bluuebunny/crossref_metadata_2025 huggingface dataset:
CRCM + BATS‒R‒US two‒way coupling
Authors: Glocer A. , Fok M. , Meng X. , Toth G. , Buzulukova N. , Chen S. , Lin K.
Published: April 2013

We present the coupling methodology and validation of a fully coupled inner and global magnetosphere code using the infrastructure provided by the Space Weather Modeling Framework (SWMF). In this model, the Comprehensive Ring Current Model (CRCM) represents the inner magnetosphere, while the Block‒Adaptive‒Tree Solar‒Wind Roe‒Type Upwind Scheme (BATS‒R‒US) represents the global magnetosphere. The combined model is a global magnetospheric code with a realistic ring current and consistent electric and magnetic fields. The computational performance of the model was improved to surpass real‒time execution by the use of the Message Passing Interface (MPI) to parallelize the CRCM. Initial simulations under steady driving found that the coupled model resulted in a higher pressure in the inner magnetosphere and an inflated closed field‒line region as compared to simulations without inner‒magnetosphere coupling. Our validation effort was split into two studies. The first study examined the ability of the model to reproduce Dst for a range of events from the Geospace Environment Modeling (GEM) Dst Challenge. It also investigated the possibility of a baseline shift and compared two approaches to calculating Dst from the model. We found that the model did a reasonable job predicting Dst and Sym‒H according to our two metrics of prediction efficiency and predicted yield. The second study focused on the specific case of the 22 July 2009 moderate geomagnetic storm. In this study, we directly compare model predictions and observations for Dst, THEMIS energy spectragrams, TWINS ENA images, and GOES 11 and 12 magnetometer data. The model did an adequate job reproducing trends in the data. Moreover, we found that composition can have a large effect on the result.
Study of small‐scale plasmoid structures in the magnetotail using Cluster observations and Hall MHD simulations
Authors: Liu Chaoxu , Feng Xueshang , Guo Jianpeng , Ye Yudong
Published: April 2013

The existence of small‐scale plasmoids associated with the Hall effect has been often observed in the magnetotail. They are considered as the signature of multiple X‐line collisionless reconnection. To study these plasmoids structures, we present some Cluster observations and Hall MHD simulations of their features. In this study, the observation survey is divided into two types. The first one is the isolated plasmoid with two typical plasmoid events the flux‐rope‐like plasmoid on 3 August 2001 and the closed‐loop‐like plasmoid on 22 August 2001. The second type contains multiple successive plasmoids, on 12 September 2001 with three neighboring plasmoids structures observed during a substorm. Especially for the second plasmoid, three main features were observed, including a core field in the plasmoid, a quadrupole magnetic field near the X line, and a local plasma convection within the plasmoid. The Grad‐Shafranov reconstruction method was used to recover the two‐dimensional magnetic field maps for this plasmoid. These results may provide evidence that the small‐scale plasmoids frequently observed in the magnetotail may be produced by multiple X‐line collisionless reconnection. To study the impact of crosstail magnetic field on the structures of small‐scale plasmoids, a 2.5‐D Hall MHD simulation was performed. In the case with a guide field By0, the in‐plane plasma inflows carrying Byflux enter into the plasmoid due to magnetic reconnection. However, there is no such By flux transport process for the case without guide field. These results demonstrate that a crosstail magnetic field is an important factor in the formation of flux‐rope‐like plasmoids.
Non‐migrating tides in the ionosphere‐thermosphere: In situ versus tropospheric sources
Authors: Jones M. , Forbes J. M. , Hagan M. E. , Maute A.
Published: April 2013

In this paper we demonstrate how magnetic control of ion‐neutral interactions in the ionosphere‐thermosphere (IT) system effectively produces source terms for non‐migrating solar tides in the neutral momentum equations for the thermosphere. The National Center for Atmospheric Research (NCAR) Thermosphere‐Ionosphere‐Mesosphere‐Electrodynamics General Circulation Model (TIME‐GCM) is utilized to quantify these tides, and to assess their importance relative to those that propagate upward from lower atmospheric regions. The primary diurnal tides excited in situ by the above mechanism include DE1, D0 and DW2, with zonal wind amplitudes on the order of 20 m s−1 (5–10 m s−1) at ∼500 km (∼350 km) under solar maximum (minimum) conditions. Smaller amplitude semidiurnal non‐migrating tides, mainly SE1, S0, SW1, and SW3, are also generated under solar maximum conditions. The aggregate effect of these tidal components is to produce extrema ranging from −110 to +140 m s−1 in a typical illustration of latitude versus longitude at a constant local time. The associated wind circulations include vertical wind perturbations that drive temperature perturbations through adiabatic heating and cooling effects. At high latitudes, hydromagnetic coupling effects generate non‐migrating tidal components including DE1, D0, DW2, SE1, S0, and SW1, which show interhemispheric differences in both amplitude and latitudinal structure due to interhemispheric differences in the offset between the geographic and geomagnetic poles. Our computational results show that the in situ generated non‐migrating tidal components dominate some parts of the tidal spectrum at high levels of solar activity and suggest that in situ generated non‐migrating tides must be taken into account in order to reconcile differences in data‐model comparisons.
Response of the equatorial and low‐latitude ionosphere to an intense X‐class solar flare (X7/2B) as observed on 09 August 2011
Authors: Sripathi S. , Balachandran N. , Veenadhari B. , Singh R. , Emperumal K.
Published: April 2013

In this paper, we present response of equatorial and low‐latitude ionosphere to an intense solar flare of class X7/2B that peaked at 08:05 UT on 09 August 2011 in the solar cycle 24. Global positioning system total electron content (TEC) observations in the sunlit hemisphere show enhancement of ~3 TEC units, while geomagnetic H component observations indicate sudden decrease and increase in their strength at equatorial and low‐latitude stations, respectively, at several stations in the sunlit hemisphere. In addition, equatorial electrojet strength over Indian region reveals commencement of counter electrojet. Simultaneous Canadian Advanced Digital Ionosonde observations at Tirunelveli, an equatorial station in India, show the disappearance of ionogram echoes during the flare event indicating absorption of radio signals in the D region. Strong equatorial blanketing type Es layer was observed in the ionogram records at Tirunelveli prior to the occurrence of the solar flare that continued for several hours though it became weak/absent during the flare event. Ionogram records on the control day show regular F layer movement without any blanketing type Es layer. Very low frequency (VLF) observations at Allahabad, an Indian low‐latitude station, show enhanced VLF amplitude signal during the same time revealing the sudden enhancement of D region ionization. Using the observations presented here, an attempt has been made to study the impact of the solar flares on the electrodynamics of the equatorial and low‐latitude ionosphere.
Sporadic E layer development and disruption at low latitudes by prompt penetration electric fields during magnetic storms
Authors: Abdu M. A. , Souza J. R. , Batista I. S. , Fejer B. G. , Sobral J. H. A.
Published: April 2013

An investigation of low‐latitude sporadic E layers during magnetic storms shows that the formation and disruption of these layers are strongly controlled by the magnetospheric electric fields that penetrate to equatorial ionosphere. It is observed that a prompt penetration electric field (PPEF) of westward polarity that dominates the nightside ionosphere can cause formation of sporadic E layers near 100 km, while a PPEF of eastward polarity that dominates the dayside and eveningside can lead to disruption of an Es layer in progress. It is shown that a vertical Hall electric field, induced by the primary zonal PPEF, in the presence of the storm‐associated enhanced conductivity of the night E layer, can be responsible for vertical ion velocity convergence sufficient to influence the Es layer formation. A downward polarity of the Hall electric field leads to Es layer formation, while an upward polarity causes the Es layer disruption. An interplay of magnetic storm associated prompt penetration electric field and energetic particle precipitation is evident in the observed Es layer response features in the region of the South Atlantic/American magnetic anomaly reported here.
A decade‐long climatology of terdiurnal tides using TIMED/SABER observations
Authors: Moudden Y. , Forbes J. M.
Published: April 2013

In this paper, we globally characterize the solar terdiurnal tide in the 80–110 km region of Earth's atmosphere through analysis of 10 years of temperature measurements made by the Sounding of the Atmosphere using Broadband Emission Radiometry instrument on the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics spacecraft. The Sun‐synchronous (“migrating”) component (TW3), which is longitude‐independent and achieves maximum amplitudes of order of 5 K (10 K) at 90 km (110 km), not too different than the 7–15 K amplitudes that are typical of the migrating diurnal and semidiurnal tides in this region. Significant longitude variability (∼ 20–25%) in terdiurnal temperature amplitudes also exists, which is decomposed into zonal wave number components. The largest of these (TE1, TW4, and TW5) reveal distinct seasonal‐latitudinal and height versus latitude patterns and interannual consistency. In addition, it is demonstrated that these particular components vary in ways that suggest that they originate from nonlinear interactions between diurnal and semidiurnal tides, specifically between DE3 and SW2 for TE1, between DW2 and SW2 for TW4, and between DW1 and SW4 for TW5. We also demonstrate that the terdiurnal tides derived here are not influenced to any significant degree by aliasing due to the presence of other waves.
Origins of oblique‐slip faulting during caldera subsidence
Authors: Holohan Eoghan P. , Walter Thomas R. , Schöpfer Martin P.J. , Walsh John J. , van Wyk de Vries Benjamin , Troll Valentin R.
Published: April 2013

Although conventionally described as purely dip‐slip, faults at caldera volcanoes may have a strike‐slip displacement component. Examples occur in the calderas of Olympus Mons (Mars), Miyakejima (Japan), and Dolomieu (La Reunion). To investigate this phenomenon, we use numerical and analog simulations of caldera subsidence caused by magma reservoir deflation. The numerical models constrain mechanical causes of oblique‐slip faulting from the three‐dimensional stress field in the initial elastic phase of subsidence. The analog experiments directly characterize the development of oblique‐slip faulting, especially in the later, non‐elastic phases of subsidence. The combined results of both approaches can account for the orientation, mode, and location of oblique‐slip faulting at natural calderas. Kinematically, oblique‐slip faulting originates to resolve the following: (1) horizontal components of displacement that are directed radially toward the caldera center and (2) horizontal translation arising from off‐centered or “asymmetric” subsidence. We informally call these two origins the “camera iris” and “sliding trapdoor” effects, respectively. Our findings emphasize the fundamentally three‐dimensional nature of deformation during caldera subsidence. They hence provide an improved basis for analyzing structural, geodetic, and geophysical data from calderas, as well as analogous systems, such as mines and producing hydrocarbon reservoirs.
The importance of microearthquakes in crustal extension of an active rift: A case study from New Zealand
Authors: Mouslopoulou Vasiliki , Hristopulos Dionissios T. , Nicol Andrew , Walsh John J. , Bannister Stephen
Published: April 2013

The amount of extension accommodated in active rifts by earthquakes that do not rupture the ground surface (e.g., <Mw 5.5) is often poorly constrained. The Matata Earthquake Sequence (MES), a high‐quality dataset of 2563 relocated microearthquakes (1 < Mw < 4.7) that ruptured the Taupo Rift in New Zealand over a period of 49 months, has been used to quantify the proportion of extension produced by small to moderate sized earthquakes. Analysis shows that the extension rate across the rift due to the MES is 2.4±0.7 mm/yr (at 1σ standard deviation), an average extension rate for small to moderate magnitude earthquakes which also prevailed during the preceding 28 years (1977–2004) and represents up to ~30% of the total contemporary deformation recorded across the north Taupo Rift by GPS (15 ± 5 mm/yr). The bulk of the MES (94%) occurred at depths of 1.5 to 6.5 km and may not be observed in geological datasets (e.g., as displacements of the ground‐ or near‐surface horizons). Small‐scale faulting associated with microseismicity may thus record strains not measured by geological datasets and constitute an important component of the ~3–10 mm/yr disparity between geological and GPS rates of extension across the Taupo Rift.
Internal structure of Erebus volcano, Antarctica imaged by high‐resolution active‐source seismic tomography and coda interferometry
Authors: Zandomeneghi D. , Aster R. , Kyle P. , Barclay A. , Chaput J. , Knox H.
Published: April 2013

Erebus volcano, Antarctica has hosted a persistent convecting phonolite lava lake for over 40 years. The lake produces small (VEI 0–1) Strombolian eruptions resulting from gas slugs rising through the upper conduit system. High‐resolution (to scale lengths of several hundreds of meters) three‐dimensional P‐wave tomographic velocity images were obtained to a depth of approximately 600 m below the volcano surface. Data were collected using 91 seismographs deployed over an approximately 4 by 4 km area of the summit region. Seismic illumination was provided by 12 chemical shots emplaced in shallow snow and ice boreholes. P‐wave direct arrival travel‐time measurements were used to invert for strong velocity anomalies (with spatial variations in Vp exceeding ±1 km/s) associated with the uppermost few km. Shallow anomalies correlate with fumarolic ice caves, a prominent radial chilled dike, and ring structures associated with the caldera rim. Conduit structures feeding the lava lake and other vents within the Inner Crater are evidently too small (e.g., less than many 10 s of meters) to be imaged under the resolution limits of this experiment. However, combined velocity and coda interferometry scattering intensity images identify near‐summit regions with both low velocity and high scattering that are candidates for magma accommodation. Results indicate a nonaxisymmetric near‐summit magmatic system that is likely constrained by heterogeneous structures in the uppermost volcano. The most extensive volume of near‐summit magma likely resides approximately 500 m NW of the active Inner Crater vents at depths of 500 m and more below the surface.
Determination of stress state in deep subsea formation by combination of hydraulic fracturing in situ test and core analysis: A case study in the IODP Expedition 319
Authors: Ito Takatoshi , Funato Akio , Lin Weiren , Doan Mai‐Linh , Boutt David F. , Kano Yasuyuki , Ito Hisao , Saffer Demian , McNeill Lisa C. , Byrne Timothy , Moe Kyaw Thu
Published: April 2013

In situ test of hydraulic fracturing (HF) provides the only way to observe in situ stress magnitudes directly. The maximum and minimum horizontal stresses, SHmax and Shmin, are determined from critical borehole pressures, i.e., the reopening pressure Pr and the shut‐in pressure Ps, etc, observed during the test. However, there is inevitably a discrepancy between actual and measured values of the critical pressures, and this discrepancy is very significant for Pr. For effective measurement of Pr, it is necessary for the fracturing system to have a sufficiently small compliance. A diagnostic procedure to evaluate whether the compliance of the employed fracturing system is appropriate for SHmax determination from Pr was developed. Furthermore, a new method for stress measurement not restricted by the system compliance and Pr is herein proposed. In this method, the magnitudes and orientations of SHmax and Shmin are determined from (i) the cross‐sectional shape of a core sample and (ii) Ps obtained by the HF test performed near the core depth. These ideas were applied for stress measurement in a central region of the Kumano fore‐arc basin at a water depth of 2054 m using a 1.6 km riser hole drilled in the Integrated Ocean Drilling Program (IODP) Expedition 319. As a result, the stress decoupling through a boundary at 1285 m below seafloor was detected. The boundary separates new upper layers and old lower ones with an age gap of ~1.8 Ma, which is possibly the accretionary prism. The stress state in the lower layers is consistent with that observed in the outer edge of accretionary prism.
The rheological transition in plagioclase‐bearing magmas
Authors: Picard D. , Arbaret L. , Pichavant M. , Champallier R. , Launeau P.
Published: April 2013

Volcanological processes, such as melt segregation, ascent, and eruption, are directly dependent on the rheological behavior of magmatic suspensions. An increase of the crystal fraction of the suspension leads to the formation of a solid‐particle network, which abruptly increases magma viscosity. The crystal fraction at which this rheological transition occurs depends on parameters such as the strain rate and the size, shape, and sorting of particles. To determine the influence of the crystal shape on the rheological transition, suspensions of plagioclase, a representative anisometric crystal, have been investigated at high temperatures and pressures. Synthetic suspensions with crystal fractions (ϕs) ranging from 0.38 to 0.75 were deformed both in compression and torsion in a Paterson apparatus at 300 MPa, 900°C and 800°C, and for strain rates between 1.0 × 10‐5 and 1.0 × 10‐3 s‐1. All suspensions exhibit a non‐Newtonian shear thinning rheological behavior. The experimental results, coupled with existing data and models at low crystal fractions (ϕs < 0.3), allow several rheological domains to be identified, from steady‐state flow to strain weakening, each characterized by a specific microstructure. In particular, a progressive evolution from a pervasive to a strain partitioning fabric is found when increasing the crystal fraction. Our results highlight the influence of both the strain rate and the shape of crystals on the rheological behavior of magmas. During crystallization, magmatic suspensions of anisometric minerals such as plagioclase would develop a solid‐particle network earlier (ϕs ~ 0.3) than suspensions of isometric minerals (ϕs ~ 0.5). Our study shows that localization of strain early in the crystallization history of mushy zones in the magma chamber, near the conduit margins, and at the base of lava flows would facilitate the mobilization, the transfer, and the final emplacement at the surface of highly viscous, feldspar‐rich magmas.
Coupled subdaily and multiweek cycles during the lava dome eruption of Soufrière Hills Volcano, Montserrat
Authors: Costa A. , Wadge G. , Stewart R. , Odbert H.
Published: April 2013

Observations of volcanoes extruding andesitic lava to produce lava domes often reveal cyclic behavior. At Soufrière Hills Volcano, Montserrat, cycles with subdaily and multiweek periods have been recognized on many occasions. Observations clearly show that the period of subdaily cycles is modulated by the multiweek cycle. The subdaily and multiweek cycles have been modeled separately as stick‐slip magma flow at the junction between a dyke and an overlying cylindrical conduit and as the filling and discharge of magma through the elastic‐walled dyke, respectively. Here, we couple these two models to describe the behavior over a period of well‐observed multiweek cycles, with accompanying subdaily cycles, from 13 May to 21 September 1997. The coupled model captures well the asymmetrical first‐order behavior: the first 40% of the multiweek cycle consists of high rates of lava extrusion during short period/high amplitude subdaily cycles as the dyke reservoir discharges itself. The remainder of the cycle involves increasing pressurization as more magma is stored, and extrusion rate falls, followed by a gradual increase in the period of the subdaily cycles.
Heterogeneous rupture in the great Cascadia earthquake of 1700 inferred from coastal subsidence estimates
Authors: Wang Pei‐Ling , Engelhart Simon E. , Wang Kelin , Hawkes Andrea D. , Horton Benjamin P. , Nelson Alan R. , Witter Robert C.
Published: April 2013

Past earthquake rupture models used to explain paleoseismic estimates of coastal subsidence during the great A.D. 1700 Cascadia earthquake have assumed a uniform slip distribution along the megathrust. Here we infer heterogeneous slip for the Cascadia margin in A.D. 1700 that is analogous to slip distributions during instrumentally recorded great subduction earthquakes worldwide. The assumption of uniform distribution in previous rupture models was due partly to the large uncertainties of then available paleoseismic data used to constrain the models. In this work, we use more precise estimates of subsidence in 1700 from detailed tidal microfossil studies. We develop a 3‐D elastic dislocation model that allows the slip to vary both along strike and in the dip direction. Despite uncertainties in the updip and downdip slip extensions, the more precise subsidence estimates are best explained by a model with along‐strike slip heterogeneity, with multiple patches of high‐moment release separated by areas of low‐moment release. For example, in A.D. 1700, there was very little slip near Alsea Bay, Oregon (~44.4°N), an area that coincides with a segment boundary previously suggested on the basis of gravity anomalies. A probable subducting seamount in this area may be responsible for impeding rupture during great earthquakes. Our results highlight the need for more precise, high‐quality estimates of subsidence or uplift during prehistoric earthquakes from the coasts of southern British Columbia, northern Washington (north of 47°N), southernmost Oregon, and northern California (south of 43°N), where slip distributions of prehistoric earthquakes are poorly constrained.
Numerical investigation of the interplay between wall geometry and friction in granular fault gouge
Authors: Rathbun Andrew P. , Renard François , Abe Steffen
Published: April 2013

The influence of surface roughness is central in understanding the behavior of various types of shear zones including faults, landslides, and deformation in glacial till. All of these systems contain a non‐planar rough wall, which interacts with either a gouge zone or another wall. We use the 3‐D discrete element method (DEM) to investigate both the effect of boundary roughness and friction. Granular non‐cohesive gouge is sandwiched between rough walls with large grooves, or smooth walls composed of spherical particles that can be adjusted to control roughness. Roughness and gouge properties are scaled to laboratory friction experiments. We vary friction between the particles and the wall and monitor shear strength, height, coordination number, distribution, and orientation of particle forces, localization, and porosity distribution in the shear zone. We find that, on the first‐order, strength is controlled by particle‐particle friction and mechanical coupling of the fault zone wall to the gouge. Rough boundaries (RMS roughness > grain radius) force more shear within the gouge zone, dilating the layer and sliding more grains, which leads to large stress necessary to shear the layer. When large amplitude roughness is removed, and roughness is at the grain‐scale, the coupling, and thus the strength, is controlled by both wall and particle friction as well as fine‐scale boundary roughness. These differences are reflected in profiles of shear within the gouge zone and offset at the boundary in smooth models. From our simulations, we quantify how and why rough natural faults will have a higher overall strength.
In situ calibration of and algorithm for strain monitoring using four‐gauge borehole strainmeters (FGBS)
Authors: Qiu Zehua , Tang Lei , Zhang Baohong , Guo Yanping
Published: April 2013

Borehole strainmeters have proved very useful in geodynamic research. Because the sensors are imbedded in rock, their in situ calibration is of crucial importance. The four‐gauge borehole strainmeter (FGBS) is a Chinese invention to monitor the temporal variation in horizontal strain. The four gauges in the FGBS are arranged at 45° intervals to bring about a simple self‐consistency equation, which serves as a means of checking that the measurements obtained from the FGBS are correct. The instruments currently used in China are usually placed at depths of several tens of meters to avoid disturbances at the surface, while still being sufficiently near the surface for the vertical stress to be regarded as zero—the premise on which the theoretical model of this observation is based. In this paper, an index of data credibility is established, based on the self‐consistency equation, to allow evaluation of the observations. A relative in situ calibration has been developed to calculate a relative correction factor for each gauge's sensitivity, termed the gauge weight, and this has proven effective in enhancing data credibility. Parameters for deriving strain from readings are determined by a concise absolute in situ calibration with the aid of the theoretical Earth tide. Instead of averaging four groups of solutions, a simpler comprehensive algorithm is developed to transform readings into strain. Data from 24 Chinese sites of YRY‐4‐type FGBS are processed and evaluated to be fairly good.
Imaging the shallow crust with local and regional earthquake tomography
Authors: Biryol Cemal B. , Leahy Garrett M. , Zandt George , Beck Susan L.
Published: April 2013

While active‐source imaging (seismic reflection, refraction) is typically used to image the shallow crust, these techniques tend to suffer from energy penetration problems in complex tectonic regimes, resulting in poor imaging. Further, active sources (such as air guns or vibroseis) tend to be band limited, resulting in poor signal‐to‐noise ratio at low frequencies (1–10 Hz). Recent studies suggest that earthquake data may be able to solve these imaging problems. However, conventional earthquake tomography typically aims to image the upper mantle and lithosphere, where typical station spacing (tens of km) and array aperture (hundreds of km) have resulted in the maximum resolution. In this study, we take advantage of the small‐scale and 250 m station spacing of the LaBarge Passive Seismic Experiment to determine whether local and regional earthquake tomographies can be used to constrain structure in the top 5 km of the crust. We also study how the inclusion of finite‐frequency effects impacts the final images. Our results indicate that local and regional events provide substantial improvements over teleseismic events alone, with approximately 500 m resolution both vertically and laterally in the upper most 5 km. We also find that inclusion of finite‐frequency data between 1 and 10 Hz plays a key role in maintaining resolution in the shallowest portion of the model.
Ages and relative sizes of pre‐2004 tsunamis in the Bay of Bengal inferred from geologic evidence in the Andaman and Nicobar Islands
Authors: Rajendran C. P. , Rajendran Kusala , Andrade Vanessa , Srinivasalu S.
Published: April 2013

Geologic evidence along the northern part of the 2004 Aceh‐Andaman rupture suggests that this region generated as many as five tsunamis in the prior 2000 years. We identify this evidence by drawing analogy with geologic records of land‐level change and the tsunami in 2004 from the Andaman and Nicobar Islands (A&N). These analogs include subsided mangrove swamps, uplifted coral terraces, liquefaction, and organic soils coated by sand and coral rubble. The pre‐2004 evidence varies in potency, and materials dated provide limiting ages on inferred tsunamis. The earliest tsunamis occurred between the second and sixth centuries A.D., evidenced by coral debris of the southern Car Nicobar Island. A subsequent tsunami, probably in the range A.D. 770–1040, is inferred from deposits both in A&N and on the Indian subcontinent. It is the strongest candidate for a 2004‐caliber earthquake in the past 2000 years. A&N also contain tsunami deposits from A.D. 1250 to 1450 that probably match those previously reported from Sumatra and Thailand, and which likely date to the 1390s or 1450s if correlated with well‐dated coral uplift offshore Sumatra. Thus, age data from A&N suggest that within the uncertainties in estimating relative sizes of paleo‐earthquakes and tsunamis, the 1000 year interval can be divided in half by the earthquake or earthquakes of A.D. 1250–1450 of magnitude >8.0 and consequent tsunamis. Unlike the transoceanic tsunamis generated by full or partial rupture of the subduction interface, the A&N geology further provides evidence for the smaller‐sized historical tsunamis of 1762 and 1881, which may have been damaging locally.
Role of heat advection in a channeled lava flow with power law, temperature‐dependent rheology
Authors: Filippucci Marilena , Tallarico Andrea , Dragoni Michele
Published: April 2013

The cooling of a lava flow, both in the transient and the steady state, is investigated considering that lava rheology is pseudoplastic and dependent on temperature. Lava exits from the vent with constant velocity and flows down a slope under the effect of gravity force inside a channel of rectangular cross section. We consider that cooling of lava is caused by thermal radiation into the atmosphere and thermal conduction at the channel walls and at the ground. The heat equation is solved numerically in a 3‐D computational domain, and the solution is tested to evaluate the numerical errors. We study the steady state and the initial transient period of lava cooling. Results indicate that the advective heat transport significantly modifies the cooling rate of lava, slowing down the cooling process. Since the lava velocity depends on temperature, the cooling rate depends on the effusion temperature. Velocity profiles are modified during cooling showing two marginal static zones where the crust can form and remain stable. The fraction of crust coverage is calculated under the assumption that the solid lava is a plastic body with temperature‐dependent yield strength. We numerically confirm that heat advection cannot be neglected in the mechanism of formation of lava tubes.
Chemical controls on fault behavior: Weakening of serpentinite sheared against quartz‐bearing rocks and its significance for fault creep in the San Andreas system
Authors: Moore Diane E. , Lockner David A.
Published: April 2013

The serpentinized ultramafic rocks found in many plate‐tectonic settings commonly are juxtaposed against crustal rocks along faults, and the chemical contrast between the rock types potentially could influence the mechanical behavior of such faults. To investigate this possibility, we conducted triaxial experiments under hydrothermal conditions (200–350°C), shearing serpentinite gouge between forcing blocks of granite or quartzite. In an ultramafic chemical environment, the coefficient of friction, μ, of lizardite and antigorite serpentinite is 0.5–0.6, and μ increases with increasing temperature over the tested range. However, when either lizardite or antigorite serpentinite is sheared against granite or quartzite, strength is reduced to μ ~ 0.3, with the greatest strength reductions at the highest temperatures (temperature weakening) and slowest shearing rates (velocity strengthening). The weakening is attributed to a solution‐transfer process that is promoted by the enhanced solubility of serpentine in pore fluids whose chemistry has been modified by interaction with the quartzose wall rocks. The operation of this process will promote aseismic slip (creep) along serpentinite‐bearing crustal faults at otherwise seismogenic depths. During short‐term experiments, serpentine minerals reprecipitate in low‐stress areas, whereas in longer experiments, new Mg‐rich phyllosilicates crystallize in response to metasomatic exchanges across the serpentinite‐crustal rock contact. Long‐term shear of serpentinite against crustal rocks will cause the metasomatic mineral assemblages, which may include extremely weak minerals such as saponite or talc, to play an increasingly important role in the mechanical behavior of the fault. Our results may explain the distribution of creep on faults in the San Andreas system.
Precise relative earthquake location using surface waves
Authors: Michael Cleveland K. , Ammon Charles J.
Published: April 2013

Earthquake locations provide a fundamental tool for seismological investigations. While dense seismic networks can provide robust locations, accuracy and precision of these locations suffer outside dense networks. This is particularly true in offshore areas, where location analysis relies heavily on distant seismic observations. We present a method for estimating precise relative seismic source epicentroid locations using surface waves. Several reasons, including lower velocities and strength of the signal at distance, make use of surface waves for event location appealing. We focus on the Panama Fracture Zone region and relocate 81 strike‐slip earthquakes to produce tectonically consistent epicentroid locations. The resulting pattern of earthquakes more clearly delineates recently active regional structures than original body‐wave locations. The mean shift between the US Geological Survey National Earthquake Information Center epicenter and our epicentroids is about 14 km (the median is about 11 km), and typical origin time changes are generally less than ±2 s. We find that north of 6.5°N, the plate boundary motion is split across two roughly north‐south striking structures, the Panama and Balboa Fracture zones. For the last 36 years, slip along these two structures roughly matches slip along the Panama Fracture Zone to the south (from 4.5°N to 6.25°N), but the Balboa Fracture zone has roughly three times the moment than the northern Panama Fracture Zone. Our analyses show that observed Rayleigh‐wave signal‐to‐noise ratios for moderate‐to‐large shallow earthquakes are suitable for applying the procedure and that Rayleigh‐wave observations form a self‐consistent set of constraints on the relative location of earthquake centroids.
A decade of horizontal deformation from great earthquakes
Authors: Tregoning P. , Burgette R. , McClusky S. C. , Lejeune S. , Watson C. S. , McQueen H.
Published: April 2013

The 21st Century has seen the occurrence of 17 great earthquakes (Mw >8), including some of the largest earthquakes ever recorded. Numerical modeling of the earthquakes shows that nearly half of the Earth's surface has undergone horizontal coseismic deformation >1 mm, with the 2004 Sumatra‐Andaman earthquake dominating the global deformation field. This has important implications for both the realization of a terrestrial reference frame and in the interpretation of regional tectonic studies based on GPS velocities. We show that far‐field coseismic deformations from great earthquakes will, if unaccounted for, introduce errors in estimates of linear site velocities of at least 0.1‐0.3 mm/yr across most of the surface of the Earth. The accumulated global deformation field shows that two regions, Australia and the north Atlantic/Arctic Ocean, have been largely undeformed by these great earthquakes, with accumulated deformations generally <0.5 mm. Using GPS estimates of surface deformation, we show that the majority of the Australian continent is deforming at <0.2 mm/yr, the northern part of New Zealand is rotating clockwise relative to the Australian Plate with relative horizontal velocities of ∼2 mm/yr, while the southeastern coast of Australia is undergoing post‐seismic relaxation caused by the 2004 Mw = 8.1 Macquarie Ridge earthquake. The presence of ongoing post‐seismic relaxation thousands of kilometers from plate margins violates the secular/linear assumption made in current terrestrial reference frame definitions. These effects have significant ramifications for regional tectonic interpretations and global studies such as sea level rise that require reference frame accuracy greater than this level.
The difficulty for subducted oceanic crust to accumulate at the Earth's core‐mantle boundary
Authors: Li Mingming , McNamara Allen K.
Published: April 2013

Seismic tomography has revealed two large low shear velocity provinces (LLSVPs) in the lowermost mantle beneath the central Pacific and Africa. The LLSVPs are further shown to be compositionally different from their surroundings. Among several hypotheses put forth in recent years to explain the cause of the LLSVPs, one postulates that they are thermochemical piles caused by accumulation of subducted oceanic crust at the core‐mantle boundary (CMB). Mineral physics experiments indicate that oceanic crust becomes denser than the surrounding mantle at lower mantle pressures. In addition, seismic observations provide evidence of subducted slabs arriving at the CMB. However, a major question pertains to whether subducted oceanic crust can survive viscous stirring associated with mantle plumes and accumulate into piles with the same spatial scale as LLSVPs. We perform a set of high‐resolution convection calculations to examine this hypothesis by investigating the interaction of thin oceanic crust (6 km) with mantle plumes. Our results show that as subducted oceanic crust is swept toward upwelling plume regions, the majority of it is viscously stirred into the surrounding mantle. Only a small amount of oceanic crust may accumulate at the base of plumes, but it is consistently entrained away into the plume at a rate equal to or greater than it is accumulated. We find that it is difficult for subducted oceanic crust to accumulate into large thermochemical piles at the CMB.
Graphite as a lubricating agent in fault zones: An insight from low‐ to high‐velocity friction experiments on a mixed graphite‐quartz gouge
Authors: Oohashi Kiyokazu , Hirose Takehiro , Shimamoto Toshihiko
Published: April 2013

Graphite is a very low friction material, often enriched within fault zones due to mechanical or chemical processes. The effects of weak minerals on the strength of faults have been examined by friction experiments on bimineralic mixtures. However, previous experiments were conducted with limited shear strains, even though applied shear strains and textural developments had already been signaled as significant factors in the weakening of faults. We therefore conducted large‐displacement, low‐ to high‐velocity friction experiments with graphite‐quartz gouges, to determine how much graphite is needed to reduce frictional strength, and to examine how textures contribute to the strength reduction of a mature fault at various slip rates. We found that the coefficients of friction of the gouges decrease nonlinearly with increasing graphite fraction for any given shear strain and slip rate, decreasing first with 5–20 vol% graphite, then reaching similar frictional levels to pure graphite with 30–50 vol% graphite. The nonlinear weakening trends can be fitted by sigmoidal curves. The weakening with 10–30 vol% graphite is associated with zones of slip‐localization and the development of a graphite‐lubricated penetrative slip surface(s). With increasing shear strain, the relationship between strength and graphite fraction evolves abruptly from an early gentle curve to a sigmoidal curve, and the frictional strength drops significantly even with small amounts of graphite (~10 vol%). Our results highlight the importance of shear strain and textural developments on weak faults, not only with respect to graphite, but also other fault lubricants such as the phyllosilicates.
Probabilistic modeling of future volcanic eruptions at Mount Etna
Authors: Cappello Annalisa , Bilotta Giuseppe , Neri Marco , Negro Ciro Del
Published: April 2013

The statistical analysis of volcanic activity at Mt Etna was conducted with the twofold aim of (1) constructing a probability map for vent opening of future flank eruptions and (2) forecasting the expected number of eruptive events at the summit craters. The spatiotemporal map of new vent opening at Etna volcano is based on the analysis of spatial locations and frequency of flank eruptions starting from 1610. Thanks to the completeness and accuracy of historical data over the last four centuries, we examined in detail the spatial and temporal distribution of flank eruptions showing that effusive events follow a nonhomogenous Poisson process with space‐time varying intensities. After demonstrating the spatial nonhomogeneity and the temporal nonstationarity of flank eruptions at Etna, we calculated the recurrence rates (events expected per unit area per unit time) and produced different spatiotemporal probability maps of new vent opening in the next 1, 10 and 50 years. These probabilistic maps have an immediate use in evaluating the future timing and areas of Etna prone to volcanic hazards. Finally, the results of the analysis of the persistent summit activity during the last 110 years indicate that the hazard rate for eruptive events is not constant with time, differs for each summit crater of Mt Etna, highlighting a general increase in the eruptive frequency starting from the middle of last century and particularly from 1971, when the SE crater was formed.
Export of nutrients from the Arctic Ocean
Authors: Torres‐Valdés Sinhué , Tsubouchi Takamasa , Bacon Sheldon , Naveira‐Garabato Alberto C. , Sanders Richards , McLaughlin Fiona A. , Petrie Brian , Kattner Gerhard , Azetsu‐Scott Kumiko , Whitledge Terry E.
Published: April 2013

This study provides the first physically based mass‐balanced transport estimates of dissolved inorganic nutrients (nitrate, phosphate, and silicate) for the Arctic Ocean. Using an inverse model‐generated velocity field in combination with a quasi‐synoptic assemblage of hydrographic and hydrochemical data, we quantify nutrient transports across the main Arctic Ocean gateways: Davis Strait, Fram Strait, the Barents Sea Opening (BSO), and Bering Strait. We found that the major exports of all three nutrients occur via Davis Strait. Transports associated with the East Greenland Current are almost balanced by transports associated with the West Spitsbergen Current. The most important imports of nitrate and phosphate to the Arctic occur via the BSO, and the most important import of silicate occurs via Bering Strait. Oceanic budgets show that statistically robust net silicate and phosphate exports exist, while the net nitrate flux is zero, within the uncertainty limits. The Arctic Ocean is a net exporter of silicate (−15.7 ± 3.2 kmol s−1) and phosphate (−1.0 ± 0.3 kmol s−1; net ± 1 standard error) to the North Atlantic. The export of excess phosphate (relative to nitrate) from the Arctic, calculated at −1.1 ± 0.3 kmol s−1, is almost twice as large as previously estimated. Net transports of silicate and phosphate from the Arctic Ocean provide 12% and 90%, respectively, of the net southward fluxes estimated at 47°N in the North Atlantic. Additional sources of nutrients that may offset nutrient imbalances are explored, and the relevance and the pathway of nutrient transports to the North Atlantic are discussed.
The settling velocity of mineral, biomineral, and biological particles and aggregates in water
Authors: Maggi Federico
Published: April 2013

A new equation was developed to relate the size and settling velocity of particulate matter commonly recurring in aqueous ecosystems. This equation explicitly balanced the gravitational, buoyancy, viscous, and inertial forces as in Rubey () but was amended to describe in one instance both individual particles and granular aggregates with an internal fractal architecture. This approach allowed for an algebraic solution of the settling velocity, thus overcoming earlier approaches that required iterative numerical solutions. The equation was tested with mineral, biomineral, and biological suspended particles and granular aggregates from 52 existing experimental data sets, and resulted in average correlation coefficients R between 71% and 93.9%, and normilized residuals between 14.3% and 24.8% over Reynolds numbers ranging within 10−7 and 102. Accuracy of these results was generally better than for the Stokes' law, the Stokes' law modified with the Schiller‐Naumann drag coefficient, and Rubey's equation. Estimated parameters ranged within observed ones, thus suggesting that the equation was robust. An analysis of the drag showed that inertial force was negligible only for biological cells (isolated cysts), whereas it contributed by not less than 5% to the drag on large mineral particles and up to 20% for biomineral and biological aggregates. Finally, a correlation was found between the organic matter content and fractal properties of granular aggregates, which were described by empirical equations proposed here for the first time. The hypothesis that the settling velocity is a function of linear and nonlinear drag, and is ultimately determined by physical characteristics as much as biological composition and internal aggregate geometry, is supported here by quantitative analyses.
Air‐sea and oceanic heat flux contributions to the heat budget of the northern Gulf of Alaska shelf
Authors: Janout Markus A. , Weingartner Thomas J. , Stabeno Phyllis J.
Published: April 2013

We constructed annual cycles of National Centers for Environmental Prediction air‐sea fluxes and temporal oceanic heat content change from Seward Line hydrographic surveys to quantify the different contributions to the oceanic heat budget within the Alaska Coastal Current (ACC) on the northern Gulf of Alaska shelf. The deficit between air‐sea fluxes and the temporal change in oceanic heat content throughout the cooling season (October–April) varies from ~40 to 110 W m−2 and is balanced by ocean heat flux convergence. Cross‐shelf heat flux convergence is insignificant on annual average, and the nearshore heat budget is likely entirely balanced by the ACC, which resupplies ~15%–50% of the heat removed by air‐sea fluxes during the cooling season. Furthermore, we estimated spatial heat flux gradients and conclude that air‐sea fluxes increase from east to west and from offshore to onshore. The cross‐shore gradients are governed by wind speed gradients, likely due to ageostrophic nearshore wind events during the cooling season, while the along‐shelf heat flux gradients are governed by the occurrence of low‐pressure systems in the northern GOA that result in cold northerly winds over the northwestern GOA. These results underline the ACC's role as the dominant oceanic heat source to the northern GOA shelf and further imply an increased cooling rate of the ACC west of the Seward Line. Furthermore, our analysis showed that nearshore regions, particularly waters in the ACC, are subjected to stronger winter cooling than the middle and outer shelves.