

Hundreds of earthquakes were recorded during a nine‐month ocean bottom seismometer deployment surrounding Lō'ihi submarine volcano, Hawai'i. The 12‐station ocean bottom seismometer network widened the aperture of earthquake detection around the Big Island, allowing better constraints on the location of seismicity offshore Hawai'i. Although this deployment occurred during a time of volcanic quiescence for Lō'ihi, it establishes an important basis for background seismicity of the volcano. Offshore seismicity during this study was dominated by events located in the mantle fault zone at depths of 25–40 km. These events reflect rupture on preexisting faults in the lower lithosphere caused by stresses induced by volcano loading and flexure of the Pacific Plate (Pritchard et al., 2007, https://doi.org/10.1111/j.1365‐246X.2006.03169.x; Wolfe et al., 2004, https://doi.org/10.1029/2003GC000618). Tomography was performed using double‐difference seismic tomography and showed shallow velocities to be slower than the regional velocity model (HG50; Klein, 1981, https://pubs.geoscienceworld.org/ssa/bssa/article/71/5/1503/118231/A‐linear‐gradient‐crustal‐model‐for‐south‐Hawaii). A broad, low‐velocity anomaly was observed from 20–40‐km depth, and is suggestive of the central plume conduit that supplies magma to Lō'ihi and the active volcanoes of the Big Island. A localized high‐velocity body is observed 4–6‐km depth beneath Lō'ihi's summit, extending 10 km to the north and south. Following Lō'ihi's active rift zones and crossing the summit, this high‐velocity body is characteristic of intrusive material. Two low‐velocity anomalies are observed below the oceanic crust, interpreted as melt accumulation beneath Lō'ihi and magmatic underplating beneath Hawai'i Island. Plain Language Summary: We deployed a network of ocean bottom seismometers on Lō'ihi submarine volcano for a nine‐month period in 2010–2011. During that time, there were few earthquakes recorded within Lō'ihi, but many in the surrounding area. These earthquakes are thought to be associated with the bending of the Pacific Plate under the weight of Hawai'i Island rather than volcanic processes. Data from these earthquakes were used to examine the speed of seismic waves within Lō'ihi, providing clues into the volcano's internal structure. We find that Lō'ihi has many of the same internal features as the older volcanoes on Hawai'i Island. Slower velocities surrounding Lō'ihi reveal the boundary between Lō'ihi and the underlying flank of Mauna Loa volcano. Slow seismic velocities within the mantle beneath the region may be associated with the Hawaiian hot spot itself. Key Points: An ocean bottom seismic network was deployed on Lō'ihi submarine volcano in 2010–2011Lō'ihi was seismically quiescent during the nine‐month deployment periodP wave tomography reveals low velocities beneath Lō'ihi and high velocities within its rifts and beneath the summit [ABSTRACT FROM AUTHOR]

At extensional volcanic arcs, faulting often acts to localize magmatism. Santorini is located on the extended continental crust of the Aegean microplate and is one of the most active volcanoes of the Hellenic arc, but the relationship between tectonism and magmatism remains poorly constrained. As part of the Plumbing Reservoirs Of The Earth Under Santorini experiment, seismic data were acquired across the Santorini caldera and the surrounding region using a dense amphibious array of >14,300 marine sound sources and 156 short‐period seismometers, covering an area 120 km by 45 km. Here a P wave velocity model of the shallow, upper‐crustal structure (<3‐km depth), obtained using travel time tomography, is used to delineate fault zones, sedimentary basins, and tectono‐magmatic lineaments. Our interpretation of tectonic boundaries and regional faults are consistent with prior geophysical studies, including the location of basin margins and E‐W oriented basement faults within the Christiana Basin west of Santorini. Reduced seismic velocities within the basement east of Santorini, near the Anydros and Anafi Basins, are coincident with a region of extensive NE‐SW faulting and active seismicity. The structural differences between the eastern and western sides of Santorini are in agreement with previously proposed models of regional tectonic evolution. Additionally, we find that regional magmatism has been localized in NE‐SW trending basin‐like structures that connect the Christiana, Santorini, and Kolumbo volcanic centers. At Santorini itself, we find that magmatism has been localized along NE‐SW trending lineaments that are subparallel to dikes, active faults, and regional volcanic chains. These results show strong interaction between magmatism and active deformation. Key Points: P wave tomography of the upper crust (<3 km) around Santorini delineates basin structures including buried margins and basement faultsThese structures, also seen in seismic reflection images, result from both the integrated tectonic history, as well as ongoing deformationTectonic structures and stresses localize volcanism in regional NE‐SW basins, and, within Santorini caldera, along local NE‐SW lineaments [ABSTRACT FROM AUTHOR]