bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct detection of wavelike spatial structure at the bottom of the F region and its role on the formation of equatorial plasma bubble
Authors: Patra A. K. , Taori A. , Chaitanya P. P. , Sripathi S.
Published: April 2013

We present hitherto unrevealed wavelike spatial structures, as seen in the 630 nm airglow intensity using a scanning photometer, prior to the occurrence of equatorial plasma bubble (EPB). The dominant wavelengths in the spatial variation of airglow intensity have excellent correspondence with the spatial separations of periodic plume structures in the radar observations, and the amplitudes of the wave perturbations (6–12%) are significantly more than the commonly quoted 5% seed perturbation used in simulation studies. In contrast, no such airglow structure was observed on EPB free evening. Concurrent ionosonde observations made from the magnetic equator, although displayed satellite trace on some occasions, did not show one‐to‐one correspondence between the occurrence of satellite trace and EPB, a finding that is inconsistent with a recent suggestion that satellite trace can be used as a precursor of EPB occurrence. The new findings are discussed in the light of current understanding of the day‐to‐day variability in EPB including its prediction.
Thermospheric mass density specification using an ensemble Kalman filter
Authors: Matsuo Tomoko , Lee I‐Te , Anderson Jeffrey L.
Published: April 2013

This paper presents an application of ensemble Kalman filtering (EnKF) to a general circulation model of the thermosphere and ionosphere. It is designed to incorporate the feedback between plasma and neutral variables in both the analysis and forecast steps of filtering so that thermospheric parameters can be inferred from ionospheric observations and vice versa. We make a case that the global neutral density specification can greatly benefit from this approach based on a number of filtering experiments conducted under the assumption of no model bias. Specific observations considered are (i) neutral mass densities obtained from the accelerometer experiment on board the CHAMP satellite and (ii) electron density profiles obtained from the COSMIC/FORMOSAT‐3 mission. Assimilation of the neutral mass density obtained from the CHAMP mission into the TIEGCM is shown to improve the neutral density specification in the vicinity of satellite orbits, but is short of making a global impact unless accompanied by the estimation of the primary driver of the density variability such as solar EUV flux. On the other hand, assimilation of the COSMIC/FORMOSAT‐3 electron density profiles into the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) is far more effective than the CHAMP neutral density in terms of improving the global neutral mass density specification. This suggests a synthesis of thermospheric and ionospheric observations into the general circulation model, brought about with the help of the latest EnKF techniques, can effectively increase the geophysical information content of observations.
Compressible f‐plane solutions to body forces, heatings, and coolings, and application to the primary and secondary gravity waves generated by a deep convective plume
Authors: Vadas S. L.
Published: April 2013

We derive the analytic, linear, f‐plane compressible solutions to local, interval, 3‐D horizontal and vertical body forces, and heat/coolings in an isothermal, unsheared, and nondissipative atmosphere. These force/heat/coolings oscillate at the frequency and turn on and off smoothly over a finite interval in time. The solutions include a mean response, gravity waves (GWs), and acoustic waves (AWs). The excited waves span a large range of horizontal/vertical scales and frequencies ω. We find that the compressible solutions are important for GWs with vertical wavelengths if the depth of the force/heat/cooling is greater than the density scale height . We calculate the primary GWs excited by a deep convective plume, ray trace them into the thermosphere, and calculate the body force/heat/coolings which result where the GWs dissipate. We find that the force/heat/cooling amplitudes are up to ∼40% smaller using the compressible (as compared to the Boussinesq) GW spectra. For a typical plume, the force/heat/coolings are deeper than and have maximum amplitudes of ∼0.2 to 0.6 m/s2and ∼0.06 to 0.15 K/s for solar maximum to minimum, respectively. The heat/cooling consists of dipoles at z∼150–200 km and a heating at z∼240–260 km. We find that the compressible solutions are necessary for calculating the secondary GWs excited by these thermospheric force/heat/coolings.
Ion gyro‐harmonic structuring in the stimulated radiation spectrum and optical emissions during electron gyro‐harmonic heating
Authors: Mahmoudian A. , Scales W. A. , Bernhardt P. A. , Samimi A. , Kendall E. , Ruohoniemi J. M. , Isham B. , Vega‐Cancel O. , Bordikar M.
Published: April 2013

Stimulated electromagnetic emissions (SEEs) are secondary radiation produced during active space experiments in which the ionosphere is actively heated with high power high frequency (HF) ground‐based radio transmitters. Recently, there has been significant interest in ion gyro‐harmonic structuring the SEE spectrum due to the potential for new diagnostic information available such as electron acceleration and creation of artificial ionization layers. These relatively recently discovered gyro‐harmonic spectral features have almost exclusively been studied when the transmitting frequency is near the second electron gyro‐harmonic frequency. The first extensive systematic experimental investigations of the possibility of these spectral features for third electron gyro‐harmonic heating are provided here. Discrete spectral features shifted from the transmit frequency ordered by harmonics of the ion gyro‐frequency were observed for third electron gyro‐harmonic heating for the first time at a recent campaign at the High Frequency Active Auroral Research Program (HAARP) facility. These features were also closely correlated with a broader band feature at a larger frequency shift from the transmit frequency known as the downshifted peak (DP). The power threshold of these spectral features was measured, as well as their behavior with heater beam angle, and proximity of the transmit frequency to the third electron gyro‐harmonic frequency. Comparisons were also made with similar spectral features observed during second electron gyro‐harmonic heating during the same campaign. A theoretical model is provided that interprets these spectral features as resulting from parametric decay instabilities in which the pump field ultimately decays into high frequency upper hybrid/electron Bernstein and low frequency neutralized ion Bernstein IB and/or obliquely propagating ion acoustic waves at the upper hybrid interaction altitude. Coordinated optical and SEE observations were carried out in order to provide a better understanding of electron acceleration and precipitation processes. Optical emissions were observed associated with SEE gyro‐harmonic features for pump heating near the second electron gyro‐harmonic during the campaign. The observations affirm strong correlation between the gyro‐structures and the pump‐induced optical emissions.
Midlatitude propagation of VLF to MF waves through nighttime ionosphere above powerful VLF transmitters
Authors: Lefeuvre F. , Pinçon J. L. , Parrot M.
Published: April 2013

Midlatitude nighttime observations made by the DEMETER satellite in the very low frequency (VLF) to medium frequency (MF) bands (3 kHz to 3 MHz) have demonstrated the propagation of radio waves from the bottom of ionosphere up to the satellite altitude (~700 km). Propagation characteristics derived from the magneto‐ionic theory [Budden, ] are used to explain the absence of wave observations between ~1 and 2 MHz. Under hypotheses made for the Appleton and Hartree (or Appleton and Lassen) formula, studies of the vertical variations of the real and imaginary parts of the refractive index are performed to point out modifications in the propagation characteristics of the waves: (i) at the crossing of the plasma cutoffs regions, (ii) at the crossing of the ordinary and extraordinary mode resonance regions, and (iii) in the region where the product of the collision frequency (ν) and the electronic density (Ne) is maximum. It is shown that enhancements in the collision frequencies, produced by powerful VLF transmitters in the region where the product of ν and Ne is maximum, open the half angle of the MF wave transmission cones and increase the power densities of those waves at the DEMETER altitude.
Orientation of solar wind dynamic pressure phase fronts
Authors: Jackel Brian J. , Cameron Taylor , Weygand James M.
Published: April 2013

Orientation of structures in the solar wind plays an important role when attempting to use upstream observations at L1 for prediction of subsequent conditions near the Earth. In this study, the relationship between solar wind dynamic pressure forcing and geosynchronous magnetic field response is used to determine a very large set of lagged correlations between the ACE and GOES satellites. Effects due to tilted solar wind structures are explored using the dispersion of arrival times relative to a simple phase plane model. Assuming that structure phase‐front normal vectors were located in the GSE‐xy plane, we found a characteristic azimuth of 15°. Similar analysis carried out with velocity scaling according to the Parker spiral model did not produce an improved fit. Binning by average interplanetary magnetic field (IMF) orientation produced a clear pattern in characteristic azimuth, with phase‐front normals perpendicular to both the predominant Parker spiral orientation and the less common ortho‐spiral configuration. An empirical relationship was found to predict phase‐front normal azimuth over the entire range of observed IMF azimuths. The effects of lateral displacement from the Sun‐Earth line in the GSE‐z direction are comparable to those for GSE‐y, indicating that solar wind structures are often significantly inclined with respect to the ecliptic plane.
The substorm current wedge in MHD simulations
Authors: Birn J. , Hesse M.
Published: April 2013

Using magnetohydrodynamic (MHD) simulations of magnetotail dynamics, we investigate the build‐up and evolution of the substorm current wedge (SCW) and its association with plasma flows from the tail. Three different scenarios are considered: the propagation of magnetic flux ropes of artificially reduced entropy (bubbles), and the formation and propagation of bubbles resulting from magnetic reconnection in the near and far tail. The simulations confirm the important role of the entropy reduction in the earthward penetration of bubbles, as well as in the build‐up of field‐aligned current signatures attributed to the SCW. Low‐entropy flow channels can indeed propagate close to the Earth from the distant tail, as suggested recently. However, this requires substantial entropy reduction, presumably from progression of reconnection into the lobes. The major SCW and pressure build‐up occurred when the low‐entropy flow channels were braked and the flow diverted azimuthally in the near‐Earth region. The flows commonly exhibit multiple narrow channels, separated in space and time, whereas the associated increases in Bz (dipolarization) accumulate over a wider spatial range, spreading both azimuthally and radially. This suggests a picture of the SCW as being composed of multiple smaller “wedgelets,” rather than one big wedge.
Low‐latitude Es capable of controlling the onset of equatorial spread F
Authors: Joshi L. M. , Patra A. K. , Rao S. V. B.
Published: April 2013

To investigate their role on the occurrence of equatorial spread F (ESF), the low‐latitude E region in the evening hours has been characterized in terms of sporadic E (Es) activity based on ionosonde observations from Sriharikota, a low‐latitude station in India. While prior to the sunset, either Chapman type E layer, or Es, or a combination of the two was observed, after sunset, when observed, the E layer was exclusively Es. Es, however, could be either blanketing or nonblanketing type. Observations revealed that while the occurrence of ESF had a broad relationship with the type of Es, a detailed investigation suggested that (1) ESF occurred (did not occur) irrespective of Es type if the virtual height of the F layer base (h'F) was greater than ~360 km (less than ~290 km), and (2) ESF occurrence depended strongly on the type of Es if 290 km < h'F < 360 km. Model computation shows that low‐latitude Es can significantly influence the growth of the Rayleigh‐Taylor instability for the cases corresponding to category 2 by increasing the total field‐line integrated Pedersen conductivity, consistent with the observations. The results presented in this paper highlight under what circumstances the low‐latitude Es would play a critical role in the occurrence of ESF, which can be used to understand the day‐to‐day variability of ESF.
Enhancements of nighttime neutral and ion temperatures in the F region over Millstone Hill
Authors: Ruan Haibing , Lei Jiuhou , Dou Xiankang , Zhang Shun‐Rong , Noto John , Kapali Sudha
Published: April 2013

Nighttime ion temperature in the F region is generally thought to be close to neutral temperature at a similar altitude. However, few studies have been conducted that explore the temporal variation between ion and neutral temperatures at night in the F region. In this paper, the similarities and differences of the nighttime ion and neutral temperatures in the F region over Millstone Hill (42.5°N, 71.5°W) were examined using the incoherent scatter radar and Fabry‐Pérot interferometer observations during the periods of 1–11 August 2011 and 12–23 January 2012, which represented summer and winter seasons, respectively. Our results showed that in winter, both neutral and ion temperatures exhibited the postmidnight enhancements, which occurred during 3–4 LT and had amplitudes of about 30–90 K. However, the situation was different during the August observation period when the postmidnight enhancement was only observed in the neutral temperatures so that the variation of the nighttime ion and neutral temperatures was almost out of phase. The causes for the postmidnight enhancements of neutral temperature in either winter or summer remain debatable. Our preliminary analysis indicates that the extension of midnight temperature maximum to the latitude of Millstone Hill is the preferred mechanism for the observed postmidnight enhancements of neutral temperature in winter and summer.
Terrestrial gamma‐ray flashes in the Fermi era: Improved observations and analysis methods
Authors: Briggs Michael S. , Xiong Shaolin , Connaughton Valerie , Tierney Dave , Fitzpatrick Gerard , Foley Suzanne , Grove J. Eric , Chekhtman Alexandre , Gibby Melissa , Fishman Gerald J. , McBreen Shelia , Chaplin Vandiver L. , Guiriec Sylvain , Layden Emily , Bhat P. N. , Hughes Maximilian , Greiner Jochen , von Kienlin Andreas , Kippen R. Marc , Meegan Charles A. , Paciesas William S. , Preece Robert D. , Wilson‐Hodge Colleen , Holzworth Robert H. , Hutchins Michael L.
Published: April 2013

A new data mode and new analysis methods are used to detect Terrestrial Gamma‐ray Flashes (TGFs) with the Fermi Gamma‐ray Burst Monitor (GBM) 10 times more frequently than previously. In 1037 h of observations at times and over regions for which TGFs are expected, 384 new TGFs were found in addition to the 39 TGFs and two Terrestrial Electron Beam events already detected without the new data mode and methodology. Cosmic ray showers were found to be an important background; they show characteristic signatures in the data of both GBM and the Fermi Large Area Telescope Calorimeter that enable their removal, leaving a sample estimated to consist of ≈98% TGFs. The sample includes shorter TGFs than previously found with GBM. The true duration distribution likely contains additional short TGFs because their detection by GBM is limited by detector dead time. One‐third of this sample has matches with locations from the World Wide Lightning Location Network (WWLLN)—maps of these locations show the geographic and meteorological features more clearly than maps of spacecraft locations. The intrinsic TGF rate is evaluated using the lightning rate maps of the Lightning Imaging Sensor, accounting for the detection efficiency of GBM as a function of spacecraft‐source offset, from which we estimate a global TGF rate of ≈400,000 per year. With continuous production of data in the new mode we estimate that GBM will detect ≈850 TGFs per year.
Prediction of solar activity for the next 500 years
Authors: Steinhilber Friedhelm , Beer Jürg
Published: April 2013

Recently, a new low‐noise record of solar activity has been reconstructed for the past 9400 years by combining two 10Be records from Greenland and Antarctica with 14C from tree rings [Steinhilber et al., ]. This record confirms earlier results, namely, that the Sun has varied with distinct periodicities in the past. We present a prediction of mean solar magnetic activity averaged over 22 years for the next 500 years mainly based on the spectral information derived from the solar activity record of the past. Assuming that the Sun will continue to vary with the same periodicities for the next centuries, we extract the spectral information from the past and apply it to two different methods to predict the future of solar magnetic activity. First, the two methods are tested by predicting past changes. Our methods are able to predict periods of high and low solar activities for a few centuries in the past. However, they are less successful in predicting the correct amplitude. Then, the methods were used to predict the period 2000–2500. Both methods predict a period of low activity around 2100 A.D. Between 2100 and 2350 A.D., the results are inconsistent regarding the duration of the low‐activity state in 2100 A.D. and the level of activity until 2250 A.D. Around 2250 A.D., both methods predict a period of moderate activity. After 2350 A.D., both methods point to a period of high activity. The period of high activity will end around 2400 A.D. and will be followed by a period of moderate activity.
Mercury's magnetopause and bow shock from MESSENGER Magnetometer observations
Authors: Winslow Reka M. , Anderson Brian J. , Johnson Catherine L. , Slavin James A. , Korth Haje , Purucker Michael E. , Baker Daniel N. , Solomon Sean C.
Published: April 2013

We have established the average shape and location of Mercury's magnetopause and bow shock from orbital observations by the MESSENGER Magnetometer. We fit empirical models to midpoints of boundary crossings and probability density maps of the magnetopause and bow shock positions. The magnetopause was fit by a surface for which the position R from the planetary dipole varies as [1 + cos(θ)]−α, where θ is the angle between R and the dipole‐Sun line, the subsolar standoff distance Rss is 1.45 RM (where RM is Mercury's radius), and the flaring parameter α = 0.5. The average magnetopause shape and location were determined under a mean solar wind ram pressure PRam of 14.3 nPa. The best fit bow shock shape established under an average Alfvén Mach number (MA) of 6.6 is described by a hyperboloid having Rss = 1.96 RM and an eccentricity of 1.02. These boundaries move as PRam and MA vary, but their shapes remain unchanged. The magnetopause Rss varies from 1.55 to 1.35 RM for PRam in the range of 8.8–21.6 nPa. The bow shock Rss varies from 2.29 to 1.89 RM for MA in the range of 4.12–11.8. The boundaries are well approximated by figures of revolution. Additional quantifiable effects of the interplanetary magnetic field are masked by the large dynamic variability of these boundaries. The magnetotail surface is nearly cylindrical, with a radius of ~2.7 RM at a distance of 3 RM downstream of Mercury. By comparison, Earth's magnetotail flaring continues until a downstream distance of ~10 Rss.
Long‐lasting daytime equatorial plasma bubbles observed by the C/NOFS satellite
Authors: Huang Chao‐Song , de La Beaujardiere O. , Roddy P. A. , Hunton D. E. , Ballenthin J. O. , Hairston M. R.
Published: April 2013

We report the first observations of long‐lasting daytime equatorial plasma bubbles with the Communication/Navigation Outage Forecasting System (C/NOFS) satellite. The most unusual features of the plasma bubbles are the persistence from the post‐midnight sector through the afternoon sector and the extremely long lifetime of 12 h. In one case, the plasma bubbles were generated at 02:00–03:00 LT near the end of the main phase of a moderate magnetic storm and detected by C/NOFS over eight successive orbits, and the decrease of the ion density inside the bubbles was still as large as ~30% at 14:00–15:00 LT. In another case, one group of plasma bubbles was generated near the sunset terminator and existed over the entire nighttime until the post‐sunrise sector (06:00–08:00 LT), and another group of plasma bubbles was first detected at 04:00–06:00 LT and lasted until ~11:00 LT. The latter group of bubbles occurred following a sharp northward turning of the interplanetary magnetic field (IMF) near the end of the main phase of a weak magnetic storm, and the overshielding electric field caused by the IMF northward turning and the storm time disturbance dynamo might both have contributed to the generation of the bubbles. The plasma bubbles reached 800 km or higher in altitude during daytime. The high altitudes may be critical for the long lifetime of the bubbles: the photo‐ionization rate decreases rapidly with altitude. The photo‐ionization process may take a long time to produce enough new plasma particles to fill the daytime bubbles at high altitudes.
Physical mechanisms of the ionospheric storms at equatorial and higher latitudes during the recovery phase of geomagnetic storms
Authors: Balan N. , Otsuka Y. , Nishioka M. , Liu J. Y. , Bailey G. J.
Published: April 2013

The paper studies the physical mechanisms of the ionospheric storms at equatorial and higher latitudes, which are generally opposite both during the main phase (MP) and recovery phase (RP) of geomagnetic storms. The mechanisms are based on the natural tendency of physical systems to occupy minimum energy state which is most stable. The paper first illustrates the recent developments in the understanding of the mechanisms during daytime MPs when generally negative ionospheric storms (in Nmax and TEC) develop at equatorial latitudes and positive storms occur at higher latitudes, including why the storms are severe only in some cases. The paper then investigates the relative importance of the physical mechanisms of the positive ionospheric storms observed at equatorial latitudes (within ±15°) during daytime RPs when negative storms occur at higher latitudes using CHAMP Ne and GPS‐TEC data and Sheffield University Plasmasphere Ionosphere Model. The results indicate that the mechanical effect of the storm‐time equatorward neutral winds that causes plasma convergence at equatorial F region could be a major source for the positive storms, with the downwelling effect of the winds and zero or westward electric field, if present, acting as minor sources.
Lithospheric cooling trends and deviations in oceanic PP‐P and SS‐S differential traveltimes
Authors: Goes Saskia , Eakin Caroline M. , Ritsema Jeroen
Published: April 2013

The thermal and compositional structure of oceanic lithosphere, which exerts an important control on plate behavior, is still debated. Our set of 60,000 PP‐P and SS‐S traveltime differences with oceanic PP and SS bounce points provides a good constraint on both compressional‐ and shear‐wave velocity. By calculating traveltimes for thermal models that are converted to seismic structures with a thermodynamic approach, we test whether lithospheric cooling can explain PP‐P and SS‐S traveltime variations with plate age. The PP‐P and SS‐S traveltimes have substantial scatter but, on average, decrease by 0.2 and 0.7 s/Myr½, respectively, when the PP and SS waves reflect off progressively older oceanic crust. Both a half‐space and a plate cooling model with a mid‐ocean ridge basalt‐source mantle potential temperature (1315° ± 50°C) explain the average values of the PP‐P and SS‐S anomalies and their decrease with plate age. Residual PP‐P and SS‐S anomalies relative to a cooling model reveal large‐scale patterns. Along a few paths (e.g., Tonga–Fiji to western North America), seismic heterogeneity in the deep mantle is responsible for a significant fraction of the PP‐P and SS‐S traveltime variation. Most anomalies probably correspond to broad temperature variations in the upper mantle, such as a very slow central–northern Pacific (which may require a 100°C excess temperature) and high‐ and low‐velocity anomalies along the ridges that correlate with deep and shallow bathymetry, respectively.
Low shear velocities in the sub‐lithospheric mantle beneath the Indian shield?
Authors: Kumar M. Ravi , Saikia Dipankar , Singh Arun , Srinagesh D. , Baidya P.R. , Dattatrayam R. S.
Published: April 2013

Ever since its breakup from the Gondwanaland ~140 Myr ago, the Indian plate was ravaged by four hot spots. Although the surface manifestations of such deep processes are evident in terms of large igneous provinces like the Deccan and the fast drift of the Indian plate, the modifications to the deep structure remain to be grasped. In this study, we investigate the mantle transition zone (TZ) structure beneath the Indian shield region using ~14,000 teleseismic receiver functions from 77 broadband stations sited on diverse geologic terrains. The arrival times of the P‐to‐s (Ps) conversions from the 410 km discontinuity at most cratonic stations appear to be delayed by ~2 s in comparison with the times observed for other Precambrian shield regions like Africa, Australia, and Canada. Such delays in the conversions from the 410 km discontinuity below the Indian shield suggest low shear wave speeds in the lithospheric and sub‐lithospheric mantles due to higher temperatures, together with a thinner high velocity lid that contrasts with a thicker one found beneath most Archean cratons. A thin transition zone beneath most of the cratonic stations lends support to the enhanced temperatures within the TZ itself. Also, a further delay of the TZ discontinuities is observed for stations on the southern granulite terrain, which was under the influence of the Marion plume that is responsible for the separation of Madagascar from India. Although the data do not conclusively show evidence for a 520 km discontinuity, an LVL atop the 410 cannot be ruled out beneath certain geological provinces of the Indian shield.
Permanent upper plate deformation in western Myanmar during the great 1762 earthquake: Implications for neotectonic behavior of the northern Sunda megathrust
Authors: Wang Yu , Shyu J. Bruce H. , Sieh Kerry , Chiang Hong‐Wei , Wang Chung‐Che , Aung Thura , Lin Yu‐nung Nina , Shen Chuan‐Chou , Min Soe , Than Oo , Lin Kyaw Kyaw , Tun Soe Thura
Published: April 2013

The 1762 Arakan earthquake resulted from rupture of the northern Sunda megathrust and is one of those rare preinstrumental earthquakes for which early historical accounts document ground deformations. In order to obtain more comprehensive and detailed measurements of coseismic uplift, we conducted comprehensive field investigations and geochronological analyses of marine terraces on the two largest islands in western Myanmar. We confirm 3–4 m of coseismic coastal emergence along southwestern Cheduba Island, diminishing northeastward to less than 1 m. Farther northeast, uplift associated with the earthquake ranges from slightly more than 1 m to 5–6 m along the western coast of Ramree Island but is insignificant along the island's eastern coast. This double‐hump pattern of uplift coincides with the long‐term anticlinal growth of these two islands. Thus, we propose that the 1762 earthquake resulted from slip on splay faults under the islands, in addition to rupture of the megathrust. Elastic modeling implies that fault slip during the 1762 earthquake ranges from about 9 to 16 m beneath the islands and corresponds to a magnitude ofMw8.5 if the rupture length of the megathrust is ~500 km. The island's uplift histories suggest recurrence intervals of such events of about 500–700 years. Additional detailed paleoseismological studies would add significant additional detail to the history of large earthquakes in this region.
3‐D multi‐observable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle. II: General methodology and resolution analysis
Authors: Afonso J.C. , Fullea J. , Yang Y. , Connolly J.A.D. , Jones A.G.
Published: April 2013

Here we present a 3‐D multi‐observable probabilistic inversion method, particularly designed for high‐resolution (regional) thermal and compositional mapping of the lithosphere and sub‐lithospheric upper mantle that circumvents the problems associated with traditional inversion methods. The key aspects of the method are as follows: (a) it exploits the increasing amount and quality of geophysical datasets; (b) it combines multiple geophysical observables (Rayleigh and Love dispersion curves, body‐wave tomography, magnetotelluric, geothermal, petrological, gravity, elevation, and geoid) with different sensitivities to deep/shallow, thermal/compositional anomalies into a single thermodynamic‐geophysical framework; (c) it uses a general probabilistic (Bayesian) formulation to appraise the data; (d) no initial model is needed; (e) compositional a priori information relies on robust statistical analyses of a large database of natural mantle samples; and (f) it provides a natural platform to estimate realistic uncertainties. In addition, the modular nature of the method/algorithm allows for incorporating or isolating specific forward operators according to available data. The strengths and limitations of the method are thoroughly explored with synthetic models. It is shown that the a posteriori probability density function (i.e., solution to the inverse problem) satisfactorily captures spatial variations in bulk composition and temperature with high resolution, as well as sharp discontinuities in these fields. Our results indicate that only temperature anomalies of ΔT ⪆150°C and large compositional anomalies of ΔMg# > 3 (or bulk ΔAl2O3 > 1.5) can be expected to be resolved simultaneously when combining high‐quality geophysical data. This resolving power is sufficient to explore some long‐standing problems regarding the nature and evolution of the lithosphere (e.g., vertical stratification of cratonic mantle, compositional versus temperature signatures in seismic velocities, etc) and offers new opportunities for joint studies of the structure of the upper mantle with unprecedented resolution.
Geometry of the Pamir‐Hindu Kush intermediate‐depth earthquake zone from local seismic data
Authors: Sippl C. , Schurr B. , Yuan X. , Mechie J. , Schneider F. M. , Gadoev M. , Orunbaev S. , Oimahmadov I. , Haberland C. , Abdybachaev U. , Minaev V. , Negmatullaev S. , Radjabov N.
Published: April 2013

We present new seismicity images based on a two‐year seismic deployment in the Pamir and SW Tien Shan. A total of 9532 earthquakes were detected, located, and rigorously assessed in a multistage automatic procedure utilizing state‐of‐the‐art picking algorithms, waveform cross‐correlation, and multi‐event relocation. The obtained catalog provides new information on crustal seismicity and reveals the geometry and internal structure of the Pamir‐Hindu Kush intermediate‐depth seismic zone with improved detail and resolution. The relocated seismicity clearly defines at least two distinct planes: one beneath the Pamir and the other beneath the Hindu Kush, separated by a gap across which strike and dip directions change abruptly. The Pamir seismic zone forms a thin (approximately 10 km width), curviplanar arc that strikes east‐west and dips south at its eastern end and then progressively turns by 90° to reach a north‐south strike and a due eastward dip at its southwestern termination. Pamir deep seismicity outlines several streaks at depths between 70 and 240 km, with the deepest events occurring at its southwestern end. Intermediate‐depth earthquakes are clearly separated from shallow crustal seismicity, which is confined to the uppermost 20–25 km. The Hindu Kush seismic zone extends from 40 to 250 km depth and generally strikes east‐west, yet bends northeast, toward the Pamir, at its eastern end. It may be divided vertically into upper and lower parts separated by a gap at approximately 150 km depth. In the upper part, events form a plane that is 15–25 km thick in cross section and dips sub‐vertically north to northwest. Seismic activity is more virile in the lower part, where several distinct clusters form a complex pattern of sub‐parallel planes. The observed geometry could be reconciled either with a model of two‐sided subduction of Eurasian and previously underthrusted Indian continental lithosphere or by a purely Eurasian origin of both Pamir and Hindu Kush seismic zones, which necessitates a contortion and oversteepening of the latter.
Comment on “Self‐similar earthquake triggering, Båth's law, and foreshock/aftershock magnitudes: Simulations, theory, and results for southern California” by P. M. Shearer
Authors: Hainzl Sebastian
Published: April 2013

Shearer [2012] finds three differences of the seismicity clustering in southern California compared to self‐similar triggering models: (i) a significantly lower b‐value for the aftershocks, (ii) a too large aftershock number, and (iii) a too large foreshock‐aftershock ratio to be consistent with the Båth law. Based on these observations, the author concluded that the observed seismicity is not in agreement with self‐similarity triggering and/or the observed clustering is not primarily caused by earthquake‐to‐earthquake triggering. However, I show that the observed lower b‐value is likely related to incomplete recordings after mainshocks and that the apparently too large aftershock number does not disprove the self‐similarity. Thus, only the enhanced foreshock‐to‐aftershock ratio seems to indicate some discrepancy to self‐similar triggering.
The stress shadow induced by the 1975–1984 Krafla rifting episode
Authors: Maccaferri F. , Rivalta E. , Passarelli L. , Jónsson S.
Published: April 2013

It has been posited that the 1975–1984 Krafla rifting episode in northern Iceland was responsible for a significant drop in the rate of earthquakes along the Húsavík‐Flatey Fault (HFF), a transform fault that had previously been the source of several magnitude 6–7 earthquakes. This compelling case of the existence of a stress shadow has never been studied in detail, and the implications of such a stress shadow remain an open question. According to rate‐state models, intense stress shadows cause tens of years of low seismicity rate followed by a faster recovery phase of rate increase. Here, we compare the long‐term predictions from a Coulomb stress model of the rifting episode with seismological observations from the SIL catalog (1995–2011) in northern Iceland. In the analyzed time frame, we find that the rift‐induced stress shadow coincides with the eastern half of the fault where the observed seismicity rates are found to be significantly lower than expected, given the historical earthquake activity there. We also find that the seismicity rates on the central part of the HFF increased significantly in the last 17 years, with the seismicity progressively recovering from west to east. Our observations confirm that rate‐state theory successfully describes the long‐term seismic rate variation during the reloading phase of a fault invested by a negative Coulomb stress. Coincident with this recovery, we find that the b‐value of the frequency‐magnitude distribution changed significantly over time. We conclude that the rift‐induced stress shadow not only decreased the seismic rate on the eastern part of the HFF but also temporarily modified how the system releases seismic energy, with more large magnitude events in proportion to small ones. This behavior is currently being overturned, as rift‐induced locking is now being compensated by tectonic forcing.
DynEarthSol2D: An efficient unstructured finite element method to study long‐term tectonic deformation
Authors: Choi E. , Tan E. , Lavier L. L. , Calo V. M.
Published: April 2013

Many tectonic problems require to treat the lithosphere as a compressible elastic material, which can also flow viscously or break in a brittle fashion depending on the stress level applied and the temperature conditions. We present a flexible methodology to address the resulting complex material response, which imposes severe challenges on the discretization and rheological models used. This robust, adaptive, two‐dimensional, finite element method solves the momentum balance and the heat equation in Lagrangian form using unstructured meshes. An implementation of this methodology is released to the public with the publication of this paper and is named DynEarthSol2D (available at http://bitbucket.org/tan2/dynearthsol2). The solver uses contingent mesh adaptivity in places where shear strain is focused (localization) and a conservative mapping assisted by marker particles to preserve phase and facies boundaries during remeshing. We detail the solver and verify it in a number of benchmark problems against analytic and numerical solutions from the literature. These results allow us to verify and validate our software framework and show its improved performance by an order of magnitude compared against an earlier implementation of the Fast Lagrangian Analysis of Continua algorithm.
Real episodic growth of continental crust or artifact of preservation? A 3‐D geodynamic model
Authors: Walzer Uwe , Hendel Roland
Published: April 2013

We investigate whether the observed zircon age distribution of continental crust (CC) is produced by real crustal growth episodes or is only an artifact of preservation. In connection with the second alternative of this question, other authors proposed that there was little episodicity in the production of new CC and that modeling corroborates this opinion. We conclude that a combination of the two answers might be possible. In matters of modeling, however, we ascertain that adynamicmodeling of the convection‐differentiation system of the mantle reveals the high probability of magmatic episodes. We solve the full set of balance equations in a 3‐D spherical‐shell mantle. The heat‐producing elements are redistributed by chemical differentiation. A realistic solidus model of mantle peridotite is essential for an applicable model. The solidus depends not only on depth but also on the volatile concentration. Furthermore, we introduced realistic profiles of Grüneisen parameter, viscosity, adiabatic temperature, thermal expansivity, and specific heat. Our model automatically produces lithospheric plates and growing continents. Regarding number, size, form, distribution, and surface velocity of the continents, no constraints have been prescribed. Regions of the input parameter space (Ra,σy,k,f3) that are favorable with respect to geophysical quantities showsimultaneouslynot only episodicity of CC growth but also a reproduction of the observed zircon‐age maxima referring to the instants of time. We also obtain Archean events for ages greater than 3000 Ma that are not or scarcely visible in the observed zircon ages. Sinusoidal parts of the evolution curve ofqob,Ur, andEkinare superposed with a monotonous decrease. The volumetrically averaged mantle temperature,Tmean, however, decreases smoothly and slowly, nearly without pronounced variations. Therefore, we can dismiss catastrophic mechanisms thatsimultaneouslyincorporate thewholemantle.
Evidence for Late Eocene emplacement of the Malaita Terrane, Solomon Islands: Implications for an even larger Ontong Java Nui oceanic plateau
Authors: Musgrave Robert J.
Published: April 2013

Most tectonic models for the Solomon Islands Arc invoke a Miocene collision with the Ontong Java Plateau (OJP) to halt cessation of Pacific Plate subduction, initiate Australian Plate subduction, and emplace the Malaita Terrane, which shares the characteristic basement age and geochemistry of OJP. Existing paleomagnetic evidence, however, required the Malaita Terrane to have been fixed to the arc from at least the Late Eocene. New sampling has yielded a paleomagnetic pole from Aptian–Albian limestones and mudstones that falls between the apparent polar wander paths for the Australian Plate and OJP, confirming the extended period of residence of the Malaita Terrane on the arc. Arc‐derived turbidities within Late Eocene through Miocene limestones on Malaita and Santa Isabel, and related clasts in broadly contemporary sandstones and conglomerates on Santa Isabel, also attest to early emplacement. Modeling the emplacement at 35 Ma satisfies both the paleomagnetic data and the sediment provenance. Continuing the reconstruction to 125 Ma leaves the Malaita Terrane far from OJP at the time of plateau formation. OJP is now understood to have formed as part of a larger Ontong Java Nui, also comprising the Hikurangi and Manihiki plateaus, separated by spreading during the Cretaceous. Restoring the separation of the known elements, and invoking an additional triple junction, unites the (now largely subducted) Malaita Terrane with the rest of Ontong Java Nui. Subduction of substantial areas of the Ontong Java Nui plateau, with little geological signal other than a reduction in arc volcanism, is a corollary.
Comparisons of atmospheric data and reduction methods for the analysis of satellite gravimetry observations
Authors: Forootan E. , Didova O. , Kusche J. , Löcher A.
Published: April 2013

The Gravity Recovery and Climate Experiment (GRACE) derived gravity solutions contain errors mostly due to instrument noise, anisotropic spatial sampling, and temporal aliasing. Improving the quality of satellite gravimetry observations, in terms of using more sensitive sensors and/or increasing the spatial isotropy, has been discussed in the context of the designed scenarios of future satellite gravimetry missions. Temporal aliasing caused by incomplete reducing of background models, however, is still a factor that affects the quality of the gravity field solutions. This paper specifically explores the possible physical, geometrical, and numerical modifications of the three‒dimensional (3‒D) integration approach to eliminate the high‒frequency atmospheric effects from satellite gravimetry observations. The new modified 3‒D approach is then applied to compute new sets of atmospheric dealiasing products, using atmospheric fields from the European Centre for Medium‒Range Weather Forecasts (ECMWF) operational analysis model and ERA‒Interim reanalysis. Impacts of modifications are compared to the prelaunch baseline and the current error‒curve of GRACE as well as an error‒curve of a Bender‒type multiorbit satellite configuration. Specifically, we found that using latitude‒dependent radius, latitude‒ and altitude‒dependent gravity accelerations along with numerical modifications have a considerable impact on the 3‒D integral. Comparing the new products to those of GRACE Atmosphere and Ocean Dealiasing level‒1B shows a nonnegligible difference with respect to the prelaunch baseline of GRACE and a possible Bender‒type mission up to harmonic degrees 13 and 50, respectively. A big difference is also found between the derived dealiasing products from ECMWF operational analysis and ERA‒Interim indicating the importance of input parameters on the final atmospheric dealiasing products.
Quasi‐periodic slow slip events in the afterslip area of the 1996 Hyuga‐nada earthquakes, Japan
Authors: Yarai Hiroshi , Ozawa Shinzaburo
Published: April 2013

The time evolution of afterslip on a plate boundary experiencing interplate earthquakes is expected to show logarithmic decay. The global positioning system network in Japan has been monitoring transient deformation since the occurrence of two large interplate earthquakes with moment magnitudes of 6.8 and 6.7 in the Hyuga‐nada area, southwest Japan, in 1996. The spatial and temporal evolution of aseismic interplate slip based on crustal deformation data indicates that afterslip followed the two earthquakes and gradually declined to background rates by around 2004 with total moment magnitude of 7.3. However, quasi‐periodic slow slip events suddenly began within the afterslip area in 2005 with approximately one year duration and two‐year recurrence interval. The moment magnitudes of the three slow slip events since January 2005 range from 6.7 to 6.8. This differs greatly from the expected behavior of logarithmic decay over time. Both velocity‐strengthening and velocity‐weakening rate‐and‐state modes have been implicated as the cause of afterslip, whose location is complementary to the main shock area of velocity‐weakening, while a slow slip event occurs in the velocity‐weakening area with different frictional properties from those of an afterslip area. In light of the seemingly different frictional properties, the coexistence of afterslip and slow slip events in the same area would provide additional information about precisely how the plate interface is behaving. The monitoring of these slow slip events should give the clues to understanding the coexistence of long‐term afterslip and slow slip events and the increasing risk of earthquakes in neighboring areas.
Anisotropic amplitude variation of the bottom‐simulating reflector beneath fracture‐filled gas hydrate deposit
Authors: Sriram G. , Dewangan P. , Ramprasad T. , Rama Rao P.
Published: April 2013

For the first time, we report the amplitude variation with angle (AVA) pattern of bottom‐simulating reflectors (BSRs) beneath fracture‐filled gas hydrate deposits when the effective medium is anisotropic. The common depth point (CDP) gathers of two mutually perpendicular multichannel seismic profiles, located in the vicinity of Site NGHP‐01‐10, are appropriately processed such that they are fit for AVA analysis. AVA analysis of the BSR shows normal‐incidence reflection coefficients of −0.04 to −0.11 with positive gradients of 0.04 to 0.31 indicating class IV pattern. The acoustic properties from isotropic rock physics model predict class III AVA pattern which cannot explain the observed class IV AVA pattern in Krishna‐Godavari basin due to the anisotropic nature of fracture‐filled gas hydrate deposits. We modeled the observed class IV AVA of the BSR by assuming that the gas hydrate bearing sediment can be represented by horizontally transversely isotropic (HTI) medium after accounting for anisotropic wave propagation effects on BSR amplitudes. The effective medium properties are estimated using Backus averaging technique, and the AVA pattern of BSRs is modeled using the properties of overlying HTI and underlying isotropy/HTI media with or without free gas. Anisotropic AVA analysis of the BSR from the inline seismic profile shows 5–30% gas hydrate concentration (equivalent to fracture density) and the azimuth of fracture system (fracture orientation) with respect to the seismic profile is close to 45°. Free gas below the base of gas hydrate stability zone is interpreted in the vicinity of fault system (F1).