bluuebunny/crossref_metadata_2025 huggingface dataset:
Modification of continental lithosphere by tectonic processes: A tomographic image of central North America
Authors: Frederiksen A. W. , Bollmann T. , Darbyshire F. , van der Lee S.
Published: April 2013

The assembly of Laurentia by Precambrian accretion is also believed to have formed the underlying lithosphere. Accretionary signatures are detectable by seismic observations but subject to modification by later processes, e.g., orogeny, rifting, and plumes. We examine the Archean Superior Province (SP) and environs using a set of teleseismic P wave arrivals from Canadian and American instruments. The resulting tomographic model has high resolution beneath the Dakotas and Minnesota, provides a first look at the lithosphere beneath Manitoba, and sharpens previously documented features in Ontario. From the model and previous anisotropy observations, we detect (i) a large high‐velocity feature beneath the western SP, associated with elevated lithospheric anisotropy. The high‐velocity feature does not match crustal boundaries; notably, its western edge lies approximately 200 km east of the contact with the Proterozoic Trans‐Hudson Orogen (THO). (ii) A low‐velocity channel‐shaped feature strikes northwest through Minnesota and the Dakotas, associated with weakening anisotropy. (iii) High velocities southwest of (ii), beneath the Minnesota River Valley terrane (MRVT), associated with low anisotropy. We interpret (i) to be accretionary, and contemporaneous with Superior assembly; similar velocity but weaker anisotropy of the MRVT is consistent with vertical‐tectonic mechanisms. The inboard location of the THO contact may indicate modification of the Superior root. The low‐velocity channel has no obvious crustal expression but connects to an offset in the Proterozoic Mid‐Continent Rift (MCR) and may be rift related. The MCR is not well imaged but will be examined via the temporary Superior Province Rifting EarthScope Experiment, in progress.
Volcanic eruption monitoring by thermal image correlation: Pixel offsets show episodic dome growth of the Colima volcano
Authors: Walter Thomas R. , Legrand Denis , Granados Hugo Delgado , Reyes Gabriel , Arámbula Raúl
Published: April 2013

Estimating the magnitude of dome eruptions is one of the main challenges in volcano monitoring. Although modern monitoring networks are in place at many dome‐building volcanoes, the type and occurrence of explosive activity and the scale of the eruptions are commonly estimated by visual inspection. Quantifying the deformation of dome‐building volcanoes and the occurrence of explosions is highly valuable, not only for enabling the provision of early warnings but also for facilitating an understanding of the physics of explosive volcanoes, as demonstrated by this study of one of the most active volcanoes in Mexico. The Volcán de Colima is currently experiencing a phase of viscous dome growth, which is associated with episodic “Vulcanian” eruptions and rock falls. Little is known about the dynamics of this dome, its growth rates, or the scale of the associated explosions. We present the results from an analysis of nighttime time‐lapse infrared images and compare these data with local seismic amplitude recordings. By digital image correlation, we track temperature features in infrared images. Images taken before and after the explosions reveal the location of the hot dome to be subject to significant and systematic lateral pixel offsets. Dome deformation is shown to occur intermittently every 3–4 h, with lateral displacements exceeding 0.3 m within periods of less than 120 s. Only the thermally elevated regions of the western dome, which may represent a coulée‐like flow, are displaced. This movement is often, but not always, associated with seismic amplitude peaks. Therefore, our analysis of the infrared image correlation suggests the occurrence of aseismic dome‐deformation episodes, thereby challenging the current understanding of dome growth and/or the appropriateness of commonly used volcano surveillance techniques.
The breakup of East Gondwana: Assimilating constraints from Cretaceous ocean basins around India into a best‐fit tectonic model
Authors: Gibbons Ana D. , Whittaker Joanne M. , Müller R. Dietmar
Published: April 2013

Published models for the Cretaceous seafloor‐spreading history of East Gondwana result in unlikely tectonic scenarios for at least one of the plate boundaries involved and/or violate particular constraints from at least one of the associated ocean basins. We link East Gondwana spreading corridors by integrating magnetic and gravity anomaly data from the Enderby Basin off East Antarctica within a regional plate kinematic framework to identify a conjugate series of east‐west‐trending magnetic anomalies, M4 to M0 (~126.7–120.4 Ma). The mid‐ocean ridge that separated Greater India from Australia‐Antarctica propagated from north to south, starting at ~136 Ma northwest of Australia, and reached the southern tip of India at ~126 Ma. Seafloor spreading in the Enderby Basin was abandoned at ~115 Ma, when a ridge jump transferred the Elan Bank and South Kerguelen Plateau to the Antarctic plate. Our revised plate kinematic model helps resolve the problem of successive two‐way strike‐slip motion between Madagascar and India seen in many previously published reconstructions and also suggests that seafloor spreading between them progressed from south to north from 94 to 84 Ma. This timing is essential for tectonic flow lines to match the curved fracture zones of the Wharton and Enderby basins, as Greater India gradually began to unzip from Madagascar from ~100 Ma. In our model, the 85‐East Ridge and Kerguelen Fracture Zone formed as conjugate flanks of a “leaky” transform fault following the ~100 Ma spreading reorganization. Our model also identifies the Afanasy Nikitin Seamounts as products of the Conrad Rise hotspot.
Outgassing: Influence on speed of magma fragmentation
Authors: Richard Dominique , Scheu Bettina , Mueller Sebastian P. , Spieler Oliver , Dingwell Donald B.
Published: April 2013

Predicting explosive eruptions remains an outstanding challenge. Knowledge of the controlling parameters and their relative importance is crucial to deepen our understanding of conduit flow dynamics and accurately model the processes involved. This experimental study sheds light on one important parameter—outgassing—and evaluates its influence on magma fragmentation behavior. We perform fragmentation experiments based on the shock tube theory at room temperature on natural pyroclastic material with a connected porosity ranging from 15% to 78%. For each sample series, we determine the initial pressure (P) required to initiate magma fragmentation (fragmentation threshold, Pth). Furthermore, we measure the permeability of each sample for P < Pth and the fragmentation speed for P > Pth. A significant loss of initial pressure, caused by outgassing in samples with permeability ≥1e–12 m2, is observed within the fragmentation time scale (a few milliseconds). The samples are classified into: (a) dome/conduit wall rocks and (b) pumice/scoria. Substantial outgassing during fragmentation leads to higher fragmentation thresholds. Experimental fragmentation speeds are significantly higher than the modeled fragmentation speeds for high‐permeability dome/conduit wall rocks, but lower for high‐permeability pumices. Experimental fragmentation speeds for low‐permeability dome/conduit wall rocks and low‐permeability pumice/scoria are as expected. We also find that low‐porosity, low‐permeability, altered dome/conduit wall rocks fragment at significantly higher speeds than expected. Because fragmentation threshold and fragmentation speed are among the determining parameters for the initiation, sustainment and cessation of an eruption, outgassing should be considered in the modeling of magma fragmentation dynamics.
Missing great earthquakes
Authors: Hough Susan E.
Published: April 2013

The occurrence of three earthquakes with moment magnitude (Mw) greater than 8.8 and six earthquakes larger than Mw 8.5, since 2004, has raised interest in the long‐term global rate of great earthquakes. Past studies have focused on the analysis of earthquakes since 1900, which roughly marks the start of the instrumental era in seismology. Before this time, the catalog is less complete and magnitude estimates are more uncertain. Yet substantial information is available for earthquakes before 1900, and the catalog of historical events is being used increasingly to improve hazard assessment. Here I consider the catalog of historical earthquakes and show that approximately half of all Mw ≥ 8.5 earthquakes are likely missing or underestimated in the 19th century. I further present a reconsideration of the felt effects of the 8 February 1843, Lesser Antilles earthquake, including a first thorough assessment of felt reports from the United States, and show it is an example of a known historical earthquake that was significantly larger than initially estimated. The results suggest that incorporation of best available catalogs of historical earthquakes will likely lead to a significant underestimation of seismic hazard and/or the maximum possible magnitude in many regions, including parts of the Caribbean.
Causes and mechanisms of the 2011–2012 El Hierro (Canary Islands) submarine eruption
Authors: Martí Joan , Pinel Virginie , López Carmen , Geyer Adelina , Abella Rafael , Tárraga Marta , Blanco María José , Castro Antonio , Rodríguez Carmen
Published: April 2013

El Hierro eruption started on 10 October 2011 after an unrest episode that initiated on 17 July 2011. This is the first eruption in the Canary Islands that has been tracked in real time. Although being submarine and not directly observable, the data recorded allowed its reconstruction and to identify its causes and mechanisms. Seismicity, surface deformation, and petrological data indicate that a batch of basanitic magma coming from a reservoir located at a depth of about 25 km below the El Hierro Island was emplaced at shallower depth creating a new reservoir about 10–12 km above, where magma evolved until the initiation of the eruption. The characteristics of seismicity and surface deformation suggest that the necessary space to accumulate magma at this shallower position, which coincides with the crust/mantle boundary beneath El Hierro, was created in about 2 months by elastic deformation and magma‐driven fracturing of the crust. After this first intrusion episode, part of the magma started to migrate laterally toward the SE for nearly 20 km, always keeping the same depth and following a path apparently controlled by stress barriers created by tectonic and rheological contrasts in the upper lithosphere. This lateral migration of magma ended with a submarine eruption at about 5 km offshore from the southern corner of El Hierro Island. The total seismic energy released during the unrest episode was of 8.1 × 1011 J, and the total uplift previous to the onset of the eruption was of 40 mm. Combining geological, geophysical, and petrological data and numerical modeling, we propose a volcanological model of the causes and mechanisms of El Hierro eruption that shows how the stress distribution in the crust beneath El Hierro, which was influenced by rheological contrasts, tectonic stresses, and gravitational loading, controlled the movement and eruption of magma. We also discuss the implications of this model in terms of eruption forecast in the Canary Islands.
Rupture dynamics and ground motion from 3‐D rough‐fault simulations
Authors: Shi Zheqiang , Day Steven M.
Published: April 2013

We perform three‐dimensional (3‐D) numerical calculations of dynamic rupture along non‐planar faults to study the effects of fault roughness on rupture propagation and resultant ground motion. The fault roughness model follows a self‐similar fractal distribution over length scales spanning three orders of magnitude, from ~102 to ~105 m. The fault is governed by a strongly rate‐weakening friction, and the bulk material is subject to Drucker‐Prager viscoplasticity. Fault roughness promotes the development of self‐healing rupture pulses and a heterogeneous distribution of fault slip at the free surface and at depth. The inelastic deformation, generated by the large dynamic stress near rupture fronts, occurs in a narrow volume around the fault with heterogeneous thickness correlated to local roughness slopes. Inelastic deformation near the free surface, however, is induced by the stress waves originated from dynamic rupture at depth and spreads to large distances (>10 km) away from the fault. The present simulations model seismic wave excitation up to ~10 Hz with rupture lengths of ~100 km, permitting comparisons with empirical studies of ground‐motion intensity measures of engineering interest. Characteristics of site‐averaged synthetic response spectra, including the distance and period dependence of the median values, absolute level, and intra‐event standard deviation, are comparable to appropriate empirical estimates throughout the period range 0.1–3.0 s. This class of model may provide a viable representation of the ground‐motion excitation process over a wide frequency range in a large spatial domain, with potential applications to the numerical prediction of source‐ and path‐specific effects on earthquake ground motion.
Quartz grain boundaries as fluid pathways in metamorphic rocks
Authors: Kruhl Jörn H. , Wirth Richard , Morales Luiz F. G.
Published: April 2013

TEM and SEM/FIB sequential imaging of quartz grain boundaries from contact and regional metamorphic rocks show that most of the grain boundaries are open on the nanometer scale. Three types of voids occur. (i) Roughly 40–500 nm wide open zones parallel to the grain boundaries. They are suggested to be caused by general volume reduction as a result of anisotropic cooling contraction at temperatures decreasing below ca. 300°C, the threshold temperature of diffusion in quartz and of decompression expansion at pressures decreasing below several hundred MPa. (ii) Cavities of variable shape and up to micrometer size along the open grain boundaries and (iii) cone‐shaped, nanometer‐sized depressions at sites where dislocation lines meet the open grain boundaries. The latter two types are generated by dissolution–precipitation processes. Open grain boundaries, cavities, and depressions form a connected network of porosity, which allows fluid circulation and may affect physical properties of the rocks. The same process is suggested to occur along grain and phase boundaries in other rocks as exemplified in this study, and it should be expected along intracrystalline cracks or cleavage planes.
Waveform tomography in 2.5D: Parameterization for crooked‐line acquisition geometry
Authors: Smithyman B. R. , Clowes R. M.
Published: April 2013

A method for 2.5D viscoacoustic waveform tomography that can be applied to generate 2D models of velocity and attenuation from inversion of refraction waveforms on land seismic reflection data acquired along crooked roads is developed. It is particularly useful for typical crustal reflection surveys. First‐arrival travel time tomography is applied using a 3D method, but with constraints on the intermediate 3D velocity model; the result is the starting model for the next step. A 2.5D frequency‐domain full‐waveform inversion stage parameterizes 3D geometry in the seismic source and receiver arrays, with the assumption that the velocity and attenuation models are homogeneous in the out‐of‐plane direction. This approach results in superior results compared to a strictly 2D approach when the acquisition line is crooked, with a moderate increase in computational cost. A case study using data acquired in the Nechako Basin in south‐central British Columbia, Canada, exemplifies and validates the procedure. The velocity model derived from 2.5D waveform tomography is compared with that from a previous study in which 2D waveform tomography was applied to the same data set and with results from 3D travel time tomography. The resolution and accuracy of the velocity model from 2.5D waveform tomography are demonstrated to be greater than those from travel time or 2D waveform tomography. A model of viscoacoustic attenuation, which was not possible in the 2D case, is also generated. These models are interpreted jointly to highlight features of geological interest, such as a sedimentary basin, basement rocks, and faults, from surface to about 3 km depth.
Short‐time scale correlation between slow slip and tremor in Cascadia
Authors: Hawthorne J. C. , Rubin A. M.
Published: April 2013

We use borehole strain and seismic data to show that slow slip and tremor in central Cascadia are correlated on a range of time scales shorter than 1 day. The recorded strain rate is our proxy for the slow slip moment rate, and the seismic amplitude is our proxy for the tremor amplitude. We find that, on average, the strain rate is higher when the seismic amplitude is larger. This correlation persists on time scales between 15 min and 16 h, and it can be seen in each of the five slow slip events between 2007 and 2011. Our results imply that the slow slip moment rate varies by a large amount even at these short time scales. For instance, we observe a factor of 2 variation on time scales shorter than 4 h. This apparently aperiodic variation is larger than the previously observed variation in moment rate resulting from tidal forcing. It is a lower bound on the actual moment rate variation, as we detect only changes in slow slip that are correlated with tremor amplitude.
Modified Jeffery model: Influence of particle concentration on mineral fabric in moderately concentrated suspensions
Authors: Jezek Josef , Schulmann Karel , Paterson Scott
Published: April 2013

A modified Jeffery model involving particle interactions allows the simulation of preferred orientation of crystals and the anisotropy of magnetic susceptibility in magmatic flow with up to 40% of rigid particle fraction. Numerical simulations are compared to analogue and natural fabrics. A study of the role of particle concentration shows that the increasing percentage of crystal fraction in dilute suspensions leads to progressive attenuation of the fabric intensity and cyclicity. At high strains in simple shear flow, the lineation converges to a constant value close to the shear plane. With increasing particle concentration, the fabric diverges from the shear direction reflecting the effect of increased crystal tiling. The model can be applied to quantify mineral preferred orientations of crystal mushes in magmatic fabric studies.
How do long‐offset oceanic transforms adapt to plate motion changes? The example of the Western Pacific‐Antarctic plate boundary
Authors: Lodolo Emanuele , Coren Franco , Ben‐Avraham Zvi
Published: April 2013

Oceanic transform faults respond to changes in the direction of relative plate motion. Studies have shown that short‐offset transforms generally adjust with slight bends near the ridge axis, while long‐offset ones have a remarkably different behavior. The western Pacific‐Antarctic plate boundary highlights these differences. A set of previously unpublished seismic profiles, in combination with magnetic anomaly identifications, shows how across a former, ~1250 km long transform (the Emerald Fracture Zone), plate motion changes have produced a complex geometric readjustment. Three distinct sections are recognized along this plate boundary: an eastern section, characterized by parallel, multiple fault strand lineaments; a central section, shallower than the rest of the ridge system, overprinted by a mantle plume track; and a western section, organized in a cascade of short spreading axes/transform lineaments. This configuration was produced by changes that occurred since 30 Ma in the Australia‐Pacific relative plate motion, combined with a gradual clockwise change in Pacific‐Antarctic plate motion. These events caused extension along the former Emerald Fracture Zone, originally linking the Pacific‐Antarctic spreading ridge system with the Southeast Indian ridge. Then an intra‐transform propagating ridge started to develop in response to a ~6 Ma change in the Pacific‐Antarctic spreading direction. The close proximity of the Euler poles of rotation amplified the effects of the geometric readjustments that occurred along the transform system. This analysis shows that when a long‐offset transform older than 20 Ma is pulled apart by changes in spreading velocity vectors, it responds with the development of multiple discrete, parallel fault strands, whereas in younger lithosphere, locally modified by thermal anisotropies, tensional stresses generate an array of spreading axes offset by closely spaced transforms.
Seismic velocities, anisotropy, and shear‐wave splitting of antigorite serpentinites and tectonic implications for subduction zones
Authors: Ji Shaocheng , Li Awei , Wang Qian , Long Changxing , Wang Hongcai , Marcotte Denis , Salisbury Matthew
Published: April 2013

Antigorite, the high‐temperature (HT) form of serpentinite, is believed to play a critical role in various geological processes of subduction zones. We have measured P‐ and S‐wave velocities (Vp and Vs), anisotropy and shear‐wave splitting of 17 serpentinite samples containing >90% antigorite at pressures up to 650 MPa. The new results, combined with data for low‐temperature (LT) lizardite and/or chrysolite, reveal distinct effects of LT and HT serpentinization on the seismic properties of mantle rocks. At 600 MPa, Vp = 5.10 and 6.68 km/s, Vs = 2.32 and 3.67 km/s, and Vp/Vs = 2.15 and 1.81 for pure LT and HT serpentinites, respectively. Above the crack‐closure pressure (~150 MPa), the velocity ratio of antigorite serpentinites displays little dependence on pressure or temperature. Serpentine contents within subduction zones and forearc mantle wedges where temperature is >300°C should be at least twice that of previous estimates based on LT serpentinization. The presence of seismic anisotropy, high‐pressure fluids, or partial melt is also needed to interpret HT serpentinized mantle with Vp < 6.68 km/s, Vs < 3.67 km/s, and Vp/Vs > 1.81. The intrinsic anisotropy of the serpentinites (3.8–16.9% with an average value of 10.5% for Vp, and 3.6–18.3% with an average value of 10.4% for Vs) is caused by dislocation creep‐induced lattice‐preferred orientation of antigorite. Three distinct patterns of seismic anisotropy correspond to three types of antigorite fabrics (S‐, L‐, and LS‐tectonites) formed by three categories of strain geometry (i.e., coaxial flattening, coaxial constriction, and simple shear), respectively. Our results are thought to provide a new explanation for various anisotropic patterns of subduction systems observed worldwide.
Subsidence history, crustal structure, and evolution of the Somaliland‐Yemen conjugate margin
Authors: Ali M. Y. , Watts A. B.
Published: April 2013

We have used biostratigraphic data from deep exploration wells to determine the tectonic subsidence history of the Somaliland (northwestern Somalia)‐Yemen conjugate margin, a poorly known margin in the central part of the Gulf of Aden. Bathymetry and magnetic anomaly data suggest the Gulf of Aden is a young feature that formed following the rifting apart and breakup of the African and Arabian plates ~32 Ma. Our tectonic subsidence data suggest, however, that the present‐day Gulf of Aden developed on an earlier Mesozoic rift system. The oldest episode of rifting initiated at ~156 Ma and lasted for ~10 Ma and had a NW‐SE trend. We interpret the rift as a late stage event associated with the breakup of Gondwana and the separation of Africa and Madagascar. At ~80 Ma, there is evidence of an intermediate rift event which correlates with a rapid increase in spreading rate on the ridges separating the African and Indian and African and Antarctica plates and a contemporaneous slowing down of Africa's plate motion. The combined effect of all three rifting events has been to thin the crust and upper mantle by up to a factor of 2. The amount of thinning deduced from the wells is in accord with the crustal structure inferred from available seismic refraction data and process‐oriented gravity and flexure modeling. The margin is asymmetric with a steeper gradient in the Moho on the Yemen side than the Somaliland side. The main discrepancy is on the Yemen side where the gravity‐derived Moho is 10 km deeper than the well‐derived Moho. We attribute the discrepancy to the addition of material at the base of the crust since rifting, possibly magma sourced from the Afar plume.
InSAR Evidence for an active shallow thrust fault beneath the city of Spokane Washington, USA
Authors: Wicks Charles , Weaver Craig , Bodin Paul , Sherrod Brian
Published: April 2013

In 2001, a nearly five month long sequence of shallow, mostly small magnitude earthquakes occurred beneath the city of Spokane, a city with a population of about 200,000, in the state of Washington. During most of the sequence, the earthquakes were not well located because seismic instrumentation was sparse. Despite poor‐quality locations, the earthquake hypocenters were likely very shallow, because residents near the city center both heard and felt many of the earthquakes. The combination of poor earthquake locations and a lack of known surface faults with recent movement make assessing the seismic hazards related to the earthquake swarm difficult. However, the potential for destruction from a shallow moderate‐sized earthquake is high, for example Christchurch New Zealand in 2011, so assessing the hazard potential of a seismic structure involved in the Spokane earthquake sequence is important. Using interferometric synthetic aperture radar (InSAR) data from the European Space Agency ERS2 and ENVISAT satellites and the Canadian Space Agency RADARSAT‐1, satellite we are able to show that slip on a shallow previously unknown thrust fault, which we name the Spokane Fault, is the source of the earthquake sequence. The part of the Spokane Fault that slipped during the 2001 earthquake sequence underlies the north part of the city, and slip on the fault was concentrated between ~0.3 and 2 km depth. Projecting the buried fault plane to the surface gives a possible surface trace for the Spokane Fault that strikes northeast from the city center into north Spokane.
Real‐time prediction of ground motion by Kirchhoff‐Fresnel boundary integral equation method: Extended front detection method for Earthquake Early Warning
Authors: Hoshiba Mitsuyuki
Published: April 2013

A method of real‐time prediction of ground motion is proposed for application for the Earthquake Early Warning (EEW). In many methods of the present EEW systems, hypocenter and magnitude are determined quickly, after which ground motions are predicted. Although these methods can predict the strength of ground motions by using a few parameters (e.g., hypocenter, magnitude, and site factors), error in the parameters leads directly to error in the prediction, and it is not easy to take the effects of rupture directivity and source extent into account. During the aftershock activity of the 2011Tohoku earthquake (Mw 9.0), multiple events occurred simultaneously, which made it difficult to accurately determine the hypocenters and magnitudes, and led to some false alarms. To address these problems, a new method is proposed that applies the Kirchhoff‐Fresnel boundary integral equation. Ground motion is predicted from real‐time ground motion observation at front stations in the direction of incoming seismic waves. The real‐time monitoring of wavefield and propagation direction are important for this method, but a hypocenter and magnitude are not required. It is possible to predict ground motion without a hypocenter and magnitude, and the precision of the prediction is not affected by error in the source parameters. The effects of rupture directivity, source extent, and simultaneous multiple events are substantially included in this method. The method is a quantitative extension of the front detection technique of EEW.
3‐D multiobservable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle. I:a prioripetrological information and geophysical observables
Authors: Afonso J. C. , Fullea J. , Griffin W. L. , Yang Y. , Jones A. G. , D. Connolly J. A. , O'Reilly S. Y.
Published: April 2013

Traditional inversion techniques applied to the problem of characterizing the thermal and compositional structure of the upper mantle are not well suited to deal with the nonlinearity of the problem, the trade‐off between temperature and compositional effects on wave velocities, the nonuniqueness of the compositional space, and the dissimilar sensitivities of physical parameters to temperature and composition. Probabilistic inversions, on the other hand, offer a powerful formalism to cope with all these difficulties, while allowing for an adequate treatment of the intrinsic uncertainties associated with both data and physical theories. This paper presents a detailed analysis of the two most important elements controlling the outputs of probabilistic (Bayesian) inversions for temperature and composition of the Earth's mantle, namely thea prioriinformation on model parameters,ρ(m), and the likelihood function,L(m). The former is mainly controlled by our current understanding of lithosphere and mantle composition, while the latter conveys information on the observed data, their uncertainties, and the physical theories used to relate model parameters to observed data.

The benefits of combining specific geophysical datasets (Rayleigh and Love dispersion curves, body wave tomography, magnetotelluric, geothermal, petrological, gravity, elevation, and geoid), and their effects onL(m), are demonstrated by analyzing their individual and combined sensitivities to composition and temperature as well as their observational uncertainties. The dependence of bulk density, electrical conductivity, and seismic velocities to major‐element composition is systematically explored using Monte Carlo simulations. We show that the dominant source of uncertainty in the identification of compositional anomalies within the lithosphere is the intrinsic nonuniqueness in compositional space. A general strategy for definingρ(m) is proposed based on statistical analyses of a large database of natural mantle samples collected from different tectonic settings (xenoliths, abyssal peridotites, ophiolite samples, etc.). This strategy relaxes more typical and restrictive assumptions such as the use of local/limited xenolith data or compositional regionalizations based on age‐composition relations. We demonstrate that the combination of ourρ(m) with aL(m) that exploits the differential sensitivities of specific geophysical observables provides a general and robust inference platform to address the thermochemical structure of the lithosphere and sublithospheric upper mantle. An accompanying paper deals with the integration of these two functions into a general 3‐D multiobservable Bayesian inversion method and its computational implementation.
The effects of magma flux variations on the formation and lifetime of large silicic magma chambers
Authors: Schöpa A. , Annen C.
Published: April 2013

Incremental emplacement of large silicic intrusions limits the size and lifetime of any magma chamber if earlier injected magma pulses solidify before the next pulse is emplaced. Geochronology indicates that long‐term average magma fluxes of plutons are too low for the accumulation of magma volumes larger than a single pulse. To better constrain the formation of large‐volume magma chambers, we investigate the influence of a changing emplacement rate of successive sill injections that form a composite intrusion in the upper crust. A thermal model with an explicit finite difference scheme simulates periods of transient high magma fluxes keeping the long‐term average flux small. Several scenarios regarding how fluxes vary were analyzed. A progressive increase in flux does not result in magma accumulation. Only a step‐like flux increase of at least one order of magnitude above the background flux produces a large magma reservoir. To generate a magma chamber of a melt‐crystal mix above solidus temperature with a volume of 500–2000 km3, transient high fluxes of at least 1.25 × 10 − 2km3/a, equivalent to vertical, one‐dimensional accretion rates of a few cm/a, are required. The transient high flux range where magma accumulates is largest for model scenarios with one period of transient high flux, and therefore, this emplacement style is favored for the built‐up of substantial magma chambers during pluton growth. Intrusion scenarios with three and four periods of transient high fluxes do not generate reservoirs of mobile magma, but magma mushes can be present for some hundred thousand years.
Magnetostratigraphy of the Dali Basin in Yunnan and implications for late Neogene rotation of the southeast margin of the Tibetan Plateau
Authors: Li Shihu , Deng Chenglong , Yao Haitao , Huang Sheng , Liu Chengying , He Huaiyu , Pan Yongxin , Zhu Rixiang
Published: April 2013

The rotation pattern and fault activity in the southeast margin of the Tibetan Plateau (SEMTP) provide meaningful constraints on the geodynamic evolution of the plateau. However, the lack of Cenozoic paleomagnetic studies and accurate age constraints on Neogene sediments prevents a better understanding of the late Cenozoic tectonic activity in this area. To clarify the tectonic rotation pattern and deformation history of the SEMTP, we report new magnetostratigraphic data from a late Neogene sedimentary sequence in the Dali Basin, northwestern Yunnan Province, China. Rock magnetic analyses indicate that both magnetite and hematite are the main carriers of the characteristic remanent magnetizations (ChRMs). Magnetostratigraphic results show that the sedimentary profile spans from Chron C4n.1r to Chron C2n. The age of the sedimentary sequence in the Dali Basin can thus be paleomagnetically constrained to an interval from late Miocene to early Pleistocene. The basal age of the sediments is ~7.6 Ma, which indicates that the unroofing of Diancang Shan and activation of the Dali fault system were initiated at this time. The appearance of conglomerates and syntectonic sediments suggests the reactivation of the Dali fault system at ~2.5 Ma. Moreover, the overall mean ChRM direction suggests that the Dali Basin experienced significant (4.4 ± 2.5°), but minor post–late Miocene rotation. This indicates that most of the clockwise rotation demonstrated by previous paleomagnetic studies in the SEMTP occurred prior to late Miocene and may be concentrated between Eocene and Miocene, which is contemporaneous with the sinistral slip of the Ailao Shan‐Red River fault.
A new velocity field for Africa from combined GPS and DORIS space geodetic Solutions: Contribution to the definition of the African reference frame (AFREF)
Authors: Saria E. , Calais E. , Altamimi Z. , Willis P. , Farah H.
Published: April 2013

We analyzed 16 years of GPS and 17 years of Doppler orbitography and radiopositioning integrated by satellite (DORIS) data at continuously operating geodetic sites in Africa and surroundings to describe the present‐day kinematics of the Nubian and Somalian plates and constrain relative motions across the East African Rift. The resulting velocity field describes horizontal and vertical motion at 133 GPS sites and 9 DORIS sites. Horizontal velocities at sites located on stable Nubia fit a single plate model with a weighted root mean square residual of 0.6 mm/yr (maximum residual 1 mm/yr), an upper bound for plate‐wide motions and for regional‐scale deformation in the seismically active southern Africa and Cameroon volcanic line. We confirm significant southward motion ( ∼ 1.5 mm/yr) in Morocco with respect to Nubia, consistent with earlier findings. We propose an updated angular velocity for the divergence between Nubia and Somalia, which provides the kinematic boundary conditions to rifting in East Africa. We update a plate motion model for the East African Rift and revise the counterclockwise rotation of the Victoria plate and clockwise rotation of the Rovuma plate with respect to Nubia. Vertical velocities range from − 2 to +2 mm/yr, close to their uncertainties, with no clear geographic pattern. This study provides the first continent‐wide position/velocity solution for Africa, expressed in International Terrestrial Reference Frame (ITRF2008), a contribution to the upcoming African Reference Frame (AFREF). Except for a few regions, the African continent remains largely under‐sampled by continuous space geodetic data. Efforts are needed to augment the geodetic infrastructure and openly share existing data sets so that the objectives of AFREF can be fully reached.
Vertical and horizontal surface displacements near Jakobshavn Isbræ driven by melt‐induced and dynamic ice loss
Authors: Nielsen Karina , Khan Shfaqat A. , Spada Giorgio , Wahr John , Bevis Michael , Liu Lin , van Dam Tonie
Published: April 2013

We analyze Global Positioning System (GPS) time series of relative vertical and horizontal surface displacements from 2006 to 2012 at four GPS sites located between ∼5 and ∼150 km from the front of Jakobshavn Isbræ (JI) in west Greenland. Horizontal displacements during 2006–2010 at KAGA, ILUL, and QEQE, relative to the site AASI, are directed toward north‐west, suggesting that the main mass loss signal is located near the frontal portion of JI. The directions of the observed displacements are supported by modeled displacements, derived from NASA's Airborne Topographic Mapper (ATM) surveys of surface elevations from 2006, 2009, and 2010. However, horizontal displacements during 2010–2012 at KAGA and ILUL are directed more towards the west suggesting a change in the spatial distribution of the ice mass loss. In addition, we observe an increase in the uplift rate during 2010–2012 as compared to 2006–2010. The sudden change in vertical and horizontal displacements is due to enhanced melt‐induced ice loss in 2010 and 2012.
Time‐dependent model of creep on the Hayward fault from joint inversion of 18 years of InSAR and surface creep data
Authors: Shirzaei M. , Bürgmann R.
Published: April 2013

Spatial and temporal variations of aseismic fault creep influence the size and timing of large earthquakes along partially coupled faults. To solve for a time‐dependent model of creep on the Hayward fault, we invert 18 years of surface deformation data (1992–2010), obtained by interferometric processing of 52 and 50 SAR images acquired by the ERS1/2 and Envisat satellites, respectively, and surface creep data obtained at 19 alinement and 4 creepmeter stations. For multi‐temporal analysis of the SAR data we developed a method for identifying stable pixels using wavelet multi‐resolution analysis. We also implement a variety of wavelet‐based filters for reducing the effects of environmental artifacts. Using a reweighted least squares approach, we inverted the interferometric data to generate a time series of surface deformation over the San Francisco Bay Area with a precision of better than a few millimeters. To jointly invert the InSAR displacement time series and the surface creep data for a time‐dependent model of fault creep, we use a robust inversion approach combined with a Kalman filter. The time‐dependent model constrains a zone of high slip deficit that may represent the locked rupture asperity of past and future M≈7 earthquakes. We identify several additional temporal variations in creep rate along the Hayward fault, the most important one being a zone of accelerating slip just northwest of the major locked zone. We estimate that a slip‐rate deficit equivalent to Mw 6.3–6.8 has accumulated on the fault, since the last event in 1868.
Detecting offsets in GPS time series: First results from the detection of offsets in GPS experiment
Authors: Gazeaux Julien , Williams Simon , King Matt , Bos Machiel , Dach Rolf , Deo Manoj , Moore Angelyn W , Ostini Luca , Petrie Elizabeth , Roggero Marco , Teferle Felix Norman , Olivares German , Webb Frank H.
Published: April 2013

The accuracy of Global Positioning System (GPS) time series is degraded by the presence of offsets. To assess the effectiveness of methods that detect and remove these offsets, we designed and managed the Detection of Offsets in GPS Experiment. We simulated time series that mimicked realistic GPS data consisting of a velocity component, offsets, white and flicker noises (1/f spectrum noises) composed in an additive model. The data set was made available to the GPS analysis community without revealing the offsets, and several groups conducted blind tests with a range of detection approaches. The results show that, at present, manual methods (where offsets are hand picked) almost always give better results than automated or semi‒automated methods (two automated methods give quite similar velocity bias as the best manual solutions). For instance, the fifth percentile range (5% to 95%) in velocity bias for automated approaches is equal to 4.2 mm/year (most commonly ±0.4 mm/yr from the truth), whereas it is equal to 1.8 mm/yr for the manual solutions (most commonly 0.2 mm/yr from the truth). The magnitude of offsets detectable by manual solutions is smaller than for automated solutions, with the smallest detectable offset for the best manual and automatic solutions equal to 5 mm and 8 mm, respectively. Assuming the simulated time series noise levels are representative of real GPS time series, robust geophysical interpretation of individual site velocities lower than 0.2–0.4 mm/yr is therefore certainly not robust, although a limit of nearer 1 mm/yr would be a more conservative choice. Further work to improve offset detection in GPS coordinates time series is required before we can routinely interpret sub‒mm/yr velocities for single GPS stations.
Particle sedimentation and diffusive convection in volcanic ash‐clouds
Authors: Carazzo G. , Jellinek A. M.
Published: April 2013

Understanding the longevity of volcanic ash‐clouds generated by powerful explosive eruptions is a long standing problem for assessing volcanic hazards and the nature and time scale of volcanic forcings on climate change. It is well known that the lateral spreading and longevity of these clouds is influenced by stratospheric winds, particle settling and turbulent diffusion. Observations of the recent 2010 Eyjafjallajökull and 2011 Grimsvötn umbrella clouds, as well as the structure of atmospheric aerosol clouds from the 1991 Mt Pinatubo event, suggest that an additional key process governing the cloud dynamics is the production of internal layering. Here, we use analog experiments on turbulent particle‐laden umbrella clouds to show that this layering occurs where natural convection driven by particle sedimentation and the differential diffusion of primarily heat and fine particles give rise to a large scale instability. Where umbrella clouds are particularly enriched in fine ash, this “particle diffusive convection” strongly influences the cloud longevity. More generally, cloud residence time will depend on fluxes due to both individual settling and diffusive convection. We develop a new sedimentation model that includes both sedimentation processes, and which is found to capture real‐time measurements of the rate of change of particle concentration in the 1982 El Chichon, 1991 Mt Pinatubo and 1992 Mt Spurr ash‐clouds. A key result is that these combined sedimentation processes enhance the fallout of fine particles relative to expectations from individual settling suggesting that particle aggregation is not the only mechanism required to explain volcanic umbrella longevity.
The tidal displacement field at Earth's surface determined using global GPS observations
Authors: Yuan Linguo , Chao Benjamin Fong , Ding Xiaoli , Zhong Ping
Published: April 2013

We investigate the 3‐D tidal displacement field on Earth's surface recorded globally by 456 continuous global positioning system (GPS) stations of IGS spanning 1996–2011, for eight principal diurnal and semidiurnal tidal constituents. In‐phase and quadrature amplitudes of the residual tidal displacements, after removal of an a priori body tide model, are estimated using the precise point positioning (PPP) technique on the daily GPS data; the resultant daily estimates are combined to derive final estimates for each tide at each station. The results are compared with the predictions of eight recent global ocean tide models, separately for coastal (307) and inland (149) stations. We show that GPS can provide tidal displacement estimates accurate to the level of 0.12 mm (horizontal) and 0.24 mm (vertical) for the lunar‐only constituents (M2, N2, O1, and Q1) and less favorably for solar‐related tidal constituents (S2, K2, K1, and P1), although improved by ambiguity resolution. Most recent ocean tide models fit the GPS estimates equally well on the global scale but do not agree well between them in certain coastal areas, especially for the vertical displacements, suggesting the existence of model uncertainties near shallow seas. The tidal residuals for the inland stations after removing both body tides and ocean tidal loading (OTL) furthermore show clear continental‐scale spatial coherence, implying deficiencies of the a priori body tide modeling in catching lateral heterogeneity in elastic as well as inelastic properties in the Earth's deep interior. We assert that the GPS tidal displacement estimates now achieve sufficient accuracy to potentially provide constraints on the Earth's structure.
Imaging lithospheric structure of the eastern Himalayan syntaxis: New insights from receiver function analysis
Authors: Xu Qiang , Zhao Junmeng , Pei Shunping , Liu Hongbing
Published: April 2013

We employ the P and S receiver function technique to data from the 44 seismic stations deployed in the eastern Himalayan syntaxis to investigate the crustal thickness, the average Poisson's ratio, and the depth of the lithosphere‐asthenosphere boundary (LAB). The observed crustal thickness exhibits an overall NE‐deepening trend, varying from 55 to 75 km. Two anomalous areas lie in the west and east of the Namche Barwa syntaxis characterized by thinner and thicker crust, respectively. The average Poisson's ratios within the study area are low in the north and moderate elsewhere with some high values in the south, consistent with felsic and intermediate rocks forming the crust. Our migrated images reveal that (1) the LAB of the Tibetan plate exists at relatively shallow depths (~110 km) and exhibits a gap beneath the Namche Barwa syntaxis, which may have formed by the delamination of mantle lithosphere due to local mantle upwelling, and (2) the LAB of the Asian plate is observed at a depth of ~180 km, which implies that the Asian plate has advanced southward to about 30°N under the Lhasa terrane. Our results provide new insights into the understanding of continental subduction and lithospheric deformation of the eastern Himalayan syntaxis.
Seismic attenuation in the Middle America Region and the frequency dependence of intrinsicQ
Authors: Dominguez L. A. , Davis P. M.
Published: April 2013

We evaluate the mechanisms of attenuation for the Middle America (MA) region in the range of 1.0≤f≤15 Hz. Our analysis focuses on reconciling laboratory experiments, which suggest that in this frequency range, intrinsic attenuation has a weak to zero frequency dependence, with results obtained using the Multiple Lapse Time Widow (MLTW) method. The MLTW method is designed to separate the contributions of scattering by heterogeneities and intrinsic attenuation from total attenuation assuming spherical geometrical spreading and isotropic scattering. Application of the MLTW method suggests that total attenuation in the MA region is concentrated towards the upper crust and is relatively constant throughout the region. It also shows a strong frequency dependence of the energy loss component of the attenuation (apparent intrinsic attenuation). We test the effect that forward and back scattering operators, distribution of the sources, and absorption of energy into the mantle has on the separation of the components of attenuation. We conclude that the inclusion of the frequency‐dependent leakage of energy towards the mantle provides a mechanism to satisfy both laboratory experiments and estimates based on the MLTW method. For the Middle America region, we find that the intrinsic attenuation.