scholarly journals The signal of outermost-core stratification in body-wave and normal-mode data

2020 ◽  
Vol 223 (2) ◽  
pp. 1338-1354
Author(s):  
Rûna van Tent ◽  
Arwen Deuss ◽  
Satoshi Kaneshima ◽  
Christine Thomas
Keyword(s):  
Normal Mode ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Body Wave ◽  
Normal Modes ◽  
Outer Core ◽  
Core Structure ◽  
Seismic Velocities ◽  
Data Types ◽  
Additional Information

Summary Seismological models of the outer core’s radial velocity structure show that the outermost core is slower than PREM. For models derived from body-wave data these low velocities are confined to the top of the outer core, while normal-mode data prefer a velocity gradient that deviates from PREM throughout the entire outer core. These different models have led to conflicting interpretations regarding the presence of stratification at the top of the outer core. While body-wave based models have been shown to require a compositionally stratified outermost core, the velocity and density profiles obtained from normal-mode data correspond to a homogeneous outer core. In addition, the observed low velocities in the outermost core are difficult to reconcile with compositional models of stratification, as the required enrichment in light elements would generally increase seismic velocities. Here, we investigate how well-suited both seismic body-wave and normal-mode data are to constrain the velocity and density structure of the outer core. To this end, we model and compare the effects of outer-core structure and D″ structure on the differential traveltimes of body-wave phases SmKS and on the centre frequencies of normal modes. We find that a trade-off between outer-core structure and D″ structure exists for both data types, but neither data can be readily explained by reasonable D″ velocities and densities. Low outermost-core velocities are therefore still required by seismological data. Using additional information from the centre frequencies of Stoneley modes—normal modes that are particularly sensitive to variations in velocity and density at the top of the outer core—we confirm that normal-mode data indeed require low velocities with respect to PREM in the outermost core, similar to a recent normal-mode model, and an overall higher outer-core density. The presence of buoyant stratification in the outermost core is therefore not immediately supported by the centre frequencies of Stoneley modes. Stratification with high seismic velocity, as one would expect from most straightforward stratification-forming processes, is directly contradicted by our results.

Quaternary sodic and potassic intraplate volcanism in northeast China controlled by the underlying heterogeneous lithospheric structures

Geology ◽  
2021 ◽  
Author(s):  
Xingli Fan ◽  
Qi-Fu Chen ◽  
Yinshuang Ai ◽  
Ling Chen ◽  
Mingming Jiang ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Lithospheric Mantle ◽  
Northeast China ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Subcontinental Lithospheric Mantle ◽  
Seismic Velocities ◽  
S Wave ◽  
Crust And Upper Mantle ◽  
Potassic Volcanism

The origin and mantle dynamics of the Quaternary intraplate sodic and potassic volcanism in northeast China have long been intensely debated. We present a high-resolution, three-dimensional (3-D) crust and upper-mantle S-wave velocity (Vs) model of northeast China by combining ambient noise and earthquake two-plane wave tomography based on unprecedented regional dense seismic arrays. Our seismic images highlight a strong correlation between the basalt geochemistry and upper-mantle seismic velocity structure: Sodic volcanoes are all characterized by prominent low seismic velocities in the uppermost mantle, while potassic volcanoes still possess a normal but thin upper-mantle “lid” depicted by high seismic velocities. Combined with previous petrological and geochemical research findings, we propose that the rarely erupted Quaternary potassic volcanism in northeast China results from the interaction between asthenospheric low-degree melts and the overlying subcontinental lithospheric mantle. In contrast, the more widespread Quaternary sodic volcanism in this region is predominantly sourced from the upwelling asthenosphere without significant overprinting from the subcontinental lithospheric mantle.

Seismic structure of basalt flows from surface seismic data, borehole measurements, and synthetic seismogram modeling

Geophysics ◽  
2001 ◽  
Vol 66 (6) ◽  
pp. 1925-1936 ◽  
Author(s):  
Moritz M. Fliedner ◽  
Robert S. White
Keyword(s):  
Velocity Structure ◽  
Seismic Velocity ◽  
Synthetic Seismogram ◽  
P Wave ◽  
Wide Angle ◽  
Seismic Velocities ◽  
Seismic Structure ◽  
Low Velocity ◽  
Amplitude Variation With Offset ◽  
Velocity Gradients

We use the wide‐angle wavefield to constrain estimates of the seismic velocity and thickness of basalt flows overlying sediments. Wide angle means the seismic wavefield recorded at offsets beyond the emergence of the direct wave. This wide‐angle wavefield contains arrivals that are returned from within and below the basalt flows, including the diving wave through the basalts as the first arrival and P‐wave reflections from the base of the basalts and from subbasalt structures. The velocity structure of basalt flows can be determined to first order from traveltime information by ray tracing the basalt turning rays and the wide‐angle base‐basalt reflection. This can be refined by using the amplitude variation with offset (AVO) of the basalt diving wave. Synthetic seismogram models with varying flow thicknesses and velocity gradients demonstrate the sensitivity to the velocity structure of the basalt diving wave and of reflections from the base of the basalt layer and below. The diving‐wave amplitudes of the models containing velocity gradients show a local amplitude minimum followed by a maximum at a greater range if the basalt thickness exceeds one wavelength and beyond that an exponential amplitude decay. The offset at which the maximum occurs can be used to determine the basalt thickness. The velocity gradient within the basalt can be determined from the slope of the exponential amplitude decay. The amplitudes of subbasalt reflections can be used to determine seismic velocities of the overburden and the impedance contrast at the reflector. Combining wide‐angle traveltimes and amplitudes of the basalt diving wave and subbasalt reflections enables us to obtain a more detailed velocity profile than is possible with the NMO velocities of small‐offset reflections. This paper concentrates on the subbasalt problem, but the results are more generally applicable to situations where high‐velocity bodies overlie a low‐velocity target, such as subsalt structures.

scholarly journals Seismic velocity model of the crust in the northern Canadian Cordillera from Rayleigh wave dispersion data

2017 ◽  
Vol 54 (2) ◽  
pp. 163-172 ◽  
Author(s):  
Shutian Ma ◽  
Pascal Audet
Keyword(s):  
Rayleigh Wave ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Group Velocity Dispersion ◽  
Velocity Model ◽  
Canadian Cordillera ◽  
Seismic Velocities ◽  
Crustal Layer ◽  
Velocity Models ◽  
Moderate Earthquake

Models of the seismic velocity structure of the crust in the seismically active northern Canadian Cordillera remain poorly constrained, despite their importance in the accurate location and characterization of regional earthquakes. On 29 August 2014, a moderate earthquake with magnitude 5.0, which generated high-quality Rayleigh wave data, occurred in the Northwest Territories, Canada, ∼100 km to the east of the Cordilleran Deformation Front. We carefully selected 23 seismic stations that recorded the Rayleigh waves and divided them into 13 groups according to the azimuth angle between the earthquake and the stations; these groups mostly sample the Cordillera. In each group, we measured Rayleigh wave group velocity dispersion, which we inverted for one-dimensional shear-wave velocity models of the crust. We thus obtained 13 models that consistently show low seismic velocities with respect to reference models, with a slow upper and lower crust surrounding a relatively fast mid crustal layer. The average of the 13 models is consistent with receiver function data in the central portion of the Cordillera. Finally, we compared earthquake locations determined by the Geological Survey of Canada using a simple homogenous crust over a mantle half space with those estimated using the new crustal velocity model, and show that estimates can differ by as much as 10 km.

scholarly journals Seismic Velocity Structure of the Shallow Part  of the Alpine Fault and Gravity Study of the  Basement Features in the Whataroa River Flood  Plain, Central Westland, South Island

2021 ◽  
Author(s):  
◽  
Nicolas Brikke
Keyword(s):  
Velocity Structure ◽  
Seismic Velocity ◽  
River Mouth ◽  
Flood Plain ◽  
Time Interval ◽  
Seismic Velocities ◽  
Alpine Fault ◽  
River Flood ◽  
Gravity Database ◽  
Shallow Part

<p>The deep and middle sections of the Alpine fault have extensively been studied, however, the shallow part has had relatively minor geophysical attention. This study focuses on the basement geometry and the determination of the upper-crustal velocity structure of the Alpine fault in the vicinity of the Whataroa River flood plain in Central Westland, South Island. Data from a temporary gravity survey collected in November 2006, the GNS gravity database and four of the westernmost shot gathers from the SIGHT96's transect 1 were used for this project. A ray-tracing software was used to establish the velocity structure of the shallow part of the Alpine fault. Seismic velocities decrease to 3.8 km/s immediately southeast of the mylonite strip, which is adjacent to the Alpine fault's ramp heading towards the fault's surface trace from the southeast or from depth. Velocities of 5 km/s reach 2 km depth to the southeast of the Alpine fault's ramp. Results of the gravity and seismic models coincide in the positions and the dimensions of two northwest-orientated glacial overdeepings. The strike of their alignment is offset to the northeast by 3.5 km and is sub-parallel to the mouth of the Whataroa River. We propose that these kettle holes, thought to have been carved successively during the Waimea and Otira glaciations, are the beheaded river mouth of the Whataroa river. By supposing that the furthest kettle hole was carved during the Waimea glaciation, the 3.5 km offset thus corresponds to 140 Ka of dextral slip on the Alpine fault, we could approximate the mean displacement rate over the time interval of 140-18 Ka of 25 mm/yr.</p>

scholarly journals Studies of Seismic Velocities in Subduction Zones from Continuous Ocean Bottom Seismometer Data

2022 ◽  
Author(s):  
◽  
Weiwei Wang
Keyword(s):  
Ambient Noise ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Okinawa Trough ◽  
Ocean Bottom ◽  
Seismic Velocities ◽  
Slow Slip ◽  
Noise Data ◽  
Velocity Variations ◽  
Back Arc

<p><b>This thesis uses continuous ambient noise data recorded by Ocean Bottom Seismometers (OBSs) to study seismic velocities in the upper crust of the overriding plate. The first and second projects (Chapters 3 and 4) focus on temporal seismic velocity variations in the northern Hikurangi subduction zone offshore the North Island, New Zealand, while the third project (Chapter 5) focuses on shear wave velocities in the southwestern Okinawa Trough offshore northeastern Taiwan. In the first project (Chapters 3), we investigate a region of frequent slow slip events (SSEs) offshore Gisborne, North Island, New Zealand. From September to October 2014, an SSE occurred with a slip over 250 mm and was recorded successfully by the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip deployment II (HOBITSS II). We apply coda wave interferometry on the ambient noise data acquired by nine OBSs deployed by the HOBITSS II to study the seismic velocity variations related to the SSE. The average velocity variations display a decrease on the order of 0.05% during the SSE, followed by an increase of similar magnitude afterwards. Two hypotheses are proposed to explain our observation. The first hypothesis, which has been suggested by previous studies, considers that the velocity decrease during the SSE is caused by more fluids migrating into the upper plate as the SSE breaks a low-permeability seal on the plate boundary. After the SSE, the fluids in the upper plate diffuse gradually and the velocity increases; The second hypothesis is that before the SSE, elastic strain accumulates causing contraction and reduction of porosity and therefore increase of velocity (the velocity increase between SSEs). During the SSE, the velocity decrease is caused by increased porosity as the SSE relieves the accumulated elastic strain on the plate interface, which results in dilation. After the SSE, stress and strain accumulate again, causing a porosity decrease and a velocity increase back to the original value. This study demonstrates that the velocity variations related to SSEs are observable and provides evidence for slow slip mechanism hypotheses.</b></p> <p>The second project (Chapter 4) focuses on the temporal seismic velocity variations associated with an SSE in 2019 offshore Gisborne, North Island, New Zealand. This is a later SSE in the same area as the first project (Chapters 3). Based on the success of the HOBITSS II, more ocean bottom instruments were deployed in the northern Hikurangi subduction zone from 2018 to 2019 (HOBITSS V). An SSE lasting approximately one month from the end of March to the beginning of May 2019 occurred during the deployment and was recorded by the network. The main slip was south of the deployment and the slip beneath the deployment was up to 150 mm. This study applies coda wave interferometry on the ambient noise data acquired by five OBSs and computes seismic velocity variations to investigate their relation to the SSE. A velocity decrease on the order of 0.015% during the SSE and an increase back to the original velocity value are observed at 1–2.5 s. This supports the two hypotheses proposed in Chapters 3: fluid migration and/or stain changes through the SSE cycle. In addition, velocity variations computed from individual stations show velocity increases before the SSE, which are destructively interfered in their average. Such a situation could occur if the SSE migrated across the network. If the velocity increases before the SSE from individual stations are real, they can be only explained by the hypothesis of crustal strain changes (the second hypothesis in project 1). However, fluid migration (the first hypothesis in project 1) may still happen concomitantly.</p> <p>The third project focuses on the tectonics in southwestern Okinawa Trough offshore northeastern Taiwan. The southwestern Okinawa Trough is an active back-arc basin, extending and rifting within the continental lithosphere. The tectonic development of the back-arc basin is still not well-understood. This study uses continuous ambient noise data recorded by 34 OBSs deployed by Academia Sinica at various periods from 2010 to 2018. Cross-correlations on vertical seismic components and pressure gauges are computed to construct Rayleigh/Scholte waves to study the shear wave velocity structure in the southwestern Okinawa Trough. Phase velocities are measured from the Rayleigh/Scholte waves. Shear velocities are inverted from the phase velocities. Results show the velocity in the south of the back-arc rifting axis near the axis is slower than the velocity in the north of the rifting axis, suggesting the velocity structure in the southwestern Okinawa Trough is asymmetric along the rifting axis. Previous studies have shown high heat flows (about 110mW/m 2 on average) in the south of the rifting axis. The low velocity in the south could be caused by the high heat flow that may be related to asymmetric back-arc extension and/or rifting. This study presents the shear wave velocity structure in the southwest Okinawa Trough is asymmetric along the rifting axis, which implies the back-arc extending/rifting is asymmetric in the study region. This study also suggests effective techniques for OBS noise corrections and unwrapping the cycle skipping of phase velocity measurements.</p> <p>In summary, this thesis represents three projects focusing on seismic velocities in two subduction zones using ambient noise data collected by OBSs. The first and second projects study the temporal velocity variations and the relation to SSEs. Both studies observe velocity decreases during the SSEs and increases after the SSEs, supporting two hypotheses of fluid migration and/or stain changes through the SSE cycle. The third project finds the shear velocity structure in the southwestern Okinawa Trough is asymmetric along the rifting center, which may imply the back-arc extension is asymmetric.</p>

Towards consistent seismological models of the core-mantle boundary landscape

2020 ◽  
Author(s):  
Paula Koelemeijer
Keyword(s):  
Normal Mode ◽  
Lower Mantle ◽  
Seismic Velocity ◽  
Body Wave ◽  
P Wave ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
Trade Offs ◽  
Multiple Data Sets

&lt;p&gt;The dynamic topography of the core-mantle boundary (CMB) provides important constraints on dynamic processes in the mantle and core. However, inferences on CMB topography are complicated by the uneven coverage of data with sensitivity to different length scales and strong heterogeneity in the lower mantle. Particularly, a trade-off exists with density variations, which ultimately drive mantle flow and are vital for determining the origin of mantle structures. Here, I review existing models of CMB topography and lower mantle density, focusing on seismological constraints (Koelemeijer, 2020). I develop average models and vote maps with the aim to find model consistencies and discuss what these may teach us about lower mantle structure and dynamics.&lt;/p&gt;&lt;p&gt;While most density models image two areas of dense anomalies beneath Africa and the Pacific, their exact location and relationship to seismic velocity structure differs between studies. CMB topography strongly influences the retrieved density structure, which partially helps to resolve differences between recent studies based on Stoneley modes and tidal measurements. CMB topography models vary both in pattern and amplitude and a discrepancy exists between models based on body-wave and normal-mode data. As existing models typically feature elevated topography below the Large-Low-Velocity Provinces (LLVPs), very dense compositional anomalies may be ruled out as possibility.&lt;/p&gt;&lt;p&gt;To achieve a similar consistency as observed in lower mantle models of S-wave and P-wave velocity, future studies should combine multiple data sets to break existing trade-offs between CMB topography and density. Important considerations in these studies should be the choice of theoretical approximation and parameterisation. Efforts to develop models of CMB topography consistent with both body-wave and normal-mode data should be intensified, which will aid in narrowing down possible explanations for the LLVPs and provide additional insights into mantle dynamics.&lt;/p&gt;&lt;p&gt;&lt;em&gt;Koelemeijer, P. (2020), &amp;#8220;Towards consistent seismological models of the core-mantle boundary landscape&amp;#8221;. Book chapter in revision for AGU monograph &quot;Mantle upwellings and their surface expressions&quot;, edited by Marquardt, Cottaar, Ballmer and Konter&lt;/em&gt;&lt;/p&gt;

Near-salt stress-induced seismic velocity changes and seismic anisotropy and their impacts on salt imaging: A case study in the Kuqa depression, Tarim Basin, China

2020 ◽  
Vol 8 (3) ◽  
pp. T487-T499
Author(s):  
Yunqiang Sun ◽  
Gang Luo ◽  
Yaxing Li ◽  
Mingwen Wang ◽  
Xiaofeng Jia ◽  
...  
Keyword(s):  
Salt Stress ◽  
Seismic Anisotropy ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Seismic Velocities ◽  
Kuqa Depression ◽  
Velocity Models ◽  
Salt Structure ◽  
Seismic Velocity Changes ◽  
Velocity Changes

It has been recognized that stress perturbations in sediments induced by salt bodies can cause elastic-wave velocity (seismic velocity) changes and seismic anisotropy through changing their elastic parameters, thus leading to difficulties in salt imaging. To investigate seismic velocity changes and seismic anisotropy by near-salt stress perturbations and their impacts on salt imaging, taking the Kuqa depression as an example, we have applied a 2D plane-strain static geomechanical finite-element model to simulate stress perturbations and calculate the associated seismic velocity changes and seismic anisotropy; then we used the reverse time migration and imaging method to image the salt structure by excluding and including the stress-induced seismic velocity changes. Our model results indicate that near-salt stresses are largely perturbed due to salt stress relaxation, and the stress perturbations lead to significant changes of the seismic velocities and seismic anisotropy near the salt structure: The maximum seismic velocity changes can reach approximately 20% and the maximum seismic anisotropy can reach approximately 10%. The significant changes of seismic velocities due to stress perturbations largely impact salt imaging: The salt imaging is unclear, distorted, or even failed if we exclude near-salt seismic velocity changes from the preliminary velocity structure, but the salt can be better imaged if the preliminary velocity structure is modified by near-salt seismic velocity changes. We find that the locations where salt imaging tends to fail usually occur where large seismic velocity changes happen, and these locations are clearly related to the geometric characteristics of salt bodies. To accurately image the salt, people need to integrate results of geomechanical models and stress-induced seismic velocity changes into the imaging approach. The results provide petroleum geologists with scientific insights into the link between near-salt stress perturbations and their induced seismic velocity changes and help exploration geophysicists build better seismic velocity models in salt basins and image salt accurately.

scholarly journals Seismic Velocity Structure of the Shallow Part  of the Alpine Fault and Gravity Study of the  Basement Features in the Whataroa River Flood  Plain, Central Westland, South Island

2021 ◽  
Author(s):  
◽  
Nicolas Brikke
Keyword(s):  
Velocity Structure ◽  
Seismic Velocity ◽  
River Mouth ◽  
Flood Plain ◽  
Time Interval ◽  
Seismic Velocities ◽  
Alpine Fault ◽  
River Flood ◽  
Gravity Database ◽  
Shallow Part

<p>The deep and middle sections of the Alpine fault have extensively been studied, however, the shallow part has had relatively minor geophysical attention. This study focuses on the basement geometry and the determination of the upper-crustal velocity structure of the Alpine fault in the vicinity of the Whataroa River flood plain in Central Westland, South Island. Data from a temporary gravity survey collected in November 2006, the GNS gravity database and four of the westernmost shot gathers from the SIGHT96's transect 1 were used for this project. A ray-tracing software was used to establish the velocity structure of the shallow part of the Alpine fault. Seismic velocities decrease to 3.8 km/s immediately southeast of the mylonite strip, which is adjacent to the Alpine fault's ramp heading towards the fault's surface trace from the southeast or from depth. Velocities of 5 km/s reach 2 km depth to the southeast of the Alpine fault's ramp. Results of the gravity and seismic models coincide in the positions and the dimensions of two northwest-orientated glacial overdeepings. The strike of their alignment is offset to the northeast by 3.5 km and is sub-parallel to the mouth of the Whataroa River. We propose that these kettle holes, thought to have been carved successively during the Waimea and Otira glaciations, are the beheaded river mouth of the Whataroa river. By supposing that the furthest kettle hole was carved during the Waimea glaciation, the 3.5 km offset thus corresponds to 140 Ka of dextral slip on the Alpine fault, we could approximate the mean displacement rate over the time interval of 140-18 Ka of 25 mm/yr.</p>

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