The InSight mission deployed one seismic station on Mars, providing a chance to apply single‐station‐based autocorrelation analysis to investigate Martian subsurface structures. However, recent ...analysis indicated the low‐frequency autocorrelation signals may originate from quasi‐periodic high‐amplitude instrumental “glitches” rather than the reflection response of deep Mars. In this study, we detected and removed these high‐amplitude glitches in raw seismic data and employed autocorrelation on the clean vertical component waveforms filtered between 0.05 and 0.1 Hz. We observed signals at the expected times for the olivine‐wadsleyite transition and core‐mantle boundary (CMB) as estimated by other methods. This result suggests that the low‐frequency autocorrelation signals are the reflection response from the olivine‐wadsleyite transition in the mantle and the Martian CMB region, rather than a noise phenomena. A grid search method to fit the observed PcP waveform was used to identify a layer intermediate in velocity between the Martian mantle and core at the Martian CMB.
Plain Language Summary
The InSight mission deployed one seismic station on Mars at the end of 2018 to measure the interior structure of Mars. A recent study autocorrelated the InSight ambient noise data to retrieve reflection signals from subsurface discontinuities. Two seismic signals that reflected from deep within Mars, the olivine‐wadsleyite transition, a pressure‐temperature dependent mineral phase change in the mantle, and the core‐mantle boundary (CMB), are observed. However, some analyses suggested that the arrival times of these signals coincide with the recurrence time of high‐amplitude glitches within the raw seismic data, leading to an incorrect interpretation of the autocorrelation function (ACF). To resolve this issue, we detected and removed the high‐amplitude glitches before further processing the ambient noise data. The autocorrelation analysis of clean continuous vertical‐component waveforms recovers signals at times corresponding to the olivine‐wadsleyite transition and CMB, whereas ACF of glitch‐only waveforms does not. This suggests that signals on the low‐frequency ACF are from the seismic discontinuities at olivine‐wadsleyite transition and CMB rather than being noise artifacts. We further identified velocity layering near Martian CMB by matching the complexity of the PcP signal observed in the ACF of clean data set with synthetic seismograms.
Key Points
Autocorrelation of clean Seismic Experiment for Interior Structure waveforms recovers the prominent signals reflected from deep Mars
The autocorrelation tests on raw, clean and glitch‐only data suggest the signals identified are from deep Mars rather than instrumental noise
A velocity transition model of the Martian core‐mantle boundary better fits the observed PcP phase than a sharp velocity drop
Autocorrelation R2 on Mars Deng, Sizhuang; Levander, Alan
Geophysical research letters,
09/2022, Letnik:
49, Številka:
17
Journal Article
Recenzirano
Odprti dostop
A purpose of the Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport (InSight) mission is to reveal the Martian interior structure with seismic data. In this work, ambient ...noise autocorrelation of the continuously recorded vertical‐component seismic signals has extracted the Rayleigh waves that propagate around Mars for one cycle, R2. The Mars orbiting surface waves are observed at a lag time of ∼6,000 s in the stacked autocorrelation series filtered between 0.005 and 0.01 Hz. Synthetic seismograms from a set of radially concentric velocity models were computed to find the best‐fitting one as the starting model for a Monte Carlo inversion. The starting model was randomly perturbed iteratively to increase the correlation coefficients and reduce the absolute time shifts between the synthetic and observed R2. An S‐wave low‐velocity layer in the inverted velocity model extends to ∼400 km depth, consistent with Marsquake observations, geophysical inversion, and high‐pressure experiments.
Plain Language Summary
The subsurface structures of Earth are imaged by the detailed analysis of seismic data recorded by thousands of stations deployed on the Earth's surface. The Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport (InSight) mission landed a seismograph on Mars, which was deployed at the end of 2018 to investigate the planet's interior structure and dynamic evolution. In this study, we preprocessed the continuous vertical‐component seismic data, and by autocorrelation retrieved a Rayleigh wave, one class of seismic surface wave, that orbits Mars. Rayleigh wave group velocities between 115 and 200 s period were measured from the observed Mars orbiting Rayleigh waves. Synthetic seismograms were calculated using current estimates of the velocity structure of Mars for comparisons to the observation. The spherically symmetric model was updated with a Monte Carlo algorithm, an inversion method that randomly perturbs the velocity model and determines the model that best matches the Mars orbiting surface waves through trial and error. An S‐wave low‐velocity zone is observed to the depth of ∼400 km beneath the Martian surface, consistent with other InSight seismic observations and velocity models measured from geophysical modeling and high‐pressure laboratory experiments.
Key Points
The R2 surface waves on Mars are recovered by low‐frequency ambient noise autocorrelation of Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport (InSight) seismic data
An updated upper mantle velocity model is developed using a Monte Carlo method to fit the observed R2 surface waves on Mars
The S‐wave low velocity zone extends to ∼400 km depth, consistent with the results from previous studies
The dehydration of oceanic slabs during subduction is mainly thermally controlled and is often expressed as intermediate‐depth seismicity. In warm subduction zones, shallow dehydration can also lead ...to the buildup of pore‐fluid pressure near the plate interface, resulting in nonvolcanic tremor. Along the Cascadia margin, tremor density and intermediate‐depth seismicity correlate but vary significantly from south to north despite little variation in the thermal structure of the Juan de Fuca Plate. Along the northern and southern Cascadia margin, intermediate‐depth seismicity likely corresponds to increased fluid flux, while increased tremor density may result from fluid infiltration into thick underthrust metasediments characterized by very slow shear wave velocities (<3.2 km/s). In central Cascadia, low intermediate‐depth seismicity and tremor density may indicate a lower fluid flux, and shear wave velocities indicate that the Siletzia terrane extends to the plate interface. These results indicate that the presence of thick underthrust sediments is associated with increased tremor occurrence.
Plain Language Summary
Fluids are released from subducting oceanic lithosphere as temperature and pressure within the Earth increases. The release of these fluids is manifest by seismicity within the subducting lithosphere and nonvolcanic tremor near the subduction interface. Along the northern and southern Cascadia margin, the spatial distribution of seismicity within the slab and nonvolcanic tremor correlates with very slow shear wave velocities in the lower crust of the overriding plate. These low‐velocity zones likely represent underthrust sediments containing slab‐derived fluids. In central Cascadia, however, seismicity and tremor are relatively low, and a low‐velocity zone in the lower crust of the forearc is not observed. Our results suggest that a combination of variations in the distribution of underthrust sediments and fluid flux along the margin may be the ultimate control on the tremor and seismicity distribution along the Cascadia margin.
Key Points
Low‐velocity zones in the Cascadia forearc lower crust correlate with nonvolcanic tremor density and intermediate‐depth seismicity
Low‐velocity zones are thick (>10 km) and indicate the presence of fluid within oceanic crust and underthrust metasediments
Low‐velocity zones, seismicity, and tremor can be linked to fluids sourced from a variably hydrated and/or permeable subducting slab
We have produced common conversion point (CCP) stacked Ps and Sp receiver function image volumes of the Moho and lithosphere‐asthenosphere boundary (LAB) beneath the western United States using ...Transportable Array data. The large image volumes and the diversity of tectonic environments they encompass allow us to investigate evolution of these structural discontinuities. The Moho is a nearly continuous topographic surface, whereas the LAB is not and the seismic images show a more complex expression. The first order change in LAB depth in the western U.S. occurs along the Cordilleran hingeline, the former Laurasian passive margin along the southwestern Precambrian North American terranes. The LAB is about 50% deeper to the east of the hingeline than to the west, with most of the increase in LAB thickness being in the mantle lithosphere. We infer that the Moho and the LAB are Late Mesozoic or Cenozoic everywhere west of the hingeline, modified during Farallon subduction and its aftermath. Between the hingeline and the Rocky Mountain Front, the LAB, and to a lesser extent the Moho, have been partially reset during the Cenozoic by processes that continue today. Seismicity and recent volcanism in the interior of the western U.S. are concentrated along gradients in crustal and/or lithospheric thickness, for example the hingeline, and the eastern edge of the coastal volcanic‐magmatic terranes. To us this suggests that lateral gradients in integrated lithospheric strength focus deformation. Similarly, areas conjectured to be the sites of convective downwellings and associated volcanism are located along gradients in regional lithosphere thickness.
Key Points
LAB depth is correlated to Cordilleran hinge line
Moho and LAB west of hinge line are Cenozoic
Volcanism and deformation are localized along lithospheric thickness gradients
The seismic signature of the Moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image. ...Seismic data of different types, passive source (earthquake) broad-band recordings, and controlled source seismic refraction, densely recorded wide-angle deep seismic reflection, and normal incidence reflection (using VibroseisTM, explosives, or airguns), have contributed to the description of the Moho as a relatively complex transition zone. Of critical importance for the quality and resolution of the seismic image are the acquisition parameters, used in the imaging experiments. A variety of signatures have been obtained for the Moho at different scales generally dependent upon bandwidth of the seismic source. This variety prevents the development of a single universally applicable interpretation. In this way source frequency content, and source and sensor spacing determine the vertical and lateral resolution of the images, respectively. In most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the Moho, and its relationship to a petrologic crust–mantle transition. In regional seismic studies carried out using passive source recordings the Moho is a relatively well defined structure with marked lateral continuity. The characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area. Refraction and wide-angle studies suggest the Moho to be often a relatively sharp velocity contrast, whereas the Moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity. The origin of the Moho and its relation to the crust–mantle boundary is probably better constrained by careful analysis of its internal details, which can be complex and geographically varied. Unlike the oceanic Moho which is formed in a relatively simple, well understood process, the continental Moho can be subject to an extensive variety of tectonic processes, making overarching conclusions about the continental Moho difficult. Speaking very broadly: 1) In orogenic belts still undergoing compression and active continental volcanic arcs, the Moho evolves with the mountain belt, 2) In collapsed Phanerozoic orogenic belts the Moho under the collapse structure was formed during the collapse, often by a combination of processes. 3) In regions having experienced widespread basaltic volcanism, the Moho can result from underplated basalt and basaltic residuum. In Precambrian terranes the Moho may be as ancient as the formation of the crust, in others Precambrian tectonic and magmatic processes have reset it. We note that seismic reflection data in Phanerosoic orogens as well as from Precambrian cratonic terranes often show thrust type structures extending as deep as the Moho, and suggest that even where crust and mantle xenoliths provide similar age of formation dates, the crust may be semi-allochothonous.
The current data set from USArray provides an unprecedented opportunity to investigate mantle transition zone structures beneath the western United States. We have made transition zone images with ...the Common Converted Point (CCP) stacking method. More than 9600 high quality receiver functions were stacked with reference to two different three‐dimensional tomography models and a one‐dimensional velocity model. Where the Gorda plate passes through the transition zone, the 410 discontinuity has been elevated ∼25 km and the 660 discontinuity has been depressed ∼35 km. We interpret the transition zone topography in terms of mineral physics results in several different ways, noting in particular that recent measurements on the Clapeyron slope for the ringwoodite‐to‐perovskite phase transition under dry conditions give a phase boundary slope of ∼−1.3 to −0.4 MPa/K. The ∼35 km deflection of the 660 discontinuity observed in the receiver functions seems to be the evidence that the subducted slab can carry abundant water from the surface to the transition zone, and in the transition zone the water in the slab may be fully saturated (e.g. the water content is ∼2.0 wt%). Analyses of the velocity perturbations in the tomography models and the transition zone thickness indicate that the deep water is likely well confined within the subducted slab. We infer that the presence of water in the subducted Gorda slab might have contributed ∼15 km and the thermal anomaly in the slab might have contributed ∼20 km to the depression of the 660 discontinuity.
We used cross‐correlation of ambient noise records from seismic stations in the US, Mexico, and Cuba to extract dispersion data of Rayleigh surface wave. Our derived 3D shear‐wave velocity model of ...the greater Gulf of Mexico (GOM) region captures variations in the crustal and lithospheric structures across the continental margins of the US Gulf Coast and Yucatan, Mexico. The model shows a zone of reduced velocity in the mantle lithosphere underlying the extended continental margin of the northwestern GOM. We attributed this velocity reduction to extensional deformation and melt‐induced weakening of the lithosphere during the Triassic continental rifting that preceded the seafloor spreading that formed the GOM. Melt extraction might have been hindered by the greater lithospheric thickness in the western region along the US Gulf Coast margin that resulted in the westward decrease of rift‐related volcanism/magmatism reported from previous studies. The clear asymmetry between the US Gulf Coast and its conjugate Yucatan margin suggests extension along a shear‐zone that focused more deformation on the North American plate prior to breakup. In contrast to the counterclockwise rotation of the Yucatan block during seafloor‐spreading, our analyses using deformable plate models demonstrate that continental rifting occurred in a predominantly northwest‐southeast direction. This change in plate motion is attributed to the development of mantle shear‐zones in the western part of the rift. We estimated the depth of the lithosphere‐asthenosphere boundary and determined that the extended continental and oceanic lithospheres have mostly regained their thickness since the time of breakup.
Key Points
Inherited lithospheric thickness controls spatial and temporal rate of rifting
Refertilization of the mantle lithosphere drives development of mantle shear‐zones
Deformable plate modeling shows extension toward the southeast during the Mesozoic rifting of the Gulf of Mexico continental margins
Garnet–pyroxenite xenoliths from a 25 Ma volcano on the southern edge of the Colorado Plateau in central Arizona (USA) are shown here to have crystallized as deep-seated cumulates from hydrous arc ...magmas, requiring the generation of a large complement of felsic magmas. U–Pb dating of primary titanite grains indicates that crystallization probably occurred around 60 Ma. These observations suggest that voluminous arc magmatism reached as far inland as the edge of the Colorado Plateau during the Laramide orogeny. Here, we employ a combination of petrology, petrophysics, and seismic imaging to show that the formation and subsequent removal of a thick, dense, cumulate root beneath the ancient North American Nevadaplano modified the buoyancy of the orogenic plateau, possibly resulting in two uplift events. A late Cretaceous–early Tertiary uplift event should have occurred in conjunction with thickening of the crust by felsic magmatism. Additional uplift is predicted if the pyroxenite root later foundered, but such uplift must have occurred after ∼25 Ma, the age of the xenolith host. We show that seismic velocity anomalies and seismic structures in the central part of the Colorado Plateau could represent pyroxenitic layers that still reside there. However, under the southern and western margins of the Colorado Plateau, the seismic signatures of a pyroxenite root are missing, despite xenolith records and geochemical evidence for their existence prior to 25 Ma. We suggest that these particular regions have undergone recent removal of the pyroxenite root, leading to late uplift of the plateau. In summary, our observations suggest that the Nevadaplano, west of the Colorado Plateau and now represented by the Basin and Range province, was underlain by high elevations in the late Cretaceous through early Tertiary due to magmatic thickening. This may have facilitated an east-directed drainage pattern at this time. Subsequent collapse of the Nevadaplano, culminating in Basin and Range extension and coupled with delamination-induced uplift of the margins of the Colorado Plateau in the late Cenozoic, may have reversed this drainage pattern, allowing rivers to flow west, as they do today.
•Pyroxenitic cumulate lower crust of central Arizona requires arc magmatism at 60 Ma.•Magmatic thickening and lower crustal foundering modify crustal buoyancy.•Predicted seismic signature of a cumulate lower crust is presented.•Two-stage uplift history proposed for the former Nevadaplano plateau of western USA.
We present new 3‐D P wave and S wave velocity models of the upper 20 km of the Mount St. Helens (MSH) region. These were obtained using local‐source arrival time tomography from earthquakes and ...explosions recorded at 70 broadband stations deployed as part of the imaging Magma Under St. Helens (iMUSH) project and augmented by several data sets. Principal features of our models include (1) low P wave and S wave velocities along the St. Helens seismic zone to depths of at least 20 km corresponding to high conductivity imaged by iMUSH magnetotelluric studies. This delineates a zone of weakness that magma can exploit at the location of MSH; (2) a 5‐ to 7‐km diameter, 6–15 km deep, 3–6% negative P wave and S wave velocity anomaly beneath MSH, consistent with previous estimates of the source region for recent eruptions. We interpret this as a magma storage region containing up to 15–20 km3 of partial melt, which is about 5 times more than the largest documented eruption at MSH; (3) a broad region of low P wave velocity below 10‐km depth extending between Mount Adams and Mount Rainier along and to the east of the main Cascade arc, which is likely due to high‐temperature arc crust and possible presence of fluids or melt; (4) several anomalies associated with surface‐mapped features, including high‐velocity igneous units such as the Spud Mountain and Spirit Lake plutons and low velocities in the Chehalis sedimentary basin and the Indian Heaven volcanic field. Our results place further constraints on the geometry of these features at depth.
Plain Language Summary
We deployed 70 seismometers around Mount St. Helens volcano from 2014 to 2016, which measured the surface ground motion from hundreds of small earthquakes, as well as from 23 explosions that were set off in 2014. We recorded the onset time of shaking from these sources and used a specialized computer code to model how quickly seismic waves travel through the subsurface. Seismic wave speed can be influenced by several factors, including rock type, presence of magma/fluids, temperature, pressure, and how fractured the rock is. Based on the seismic wave speeds in our model, we make several geological interpretations, including (1) increased fluids or fractures, or presence of sedimentary rocks corresponding to elevated earthquake activity to the NNW of Mount St. Helens; (2) a magma storage region beneath the volcano similar to results from previous studies. Our model places further constraints on the orientation and size of the region; (3) a large zone of high temperatures and possible fluids or magma related to regional volcanism between and to the east of Mount Adams and Mount Rainier; (4) more detailed size and depth constraints on geological features seen at the surface, including sedimentary basins and rock units related to previous regional volcanism.
Key Points
New high‐resolution P wave and S wave velocity models are calculated for the Mount St. Helens region
Velocity models place further constraints on size and location of magma storage regions, seismic zones, sedimentary basins, and plutons
These shed light on the accretionary history of the Siletzia terrane, with a transitional upper crustal boundary near Mount St. Helens
We have investigated crustal thickness and composition across the southeastern Caribbean plate boundary with the receiver function technique. We used teleseismic data recorded by a temporary ...broadband array deployed under the BOLIVAR project and the permanent national seismic network of Venezuela. We used the primary P‐to‐S conversion and crustal reverberations to estimate crustal thickness and average crustal VP/VS ratio over the region. We observe large variations in crustal thickness and Poisson's ratio. Estimated Moho depth ranges from ∼16 km beneath the southeastern Caribbean Sea to ∼52 km beneath northeastern Venezuela and the Venezuelan Andes. There is a good correlation between crustal structure and tectonic terranes. Data from the Precambrian Guayana Shield suggest that the underlying crustal structure is relatively uniform with a moderate thickness (∼37 km) and an intermediate composition. A thick crust is found below the foreland basins. The two mountain systems in northern Venezuela, the Serrania del Interior and the Serrania del Falcon, have a thin crust with arc composition and are likely dynamically supported by elastic rebound or underthrusting of the oceanic plateau that characterizes the southern Caribbean. On the other hand, the Venezuelan Andes and Perija Range on the western side of the country are probably isostatically balanced by thick crustal roots.
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