Seismic Velocity Structure Research Articles

Processing Ambient Noise Data Using Phase Cross‐Correlation and Application Toward Understanding Spatiotemporal Environmental Effects

AbstractTypical use of ambient noise interferometry focuses on longer period (>1 s) waves for exploration of subsurface structure and other applications, while very shallow structure and some environmental seismology applications may benefit from use of shorter period (<1 s) waves. We explore the potential for short‐period ambient noise interferometry to determine shallow seismic velocity structures by comparing two methodologies, the conventional amplitude‐based cross‐correlation and linear stacking (TCC‐Lin) and a more recently developed phase cross‐correlation and time‐frequency phase‐weighted‐stacking (PCC‐PWS) method with both synthetic and real data collected in a heterogeneous karst aquifer system. Our results suggest that the PCC‐PWS method is more effective in extracting short‐period wave velocities than the TCC‐Lin method, especially when using data collected in regions containing complex shallow structures such as the karst aquifer system investigated here. In addition to the different methodologies for computing the cross correlation functions, we also examine the relative importance of signal‐to‐noise ratio and number of wavelengths propagating between station pairs to determine data/solution quality. We find that the lower number of wavelengths of 3 has the greatest impact on the network‐averaged group velocity curve. Lastly, we test the sensitivity of the number of stacks used to create the final empirical Green's function, and find that the PCC‐PWS method required about half the number of cross‐correlation functions to develop reliable velocity curves compared to the TCC‐Lin method. This is an important advantage of the PCC‐PWS method when available data collection time is limited.

Seismic velocity structure of the crust and uppermost mantle beneath the Tien Shan and its adjacent areas

The Tien Shan and its adjacent areas have been a prime place to understand the process of continental collision, the mechanism of mountain building and the interaction of tectonic blocks. In this study, we collect seismic data recorded by 74 broad-band stations from the China Provincial Digital Seismic Networks and the Regional Kyrgyzstan and Kazakhstan Networks between January, 2007 and September, 2009. A joint inversion technique that combines three types of datasets (receiver functions, phase velocities of Rayleigh wave measured from both ambient noise and teleseismic earthquake data) is applied to image the crustal and upper mantle structure beneath the Tien Shan and its adjacent areas. The average crustal thickness in the study area is about 50 km, however, the Moho depth extends to ∼70 km beneath the Kyrgyz Platform near the southwestern Tien Shan. Our velocity models show a good correlation with subsurface geological features at shallow depths: low velocities are predominantly observed beneath the basins due to thick sedimentary layer, whereas high velocities are mainly distributed beneath the mountain ranges due to crystalline basement rocks. In the upper mantle a low velocity zone is obviously observed beneath the western Tien Shan. Both the crust thickness and S wave velocity structure of the Tien Shan and its adjacent regions display obvious horizontal and vertical heterogeneities from west to east, which suggests that the far-field effects of the collision between Eurasian plate and Indian plate plays an important role in the tectonic activity of the Tien Shan. The apparent velocity heterogeneities beneath the northern Tarim Basin may indicate that the Tarim Basin may have been eroded and damaged by upwelling hot materials from the upper mantle.

Present-Day Three-Dimensional Deformation across the Ordos Block, China, Derived from InSAR, GPS, and Leveling Observations

The Ordos Block in China experiences tectonic activity and frequent earthquakes due to compression from the Tibetan Plateau and extension from the North China Block. This has prompted the construction of a high-resolution three-dimensional (3D) deformation field to better understand the region’s crustal movement. Considering the limitations of the existing geodetic observations, we used InSAR, GPS, and leveling observations to create a high-precision 3D deformation field for the Ordos Block. Spherical wavelet decomposition was used to separate tectonic and non-tectonic deformation signals. Short-wavelength non-tectonic deformation fields revealed complex surface deformation patterns caused by groundwater, oil, gas extraction, and coal mining. Long-wavelength tectonic deformation fields showed subsidence in the southern margin of the block, while the interior and northeastern margins were uplifted. By combining imaging results from the seismic velocity structure and magnetotellurics, we infer that the upwelling of deep materials beneath the northeastern margin leads to surface uplift with tensile strain rates. The crustal uplift in the area south of 38°N matches the thickening of the lower crust. The weak subsidence and eastward horizontal movement disappearing near 108°E at the southern margin support the existence of asthenosphere flow beneath the Qinling orogenic belt.

High seismic velocity structures control moderate to strong induced earthquake behaviors by shale gas development

Moderate to strong earthquakes have been induced worldwide by shale gas development, however, it is still unclear what factors control their behaviors. Here we use local seismic networks to reliably determine the source attributes of dozens of M > 3 earthquakes and obtain a high-resolution shear-wave velocity model using ambient noise tomography. These earthquakes are found to occur close to the target shale formations in depth and along high seismic velocity boundaries. The magnitudes and co-seismic slip distributions of the 2018 Xingwen {M}_{{{{{{rm{L}}}}}}}5.7 and 2019 Gongxian {M}_{{{{{{rm{L}}}}}}}5.3 earthquakes are further determined jointly by seismic waveforms and InSAR data, and the co-seismic slips of these two earthquakes correlate with high seismic velocity zones along the fault planes. Thus, the distribution of high velocity zones near the target shale formations, together with the stress state modulated by hydraulic fracturing controls induced earthquake behaviors and is critical for understanding the seismic potentials of hydraulic fracturing.

Retracted: Self-consistent models of Earth’s mantle and core from long-period seismic and tidal constraints

SUMMARYIn this study, we inverted a large set of normal-mode centre-frequencies and quality (attenuation) factors, including astronomic-geodetic data (mass, moment of inertia and tidal response), using self-consistently built models of the radial elastic and anelastic seismic structure of the Earth. The mantle models are constructed using petrologic phase equilibria in combination with a laboratory-based viscoelastic model that connects dissipation from seismic to tidal periods, whereas seismic properties for a well-mixed and homogeneous core are computed using equations-of-state. Relative to the preliminary seismic reference model (PREM), we find that for the models to fit the observations, mantle P- and S-wave velocities have to be slightly faster and slower, respectively, while outer-core P-wave velocity is slower on account of a different velocity gradient, whereas inner-core velocity structure is similar, within the uncertainties of the inferred model parameters. In terms of density, we find that the lower mantle is less dense and the outer core more dense than PREM, while the inner core is similar to PREM. To study the impact of the inferred mantle seismic velocity structure, we computed P- and S-wave traveltimes and compared these to the observations of globally-averaged P- and S-wave traveltimes from the reprocessed ISC catalogue that resulted in an excellent match. In an attempt to further refine the seismic P-wave velocity structure of the outer core, we also considered multiple core–mantle-boundary underside-reflected body wave traveltime data. Although the match to the underside reflections clearly improves as a result of a steeper velocity gradient in the outer core relative to the normal-modes- and astronomic-geodetic-data-only case, subtle differences nevertheless persist that appear to support a change in velocity gradient in the outermost core, evocative of a stably-stratified layer. The laboratory-based viscoelastic model considered here resolves the anelastic response of Earth’s mantle from long-period seismic (∼100 s) to tidal (18.6 yr) periods, accounting for both normal-mode and tidal dissipation measurements. Finally, as a potential means of refining core composition, we considered the density contrast across the inner-core boundary (ICB) based on our inverted models. The most probable ICB density difference found here is 0.3–0.45 g cm−3, which is in the lower range of earlier body-wave- and normal-mode-based predictions. This suggests that the compositional heterogeneity associated with light-element partitioning, which is considered the principal driving mechanism for the compositional convection that powers the geodynamo, may be less effective than previously thought, calling for exsolution of solids from the liquid outer core as a possible additional source of energy. This would also help address the problem of a young inner core.

Insight into seismotectonics of the central-south Tanlu Fault in East China from P-wave tomography

The Tanlu Fault is a significant fault in East China that has been associated with numerous destructive earthquakes in its central-south segment. However, the mechanisms behind these earthquakes remain unclear. This study aims to shed light on the seismic velocity structures of the crust and upper mantle under East China and their influence on the region's seismotectonics. Our findings reveal that the upper mantle structures play a crucial role in regional seismotectonics. The 1668 Tancheng earthquake (M 8.5) is located above remarkable upper-mantle high-velocity bodies on both sides of the Tanlu Fault. Thus, it was likely produced by the interaction between the North China and Yangtze plates, with the lithospheric mantle significantly affecting the plate interaction. Another group of earthquakes of magnitude M 5–6 is located in the Subei–Yellow Sea Basin (SYSB). We have imaged a low-velocity body in the upper mantle, which is believed to represent hot and buoyant materials under the SYSB. This anonymous body may reduce the vertical stress in the crust and provide fluids to fault planes, thereby facilitating moderate-to-large earthquakes in the crust. These new findings provide valuable insight into the seismotectonics of East China, particularly the role of the upper mantle structures in shaping the region's seismic activity.

Seismic Tomography of Nabro Caldera, Eritrea: Insights Into the Magmatic and Hydrothermal Systems of a Recently Erupted Volcano

AbstractUnderstanding the crustal structure and the storage and movement of fluids beneath a volcano is necessary for characterizing volcanic hazard, geothermal prospects and potential mineral resources. This study uses local earthquake traveltime tomography to image the seismic velocity structure beneath Nabro, an off‐rift volcano located within the central part of the Danakil microplate near the Ethiopia‐Eritrea border. Nabro underwent its first historically documented eruption in June 2011, thereby providing an opportunity to analyze its post‐eruptive state by mapping subsurface fluid distributions. We use a catalog of earthquakes detected on a temporary seismic array using machine learning methods to simultaneously relocate the seismicity and invert for the three‐dimensional P‐ and S‐wave velocity structures (VP, VS) and the ratio between them (VP/VS). Overall, our model shows higher than average P‐ and S‐wave velocities, suggesting the presence of high‐strength, solidified intrusive magmatic rocks in the crust. We identify an aseismic region of low VP, low VS, and high VP/VS ratio at depths of 6–10 km b.s.l., interpreted as the primary melt storage region that fed the 2011 eruption. Above this is a zone of high VS, low VP, and low VP/VS ratio, representing an intrusive complex of fractured rocks partially saturated with over‐pressurized gases. Our observations identify the persistence of magma in the subsurface following the eruption, and track the degassing of this melt through the crust to the surface. The presence of volatiles and high temperatures within the shallow crust indicate that Nabro is a viable candidate for geothermal exploration.

The Northern Chile forearc constrained by 15 years of permanent seismic monitoring

In this review article, we compile seismological observations from the different constituent parts of the Northern Chile forearc: the downgoing Nazca Plate, the plate interface, the upper South American Plate as well as the mantle wedge beneath it. As Northern Chile has been monitored by a network of permanent seismic stations since late 2006, there is a wealth of observations that enables us to characterize the structure as well as ongoing processes in the forearc throughout the last 15 years. We put an emphasis on the analysis of seismicity, for which we have extended a massive earthquake catalog that now contains >180,000 events for the years 2007–2021. Moreover, we draw on published results for earthquake mechanisms, source properties, seismic velocity structure, statistical seismology and others, and discuss them in context of results from neighboring disciplines. We thus attempt to provide a comprehensive overview on the seismological knowledge about the structure and ongoing processes in the Northern Chile forearc, a breviary of which is found in the following: The Northern Chile megathrust hosted two major earthquake sequences during the analyzed time period. The 2007 Mw 7.8 Tocopilla earthquake broke the deep part of the megathrust just north of Mejillones Peninsula, whereas the 2014 Mw 8.1 Iquique earthquake ruptured the central segment in the north of the study region. The latter event has a highly interesting preparatory phase, including a significant foreshock sequence as well as aseismic slip transients. Besides these large events, background seismicity elsewhere on the megathrust may be helpful for characterizing the earthquake potential and locking state in the remaining seismic gap. The downgoing Nazca Plate in Northern Chile exhibits very high seismicity rates, with the vast majority of earthquakes occurring at depths of ∼80-140 km with downdip extensive mechanisms. While seismic tomography shows no sudden changes in slab geometry along strike, seismicity describes peculiar offsets that may be linked to subducted features on the oceanic plate. Upper plate seismicity likewise shows strong variations along strike, with the north and south of the study area showing only weak activity, whereas the central segment shows pervasive microseismicity throughout the upper plate, all the way to the plate interface. These earthquakes have thrust and strike-slip mechanisms with P-axes striking roughly N-S, indicating margin-parallel compression that may be connected to the concavity of the margin.

The Wakayama earthquake swarm in Japan

An earthquake swarm in the Wakayama prefecture, Japan, is known as the most active and persistent swarm, with > 95,000 earthquakes (M ≥ –1.3) occurring during the 2003–2020 period. However, no systematic studies have highlighted the source of this intriguing non-volcanic earthquake swarm to date. This study systematically investigates the temporal and spatial evolution of the Wakayama earthquake swarm and estimates the seismic velocity structure around the Kii peninsula, where we observe series of anomalous geophysical and geochemical signatures, such as high 3He/4He ratios, deep low-frequency earthquakes, and hot springs with high salinity and solute concentrations. We reveal that seismicity associated with the Wakayama earthquake swarm occurs almost evenly in both time and space, and that the majority of the earthquakes in the northern part of the swarm activity occur along well-defined planes that dip to the west at 30–45°. The seismic tomography results reveal that a northwestward-dipping low-velocity zone exists beneath the Wakayama swarm and the low-velocity zone is sandwiched by high-velocity anomalies in the continental crust interpreted as impermeable and rigid materials on both sides in the subduction direction. This unique tectonic setting controls a pathway of the upward migration of slab-derived fluids to the surface, with the high fluid concentration in the dipping low-velocity zone. Therefore, we infer that the location of the Wakayama swarm is controlled by deep crustal heterogeneities rather than by the major structures of geological accretionary complexes. This study suggests that the anomalous geophysical and geochemical signatures observed across the Kii peninsula are different manifestations of the frictional and hydrological processes during the upward migration of the slab-derived fluids. We further propose that the valley-shaped geometry of the Philippine Sea slab beneath the Kii peninsula is caused by the rigid materials in the continental crust.Graphical

High‐Resolution Broadband Lg Attenuation Structure of the Anatolian Crust and Its Implications for Mantle Upwelling and Plateau Uplift

AbstractThe Anatolian Plateau, currently experiencing rapid uplift and westward escape, records both the termination of oceanic subduction and the conversion to continental collision. The crustal response to the transition of the subduction environment from eastern to western Anatolia can be inferred by the seismic velocity and attenuation structures. With this study, we construct a broadband Lg‐wave attenuation model for the Anatolian Plateau and use it to constrain lateral crust heterogeneities linked to this transition. Crustal Lg attenuation links late Cenozoic magmatism with asthenospheric upwelling by characterizing the lithospheric thermal structure. The widely distributed strong attenuation observed in eastern Anatolia may be related to the crustal partial melting due to mantle upwelling after the delamination and subsequent break‐off of the Bitlis slab. Lithospheric dripping in central Anatolia likely facilitates the mantle flows through the window between the Cyprus and Aegean slabs, which results in the piecemeal low anomaly in central Anatolia.

Joint Analysis of Seismic and Electrical Observables Beneath the Central Appalachians Requires Partial Melt in the Upper Mantle

AbstractThe Central Appalachian Anomaly (CAA) is a region of the upper mantle beneath eastern North America that exhibits pronounced anomalies in its seismic velocity, seismic attenuation, and electrical conductivity structure. The CAA clearly expresses itself in low velocity, high attenuation, and high conductivity values; however, the present‐day composition and state of the asthenospheric upper mantle in the anomalous region remains imperfectly known. The collection of data from densely spaced, co‐located seismic and magnetotelluric arrays during the Mid‐Atlantic Geophysical Integrative Collaboration (MAGIC) experiment affords the opportunity to probe the structure and properties of the upper mantle in the CAA region in detail using multiple types of geophysical observations. Here, we present new observations of P and S wave travel times from teleseismic earthquakes measured at MAGIC stations, including a determination of how travel times deviate from the predictions of a standard 1‐D reference model. These observations constrain the ratio of the P to S wave travel time perturbations associated with the CAA, which in turn allows us to estimate the ratio of P and S wave velocity anomalies. We combine these observations with previously published estimates of seismic attenuation and electrical conductivity in the upper mantle beneath the MAGIC array, and carry out forward modeling to determine reasonable ranges of temperature, partial melt fraction, water content, and composition for the CAA. Our results suggest that 1%–2% partial melt is required to simultaneously explain the velocity, attenuation, and electrical conductivity observations beneath the MAGIC array.

Local Earthquake Seismic Tomography Reveals the Link Between Crustal Structure and Volcanism in Tenerife (Canary Islands)

AbstractVolcanic activity on Tenerife Island is extremely diverse. Three radial rift zones are characterized by cinder cones from basaltic fissure eruptions. A triple junction in central Tenerife exhibits a complex of merged, predominantly phonolitic, stratovolcanoes. The Las Cañadas caldera and widespread ignimbrite deposits reveal high explosive potential. We investigated the crustal and upper mantle structure beneath Tenerife using local earthquake data recorded by two dense seismic networks on the island. For our tomographic inversion, we selected >130,000 P‐ and S‐wave arrivals from ∼6,300 events that occurred during seismic unrests in 2004–2005 and 2017–2021. Synthetic tests confirmed that we could robustly resolve seismic velocity structures to ∼20 km depth. In the upper crust (down to ∼7 km) beneath central Tenerife, a prominent high‐velocity anomaly represents the rigid core of the volcanic complex; at greater depths, a strong low‐velocity anomaly reveals abrupt crustal thickening. Vp and Vs contour lines of 5.2 and 2.85 km/s, respectively, reveal Moho depth variation; crustal thickness beneath Las Cañadas reaches ∼17 km, whereas that beneath other parts of Tenerife is ∼10 km. An anomaly at ∼5 km beneath the caldera with low Vp, low Vs, and high Vp/Vs might be associated with a major phonolitic magma reservoir. Similar anomalies at ∼ sea level may represent shallow magma sources responsible for recent eruptions. Seismicity occurs in a columnar area of high Vp, high Vs, and low Vp/Vs, and may represent hydrothermal fluid migration through brittle media. Based on our results, we constructed a conceptual model of volcanic activity on Tenerife.

The Mechanical Nature of the Lithosphere Beneath the Eastern Central Atlantic Hotspots

AbstractThe Eastern Central Atlantic (ECA) region includes the Azores, Canary, Cape Verde, Great Meteor, and Madeira hotspots. These hotspots exhibit a large variety of characteristics and are rooted in the lithosphere ranging in age from newly created at the Mid Atlantic Ridge to Jurassic at the NW Africa Atlantic margin. Therefore, the ECA region represents an excellent scenario to investigate in an integrated way the effects of hotspots on the mechanical structure of oceanic lithosphere. Here, we calculate the effective elastic thickness (Te) of the lithosphere from an analysis of gravity and topography. Azores hotspot is characterized by a Te < 10 km, whereas the Great Meteor, Cape Verde, and Madeira hotspots have intermediate Te (15–30 km) values. In contrast, the Canary hotspot is characterized by a much higher Te (>50 km), forming the largest and most prominent mechanical feature in the ECA. All the hotspots except Canary show standard elastic thickness values when compared to average values for the same age lithosphere and to other oceanic areas in the world. The high strength of the Canary hotspot may be related to the highly depleted mantle composition in the area. The comparison between the elastic thickness distribution and the upper mantle seismic velocity structure shows no correlation between the Te estimated at the ECA hotspots (with the exception of Azores) and the presence of low shear‐wave velocity anomalies in the underlying mantle. This lack of correlation suggests a negligible effect of upper mantle temperature anomalies on the flexure of the ECA region.

Seismicity and décollement geometry beneath the Sikkim - Darjeeling Himalaya using local earthquake tomography: Implications for seismotectonic and Seismogenesis

In this study, we attempted to establish the plausible depth of disposition of the basal décollement through 3-D seismic imaging using local earthquake tomography to understand the linkage between seismogenesis and collisional tectonics of the Sikkim-Darjeeling Himalayan region. We analysed 2105 events recorded by 33 broadband seismograph stations during 2005–2011. Detailed 3-D seismic imaging unravelled heterogeneous velocity structures (Vp and Vs) with distinct variations in Poisson's ratio (ϭ) at different depth ranges. A well-resolved low-velocity zone is found to persist consistently at a depth of about 20 km corresponding to the gentle north dipping décollement plane, exhibiting significant velocity perturbations across this interface. The geometry and depth of the décollement surface vary laterally, and its deepest disposition is observed at a depth of ∼30 km. The shallower low-velocity zone up to 10 km with higher-ϭ suggests the presence of the quaternary piedmont sediments with high fluid saturations. In contrast, the higher velocity perturbation with low ϭ at the mid-crustal layer is indicative of the competent parts of the crust, where seismogenesis is related to the occurrence of earthquakes of varying strengths beneath the Sikkim - Darjeeling Himalayan region. Our seismic imaging corroborates with the past moderate earthquakes that were confined to the vicinity of high and low velocities (Vp and Vs) and ϭ zones, whilst the strong size earthquakes (M > 6.0) occurred mostly below the décollement plane where high-velocity basement thrust exists along with Indian subducting plate. This study clearly expresses the well-defined disposition of the décollement zone that controls the nature and extent of seismogenesis.

Connections between arc volcanoes in Central Kamchatka and the subducting slab inferred from local earthquake seismic tomography

The area of Central Kamchatka limited by latitudes of 52.5 and 54 degrees includes six active volcanoes (Avacha, Koryaksky, Zhupanovsky, Mutnovsky, Gorely and Opala), as well as a number of dormant and extinct stratovolcanoes, monogenic cones and large calderas. Furthermore, it contains the Malko-Petropavlovsk fracture zone (MPZ), which marks the boundary between two distinct subduction regimes to the south and to the north. We present a new seismic tomography model for this area, which was constructed based on the joint use of data of the Kamchatkan permanent seismic stations and a temporary network installed in the region in 2019–2020. A series of synthetic tests have demonstrated fair resolution of the derived seismic velocity structures in the crust and in the mantle wedge down to ∼150 km. The distributions of the P and S wave velocities, and especially the Vp/Vs ratio, clearly highlight the connection between the volcanic centers in Central Kamchatka and the subducting slab. At depths below 40 km depth, we observe two large low-velocity anomalies centered below Zhupanovsky and Mutnovsky volcanoes and covering all other volcanoes in the area. In the vertical sections, the corresponding anomalies of high Vp/Vs ratio have mushroom shapes with the heads spreading along the bottom of the crust, which probably represent the underplating of magma material that feeds the volcanoes of the groups. The tomography results also reveal some important tectonic features, such as a V-shaped fault system in the Avacha Graben, which is the part of the MPZ.

Tomographic evidences for hydraulic fracturing induced seismicity in the Changning shale gas field, southern Sichuan Basin, China

Seismic activity near the shale gas fields in the Sichuan Basin of China has risen significantly in the past few years. Due to the lack of industrial stimulation data to the public, it is difficult to directly link the surge in seismicity near the shale gas fields with hydraulic fracturing stimulation. Meanwhile, it is also challenging to better understand the mechanisms inducing/triggering seismicity in these areas. In the absence of stimulation data, understanding subsurface fluid distribution can go a long way toward addressing these problems. Therefore, in this study, we have applied the double-difference seismic tomography method and the focal mechanism tomography method to the monitoring data of a local seismic array in the Changning shale gas field to build high-resolution three-dimensional models of seismic velocity structure and pore pressure field, then used these models to characterize fluid distributions beneath the shale gas field. From the inverted models, the correlation of overpressure regions (>12 MPa) with moderate-to-high Vp/Vs (1.75-1.90) is clearly observed. With the aid of the exploration data (including well log, active and passive seismic data), the overpressure regions are interpreted to be created by pore pressure or fluid diffusion along pre-existing faults from the shale gas reservoirs, while variations of the Vp/Vs model partly indicate water- and gas-bearing rocks. By jointly analyzing the geophysical model features and earthquake spatio-temporal characteristics, we infer that the earthquakes are primarily caused by increased pore pressures on the seismogenic faults due to the presence of water and/or gas. Besides, the poroelastic stress perturbation and aseismic slip produced by hydraulic fracturing also contribute to the generation of certain earthquake swarms.

High-frequency S and S-coda waves at ocean-bottom seismometers

To clarify the characteristics of high-frequency (> 1 Hz) S and S-coda waves at ocean-bottom seismometers (OBSs), we analyzed seismograms observed at permanent OBSs and inland broadband seismometers around the Kii Peninsula in southwest Japan along the Nankai Trough. The coda amplitudes (both horizontal and vertical) at the OBSs were much larger than those at the inland rock-site stations. Because coda amplitudes relative to those at inland rock-site stations have been used as site-amplification factors, large site amplifications for both components can be expected due to the presence of thick oceanic sediments just below the OBSs; however, the observed maximum S-wave amplitudes in the vertical component exhibited similar attenuation trends against epicentral distances at both OBS and inland stations. To clarify the causes of this discrepancy, we conducted numerical simulations of seismic wave propagation using various three-dimensional seismic velocity structure models. The results demonstrated that coda waves at OBSs mostly comprise multiple scattered waves within a thick (> 2 km) sedimentary layer; consequently, coda amplitudes at OBSs become much larger than those at inland rock-site stations. Our numerical simulations also confirmed the generation of large coda amplitudes at regions with seawater depths ≥ 4 km, where no OBS was deployed. However, the thick sedimentary layer and seawater have limited effects on maximum S-wave amplitudes at the OBSs. Given that the effects of a thick sedimentary layer and seawater on S and S-coda waves differ, we concluded that the coda-normalization technique for site-amplification correction against a rock-site station could not be applied if stations are located within regions above the thick sedimentary layer or deeper sea depths. The site amplifications at the OBSs were corrected according to the horizontal-to-vertical ratios at each OBS; we adjusted the simulated horizontal envelopes at the OBSs using these ratios of the observed S-coda waves. As well as inland seismometers, the site-corrected simulation results practically reproduced the observed high-frequency envelopes at OBSs.Graphical

The Variable Continuous Bimaterial Interface in the San Jacinto Fault Zone Revealed by Dense Seismic Array Analysis of Fault Zone Head Waves

AbstractKey factors controlling earthquake ruptures include fault geometry, continuity, and seismic velocity structure around the fault. We present a novel tool that better informs deep bimaterial fault geometry embedded in distributed damage and seismicity, associated velocity contrasts across the fault, and their correlations with surface complexities. The method employs fault zone head and direct body waves and is applied to recordings from five spatiotemporally different seismic arrays along the complex San Jacinto fault zone (SJFZ) in southern California. We detect and distinguish these signals based on instantaneous phase coherence and relative energy in a cascading manner from one scale array to another. The analysis reveals a >70‐km long continuous bimaterial interface within the SJFZ with several deep northeast dipping fault segments. The northern SJFZ, for instance, locates ∼7 km northeast of its surface expression at 18‐km depth. P‐wave velocity contrasts range from near 0% to >15%, consistent with other bimaterial faults, and differ by a few % depending on fault‐array azimuth, implying directional‐dependent velocity contrasts. S‐wave head waves and velocity contrasts are also imaged for the first time at the southern SJFZ, averaging to 2.9% in agreement with tomography results. The imaged geometry and continuity suggest the SJFZ initiated along remnant tectonic structures and translates to a rupture potential of M > 7.2, i.e., the sizes of its largest paleo‐earthquakes. The P and S contrasts, and their ratios, have important implications for earthquake rupture speed, mode, directivity, and frictional heating along the SJFZ and other major faults globally.

Wavefield distortion imaging of Earth's deep mantle

The seismic wavefield, as recorded at the surface, carries information about the seismic source and Earth's structure along the seismic path, essential for the understanding of the interior of our planet. For 40 years seismic tomography studies have resolved the 3D seismic velocity structure in growing detail using seismic traveltimes and waveforms. These studies have been driving our understanding of the dynamics and evolution of the planet, but are limited in their spatial resolution to imaging scales of a few 100 s to 1000 km due to the constraints of the tomographic inversion. Detailed studies of seismic waveforms can resolve finer scale structure but are often reliant on serendipitous source-receiver combinations and provide very uneven coverage of the planet. Therefore, we often lack an understanding of the fine scale structure of the Earth that is important to understand structures and processes such as mantle plumes or details of slab recycling. Here we show evidence that we can exploit slowness vector deviations of the seismic wavefield to extend our knowledge of Earth structure to smaller scales using large datasets. Analysing seismic array data, we show strong and measurable focussing and defocussing effects of the teleseismic P and Pdiff wavefield sampling the deep Earth. We compare the P-wave results to additional S and Sdiff data and find good agreement between both wavetypes. We can link the wavefield deviations to strong velocity variations assuming sharp boundaries are sampled along the path in the deep mantle. The dataset samples the Pacific and Gulf of Mexico well and shows strong horizontal incidence (backazimuth) deviations in the Pacific (up to 14° westwards) and beneath the Gulf of Mexico (up to 5 to 8° east- and west-ward). The backazimuth deviations are also reflected in slowness deviations in the range of ± 0.8 s/° relating to velocity variations in the range of ± 9 km/s. Using 3D raytracing we are able to forward model the detected backazimuth variations of the P and Pdiff dataset. The high frequencies of the P-waves, density of the ray-paths, and low computational cost of our forward calculation allow us to construct a higher resolution and more detailed model of velocity anomalies under Hawaii than was possible with previous methods. The best-fitting velocity model for the Pacific contains two low-velocity regions located at N25°/W155° and N25°/W165° beneath the tip of the Hawaii Emperor chain. The Pacific anomalies have diameters (D) of 6° and 2° with velocity reductions (dVP) of 8% and 4% with heights (H) above the CMB of 70 km and at least 200 km, respectively. We also detect a fast region of 3% velocity increase in the North Pacific rising at least 300 km above the CMB with a diameter of 12° at N60°/W175°. Beneath the Gulf of Mexico we find ambiguous results with either a slow region (N25°/W85°, H = 200 km, dVP=-3%, D = 2°) or a fast region (N15°/W75°, H = 200 km, dVP = 3%, D = 4°) able to explain the data. We thus show that the directivity information of the seismic wavefield - largely underexploited - can be used to resolve the fine scale velocity structure of the Earth's interior with great accuracy and can deliver additional insight into deep Earth dynamics.

Large-scale structures in the Earth’s interior: Top-down hemispherical dynamics constrained by geochemical and geophysical approaches

Geochemical and geophysical observations for large-scale structures in the Earth’s interior, particularly horizontal variations of long wavelengths such as degree-1 and degree-2 structures, are reviewed with special attention to the cause of hemispherical mantle structure. Seismic velocity, electrical conductivity, and basalt geochemistry are used for mapping the large-scale structures to discuss thermal and compositional heterogeneities and their relations to dynamics of the Earth’s interior. Seismic velocity structure is the major source of information on the Earth’s interior and provides the best spatial resolution, while electrical conductivity is sensitive to water/hydrogen contents. The composition of young basalts reflects the mantle composition, and the formation age of large-scale structures can be inferred based on the radiogenic isotopes. Thus, these different research disciplines and methods complement each other and can be combined to more concretely constrain the structures and their origins. This paper aims to integrate observations from these different approaches to obtain a better understanding of geodynamics. Together with numerical modeling results of convection in the mantle and the core, “top-down hemispherical dynamics” model of the crust-mantle-core system is examined. The results suggest that a top-down link between the supercontinents, mantle geochemical hemisphere, and inner core seismic velocity hemisphere played an essential role in formation of the large-scale structures and dynamics of the Earth’s interior.