bluuebunny/crossref_metadata_2025 huggingface dataset:
Azimuthally small-scale Alfvén waves in magnetosphere excited by the source of finite duration
Authors: Klimushkin Dmitri Yu. , Podshibyakin Ivan Yu. , Cao Jin Bin
Published: August 2014

In this paper the spatial structure of azimuthally small-scale Alfvén waves in magnetosphere excited by the impulse source is studied. The source is suddenly switched on at a definite moment and works as e−i
ω
0
t during the finite time interval. The influence of factors which lead to the difference of toroidal and poloidal eigenfrequencies (like curvature of field lines and finite plasma pressure) is taken into account. Due to these factors, a radial component of the group velocity of Alfvén wave appears. An important value is the time moment, t
0, when a wave front moving with radial component of wave group velocity from the poloidal surface (a magnetic surface where the source frequency ω
0 coincides with the poloidal frequency) passes the given magnetic shell with the radial coordinate x. The temporal evolution at all the points, where the front has not come yet, is determined by the phase mixing of the initial disturbance. At the points through which the wave front has already passed, the wave field structure almost coincides with the structure of monochromatic wave. The region where the front propagates is bounded by the interval between the poloidal surface and the toroidal one (that is, the Alfvén resonance surface). For this reason, outside this region the evolution is always determined by the phase mixing, which leads to much smaller amplitudes than between poloidal and toroidal surfaces. After the source turned off, a back wave front is formed, which comes through the given point in direction from the poloidal surface to the toroidal one. After the back front has come, the monochromatic wave structure disappears and there is only a weak disturbance, which steadily disappears because of the phase mixing and the final conductivity of ionosphere.
Simulations of superrotation using a GCM for Venus’ middle atmosphere
Authors: Yamamoto Masaru , Takahashi Masaaki
Published: August 2014

A superrotation is simulated in a T10L100 general circulation model for Venus’ middle atmosphere (VMAGCM), in which the radiative effects of aerosols are calculated. The simulation in a domain of 30–100 km is conducted under the condition of a bottom zonal flow with a velocity of 50 m s−1 at the equator. Thermal tides contribute to the maintenance of the cloud-top superrotation together with meridional circulation and vertically propagating gravity waves. The meridional circulation and wave activity are sensitive to the vertical eddy diffusion. Although the equatorial zonal flow has a velocity of about 70 m s−1 when the vertical eddy diffusion coefficient (K
V) is set at 5.0 m s−2, it has a velocity of <100 m s−1 when K
V = 2.5 m s−2. The fully developed equatorial jet for the small K
V case is enhanced at 65 km by small-scale gravity waves emitted from the cloud top.
A study on correction equations for the effect of seafloor topography on ocean bottom magnetotelluric data
Authors: Matsuno Tetsuo , Seama Nobukazu , Baba Kiyoshi
Published: August 2014

Consideration of the effect of seafloor topography on ocean bottom magnetotelluric data is needed to estimate a reliable resistivity structure. Selection of the correction equation which precisely takes account of distortions is important when we correct the effect of seafloor topography with forward modeling. Corrections of synthetic data based on three different correction equations are carried out, and the corrected responses and the true response are compared to investigate which correction equation is the best in application to distorted magnetotelluric data on the seafloor. Differences in the corrected responses are remarkable at periods shorter than several thousands seconds. These differences are caused by differences in treatment of the distortions of magnetic field and in robustness of the correction equation to a resistivity structure assumed for the topographic correction. The results suggest that the correction equation of Nolasco et al. (1998) should be used because it better accommodates the estimation of unknown resistivity structures. We apply the correction equation of Nolasco et al. (1998) to a response observed in the Mariana area. The one-dimensional resistivity structure estimated from the corrected response explains almost all the components of the observed response.
A measurement of the lunar semidiurnal tide at Wuhan (30°40′N, 114°30′E)
Authors: Niu X. J. , Xiong J. G. , Wan W. X. , Ning B. Q. , Liu L. B.
Published: August 2014

Upper atmospheric winds have been measured at heights from 80 to 100 km overWuhan (30°40N, 114°30′E), China with a meteor radar from 2002 to 2005. The variations of lunar semidiurnal tidal amplitudes and phases with both seasons and heights are studied in detail to reveal the properties of the lunar semidiurnal tide. It is shown that the lunar semidiurnal tide is stronger in January than other months, and its second peak appears near August. For most months the eastward maximum is 3 ± 1 lunar hours later than the northward maximum, as classical theory predicts for a northern hemisphere tide. The observed seasonal and height variations are also compared with the Global Scale Wave Model (GSWM). The phases do not agree well with those of the GSWM model. The maximum amplitude occurs in a different month in the model. There are about 5 lunar hours phase difference between the observed and the model at 90 and 96 km in eastward and northward components. A comparison of the lunar and solar semidiurnal tides is also shown in this paper. The behavior of these two tides in season is different, especially for the month of appearance of maximum amplitude.
Propagation characteristics of bedrock waves traveling from pre-Tertiary basement to engineering bedrock
Authors: Kinoshita Shigeo
Published: August 2014

This study uses borehole array recordings to measure the propagation characteristics of bedrock waves traveling from pre-Tertiary basement (seismic bedrock) to engineering bedrock with anS-wave velocity of approximately 0.5–0.8 km/s. To avoid the destructive interference of surface-reflected down-going waves and incidence waves in seismic and engineering bedrock, borehole data recorded at sufficient depth levels are used in this study. This is the most important aspect of the fundamental basis of this study. The propagation characteristics, i.e., the transfer function, of bedrock motions for S-waves are well represented by a Butterworth-type low-pass filter model with a high corner frequency in excess of 15 Hz and a low decay rate of —3 power of frequency. The use of such a filter model is based on the concise representation of the transfer function from an engineering viewpoint. Simple one-dimensional ray theory with a plane wave approximation explains the characteristics of the model filter at low frequencies of less than approximately 5 Hz; however, one-dimensional ray theory with a plane wave approximation at high frequencies in excess of 5 Hz requires the unusual frequency characteristics ofQS−1(f), which increases with increasing frequency, to explain the frequency characteristics of the model filter. These facts imply that the filter gain can be determined using the impedance ratio of seismic bedrock to engineering bedrock and the attenuation characteristics of the intervening media at low frequencies less than 5 Hz. However, the cutoff frequency and decay rate of the filter must be determined from observational data.
Temporal changes in state of stress in the Tokachi Oki area after the 2003 Tokachi Oki Earthquake
Authors: Ghimire Subesh , Katsumata Kei , Kasahara Minoru
Published: August 2014

A stress tensor inversion was carried out in the aftershock area of the 2003 Tokachi Oki earthquake for a comparative study on the preseismic and postseismic stress state. The inversion technique was adopted from Gephart (1990) to pursue the study with earthquake focal mechanism data from National Research Institute for Earth Science and Disaster Prevention (NIED). Results of the inversion revealed significant change in stress field before and after the main shock of the 2003 event. The preseismic uniaxial compression in strike slip stress regime was changed to a transition from transpressional to a radial compression in a compressional regime.
On the validity of Akasofu’s ε parameter and of the Vasyliunas et al. general formula for the rate of solar wind-magnetosphere energy input
Authors: Aoki Takao
Published: August 2014

The ε parameter of Akasofu and the general formula of Vasyliunas et al. (1982) have been widely used as functions representing the rate of the solar wind energy input into the Earth’s magnetosphere. These two functions have a feature that dependence on the interplanetary magnetic field (IMF) is assumed to be expressed by the combination of B, the magnitude of the IMF, and sin (θ/2), where θ is the clock angle, or by the combination of B

T
= (By
2 + Bz
2)1/2, the magnetic field perpendicular to the Sun-Earth line, and sin(θ/2), not by the Bz component. We check the validity of this assumption by presuming that the AL index has good correlation with the solar wind energy input rate and by applying the regression analysis in a power law form to hourly values of the AL index and of solar wind parameters during the period 1966–1987. We show that Bz generally gives high correlation coefficients and shows higher consistency in exponents than the combination of B (or B

T
) and sin (θ/2). Thus we conclude that the IMF dependence of the rate of the solar wind energy input is described better by Bz than by the combination of B (or B

T
) and sin(θ/2).
An improvement of GPS height estimations: stochastic modeling
Authors: Jin Shuanggen , Wang J. , Park Pil-Ho
Published: August 2014

The results of GPS positioning depend on both functional and stochastic models. In most of the current GPS processing programs, however, the stochastic model that describes the statistical properties of GPS observations is usually assumed that all GPS measurements have the same accuracy and are statistically independent. Such assumptions are unrealistic. Although there were only a few studies modeling the effects on the GPS relative positioning, they are restricted to short baselines and short session lengths. In this paper, the stochastic modeling for IGS long-baseline positioning (with 24-hour session) is analyzed in the GAMIT software by modified stochastic models. Results show that any mis-specifications of stochastic model result in unreliable GPS baseline results, and the deviation of baseline estimations reaches as much as 2 cm in the height component. Using the stochastic model of satellite elevation angle-based cosine function, the precision of GPS baseline estimations can be improved, and the GPS baseline component is closest to the reference values, especially GPS height.
Double-difference relocations of the 2004 off the Kii peninsula earthquakes
Authors: Enescu Bogdan , Mori James , Ohmi Shiro
Published: August 2014

Two large earthquakes (Mw7.3 and Mw7.5) occurred off-shore of the Kii peninsula (Japan) on 5th September 2004. The insufficient station coverage in the area where the large events and their aftershocks occurred led to significant uncertainty regarding their depth location. By using a double-difference approach, we were able to determine more accurate hypocenter locations and better define the main characteristics of the aftershock distribution. The data we used in this study comprise of about 700 earthquakes recorded by the Japanese Meteorological Agency (JMA) from 5 to 12 September 2004. We assume several depth ranges for the initial double difference earthquake locations and select the solution with the smallest rms residual. The resulting hypocenter distribution has two distinctive features: 1) on average the depths are about 20 km shallower than the initial JMA location and 2) the hypocenters are divided into two groups: one shallow (around 12 km depth) and another deep (around 27 km depth). The epicenters of the relocated events define a finer, more clustered structure compared with the epicentral distribution of JMA. The splitting of the earthquake locations into two depth groups is still observed when using a significantly different velocity structure. Our results of the two depth groups for the hypocenters resemble the independently determined hypocentral distribution from the OBS survey.
Recursive travel-time inversion: A tool for real-time seismic tomography
Authors: Ogiso Masashi , Yomogida Kiyoshi , Katsumata Kei
Published: August 2014

A new recursive inverse scheme is applied to a currently popular problem named seismic travel-time tomography, in order to enhance the efficiency and reliability in obtaining a new velocity model if a small number of new data are added to a large data set in the past. In comparison with conventional inverse schemes in seismic tomography, either least-squares or iterative types, this scheme does not require large amounts of matrix-type computations but utilizes the amount of modification in model parameters responsible for each new data set. We also introduce the computation of a collocation travel time (i.e., from a given station to every grid point) for the reference velocity model inverted by the data for all the past events, using a ray tracing scheme called the Huygens’ method (Saito, 2001), suitable to computations prior to a new event. Combining the above information already stored with the recursive inverse scheme, we can obtain a new or updated velocity model immediately after a new event takes place, because a temporal interval between two events is usually very long in a given local area. Since the model is revised at each recursive step, we perform ray tracings with the updated reference model to get more accurate ray paths and travel times than the conventional inversion schemes that use all the ray tracings for the same reference model. We first showed the validity and stability of the proposed method with synthetic data. We then applied the new approach to the P-wave travel-time data recorded in the Hidaka, south-central Hokkaido, Japan, region, and compared our result with other previous results. Our result shares the overall feature with the previous ones. In addition, a new low-velocity zone is detected in the east of the Hidaka mountains at the depth of 10 km, corresponding to the collision zone of two arcs, due to the use of the updated reference velocity model at each recursive step. We also confirmed that the order of data does not affect the final result, so that the present approach is shown as an appropriate tool for so-called real-time seismic tomography: a updated velocity model is immediately obtained at each time that a new event takes place, in order to monitor temporal variations of model parameters such as velocity structure on the real-time basis.
Source model composed of asperities for the 2004 Mid Niigata Prefecture, Japan, earthquake (MJMA=6.8) by the forward modeling using the empirical Green’s function method
Authors: Kamae K. , Ikeda T. , Miwa S.
Published: August 2014

A preliminary source model composed of asperities for the 2004 Mid Niigata Prefecture, Japan, earthquake (MJMA=6.8) was estimated by the empirical Green’s function method. The source parameters for two asperities located on the fault plane were determined from the comparisons of the synthesized broad-band ground motions with the observed ones at several stations including near source. Furthermore, we performed the preliminary nonlinear analysis of the sedimentary soils to reproduce the observed ground motions at NIG019 of K-NET. Resultantly, we pointed out the need of more asperity in northern part on the fault plane and the importance of the quantitative analysis of the nonlinearity of the sedimentary soils at K-NET stations.
Time-dependent earthquake hazard evaluation in seismogenic systems using mixed Markov Chains: An application to the Japan area
Authors: Herrera C. , Nava F. A. , Lomnitz C.
Published: August 2014

A previous work introduced a new method for seismic hazard evaluation in a system (a geographic area with distinct, but related seismogenic regions) based on modeling the transition probabilities of states (patterns of presence or absence of seismicity, with magnitude greater or equal to a threshold magnitude M
r, in the regions of the system, during a time interval Δt) as a Markov chain. Application of this direct method to the Japan area gave very good results. Given that the most important limitation of the direct method is the relative scarcity of large magnitude events, we decided to explore the possibility that seismicity with magnitude M ≥ M
r
m
contains information about the future occurrence of earthquakes with M
M
r
m
> M
r
m
. This mixed Markov chain method estimates the probabilities of occurrence of a system state for M ≥ M
r
M
on the basis of the observed state for M ≥ M
r
m
in the previous Δt. Application of the mixed method to the area of Japan gives better hazard estimations than the direct method; in particular for large earthquakes. As part of this study, the problem of performance evaluation of hazard estimation methods is addressed, leading to the use of grading functions.
Continuous observation of seismic wave velocity and apparent velocity using a precise seismic array and ACROSS seismic source
Authors: Saiga Atsushi , Yamaoka Koshun , Kunitomo Takahiro , Watanabe Toshiki
Published: August 2014

We report the results of continuous monitoring—using a seismometer array—of the travel time of seismic waves generated by an ACROSS artificial seismic source. The seismometer array, which was deployed in a surface vault located 2.4 km from the source, recorded both directP- andS-waves and refractedP- andS-waves that traveled along a velocity boundary between the granite basement and overlying sedimentary rocks. We analyzed temporal variation in differential travel time and apparent velocity for these phases for a period of 1 month and found significant temporal variation in the differential travel time. Most of the variation can be attributed to changes in environmental conditions, such as atmospheric temperature and rainfall. Variation is even observed in the seismogram that is located 50 m from the vibration source, although much smaller variation is observed in the vibration of the foundation to which the source is attached. The spectral study revealed that the effects of temperature and rainfall depend strongly on the frequency range used by ACROSS and that a large variation occurs in the 15- to 20-Hz range, especially between 17 and 20 Hz. The environmental effect on the temporal variation is comparable to the record of refractedSwaves and that of a distance of 50 m, whereas a larger variation was observed in the directSwave. This result shows that the signal is affected by the environmental change near the vibration source. The environmental effect can be drastically reduced when the signal from the 15- to 20-Hz range is eliminated in the analysis.
Upper mantle imaging beneath the Japan Islands by Hi-net tiltmeter recordings
Authors: Tonegawa Takashi , Hirahara Kazuro , Shibutani Takuo , Shiomi Katsuhiko
Published: August 2014

We present a novel receiver-function image of the upper mantle structure around the Japan subduction zone. To increase the amount of available waveform data containing the relatively lower frequency component, we examined whether the Hi-net tiltmeter recordings are usable for imaging the upper mantle discontinuities by comparing them with broadband seismograms in different frequency bands. We found that the two are comparable at a frequency band between 0.02 and 0.16 Hz. To make receiver functions from tiltmeter data, stacked vertical components of broadband seismograms were used as source-time functions. Since such source-time functions may include biases from local structure, we also produced regional stacked source-time functions. The receiver function with the above frequency band does not seem to be affected by local structure. In the images derived from the receiver-function gathers, we were able to visualize both the oceanic Moho and the lower slab boundary, which could be traced down to depths of 400 km and 600 km, respectively. These images also show an uplift of the 410-km discontinuity and a depression of the 660-km discontinuity in the regions that are probably affected by the cold subducting Pacific slab.
Heterogeneous distribution of 60Fe in the early solar nebula: Achondrite evidence
Authors: Sugiura Naoji , Miyazaki Akiko , Yin Qing-zhu
Published: August 2014

60Fe-60Ni systematics in quenched angrites and two old eucrites were investigated by secondary ion mass spectrometry. The60Ni/62Ni isotopic compositions were normal within 2σ errors. The inferred initial60Fe/56Fe ratios for quenched angrites was (6±9)× 10−9, and similar upper limit values were also obtained from eucrites. Using the age difference of approximately 5 Ma between the quenched angrites and Ca−Al-rich inclusions, the initial60Fe/56Fe ratio at the start of the solar system was calculated to be approximately (6±9)× 10−8. This initial ratio is significantly smaller than previously published values obtained from chondritic materials, suggesting the heterogeneous distribution of60Fe in the solar nebula.
Aeromagnetic evidence for the arcuate shape of Mahanadi Delta, India
Authors: Nayak Goutam Kumar , Rao Ch. Rama , Rambabu H. V.
Published: August 2014

The stream network comprising Mahanadi and its tributaries, viz. Katjhori, Devi, Koyakhai, Kushbhadra and Bhargavi, cut transversely the ESE-WNW trending basement structure in the Cuttack-Paradip-Puri section and diverge radially from one another near and around Cuttack. The Mahanadi river flows in an ESE-WNW direction, the Katjhori and Devi rivers flow in a NW-SE direction and Koyakhai, Kushbhadra and Bhargavi River in an almost N-S direction. The general southeasterly direction of these rivers and the delta formation was thought to be continued throughout the Quaternary period and the delta was a case of superposition upon the deformational structures derived from the erosional surfaces covered by recent sediments.

Analysis of the aeromagnetic anomaly field of the Mahanadi Delta area of the on-shore portion has indicated three prominent structural trends in ESE-WNW, NW-SE and N-S directions along which the rivers Ma-hanadi/Chitratala, Katjhori and Devi, Koyakhai, Kushbhadra and Bhargavi flow, respectively. The tectonic disturbance along the N-S direction, in the western part of Bhubaneswar-Puri section, seems to have been controlling the Koyakhai and Kushbhadra and Bhargavi river system, following the possible trace of 85°E Ridge, whereas the concealed NW-SE trending Mahanadi graben trend is inferred to be the probable cause for the NW-SE flow of the Katjhori river in the middle portion south of Cuttack and the ESE-WNW flow of the Mahanadi and Chitratala rivers in the Cuttack-Paradip section controlled by the Eastern Ghat trend.
Paleomagnetism in Mesozoic rocks of the Northern Andes and its Implications in Mesozoic Tectonics of Northwestern South America
Authors: Bayona Germán , Rapalini Augusto , Costanzo-Alvarez Vincenzo
Published: August 2014

New paleomagnetic data isolated in Upper Triassic to Aptian rocks exposed in the Colombian Andes and west of the Guyana craton, in conjunction with paleomagnetic data from the Andes of Venezuela and the South American craton, permit the interpretation of along-margin northward translations of Andean Colombian terranes during the Early-Middle Jurassic. Field tests and comparison with reference paleopoles for South America indicate that characteristic components uncovered in red-siliciclastic and igneous rocks are primary, or near-depositional, and they are carried dominantly by hematite, magnetite and Ti-magnetite. Difference in declination values of characteristic components isolated in fault-bounded blocks document counter-clockwise rotations previous to syn-extensional deposition. The Jurassic tectonic scheme proposed here for the northwestern corner of South America shows an Early Jurassic, linear subduction-related magmatic arc evolving to Late Jurassic rift-related setting associated to the opening of the Proto-Caribbean Ocean and westward retreat of the subduction zone. A similar tectonic evolution for the Jurassic has been proposed for southwestern USA and the Nazas arc in Mexico.
Paleomagnetism, rock-magnetism and geochemical aspects of early Cretaceous basalts of the Paraná Magmatic Province, Misiones, Argentina
Authors: Mena Mabel , Orgeira María Julia , Lagorio Silvia
Published: August 2014

The basalts of the Posadas Formation were extruded during the huge continental volcanism that affected the Paraná Basin in the Lower Cretaceous. We have carried out a paleomagnetic and rock-magnetic study on samples collected along a basalt outcrop section in Misiones, Argentina and determined that rocks classified as tholeiitic basalts and andesi-basalts are characterized by a low to intermediate content of Ti. Paleomagnetic and rock-magnetic studies suggest that the main magnetic mineral is low-Ti titanomagnetite of superparamagnetic (SP) to single-domain (SD) sizes, and very low amounts of multi-domain (MD) particles. The stable magnetic remanence enabled us to define characteristic remanent magnetizations (ChRMs) with a maximum angular deviation (MAD) <5° in most cases; and in all the cases, a MAD <10°. The sequence has registered at least two polarity reversions, starting from a normal polarity at the base. The calculated virtual geomagnetic poles (VGPs) present an elongated distribution similar to other distributions of VGPs published for the Paraná Magmatic Province. The elongated distribution of the VGPs could be a real feature of the geomagnetic field at a time of frequent changes of polarity.
Cooling rate corrected paleointensities from the Xitle lava flow: Evaluation of within-site scatter for single spot-reading cooling units
Authors: Morales Juan , Alva-Valdivia Luis M. , Goguitchaichvili Avto , Urrutia-Fucugauchi Jaime
Published: August 2014

The historic Xitle lava, which covers a great part of southern Mexico City, is one of the most studied (magnetically) volcanic unit worldwide. Studies include detailed paleomagnetic and paleointensity investigations, which have documented an enigmatic within-flow variation of absolute paleointensity first recognized in the decade of 1960s. However, attempts to find possible explanations in terms of physical/magnetic parameters or geomagnetic effects have been unsatisfactory. As an effort to understand the over- and underestimating of geomagnetic paleointensity (PI) within the Xitle lava flow, we investigated the relation of the cooling rate upon the acquisition of thermoremanent magnetization (TRM). Contrary to archaeomagnetic investigations, most paleointensity experiments on volcanic rocks do not consider cooling rate effects in a systematic manner. Our results show that the scatter and overestimation of PI values obtained on the single Xitle lava flow are drastically reduced when using the cooling rate correction to raw selected data. The Thellier method combined with cooling rate experiments provide a higher precision compared to conventional Thellier and microwave techniques. The cooling rate effects upon acquisition of TRM in volcanic rocks seem to be as critical as in archaeomagnetic investigations.
Imaging the high b-value anomalies within the subducting Pacific plate in the Hokkaido corner
Authors: Katsumata Kei
Published: August 2014

The frequency-magnitude distribution (b-value) for seismicity within the Pacific plate is not definitely homogeneous. An anomaly of b-value higher than 0.9 is detected within the descending Pacific plate in the Hokkaido corner. In the western Hokkaido, the anomaly exists at a depth of about 150 km, which is directly beneath active volcanoes. In the eastern Hokkaido, the anomalies exist at approximately 200 and 300 km depths, which is tens of kilometers sideways from active volcanoes. The results obtained in this paper are more reliable than previous studies on b-value heterogeneity within descending plates, since I used a catalog homogeneous, both in time and space, which was obtained through re-examination of all seismograms and relocation of hypocentral parameters based on arrival times from fixed seismic stations.
A computer simulation study on the mode conversion process from slow X-mode to fast X-mode by the tunneling effect
Authors: Katoh Yuto , Iizima Masahide
Published: August 2014

The mode conversion process from slow X-mode waves to fast X-mode waves by the tunneling effect has been studied by means of computer simulation. On the basis of the new approach by the simulation, we have confirmed that the mode conversion to the fast X-mode waves is especially effective in the case where the width of the evanescent layer which exists between the local UHR frequency and the local fast X-mode cut-off frequency, is of the order of the wave length of the incident slow X-mode waves. In space plasmas, this type of mode conversion process becomes important for the case where the different plasmas form a sharp boundary or a discontinuity at their contact boundary with a scale size of the order of the wave length of the incident slow X-mode waves.
Coseismic slip distribution of the 2005 off Miyagi earthquake (M7.2) estimated by inversion of teleseismic and regional seismograms
Authors: Yaginuma Tadashi , Okada Tomomi , Yagi Yuji , Matsuzawa Toru , Umino Norihito , Hasegawa Akira
Published: August 2014

A large earthquake (M7.2) occurred along the plate boundary off Miyagi Prefecture (Miyagi-Oki), northeastern Japan, on August 16, 2005. In this area, large earthquakes (~M7.5) have occurred repeatedly at intervals of about 37 years, and more than 27 years have passed since the last event occurred. To estimate the relationship between this earthquake and the previous events, we determined coseismic slip distribution by this 2005 Miyagi-Oki earthquake by adopting the seismic waveform inversion method of Yagi et al. (2004) and compared it with that of the previous 1978 Miyagi-Oki earthquake. We performed two cases of the inversions; inversion using only far-field seismograms and that using far-field seismograms and local seismograms simultaneously. Both results show that a large slip occurred near the hypocenter and rupture extended to the westward deeper portion. Considering that the rupture area of the 2005 event partly overlapped with the southeastern part of that of the 1978 event, suggests this result the possibility that plural asperities exist which cause the sequence of Miyagi-Oki earthquakes and that the 2005 event ruptured one of such asperities, although the previous 1978 event ruptured all the asperities at one time.
Small repeating earthquakes and interplate creep around the 2005 Miyagi-oki earthquake (M=7.2)
Authors: Uchida Naoki , Matsuzawa Toru , Hirahara Satoshi , Hasegawa Akira
Published: August 2014

Taking advantage of the feature that creep around an asperity is necessary for the recurrent rupture of the same small asperity (small repeating earthquakes), we have estimated the spatio-temporal distribution of quasistatic slip (creep) around the 2005 Miyagi-oki earthquake (M=7.2) using the distribution of small repeating earthquakes. The creep was detected mainly outside of the coseismic slip areas for the 2005 Miyagi-oki, 1978 Miyagi-oki (M=7.6) and 2003 Fukushima-oki (M=6.8) earthquakes. The creep rates estimated from the recurrence intervals and slip amounts of small repeating earthquakes for 21 years were almost constant for the areas near the western limit of the interplate earthquakes but they varied temporally in the areas nearer to the Japan trench. The changes in the creep rates before and after the 2005 Miyagi-oki earthquake were not significant with the exception of small slip accelerations in some areas near the Japan trench. These results suggest that the plate boundary around the source area for the 2005 earthquake is still mostly locked.
Postseismic deformation following the 2005 West Off Fukuoka Prefecture Earthquake (M7.0) derived by GPS observation
Authors: Nakao Shigeru , Takahashi Hiroaki , Matsushima Takeshi , Kohno Yuhki , Ichiyanagi Masayoshi
Published: August 2014

We constructed a temporary GPS network around the aftershock area of the 2005 West Off Fukuoka Prefecture Earthquake (M7.0) in order to investigate the characteristics of its postseismic deformation. Our GPS network data, as well as the GEONET data, were analyzed using Bernese GPS software. We detected notable postseismic deformation in horizontal components close to the fault plane. The observed maximum displacement was 5.6 cm at the GNKI site on Genkaijima Island. A logarithmic law was adapted to the coordinate time series data, revealing decay times from 2 to 23 days, similar to those obtained for the 2003 Tokachi-oki Earthquake (M8.0). The amount of postseismic slips on the fault was assessed using the coseismic fault model proposed by the Geographical Survey Institute (GSI). We derived an optimum fault model of postseismic slip on the shallow (less than 3 km depth) portion of the fault. Our findings indicate that postseismic slip occurred only in shallow parts of the coseismic fault.
Three-dimensional seismic velocity structure as determined by double-difference tomography in and around the focal area of the 2005 West off Fukuoka Prefecture earthquake
Authors: Hori M. , Matsumoto S. , Uehira K. , Okada T. , Yamada T. , Iio Y. , Shinohara M. , Miyamachi H. , Takahashi H. , Nakahigashi K. , Watanabe A. , Matsushima T. , Matsuwo N. , Kanazawa T. , Shimizu H.
Published: August 2014

On March 20, 2005 the West off Fukuoka Prefecture earthquake (magnitude of 7.0 on the JMA scale) occurred in southeastern Japan. The earthquake fault was a left-lateral strike-slip having a nearly vertical fault plane and a strike in the WNW-ESE direction. The largest aftershock with a magnitude of 5.8 (JMA) followed 1 month later. To gain more detailed aftershock data, several teams from different Japanese universities jointly installed a number of temporary seismic stations and positioned Ocean Bottom Seismometers (OBSs) immediately above the focal area. Double-difference tomography was used to estimate the three-dimensional (3D) (Zhang and Thurber, 2003) velocity structures in and around the focal area based on the travel time data collected during seismic observations. The high-velocity regions estimated by the inversion are located on the edge of the aftershock area and on the shallow part of asperity, as inferred from the slip distribution. Conversely, the V
p/V
s ratio is not always as high as that found at the location of the asperity. This finding suggests that the construction of the medium is not uniformly elastic but complex, with different relations between elastic constants and strength.
Deep crustal structure along the Niigata-Kobe Tectonic Zone, Japan: Its origin and segmentation
Authors: Nakajima Junichi , Hasegawa Akira
Published: August 2014

A seismic tomography study has revealed a detailed three-dimensional seismic velocity structure along a highstrain-rate zone, the Niigata-Kobe Tectonic Zone (NKTZ), located in Japan. The results show that the depth extent of the low-velocity zone varies along the NKTZ. We divided the NKTZ into three regions on the basis of the velocity structure. A low-velocity anomaly observed in the lower crust beneath the southwestern part of the NKTZ is probably attributable to the fluids derived from the Philippine Sea slab, while a prominent low-velocity anomaly extending from the upper crust to the uppermost mantle in the volcanic region, the middle part of the NKTZ, may be caused by the existence of melts and a higher-temperature condition that results from magmatic activity. The northeastern part exhibits low-velocity anomalies in the upper crust and the uppermost mantle, which are probably due to the thick sediment and fluids related to the back-arc volcanism, respectively. The strength of the crust and uppermost mantle along the NKTZ may have been weakened by the concentration of the fluids, which in turn facilitates the large contraction there. The heterogeneous structures revealed in this study suggest that the origin of the high-strain-rate zone varies along the NKTZ.
Equatorial electrojet in the East Brazil anomaly region
Authors: Rastogi R. G. , Yumoto K.
Published: August 2014

An examination of the geomagnetic field variations at equatorial stations in South America has shown that the daily range of the northward field, H, is about 30% larger in Peru than in Brazil. The midday H field vector in Brazil shows a large westward tilt, corresponding to large westward declination at the station. A significant meridional trans-equatorial current system is suggested for the observed anomaly. The longitudinal variation in the equatorial electrojet current strength is suggested to be due to the corresponding variation in the ionospheric electric field rather than to the electrical conductivity.
Vertical velocity from the Korean GPS Network (2000–2003) and its role in the South Korean neo-tectonics
Authors: Hamdy Ahmed M. , Park Pil-Ho , Jo Bong Gon
Published: August 2014

In the absence of adequate leveling observations in South Korea, the vertical deformation has been investigated using the Korean Global Positioning System (GPS) Network data (2000-2003). Although the vertical components of the GPS velocities have been rarely used in crustal deformation studies because of their high noise level, the processing strategy employed here enhances the data quality and eliminates the seasonal effect. The obtained vertical velocity field shows that the maximum vertical velocity in the ITRF 97 reference frame is 3.3 mm/year (subsidence), which reflects a relatively low level of seismic activity in South Korea. Two deformation patterns were recognized; subsidence in the Okchun Basin, and uplift in its adjacent areas. This subsidence is due to the collision of Kyonggi Massif and Okchun Basin (part of the South China block) against the Yongnam Massif and Taebaeksan Basin (part of the North China block).
Volcanic origin of the 1741 Oshima-Oshima tsunami in the Japan Sea
Authors: Satake Kenji
Published: August 2014

The generation mechanism of the 1741 Oshima-Oshima tsunami, which is considered to be the most destructive tsunami that has ever originated in the Japan Sea, has been the subject of much debate. The tsunami caused about 2,000 casualties along the Hokkaido and northern Honsu coasts and inflicted damage as far as the Korean Peninsula. The tsunami source is located between recent tsunamigenic earthquakes, but there is no historical record of an earthquake in 1741. In contrast, the records indicate volcanic activity of Oshima-Oshima, including a large-scale sector collapse, although the volume change associated with the subaerial landslide is too small to explain the observed tsunami heights. Recent marine surveys indicate that the landslide extended to the ocean bottom with a volume change of about 2.5 km3, nearly an order of magnitude larger than the subaerial slide. On the basis of mapped bathymetry, the generation of the tsunami is calculated using a simple kinematic landslide model. The tsunami propagation is computed in two different grids—a 6″ grid around the source and a 1′ grid for the entire Japan Sea. A parameter search of the model shows that the observed tsunami heights are best explained by a horizontal slide velocity of 40 m/s and a rise time of 2 min. The large vertical drop of the landslide and the coupling of the submarine landslide and tsunami are the main factors responsible for the large tsunami.
Sudden commencements in the X and Y fields at equatorial stations in the African sector
Authors: Rastogi R. G.
Published: August 2014

This paper describes the mean direction of the H vector due to Sq (solar quiet) and SC (sudden commencement) at equatorial stations Addis-Ababa (geographical longitude: 39°E) and at M’Bour (17°W). The Sq (H) vector aligns fairly well with the direction of the dip declination (D), and the SC (H) vector aligns along the dipole declination (Ψ) minus the dip declination D. At M’Bour, which is situated in the Atlantic Anomaly region, both the Sq (H) and the SC (H) vectors were along 32°W of N, while the dip as well as the dipole declinations were 9°W. Thus, the normal and the disturbance current in the Atlantic region is modulated by some additional meridian currents, resulting in a large increase in the westward Y, the source of which cannot be currently estimated from only one station, M’Bour. This problem requires further investigation.
Surface wave analysis with beamforming
Authors: Tanimoto Toshiro , Prindle Kenton
Published: August 2014

It is well known that off-great-circle path propagation causes a technical difficulty for surface wave analysis in higher frequency ranges. We propose a new approach that combines a beamforming technique and two-station phase velocity measurement to resolve this problem. Beamforming allows us to determine the correct azimuth of incoming surface waves which can be taken into account in phase velocity measurement. Beamforming results also support that a plane-wave approximation is mostly acceptable for frequencies up to about 50–60 mHz (millihertz), although evidence of multipathing is occasionally recognized in beamforming results as multiple peaks. Application of this correction scheme for Rayleigh-wave data in Southern California seems to make the largest impact on the results of azimuthal anisotropy. Effects are not large for frequencies up to 30 mHz but fast velocity axes in azimuthal anisotropy maps change significantly for higher frequencies.
Geomagnetic jerks from the Earth’s surface to the top of the core
Authors: Chambodut Aude , Eymin Céline , Mandea Mioara
Published: August 2014

Rapid changes in the magnetic field characterised by an abrupt change in the secular variation have been named “secular variation impulses” or “geomagnetic jerks”. Three of these events, around 1968, 1978 and 1990, occurred during the time-span covered by the comprehensive model CM4 (Sabaka et al., 2002, 2004). This model, providing the best temporal resolution between 1960 and 2002 as well as a fine separation of the different magnetic sources, can be used to study rapid phenomena of internal origin. In order to analyse these events all over the globe, synthetic time series were obtained from the CM4 model between 1960–2002. Geomagnetic jerks are detected here as a rapid movement of the zero isoline of the second field derivative. Analysis of the area swept out by this isoline as a function of time allows us to map the spatial extent of jerks though time, and to identify an event around 1985 that is localized in the Pacific area. At the core surface, we compute the fluid flows under the frozen-flux and tangentially geostrophic assumptions. The flows do not exhibit any special pattern at jerk times, but instead show a smooth temporal evolution over the whole time period. However, the mean amplitude of the dynamical pressure associated with these flows present maxima at each jerk occurrence and helps to confirm the identification of a jerk in 1985.
Paleomagnetism of the Datong monogenetic volcanoes in China: paleodirection and paleointensity during the middle to early Brunhes Chron
Authors: Yamamoto Y. , Tsunakawa H. , Shaw J. , Kono M.
Published: August 2014

Paleomagnetic measurements were conducted on Datong volcanic rocks from China, which are thought to have formed in the mid to early Brunhes Chron. Meaningful site-mean paleodirections were obtained from 21 sites which are considered to represent 17 independent cooling units. They give a mean VGP (virtual geomagnetic pole) position of (76.5°N, 7.9°E) with A95=7.7° (N=17), which is statistically distinct from geographic north. This dataset also yields an ASD (angular standard deviation) of 17.2° around the mean VGP position. Because the paleodirections form two clusters, the samples may record the paleomagnetic field during two different short periods and therefore may not average out paleosecular variation. Paleointensity measurements were conducted using three different methods. The DHT and LTD-DHT Shaw methods, the Thellier method, and the microwave Thellier method were applied to 119, 29 and 73 specimens respectively, and they give 66, 16 and 12 successful results (success rates are 55, 55 and 16%). From the LTD-DHT Shaw dataset, eight acceptable site-mean paleointensities are obtained. They give an average VDM (virtual dipole moment) of 3.79±1.94x 1022 A m2. This is 56% lower than the average VDM of 5.91±1.74x 1022 A m2 (N=14) calculated from the selected Thellier data from the latest paleointensity database using the same criteria. One possible reason for this difference might be systematic overestimations of paleointensities by the Thellier method on volcanic rocks.
Paleointensities from Tertiary basalts, Inner Mongolia and Hebei Province, northeastern China
Authors: Tanaka Hidefumi , Takahashi Naho , Zheng Zhong
Published: August 2014

A paleointensity study using the Thellier’s method was made on Tertiary basaltic rocks in Inner Mongolia and Hebei Province, northeastern China. K-Ar ages were previously reported for all the rocks, which range around 6–8 Ma and 28–32 Ma. Sample selection was based on total quality of remanence behavior and rock magnetism. High stability to AF and thermal demagnetization, small difference between the heating and cooling curves of magnetic susceptibility vs. temperature measurements, and PSD to SD characteristics in the Day plot of the hysteresis parameters were required. Experiments in vacuum using Coe’s procedure were applied to 54 specimens from nine flows. Experiments were successful for 34 specimens, giving seven flow mean paleointensities. Excluding two flow means (in one flow, only two specimens from the same sample survived and in the other, the within-site error amounts to 42%) the final success rate was 29 out of 54 (54%), which is not low. The obtained results range from 54 to 65 μT, except for one lava which gave 23 μT. These results indicate that although the paleointensity in the Tertiary was generally smaller than the present-day value, there were large fluctuations in the dipole moment, and paleointensities of the present-day level were often attained.
Magnetic petrology and its implication for magma mixing of the 1991–1995 dacite at Unzen volcano, Japan
Authors: Saito Takeshi , Ishikawa Naoto
Published: August 2014

Magnetic petrologic analyses were carried out on lava samples from the lava dome of the 1991–1995 eruption of Unzen volcano, Japan. As a result, three kinds of titanomagnetites with different Curie temperatures were identified and the difference between the samples from the lava dome which grew exogenously and endogenously were revealed. Titanomagnetites withTc of 460–500°C and 380–400°C are the predominant magnetic minerals. In addition, a titanomagnetite withTc of ∼540°C, but in low concentration, is also present. The exogenous lava samples contain more titanomagnetite withTc of 380–400°C and less microphenocrysts of titanomagnetites than the endogenous samples. The equilibrium temperature of 900–920°C, 780–820°C and 720°C is estimated from titanomagnetite withTc of 380–400°C, 460–500°C and 540°C, respectively. We suggest that these titanomag-netites are probably formed by magma mixing and eruption processes. Titanomagnetite withTc of 460–500°C was derived from felsic magma (780–820°C) in a shallow chamber. Titanomagnetite withTc of 380–400°C was produced from the mixed magma (900–920°C). After being squeezed out from the reservoir, a small amount of titanomagnetite withTc of ∼540°C was crystallized.