Geophysical Journal International

Gharti, Hom, Nath;Langer,, Leah;Tromp,, Jeroen

doi: 10.1093/gji/ggz028pmid: N/A

Summary Although earthquake-induced gravity perturbations are frequently observed, numerical modeling of this phenomenon has remained a challenge. Due to the lack of reliable and versatile numerical tools, induced-gravity data have not been fully exploited to constrain earthquake source parameters. From a numerical perspective, the main challenge stems from the unbounded Poisson/Laplace equation that governs gravity perturbations. Additionally, the Poisson/Laplace equation must be coupled with the equation of conservation of linear momentum that governs particle displacement in the solid. Most existing methods either solve the coupled equations in a fully spherical harmonic representation, which requires models to be (nearly) spherically symmetric, or they solve the Poisson/Laplace equation in the spherical harmonics domain and the momentum equation in a discretized domain, a strategy that compromises accuracy and efficiency. We present a spectral-infinite-element approach that combines the highly accurate and efficient spectral-element method with a mapped-infinite-element method capable of mimicking an infinite domain without adding significant memory or computational costs. We solve the complete coupled momentum-gravitational equations in a fully discretized domain, enabling us to accommodate complex realistic models without compromising accuracy or efficiency. We present several coseismic and postearthquake examples and benchmark the coseismic examples against the Okubo analytical solutions. Finally, we consider gravity perturbations induced by the 1994 Northridge earthquake in a 3D model of Southern California. The examples show that our method is very accurate and efficient, and that it is stable for postearthquake simulations. Earthquake fault, moment-density tensor, Split-node, spectral-infinite-element method, gravity perturbation © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Tan,, Fengzhou;Ge,, Zengxi;Kao,, Honn;Nissen,, Edwin

doi: 10.1093/gji/ggz032pmid: N/A

Summary Back projection methods are increasingly used for mapping large earthquake ruptures in space and time. Unlike other seismological source imaging methods, back projections require no prior knowledge of earthquake source parameters or fault geometries except for the epicenter and average depth, making them a convenient tool for studying complex, multi-segmented ruptures. We developed a new Three-Dimensional Phase-Weighted Relative Back Projection method (3-D PWBP) that yields improved spatial resolutions by enacting two advances. Firstly, we exploit both the phase and amplitude of the seismogram signal to enhance the distinction of correlated signals. Secondly, we implement a 3-D velocity model to provide more accurate travel times. We vindicate these refinements with several synthetic tests and an analysis of the 1997 Mw 7.2 Zirkuh (Iran) earthquake, which we show ruptured mostly unilaterally southwards at ∼3.0 km/s along its ∼125 km-long, mostly single-stranded surface rupture. Then, we apply the new method to the more complex case of the 2016 Mw 7.8 Kaikōura (New Zealand) earthquake, which we demonstrate is divided into two major stages separated by a gap of ∼8 seconds and ∼30–40 km. The overall rupture speed is ∼1.7 km/s and the overall duration is ∼84 sec, considerably shorter than some earlier estimates. We see no clear evidence for continuous failure of the subduction interface that underlies the known, surface-rupturing crustal faults, though we cannot rule out its involvement in the second major stage in the northern part of the rupture area. The late (∼80 s) peak in relative energy is likely a high-frequency stopping phase, and the rupture appears to terminate southwest of the offshore Needles fault. 3-D Phase-Weighted Relative Back Projection, 2016 Kaikōura Earthquake, Rupture Process © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Maeda,, Yuta;Takeo,, Minoru;Kazahaya,, Ryunosuke

doi: 10.1093/gji/ggz021pmid: N/A

Summary Very-long-period (VLP) seismic events at Asama volcano in central Japan are characterised by a transient signal of 10–20 s duration. Associated with the transient motion, a high-frequency (HF) oscillation within the 5–10 Hz band is observed. We investigated the location and size of the oscillation source in the HF band (HF source) using an amplitude source location method and compared our results with those for the VLP band (VLP source) deduced in our previous waveform inversion study. We analysed 1437 VLP events recorded during an intense observation campaign during 2008–2009, and an additional 571 events surrounding an eruptive activity in 2015 (including a VLP event that immediately preceded an eruption). The HF source locations of most events were deeper than the VLP source locations by ∼150 m, although there was almost no time lag between the signals. This suggests a strong connection between the two sources, with one source immediately responding to the other. The eruptive VLP event had an HF source location close to normal VLP events but had a greater event size. We surmise that the HF signal is caused by an inflow of volcanic gas from depth into a semi-vertical crack-like cavity (as imaged in our previous waveform inversion study), and the VLP signal is caused by resultant inflation of the same cavity. The centroid of inflation (VLP source) is near the roof of the cavity, where gas accumulates, whereas the HF oscillation is emitted more intensely from lower and narrower portions of the cavity. This results in source depth differences between the two signal bands. In this model, the inflation rate of the cavity is controlled by the volume flux of the gas inflow, producing similar temporal variations between the VLP and HF signals. According to this model, the eruptive VLP is associated with a greater amount of gas inflow and accumulation, which likely played a key role in the eruption. Volcano seismology, Earthquake source observations, Eruption mechanisms and flow emplacement, Volcano monitoring, Explosive volcanism © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Sun,, Anhui;Zhao,, Dapeng;Gao,, Yuan;Tian,, Qinjian;Liu,, Ning

doi: 10.1093/gji/ggz031pmid: N/A

Summary A new crustal 3-D P-wave velocity model beneath NE Tibet is determined by jointly inverting 62,339 high-quality first P-wave and later PmP-wave arrival-time data from local earthquakes and seismic explosions. Resolution tests show that the use of the PmP data can effectively improve the resolution of crustal tomography, especially that of the middle-lower crust. Widespread but intermittent low-velocity anomalies are revealed in the lower crust beneath NE Tibet, and the Kunlun fault acts as a transfer structure. High-velocity zones are visible in most parts of the crust below the transition zones bordering the southwestern Ordos basin between 105°∼106° E longitude. We think that they form an important transpressive boundary to absorb sinistral strike-slip and thrust faulting deformation, like a western frontline of the Ordos basin, which is compatible with the latest GPS observations in the region. Considering the tectonic features and deformation of the Liupanshan fault, we need to pay much attention to the seismic risk of the fault zone from now. Our results reveal different structural features of the major blocks and their boundary faults, indicating the complexity of the Cenozoic crustal deformation in NE Tibet that partitioned between steep strike-slip shear zones and thrust faults. Crustal imaging, Seismic tomography, Seismicity and tectonics, Crustal structure © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Liu,, Shangxin;King, Scott, D

doi: 10.1093/gji/ggz036pmid: N/A

Summary A new generation mantle convection code, ASPECT (Advanced Solver for Problems in Earth ConvecTion) enables the simulation of global mantle convection with realistic parameters and complicated physical processes in highly variable resolution. ASPECT’s cutting-edge numerical features include adaptive mesh refinement as well as linear and nonlinear solvers. The fully adaptive mesh presents a challenge for calculating the geoid in the spherical harmonic domain. We develop an extension of the spectral geoid algorithm for the adaptive mesh refinement modeling and perform a series of benchmark calculations of ASPECT for 3-D spherical incompressible Stokes flow. We compare the responses of flow velocity, topography, and geoid at the surface and core-mantle boundary for different spherical harmonic wavelengths in isoviscous and radially-varying viscosity with the semi-analytical kernels determined from the propagator matrix method. The numerical results of ASPECT mostly agree within 1% error up to spherical harmonic degree 15 for 64 elements in the radial direction. We also explore the solution accuracy as a function of grid spacing. Additional calculations are performed for low Rayleigh number, steady-state thermal convection as well as instantaneous flow models from the buoyancy field derived from seismic tomography to verify the dynamic topography and geoid diagnostics with adaptive non-uniform mesh. The adaptive mesh calculation produces nearly identical results to highly-resolved uniform mesh with significant savings of computational cost. Mantle processes, Numerical approximations and analysis, Numerical modelling, Numerical solutions, Dynamics: gravity and tectonics, Dynamics of lithosphere and mantle This content is only available as a PDF. © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Schmeling,, Harro;Marquart,, Gabriele;Weinberg,, Roberto;Wallner,, Herbert

doi: 10.1093/gji/ggz029pmid: N/A

Summary The physics of magmatic systems within continental crust is poorly understood. We developed a thermo-mechanical-compositional two-phase flow formulation based on the conservation equations of mass, momentum, and energy for melt and solid, including compaction of the solid matrix, melting, melt segregation, melt ascent and freezing. We use a simplified melting law to track the enrichment or depletion in SiO2 of the advected silicic melt and solid. The non-linear visco-plastic rheology includes the effect of melt porosity. 2D-models with different heat input are carried out for cases without and with differential melt-matrix flow. The retention number, Rt, as a measure of melt mobility, is varied between 1 and infinity. In the case of no melt segregation (large Rt) our models show transient oscillatory behavior followed by stationary convection in the lower crust enforced by a solid—melt phase transition. In the case of two-phase flow (i.e. small Rt) melt separates from the solid matrix and accumulates in high melt porosity magma bodies within 10 s ka. We find a new melt ascent mechanism, termed CATMA, forCompaction/decompactionAssistedTwo-phase flowMeltAscent. This is a combination of compaction and decompaction of the contact zones between accumulated magma and solid rock that dislodges solid material from the roof which sinks through and partly dissolves in the magma. This process can be seen as an efficient microstoping mechanism and is associated with the formation of melt rich and chemically enriched channels within the magma body. The emplacement depths of magma change from > 20 km for low heat flows, to < 10 km for high heat flows. In most models with high degrees of melting, two stacked SiO2-enriched magmatic zones form, interpreted as granitic layers. Models with stronger crustal rheology show porosity waves on a few km-scale. Deviatoric stresses immediately above the evolving magma bodies are of the order of a few MPa, too small to overcome brittle or plastic yield stresses. The models predict significant chemical separation of depleted versus enriched composition, resulting in significant chemical stratification of the crust with spatial variations in solidus temperatures, and in a dual melt porosity distribution with crystal-poor magma bodies (> 60 per cent melt) on top of low melt fraction mushes (< 20 per cent). Comparison with the Altiplano-Puna magma body shows that the best agreement with observational data is obtained for a moderate (85 mW m−2 to 90 mW m−2) heat flux and retention number of order 3 to 30. Numerical modelling, Diapirism, Magma genesis and partial melting, Magma migration and fragmentation, Physics of magma and magma bodies, Pluton emplacement © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Richards, Paul, G

doi: 10.1093/gji/ggz038pmid: N/A

This content is only available as a PDF. © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Janiszewski, Helen, A;Gaherty, James, B;Abers, Geoffrey, A;Gao,, Haiying;Eilon, Zachary, C

doi: 10.1093/gji/ggz051pmid: N/A

Summary A new amphibious seismic dataset from the Cascadia subduction zone is used to characterize lithosphere structure from the Juan de Fuca ridge to the Cascades backarc. These seismic data are allowing the imaging of an entire tectonic plate from its creation at the ridge through the onset of subduction to beyond the volcanic arc, along the entire strike of the Cascadia subduction zone. We develop a tilt and compliance correction procedure for ocean-bottom seismometers that employs automated quality control to calculate robust station noise properties. To elucidate crust and upper-mantle structure, we present shoreline-crossing Rayleigh-wave phase-velocity maps for the Cascadia subduction zone, calculated from earthquake data from 20–160 s period and from ambient noise correlations from 9–20 s period. We interpret the phase-velocity maps in terms of the tectonics associated with the Juan de Fuca plate history and the Cascadia subduction system. We find that thermal oceanic plate cooling models cannot explain velocity anomalies observed beneath the Juan de Fuca plate. Instead, they may be explained by a ≤ 1 per cent partial melt region beneath the ridge and are spatially collocated with patches of hydration and increased faulting in the crust and upper mantle near the deformation front. In the forearc, slow velocities appear to be more prevalent in areas that experienced high slip in past Cascadia megathrust earthquakes and generally occur up-dip of the highest-density tremor regions and locations of intraplate earthquakes. Beneath the volcanic arc, the slowest phase velocities correlate with regions of highest magma production volume. Subduction zone processes, surface waves and free oscillations, seismic noise, seismic tomography, North America, mantle processes This content is only available as a PDF. © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Dando, B D, E;Oye,, V;Näsholm, S, P;Zühlsdorff,, L;Kühn,, D;Wuestefeld,, A

doi: 10.1093/gji/ggz017pmid: N/A

Summary Determining accurate microseismic event locations at the Groningen gas field in the Netherlands has important implications for understanding the ongoing induced seismicity and its associated seismic hazard. To improve the depth constraint of the microseismicity, downhole monitoring arrays have been deployed in the central region of the Groningen field. The observed seismicity at these receivers is characterised by significant complexity in the waveforms, due to the high velocity contrasts that exist and the acquisition geometry. Reliably identifying and picking phases for use in earthquake location algorithms is therefore particularly challenging. Using a well constrained and highly detailed 3D velocity model, we show how full waveform modelling can be used to understand the causes of the observed phase complexity. By identifying the different modelled phase arrivals in detail, we look to identify any systematic changes in waveform complexity with source location, to aid phase identification of the recorded downhole data. Theoretical travel-times are often the foundation of earthquake location algorithms. We highlight the associated problems of their computation for the downhole monitoring setup for the Groningen model by complementing the full waveform simulations with travel-time computations and their associated ray-paths from both an eikonal solver and using the wavefront construction method. We highlight large inconsistencies in the travel-times, demonstrate the limitations and sensitivity of ray-tracing in a layered velocity model, and show how the theoretical travel-times do not equate with the dominant phase arrivals observed within the modelled waveforms. Finally, we propose an approach based on computing P-wave and S-wave travel-times directly from the full-waveform modelling, such that phase picking is based on an amplitude threshold rather than individual phase identification, which can also be adjusted for any given moment tensor. Computational seismology, Induced seismicity, Wave propagation, Downhole methods, Interface waves, Guided waves © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Ojo, Adebayo, Oluwaseun;Ni,, Sidao;Xie,, Jun;Zhao,, Li

doi: 10.1093/gji/ggz008pmid: N/A

Summary To develop a new model of the Cameroon crust that would provide further constraints on its composition and help study regional tectonics, we simultaneously invert Rayleigh wave group and phase velocity dispersion, P-wave receiver function and Rayleigh wave ellipticity measurements using a joint inversion method based on the Neighbourhood Algorithm (NA). We obtain ensembles of 1-D isotropic Vs models along with layered Vp/Vs ratio and crustal thickness estimate beneath 32 broadband seismic stations in the region. Due to the addition of ellipticity data, the resolutions of shallow structures are well improved in our new model, thereby showing a better spatial correlation with known tectonic features. By interpolating the ellipticity measurements and performing joint inversions with group and phase dispersion data at several grid nodes, we are able to image the geometry and constrain the locations of mafic bodies (Vs of 3.7–4 km/s) that intruded into the upper crust (2–8 km) during the formation of Gondwana. Investigating the relationship between crustal parameters, we found a positive correlation between crustal thickness, Vp/Vs ratio, the Bouguer anomaly, and topography. Specifically, we found that the inverted Moho depth and surface topography within the Adamawa Plateau and the Garoua Rift agrees with Airy isostasy. The inverted Vp/Vs ratio shows variation that ranged from 1.67–1.85 with uncertainties that are generally less than 0.05 at crustal depths. A regional averaged Vp/Vs ratio of 1.73 and a standard deviation of ∼0.03 in the crust suggest a felsic to intermediate bulk crustal composition, indicating that the Cenozoic volcanism is most likely characterised by small volumes of mafic intrusion with limited alteration to the crust, thereby supporting plume-free hypothesis such as small-scale mantle convection for the origin of the Cameroon Volcanic Line. Joint inversion, Neighbourhood Algorithm, Cameroon, Crustal Structure, Rayleigh Wave Ellipticity This content is only available as a PDF. © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)