S. E. Hansen

Geological Survey of Alabama

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A. Nyblade

Pennsylvania State University

Douglas A. Wiens

Washington University in St. Louis

R. C. Aster

Colorado State University

S. Anandakrishnan

Pennsylvania State University

A. D. Huerta

Central Washington University

David S. Heeszel

United States Nuclear Regulatory Commission

Jordan H. Graw

Stennis Space Center

T. J. Wilson

The Ohio State University

P. Shore

Washington University in St. Louis

J. Paul Winberry

Central Washington University

Cooperative Institutions

Pennsylvania State University

Washington University in St. Louis

Planetary Science Institute

University of Alabama

Geological Survey of Alabama

New Mexico Institute of Mining and Technology

University of California, San Diego

Scripps Institution of Oceanography

Schlumberger (British Virgin Islands)

University of Leeds

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Imaging Structure beneath East Central United States Using CCP Stacking and Scattering Kernel Analysis of Ps and Sp Receiver Functions

AGU Fall Meeting Abstracts (2018)

Shangkun Deng A. Levander S. E. Hansen

Kernel (algebra)

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The deep seismic structure of the Ethiopia/Afar hotspot and the African superplume

Geophysical Journal International (2013)

S. E. Hansen A. Nyblade

The Ethiopia/Afar hotspot has been frequently explained as an upper mantle continuation of the African superplume, with anomalous material in the lower mantle under southern Africa, rising through the transition zone beneath eastern Africa. However, the significantly larger amplitude low velocity anomaly in the upper mantle beneath Ethiopia/Afar, compared to the anomalies beneath neighboring regions, has led to questions about whether or not along-strike differences in the seismic structure beneath eastern Africa and western Arabia are consistent with the superplume interpretation. Here we present a new P-wave model of the hotspot's deep structure and use it to evaluate the superplume model. At shallow (< ∼400 km) depths, the slowest velocities are centered beneath the Main Ethiopian Rift, and we attribute these low velocities to decompression melting beneath young, thin lithosphere. At deeper depths, the low velocity structure trends to the northeast, and the locus of the low velocity anomaly is found beneath Afar. The northeast-trending structure with depth is best modeled by northeastward flow of warm superplume material beneath eastern Africa. The combined effects of shallow decompression melting and northeastward flow of superplume material explain why upper mantle velocities beneath Ethiopia/Afar are significantly slower than those beneath neighboring East Africa and western Arabia. The superplume interpretation can thus explain the deep seismic structure of the hotspot if the effects of both decompression melting and mantle flow are considered.

Hotspot (geology)

10.1093/gji/ggt116

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Citations (64)

Characterizing Crustal and Upper Mantle Structure in East Antarctica with Full-Waveform Ambient Noise Tomography

Ashish Kumar Erica Emry S. E. Hansen

Seismic Tomography

Ambient noise level

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Historical Interstation Pattern Referencing (HIPR): An Application to PcP Waves Recorded in the Antarctic for ULVZ Imaging

Journal of Geophysical Research Solid Earth (2021)

S. E. Hansen Edward J. Garnero Sebastian Rost

Abstract Much of our knowledge on deep Earth structure is based on detailed analyses of seismic waveforms that often have small amplitude arrivals on seismograms; therefore, stacking is essential to obtain reliable signals above the noise level. We present a new iterative stacking scheme that incorporates Historical Interstation Pattern Referencing (HIPR) to improve data quality assessment. HIPR involves comparing travel‐time and data quality measurements between every station for every recorded event to establish historical patterns, which are then compared to individual measurements. Weights are determined based on the individual interstation measurement differences and their similarity to historical averages, and these weights are then used in our stacking algorithm. This approach not only refines the stacks made from high‐quality data but also allows some lower‐quality events that may have been dismissed with more traditional stacking approaches to contribute to our study. Our HIPR‐based stacking routine is illustrated through an application to core‐reflected PcP phases recorded by the Transantarctic Mountains Northern Network to investigate ultra‐low velocity zones (ULVZs). We focus on ULVZ structure to the east of New Zealand because this region is well‐sampled by our data set and also coincides with the boundary of the Pacific Large Low Shear Velocity Province (LLSVP), thereby allowing us to further assess possible ULVZ‐LLSVP relationships. The HIPR‐refined stacks display strong ULVZ evidence, and associated synthetic modeling suggests that the ULVZs in this region are likely associated with compositionally distinct material that has perhaps been swept by mantle convection currents to accumulate along the LLSVP boundary.

Seismogram

10.1029/2021jb022741

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Citations (4)

Connecting the African Superplume to the Anomalous Upper Mantle beneath East Africa and Western Arabia: Results from Adaptively Parameterized P-wave Tomography

AGU Fall Meeting Abstracts (2010)

S. E. Hansen A. Nyblade M. H. Benoit S. Burdick R. D. van der Hilst

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Reference seismic and thermal models for the crust and uppermost mantle beneath Antarctica from multiple datasets

Japan Geoscience Union (2017)

Weisen Shen Douglas A. Wiens Peter Gerstoft P. D. Bromirski Ralph Stephen

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Earthquake Relocation and Focal Mechanism Determination Using Waveform Cross Correlation, Nicoya Peninsula, Costa Rica

Bulletin of the Seismological Society of America (2006)

S. E. Hansen S. Y. Schwartz Heather R. DeShon Víctor González

The Nicoya Peninsula in Costa Rica directly overlies the seismogenic zone of the Middle America Trench, making it an ideal location for geophysical investigations of shallow subduction zone earthquake processes. As part of the collaborative Costa Rica Seismogenic Zone Experiment (crseize), a seismic network consisting of 20 land and 14 ocean-bottom seismometers recorded small magnitude local earthquakes along the Nicoya Peninsula from December 1999 to June 2001. Previous studies have used these data to compute local earthquake locations and 3D velocity structure to identify plate boundary seismicity and to investigate seismogenic behavior. Here we utilize waveform cross-correlation and clustering techniques in an attempt to improve earthquake relocations and determine first-motion focal mechanisms to validate, refine, and expand on existing models. Due to the high quality of the original locations and the small cross-correlation P -wave arrival time adjustments, large differences between the previously determined and the cross-correlated earthquake locations are not observed. However, focal mechanism determinations using cross-correlated P waveforms are significantly enhanced. Approximately 90% of the focal mechanisms computed for events previously identified as interplate earthquakes are consistent with underthrusting. Focal mechanisms for continental intraplate events indicate dextral strike-slip motion in the central region and normal faulting at the southern tip of the peninsula. These motions may be associated with oblique convergence and seamount subduction, respectively. Within the subducting plate, steep P and T axes of earthquakes below 50 km depth are consistent with unbending of the slab.

Relocation

Focal mechanism

Peninsula

10.1785/0120050129

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Citations (21)

Upper mantle seismic structure beneath central East Antarctica from body wave tomography: Implications for the origin of the Gamburtsev Subglacial Mountains

Geochemistry Geophysics Geosystems (2013)

Andrew Lloyd A. Nyblade Douglas A. Wiens S. E. Hansen Masaki Kanao

Abstract The Gamburtsev Subglacial Mountains (GSM), located near the center of East Antarctica, are the highest feature within the East Antarctic highlands and have been investigated seismically for the first time during the 2007/2008 International Polar Year by the Gamburtsev Mountains Seismic Experiment. Using data from a network of 26 broadband seismic stations and body wave tomography, the P and S wave velocity structure of the upper mantle beneath the GSM and adjacent regions has been examined. Tomographic images produced from teleseismic P and S phases reveal several large‐scale, small amplitude anomalies (δ V p = 1.0%, δ V s = 2.0%) in the upper 250 km of the mantle. The lateral distributions of these large‐scale anomalies are similar in both the P and S wave velocity models and resolution tests show that they are well resolved. Velocity anomalies indicate slower, thinner lithosphere beneath the likely Meso‐ or Neoproterozoic Polar Subglacial Basin and faster, thicker lithosphere beneath the likely Archean/Paleoproterozoic East Antarctic highlands. Within the region of faster, thicker lithosphere, a lower amplitude (δ V p = 0.5%, δ V s = 1.0%) slow to fast velocity pattern is observed beneath the western flank of the GSM, suggesting a suture between two lithospheric blocks possibly of similar age. These findings point to a Precambrian origin for the high topography of the GSM, corroborating other studies invoking a long‐lived highland landscape in central East Antarctica, as opposed to uplift caused by Permian/Cretaceous rifting or Cenozoic magmatism. The longevity of the GSM makes them geologically unusual; however, plausible analogs exist, such as the 550 Ma Petermann Ranges in central Australia. Additional uplift may have occurred by the reactivation of pre‐existing faults, for example, during the Carboniferous‐Permian collision of Gondwana and Laurussia.

Seismic Tomography

10.1002/ggge.20098

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Citations (28)

Crustal and upper-mantle structure beneath ice-covered regions in Antarctica fromS-wave receiver functions and implications for heat flow

Geophysical Journal International (2016)

C. Ramirez A. Nyblade S. E. Hansen Douglas A. Wiens S. Anandakrishnan

S-wave receiver functions (SRFs) are used to investigate crustal and upper-mantle structure beneath several ice-covered areas of Antarctica. Moho S-to-P (Sp) arrivals are observed at ∼6–8 s in SRF stacks for stations in the Gamburtsev Mountains (GAM) and Vostok Highlands (VHIG), ∼5–6 s for stations in the Transantarctic Mountains (TAM) and the Wilkes Basin (WILK), and ∼3–4 s for stations in the West Antarctic Rift System (WARS) and the Marie Byrd Land Dome (MBLD). A grid search is used to model the Moho Sp conversion time with Rayleigh wave phase velocities from 18 to 30 s period to estimate crustal thickness and mean crustal shear wave velocity. The Moho depths obtained are between 43 and 58 km for GAM, 36 and 47 km for VHIG, 39 and 46 km for WILK, 39 and 45 km for TAM, 19 and 29 km for WARS and 20 and 35 km for MBLD. SRF stacks for GAM, VHIG, WILK and TAM show little evidence of Sp arrivals coming from upper-mantle depths. SRF stacks for WARS and MBLD show Sp energy arriving from upper-mantle depths but arrival amplitudes do not rise above bootstrapped uncertainty bounds. The age and thickness of the crust is used as a heat flow proxy through comparison with other similar terrains where heat flow has been measured. Crustal structure in GAM, VHIG and WILK is similar to Precambrian terrains in other continents where heat flow ranges from ∼41 to 58 mW m−2, suggesting that heat flow across those areas of East Antarctica is not elevated. For the WARS, we use the Cretaceous Newfoundland–Iberia rifted margins and the Mesozoic-Tertiary North Sea rift as tectonic analogues. The low-to-moderate heat flow reported for the Newfoundland–Iberia margins (40–65 mW m−2) and North Sea rift (60–85 mW m−2) suggest that heat flow across the WARS also may not be elevated. However, the possibility of high heat flow associated with localized Cenozoic extension or Cenozoic-recent magmatic activity in some parts of the WARS cannot be ruled out.

Receiver function

Passive seismic

10.1093/gji/ggv542

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Citations (36)

Deep Mantle Influence on the Cameroon Volcanic Line

Geochemistry Geophysics Geosystems (2022)

Hesam Saeidi S. E. Hansen A. Nyblade

Abstract The origin of the Cameroon Volcanic Line (CVL), which is difficult to explain with traditional plate tectonics and mantle convection models because the volcanism does not display clear age progression, remains widely debated. Existing seismic tomography models show anomalously slow structure beneath the CVL, which some have interpreted to reflect upper mantle convective processes, possibly associated with edge‐driven flow related to the neighboring Congo Craton. However, mid‐ and lower mantle depths are generally not well resolved in these models, making it difficult to determine the extent of the anomalous CVL structure. Here, we present a new P‐wave velocity model for the African mantle, developed with the largest collection of travel‐time residuals recorded across the continent to date and an adaptive model parameterization. Our extensive data set and inversion method yield high resolution images of the mantle structure beneath western Africa, particularly at the critical mid‐ and lower mantle depths needed to further evaluate processes associated with the formation of the CVL. Our new model provides strong evidence for a connection between the African Large Low Velocity Province, centered in the lower mantle beneath southern Africa, and the continental portion of the CVL. We suggest that seismically slow material generated near the core‐mantle boundary beneath southern Africa moves northwestward under the Congo Craton. At the northern edge of the craton, the hot, buoyant material rises through the upper mantle, causing the CVL volcanism. Consequently, CVL magmatism can be linked to large‐scale mantle processes rooted in the deep mantle.

Hotspot (geology)

Core–mantle boundary

10.1029/2022gc010621

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Citations (10)