bluuebunny/crossref_metadata_2025 huggingface dataset:
Coseismic displacements and slip distribution from GPS and leveling observations for the 2006 Peinan earthquake (Mw 6.1) in southeastern Taiwan
Authors: Chen Horng-Yue , Hsu Ya-Ju , Lee Jian-Cheng , Yu Shui-Beih , Kuo Long-Chen , Jiang Yen-Lin , Liu Chi-Ching , Tsai Chun-Shyong
Published: August 2014

Since 2001, we have set up a dense geodetic network with 52 campaign-mode GPS sites and seven continuously recording GPS stations as well as six leveling routes in the Taitung area, Taiwan. Our aim was to better characterize near-fault crustal deformation of active faults at the plate suture of the Philippine Sea plate and Eurasia in southeastern Taiwan. On 1 April 2006, a moderate shallow earthquake (Mw 6.1, depth 10.8 km) occurred within this network. This earthquake resulted from rupturing of a geologically unknown or suspected fault (called the Y fault) located underneath the eastern margin of the Central Range. After removing the impacts of secular motions and postseismic slip, we estimated the coseismic displacements of the Peinan earthquake from the GPS and leveling measurements before and after the main shock. Three deformation types with distinct slip behaviors were revealed in three different regions: (1) near the epicenter—around 45 mm movement in the S-SSW direction with +20 to −20 mm vertical motion, in the northern part of the Y fault; (2) south of the epicenter across the southern part of the Y fault—approximately 35 mm in a westward movement with −60 mm subsidence (footwall side) and 40 mm in a SSW movement with at least 50 mm uplift (hanging-wall side), in the southern part of the Y fault; (3) northeast away from the epicenter—about 10 mm in a northward displacement with +15 to −10 mm vertical motions, in the Longitudinal Valley and on the western flank of the Coastal Range. This unique coseismic deformation pattern sheds new light on the characteristics of the suture zone between the Eurasian and Philippine Sea plates at the southernmost Longitudinal Valley. We used GPS and leveling measurements to invert for the fault geometry and the coseismic slip distribution. The optimal modeled fault is an 80° west-dipping fault at a depth of 0.5–20 km. The highest slip of about 0.33 m is located to the south of the hypocenter at a depth of 9–16 km. The total geodetic moment in our optimal model is 2.3 × 1018 Nt-m, which is equivalent to an earthquake of M
w 6.2. The surface coseismic displacements as well as the inferred coseismic slip distribution indicate a drastic change of slip behaviors in the middle of the Y fault. The left-lateral slippage near the hypocenter turned dramatically to reverse faulting with left-lateral component as rupturing propagated to the southern portion of the fault, suggesting that a possible right-lateral faulting occurred that coseismically cross cut the northern middle Peinanshan massif in the Longitudinal Valley.
Estimation of scan-gap limits on phase delay connections in Delta VLBI observations based on the phase structure function at a short time period
Authors: Kondo Tetsuro , Hobiger Thomas , Sekido Mamoru , Ichikawa Ryuichi , Koyama Yasuhiro , Takaba Hiroshi
Published: August 2014

The maximum scan-gap length which connects phase delays from scan to scan over a gap is an important issue in Delta Very Long Baseline Interferometry (D-VLBI), and it is affected by delay fluctuations caused by the wet troposphere. It has recently become possible to obtain near real-time fringe phases by using an e-VLBI technique that realizes real-time VLBI by connecting stations through high-speed Internet. Such real-time VLBI raises the possibility of dynamic D-VLBI scheduling, which changes scan and gap length dynamically according to the weather condition of the date. We have investigated this possibility by using phase structure functions obtained from continuous VLBI observations at S- and X-bands for 1–2 h at the Kashima, Gifu, and Koganei stations (not real-time ones). Five VLBI sessions were conducted during this study between March and July 2006 under different weather conditions. At first a simple method was developed to evaluate phase connectivity from a phase structure function. A model was also proposed to estimate a phase-structure function at longer time periods from a short time period. Finally, an available gap length was estimated using the model. Our results show that it is possible to estimate an available scan gap length by using a structure function at a time period of 10 s. This suggests that it is possible to control scan length and gap length dynamically in order to achieve the best performance of D-VLBI observations.
Coronal loops, flare ribbons and aurora during slip-running
Authors: Schmieder Brigitte , Aulanier Guillaume , Démoulin Pascal , Pariat Etienne
Published: August 2014

Solar two ribbon flares are commonly explained by magnetic field reconnections in the low corona. During the reconnection energetic particles (electrons and protons) are accelerated from the reconnection site. These particles are following the magnetic field lines down to the chromosphere. As the plasma density is higher in these lower layers, there are collisions and emission of radiation. Thus bright ribbons are observed at both ends of flare loops. These ribbons are typically observed in Hα and in EUV with SoHO and TRACE. As the time is going, these ribbons are expanding away of each other. In most studied models, the reconnection site is a separator line, where two magnetic separatrices intersect. They define four distinct connectivity domains, across which the magnetic connectivity changes discontinuously. In this paper, we present a generalization of this model to 3D complex magnetic topologies where there are no null points, but quasi-separatrices layers instead. In that case, while the ribbons spread away during reconnection, we show that magnetic field lines can quickly slip along them. We propose that this new phenomenon could explain fast extension of Hα and TRACE 1600 Å ribbons, fast moving HXR footpoints along the ribbons as observed by RHESSI, and that it is observed in soft X rays with Hinode/XRT.
Three-dimensional non-linear instability of spontaneous fast magnetic reconnection
Authors: Shimizu T. , Kondoh K. , Ugai M.
Published: August 2014

Three-dimensional instability of spontaneous fast magnetic reconnection is studied using MHD (magnetohydro- dynamic) simulation. Previous two-dimensional MHD studies have demonstrated that, if a current-driven anomalous resistivity is assumed, two-dimensional fast magnetic reconnection occurs and two-dimensional largescale magnetic loops, i.e., plasmoids, are ejected from the reconnection region. In most two-dimensional MHD studies, the structure of the current sheet is initially one-dimensinal. On the other hand, in recent space plasma observations, fully three-dimensional magnetic loops frequently appear even in the almost one-dimensional current sheet. This suggests that the classical two-dimensional fast magnetic reconnection may be unstable to any three-dimensional perturbation, resulting in three-dimensional fast magnetic reconnection. In this paper, we show that a three-dimensional resistive perturbation destabilizes two-dimensional fast magnetic reconnection and results in three-dimensional fast magnetic reconnection. The resulting three-dimensional fast reconnection repeatedly ejects three-dimensional magnetic loops downstream. The obtained numerical results are similar to the pulsating downflows observed in solar flares. According to the Fourier analysis of the ejected magnetic loops, the time evolution of this three-dimensional instability is fully non-linear.
Development of the global simulation model of the heliosphere
Authors: Kamei Satomi , Nakamizo Aoi , Tanaka Takashi , Obara Takahiro , Shimazu Hironori
Published: August 2014

The heliospheric structure ranging from the solar surface to the earth’s orbit is self-consistently reproduced from a time-stationary three-dimensional (3D) magnetohydrodynamic (MHD) simulation. The simulation model incorporates gravity, Coriolis, and centrifugal forces into the momentum equation, and coronal heating and field-aligned thermal conduction into the energy equation. The heating term in the present model has its peak at 2.8 solar radius (R
s) and exponentially falls to zero at greater distance from the solar surface. The absolute value of heating depends on the topology of the solar magnetic field so as to be in inverse proportion with the magnetic expansion factor. The results of the simulation simultaneously reproduce the plasma-exit structure on the solar surface, the high-temperature region in the corona, the open- and closed-magnetic-field structures in the corona, the fast and slow streams of the solar wind, and the sector structure in the heliosphere.
Proxy for the ionospheric peak plasma density reduced by the solar zenith angle
Authors: Gulyaeva T. L.
Published: August 2014

The F2 layer peak plasma density N
m
F
2 is reduced by the factor constructed from the relative changes in the Sun’s zenith angle ξ for a particular local time and the local noon value χ0. Proposed transformation yields a proxy for the peak plasma density which coincides with the source observation at noon but apart from the latter is gradually reduced towards the night. Hourly observations at 8 ground based ionosondes for the solar maximum (2000) and minimum (2006) are analyzed for inter-stations and inter-seasonal correlation of the peak plasma density and the proxy values. The proxy values show improved correlation between the data at different locations and improved inter-seasonal correlations for a particular location due to greater homogeneity of results throughout the year contributing to improved evaluation of the ionospheric weather indices.
A new approach to the hourly mean computation problem when dealing with missing data
Authors: Marsal S. , Curto J. J.
Published: August 2014

Geomagnetic observatory records are unavoidably affected by primary data interruptions which, in turn, may have possible effects on the accuracy of the definitive data derived from them. One of the products most widely used by the scientific community is the mean hourly values, immediately obtained from the primary minute values of the geomagnetic field. Although some precepts have already been proposed and used, a definitive criterion regarding the procedure to follow when dealing with missing data has not yet been established. This could be seen in the last IAGA meetings and workshops, where several constructive opinions were put forward in this respect. The present discussion is devoted to analyzing the effects that different amounts of missing data have upon the accuracy of the means, a necessary step before establishing a definitive rule as to how to deal with these situations. In this statistical approach, we propose a new criterion based on the relative value of the root mean square error (between actual and computed means) with respect to the natural magnetic field variations of the original hourly interval.
Co-seismic EM signals in magnetotelluric measurement—a case study during Bhuj earthquake (26th January 2001), India
Authors: Azeez K. K. Abdul , Manoj C. , Veeraswamy K. , Harinarayana T.
Published: August 2014

Significant changes in amplitude and frequency characteristics were observed in the magnetotelluric (MT) time series recorded during Bhuj earthquake (∼7.6 M
w), at a site ∼350 km from the epicenter. The telluric and magnetic signals recorded in the frequency range (10−1-101 Hz) of MT spectrum show considerable variations in their spectral characteristics during the earthquake event compared to the data recorded before and after the earthquake. The spectral analysis brings out sharp changes in amplitude of low-frequency signals during the earthquake as compared to the typical flat spectrum observed before and after the earthquake. The wavelet analysis of the electric and magnetic field data reveals two different spectral regimes; (1) the flat spectrum related to the natural MT signals, and (2) localized, high amplitude signals (in time and frequency) related to the onset of main shock. Three more high amplitude events are noted in the wavelet spectrum, after the main shock event, and can be speculated to be associated with the after-shock events. The MT impedance estimates clearly show scattered apparent resistivity and phase values during the earthquake suggesting that the high amplitude electric and magnetic signals were not related by an MT transfer function. The MT impedance estimates made before and after the earthquake are strikingly normal and smooth. The Pearson’s correlation coefficients between the orthogonal electric and magnetic fields show a drastic drop for the data measured during earthquake, while the MT fields recorded before and after the earthquake are well correlated. The observed MT signals during the seismic activity do not show any external geomagnetic origin and may be attributed to co-seismic EM phenomena. The probable mechanisms responsible for the co-seismic EM phenomena could be electro-kinetic and seismic dynamo effects.
Characteristics of gravity waves observed with intensive radiosonde campaign during November–December 2005 over western Sumatera
Authors: Ratnam M. Venkat , Alexander Simon P. , Kozu T. , Tsuda T.
Published: August 2014

Characteristics of gravity waves are studied using radiosonde campaign conducted during November–December 2005 at Koto Tabang (KT, 0.2°S, 100.32°E). Intensive sounding with hourly launches was also conducted on seven days to study the characteristics of short period (2–6 hours) waves along with EAR data. Gravity waves with period (τ) of 3 hours and vertical wavelength (λ
z
) of 10 km seemed to be generated due to localized convection around KT, which is inferred from X-band Doppler radar. The energy of the gravity wave with period of 2–3 days and vertical wavelength of 3–5 km is largest between 15 and 20 km and 25 and 30 km. We also report the comparison of the wave activity and its interaction with background wind between the three campaigns (CPEA-I, CPEA-II and Nov. 2002). Most of the time waves are propagating towards east and the source of gravity waves is strongly related to the slowly eastward-advecting tropospheric convection, implying that the wave activity was generated at far distant sources located west of KT. A key finding of this study is neither short period nor long-period gravity waves are generated during stationary type of convection. The change in the propagation direction of the short period waves within the event is observed which is not expected.
Local differences in great magnetic storms observed at middle and low latitudes
Authors: Li Qi , Gao Yufen , Wang Jianjun , Han De-Sheng
Published: August 2014

The dependence of the storm-time amplitude on longitude and latitude was analyzed by statistically investigating great magnetic storms observed at different observatories. First, we compared the storm-time ranges observed at Beijing Observatory (BJI) and San Juan Observatory (SJG) to reveal their longitudinal dependence. It was found that the difference between BJI and SJG could be fitted by the 4-order Fourier series approximation, and the storm-time H ranges at dawn were less than those at dusk. Second, we carried out a case study of two typical storms, and analyzed the focused local time when peak storm-time H ranges occurred. The results confirmed the above conclusion. Third, a statistical study was conducted for all magnetic storms observed at the 120°E magnetic chain in eastern China for 1995–2004 to reveal the latitudinal dependence of storm-time ranges. It was found the relationship depended on the activity of analyzed storms and became complicated for giant storms with Dst ≤ −300 nT.
Monitoring the global-scale winter anomaly of total electron contents using GPS data
Authors: Huo X. L. , Yuan Y. B. , Ou J. K. , Zhang K. F. , Bailey G. J.
Published: August 2014

The winter anomaly phenomenon of Total Electron Contents (TEC) at latitudes 15°–60°N and 15°S–60°S is presented using GPS carrier-phase data obtained from GPS stations during 2002. The correlation between the [O/N2] ratio estimated using the NRLMSISE-00 atmospheric model and the TEC winter anomaly is also investigated. The numerical results show that the TEC winter anomaly in different regions of the world tends to be dominated by different factors. In North America, the TEC winter anomaly is strongly affected by the magnetospheric processes in high latitudes and the [O/N2] ratio. In the Euro-Africa and Russia-Asia regions, the TEC winter anomaly depends mainly on the [O/N2] ratio at the latitude band of 30°–60°N, and the extent of the TEC winter anomaly gradually decreases from 60°N to 30°N. The extent of the TEC winter anomaly increases at the latitude band of 15°–30°N due to the influence of the meridional neutral wind and the seasonal changes of the subsolar point. However, the TEC winter anomaly was not observed in southern hemisphere in 2002. The TEC equinoctial asymmetries in the northern and southern hemisphere are also presented using GPS TEC values collected in March and September 2002.
Long-term prediction of solar and geomagnetic activity daily time series using singular spectrum analysis and fuzzy descriptor models
Authors: Mirmomeni Masoud , Kamaliha E. , Shafiee Masoud , Lucas Caro
Published: August 2014

Of the various conditions that affect space weather, Sun-driven phenomena are the most dominant. Cyclic solar activity has a significant effect on the Earth, its climate, satellites, and space missions. In recent years, space weather hazards have become a major area of investigation, especially due to the advent of satellite technology. As such, the design of reliable alerting and warning systems is of utmost importance, and international collaboration is needed to develop accurate short-term and long-term prediction methodologies. Several methods have been proposed and implemented for the prediction of solar and geomagnetic activity indices, but problems in predicting the exact time and magnitude of such catastrophic events still remain. There are, however, descriptor systems that describe a wider class of systems, including physical models and non-dynamic constraints. It is well known that the descriptor system is much tighter than the state-space expression for representing real independent parametric perturbations. In addition, the fuzzy descriptor models as a generalization of the locally linear neurofuzzy models are general forms that can be trained by constructive intuitive learning algorithms. Here, we propose a combined model based on fuzzy descriptor models and singular spectrum analysis (SSA) (FD/SSA) to forecast a number of geomagnetic activity indices in a manner that optimizes a fuzzy descriptor model for each of the principal components obtained from singular spectrum analysis and recombines the predicted values so as to transform the geomagnetic activity time series into natural chaotic phenomena. The method has been applied to predict two solar and geomagnetic activity indices: geomagneticaaand solar wind speed (SWS) of the solar wind index. The results demonstrate the higher power of the proposed method—compared to other methods—for predicting solar activity.
Influences of Venus’ topography on fully developed superrotation and near-surface flow
Authors: Yamamoto Masaru , Takahashi Masaaki
Published: August 2014

We investigate the influence of topography on Venus’ atmospheric general circulation. Based on comparative simulations with and without the Venusian topography, we elucidate the role of the topography in the fully developed superrotation. Orographically forced stationary waves are predominant over Mt. Maxwell: slightly weakening the superrotation near the cloud top. Differently from previous GCM results, the orographically forced waves do not produce significant asymmetry between the northern and southern hemispheric superrotations in the present model. Weak surface flows from mountains to lowlands are caused by the pressure dependence of the Newtonian cooling. The pattern and magnitude of the near-surface flow are largely different from those simulated in the Herrnstein and Dowling (2007) model. This implies that the parameterizations of physical processes (such as Newtonian cooling, turbulence, diffusion, and surface drag) and the model resolution could significantly influence the pattern and magnitude of the near-surface flow and the orographical forcing of planetary-scale stationary waves.
Fuzzy mathematical model for the analysis of geomagnetic field data
Authors: Sridharan M.
Published: August 2014

The Indian network of magnetometers provides an opportunity to examine the pattern of geomagnetic field variations during magnetic storms. In this study, fuzzy transitive closure analysis, which is a powerful technique for pattern recognition, has been employed. The pattern of variation differs at the nonequatorial, equatorial and the observatory situated nearer to the geomagnetic S
q focus. The results of the analysis are compared with those of classical cluster analysis. The comparison confirms the validity of applying this model for the analysis of geomagnetic storms. The superiority of fuzzy concepts over the conventional method and the analytical techniques are presented here.
Deterministic and probabilistic seismic hazard analyses in Thailand and adjacent areas using active fault data
Authors: Pailoplee Santi , Sugiyama Yuichi , Charusiri Punya
Published: August 2014

Seismic hazards in Thailand and adjacent areas were analyzed mainly on the basis of geological fault data. We identified 55 active fault zones using remote-sensing data on earthquake source parameters derived from both active fault data and earthquake catalogues. We selected strong ground-motion attenuation models by comparing the application of several candidate models with strong ground-motion data recorded in Thailand. Both deterministic (DSHA) and probabilistic (PSHA) approaches were used—DSHA for the design of critical construction and PSHA for the design of non-critical construction. We also applied two frequency-magnitude models in the PSHA approach: the exponential magnitude distribution model and the characteristic earthquake model. The seismic hazard results obtained using the deterministic and probabilistic approaches are not equivalent. The resulting DSHA map reveals extremely high seismic hazard levels in some areas of Thailand and in surrounding countries, while the PSHA map reveals a seismic hazard distribution similar to that of the DSHA but with lower seismic hazard levels. The areas of high seismic hazard include countries neighboring Thailand, such as Myanmar, Laos, Vietnam, and Indonesia (Sumatra Island), and areas within Thailand itself, primarily those areas in northern, western, and southern Thailand that are dominated by active fault zones.
Envelope syntheses of cylindrical vector-waves in 2-D random elastic media based on the Markov approximation
Authors: Sato Haruo , Korn Michael
Published: August 2014

Seismograms of microearthquakes are complex; however, their envelopes broaden as the travel distance increases. P-waves are recorded in transverse components, S-waves are recorded in the longitudinal component, and waves are observed at sites even in the nodal direction of the source radiation. These phenomena, which are typically found in short-period seismograms, can be interpreted to be the result of scattering due to lithospheric inhomogeneity. We report here our study of a simple statistical model in which the propagation of waves radiated from a point source in two-dimensional (2-D) random elastic media is characterized by a Gaussian autocorrelation function. For the case that the wavelength is shorter than the correlation distance, two methods based on the Markov approximation are introduced for the direct synthesis of vector wave envelopes. One is to analytically solve the stochastic equation for the two-frequency mutual coherence function; the validity of the solution is confirmed by using finite difference simulations. The second is to numerically solve the stochastic equation for the mutual coherence function. The two methods are equivalent, but the latter is applicable to nonisotropic source radiation. For the case of a point shear dislocation source, a peak delay from the onset and a smoothly decaying tail are found to be common to synthesized envelopes in all azimuths. Scattered waves are excited even at receivers in the nodal direction, and amplitudes become independent of the radiation pattern as lapse time increases.
High-resolution bathymetry using Alvin scanning sonar at the Southern East Pacific Rise and its implication to the formation of collapsed lava lakes
Authors: Tanaka Akiko , Rosat Severine , Kisimoto Kiyoyuki , Urabe Tetsuro
Published: August 2014

A high-resolution bathymetric map of a collapsed lava lake on the ridge crest of the superfast-spreading Southern East Pacific Rise was constructed. The data were acquired during the MOAI’98 Cruise using a pencil-beam scanning sonar that was installed on the submersible Alvin. The map covers an area of 200 × 350 m at the Oasis hydrothermal site (17°25.4′S, 113°12.3′W) where low-temperature fluids are venting from a collapsed lava lake. The collapsed lava lake is 250 m long, with a mean width of 10 m in parallel to the spreading axis on the western flank of the ridge crest. The estimated volume of lava that has drained out is about 7000 m3 erupted from a single event. This small amount of melt and frequent contacts between “young” and “younger” flows indicate that volcanism at the superfast-spreading ridge system is characterized by frequent eruptions of very small volumes of magma.
Determining the parameters of ionospheric perturbation caused by earthquakes using the quasi-optimum algorithm of spatiotemporal processing of TEC measurements
Authors: Kiryushkin Vladislav V. , Afraimovich Edward L.
Published: August 2014

We present the quasi-optimum algorithm of localization of a source of coseismic ionospheric perturbation based on GPS receivers’ network data processing. The initial data of the algorithm are the series of variations of ionospheric total electron content (TEC), reconstructed using the measurements of the phase delays of GPS signals. In order to select the TEC increments caused by ionospheric perturbation due to the earthquake, the TEC series are filtered using a special procedure. The algorithm realizes coherent summation of all TEC series of GPS array reasoning from the maximization of the energy of the total signal of the ionospheric response to earthquake. The quasi-optimum algorithm allows determination of the perturbation propagation velocity as well as of the coordinates, height and “switch-on” time of a source of coseismic ionospheric disturbance without prior information about the perturbation form and the site and time of the main shock of earthquake. We used the algorithm for measuring the parameters of ionospheric perturbations which accompanied the earthquake in the vicinity of Hokkaido Island on September 25, 2003 and the earthquake near the south coast of Honshu Island on September 5, 2004. The results of these experiments show the high accuracy of the perturbation source coordinates estimation (33 km and 27 km respectively with reference of the earthquakes epicenters). The estimations of perturbations propagation velocity (820 ± 60 m/s and 460 ± 40 m/s), heights (340 ± 80 km and 370 ± 130 km) and “switch-on” delay (346 s and 507 s) of a source of the perturbation obtained in both experiments are in agreement with a theory according to which coseismic atmospheric disturbance propagates within a narrow cone of zenith angles up to ionospheric heights and then diverges in the form of a spherical wave with the radial velocity close to the speed of sound at these heights. It is also in agreement with the results of earlier researches.
Evolution of source characteristics of AE events during frictional sliding
Authors: Yabe Yasuo
Published: August 2014

The evolution of source characteristics of acoustic emission (AE) events and the surface topography of the fault surface during frictional sliding were investigated with the aim of improving current understanding of the microscopic process of friction. Both the AE source dimensions and the grain-scale topographies were well conserved against abrasion during the sliding, but the magnitude of stress drop was significantly reduced. The fractal-domain topographies, the wavelength of which is shorter than the fractal limit wavelength (λc), were significantly worn during the sliding. These results suggest that the grain-scale topography determined the AE source dimension, while the fractal-domain asperities controlled the magnitude of the stress drop. Since elastic wave radiation is one of the major energy consumption processes, the grain-scale and the fractal-domain topographies may play different roles, not only in the AE source process but also in the friction of rocks.
Characteristics of the equatorial electrojet current in the central region of South America
Authors: Rastogi R. G. , Chandra H. , James M. E. , Kitamura K. , Yumoto K.
Published: August 2014

We describe here for the first time the morphology of the equatorial electrojet (EEJ) in the Central American Sector based on an analysis of the geomagnetic field components from six stations distributed on both sides of the dip equator along the 60°W geographic longitude sector. Diurnal and latitudinal variations in the horizontal and vertical components are shown to follow the Chapman model of EEJ. The horizontal component vector due to the ionospheric current aligned itself close to magnetic north, with a mean Declination of 10°W (ranging from 9°W to 14°W). There was a significant counter-electrojet effect before sunrise at stations close to the dip equator, suggesting late reversal of solar-quiet (S
q) electric field in the morning hours. The observed variations in the magnetic meridional current did not seem to be associated with EEJ currents. The centre of the electrojet was around 0.25°S of the dip equator in the morning hours and shifted gradually to 1.5°S by the evening hours. Magnetic storms occurring during the midday hours produced an exceptionally large decrease in the H (horizontal component) field at stations close to the dip equator.
Premonitory changes in seismicity prior to the Great Sumatra-Andaman earthquake of December 26, 2004
Authors: Imoto M. , Yamamoto N.
Published: August 2014

We studied changes in seismicity in a source region of the Sumatra earthquake prior to the earthquake on 26 December 2004. Rate increases were detected for earthquakes of M ≥ 5.0 using the ETAS method, with the assumption that independent events follow the Poisson process with at most only one rate change. Based on the declustered catalogue, we obtained consistent changes with simple Poisson models. Temporal changes in mean magnitude were obtained using the CUSUM method. b-values for three periods were calculated, taking into consideration the results of the CUSUM analysis, which showed a 10-year period of an anomaly high b-value followed by a 13-year period of a low b-value. The statistical significance of this change was confirmed by a bootstrap method. Both changes imply activation of seismicity in the source region.
Crustal deformation and a preliminary fault model of the 2007 Chuetsu-oki earthquake observed by GPS, InSAR, and leveling
Authors: Nishimura Takuya , Tobita Mikio , Yarai Hiroshi , Ozawa Shinzaburo , Murakami Makoto , Yutsudo Toru , Ishimoto Masayoshi , Umesawa Takeshi , Toyofuku Takashi , Kawamoto Satoshi , Amagai Tomomi , Fujiwara Midori , Suzuki Akira , Enya Syunji , Sasaki Toshiyuki , Yokokawa Masanori , Oomori Syuichi , Tanoue Setsuo , Ikeda Hisao , Nemoto Moriyuki , Suito Hisashi , Hayashi Fumi , Une Hiroshi , Koarai Mamoru , Tsuzawa Masaharu
Published: August 2014

The 2007 Chuetsu-oki earthquake occurred just west off the coast of Kashiwazaki in Niigata Prefecture, Central Japan on July 16, 2007. The permanent GPS network (GEONET) clarifies the coseismic displacement as a thrust faulting whose compressional axis lies in the NW-SE direction. Interferometric analysis of synthetic aperture radar (SAR) images acquired by “Daichi” satellite (ALOS) maps a detailed spatial pattern of the displacement toward the satellite for both ascending and descending orbits. Peak-to-peak displacement reaches approximately 400 mm in the descending orbit interferometric (In)SAR data. Repeated precise leveling shows uplift near the northeast part of the aftershock area and subsidence near the southwestern part. We construct a preliminary fault model by inverting the observed deformation. The preferred model consists of two segments of rectangular faults whose moment magnitude is 6.7 in total. From only the used geodetic data on land, it is difficult to determine which plane in two conjugate planes of the focal solution was ruptured. It is important to consider the effect of a heterogeneous medium and variable slip on the faults as well as other geophysical data to determine the fault model with confidence.
Broadband P waves transmitting through fracturing Westerly granite before and after the peak stress under a triaxial compressive condition
Authors: Yoshimitsu Nana , Kawakata Hironori , Takahashi Naoki
Published: August 2014

We analyzed temporal changes in the velocity and amplitude of P waves transmitted through a granite sample during a triaxial compression test, with the goal of monitoring the fault formation process associated with open and shear cracking. We used newly developed transducer assemblies for the broadband recording, and we continued to record transmitting waves even after the peak stress occurred. For transmitting P waves with paths parallel to the maximum compressive axis, we found that both the first wave amplitude and the velocity decreased after dilatancy started, and they kept decreasing even after the peak stress. In addition, the large nonlinear decrease in amplitude was associated with a rapid decrease in differential stress, whereas the rate of decrease in velocity remained almost constant. Thus, before the rapid decrease of differential stress, when both the amplitude and the velocity gradually decreased, open cracking was indicated to be dominant. Thereafter, shear cracking was indicated to become dominant in synchronization with the rapid decrease in differential stress. It is suggested that a main fault started to grow around the sample surface and then progressed into the sample interior; this corresponded to the rapid stress decrease. This fault acts as a strong scatterer for P waves that are parallel to the maximum compressive axis.
Strong ground motions from an Mj 6.1 inland crustal earthquake in Hokkaido, Japan: the 2004 Rumoi earthquake
Authors: Maeda Takahiro , Sasatani Tsutomu
Published: August 2014

An inland crustal earthquake (Mj6.1) occurred on December 14, 2004 in the northern part of Hokkaido, Japan. A large ground acceleration of 1127 cm/s2and a large pseudo-velocity response of over 100 cm/s were recorded at the nearest strong-motion station, HKD020, about 10 km from the hypocenter. This large ground motion is considered to be attributable to the source effect and the site effect. The site effect is investigated using the traditional spectral ratio technique and the theoretical evaluation based on the subsurface structure model. The results imply that the site effect has an insignificant effect on the large ground motion at HKD020. The source effect is investigated by constructing the source model that explains the broad-band strong-motion records at four stations around the epicenter using the empirical Green’s function method. The estimated source model satisfies the empirical relationship between the strong motion generation areas and the seismic moment for inland crustal earthquakes. The high-frequency level of the acceleration source spectrum is also consistent with the empirical relationship. These results suggest that this earthquake is a normal crustal event and that the large ground motion at HKD020 is mainly attributable to the source effect, short distance from the strong motion generation area and the forward directivity effect. Finally, the temporal change of the site response at HKD020 is examined using long duration records including the main shock and several aftershocks. The site response based on theS-wave horizontal-to-vertical spectral ratio method shows the nonlinearity for the main shock and an aftershock occurring about 20 s after the main shock. However, the site response shows linearity for other later aftershocks. This site response change is attributed to the difference in ground motion amplitude.
Remanence anisotropy effect on the palaeointensity results obtained from various archaeological materials, excluding pottery
Authors: Kovacheva Mary , Chauvin Annick , Jordanova Neli , Lanos Philippe , Karloukovski Vassil
Published: August 2014

The effect of magnetic anisotropy on the palaeointensity results has been evaluated in different materials, including samples from archaeological structures of various ages, such as baked clay from prehistoric domestic ovens or pottery kilns, burnt soil from ancient fires, and bricks and bricks or tiles used in the kiln’s construction. The remanence anisotropy was estimated by the thermoremanent (TRM) anisotropy tensor and isothermal remanence (IRM) tensor methods. The small anisotropy effect (less than 5%) observed in the palaeointensity results of baked clay from the relatively thin prehistoric oven’s floors estimated previously through IRM anisotropy was confirmed by TRM anisotropy of this material. The new results demonstrate the possibility of using IRM anisotropy evaluation to correct baked clay palaeointensity data instead of the more difficult to determine TRM anisotropy ellipsoid. This is not always the case for the palaeointensity results from bricks and tiles. The anisotropy correction to palaeointensity results seems negligible for materials other than pottery. It would therefore appear that the palaeointensity determination is more sensitive to the degree of remanence anisotropy P and the angle between the natural remanent magnetization (NRM) vector and the laboratory field direction, than to the angle between the NRM and the maximum axis of the remanence anisotropy ellipsoid (K
max).
Significant decreases in the geomagnetic indices in the ascending phase of Solar Cycle 24
Authors: Minamoto Yasuhiro , Taguchi Yosuke
Published: August 2014

Solar activity was quite low from January 2008 to February 2009, ever since the start of Solar Cycle 24. In 2008, the number of days in a year during which there were no sunspots was the fourth-largest since 1842. However, it is likely that the sunspot number would not have been able to reveal the solar inactivity in the beginning phase of cycle 24 because it hit the lower limit (zero) on more than two-thirds of the days of 2008. Geomagnetic data covers the longest span of time next to sunspot number, but its value has never reached that limit. The K-index, which is a geomagnetic index, from 2008 to the beginning of 2009, has been recorded at several observatories as being the smallest in the history of observation. The K
p-index, which characterizes the geomagnetic planetary activity, has also significantly decreased. Taking into account the relation between the K
p-index during the minimum phase of the solar cycle and sunspot numbers of the following maximum, it has been suggested that the peak solar activity in cycle 24 will be quite low. If solar activity in cycle 24 continues to be quiet, geomagnetic data are expected to be one of the key tools for monitoring space climate.
Wind observation around the tops of the midlatitude cirrus by the MU radar and Raman/Mie lidar
Authors: Yamamoto Masayuki K. , Kishi Toyohisa , Nakamura Takuji , Nishi Noriyuki , Yamamoto Mamoru , Hashiguchi Hiroyuki , Fukao Shoichiro
Published: August 2014

Using a 46.5-MHz atmospheric radar referred to as the MU radar (MUR) and a Raman/Mie lidar installed at the Shigaraki (34°51′N, 136°06′E), continuous wind motions around the tops of the midlatitude cirrus are described for the first time. The cloud system extended from the northeast to southwest (35°N-50°N) along the eastward-moving trough and passed over Shigaraki in the nighttime between 5–6 November 2004. Cloud-top altitude observed by the lidar was located at ∼10.6 km around 1900 LST 5 November, then gradually descended to ∼8.4 km around 0500 LST 6 November. The westerly wind observed by MUR with 12-min and 150-m resolutions showed a rapid increase with altitude around the cloud tops and was almost always larger than 25 m s−1 above ∼1 km higher than the cloud tops. Objective reanalysis showed that a subtropical jet whose core existed to the south of Shigaraki caused a synoptic-scale vertical increase in the westerly wind around the cloud tops. Radiosondes observed a significant vertical increase of potential temperature (greater than 4 K within several hundred meters) around the cloud tops. MUR successfully observed fine time and altitude variations of winds which showed a good correspondence with the descending cloud tops.
The seasonal and solar cycle variations of electron density gradient scale length during magnetically disturbed days: implications for Spread F
Authors: Manju G. , Devasia C. V. , Ravindran Sudha
Published: August 2014

The behaviour of electron density gradient scale length, L, around post-sunset hours during the magnetically disturbed days of the summer, winter and equinox seasons of solar maximum (2002) and minimum years (1995) has been studied, using ionosonde data of Trivandrum (8.5°N, 76.5°E, dip = 0.5°N) in the Indian longitude sector. The results indicate a clear seasonal and solar cycle variation in L. Seasonal variations of the maximum vertical drift of the F layer were also examined on these days. In particular, the seasonal variation of the Equatorial Spread F (ESF) during this period is examined in terms of the relative roles of L and the vertical drift of the F layer in the triggering of the collisional Rayleigh-Taylor instability. Our results on the clear-cut seasonal and solar cycle variation in L for disturbed days and its control of ESF occurrence are presented and discussed.
Measurement of atmospheric air-earth current density from a tropical station using improvised Wilson’s plate antenna
Authors: Anil Kumar C. P. , Panneerselvam C. , Nair K. U. , Jeeva K. , Selvaraj C. , Jeyakumar H. Johnson , Gurubaran S.
Published: August 2014

We have developed an experimental set-up to measure the atmospheric air-earth current (conduction current). Data obtained with the continuous measurements of Wilson’s plate are used to study of air-earth current density, with the aim of gaining an understanding of the experimental set-up’s response to different meteorological conditions, including fair-weather days. This paper is a part of the on-going Global Electric Circuit (GEC) studies from Tirunelveli (8.7°N, 77.8°E), a measurement site in the tropical and southern tip of the Indian peninsula. Attempts have been made in past few years to obtain the global signature in this region with this sensor, but on most of the occasions it has been impossible to obtain the global signature during fair-weather days. The data used for February–April, 2007 have the well-defined nature of this global signature, which is in agreement with the well-established classical Carnegie curve of GEC. This paper also deals with very important observations made at sunrise and during those hours when fog existed. It is noted that the resistivity of the atmosphere increased significantly with the onset of fog and later decreased as the fog disappeared, based on the measured value of conduction current density when compared with the electric field measured by horizontal passive wire antenna. Also, during fair-weather conditions, conduction current and electric field variations are similar because the conductivity during this period is more or less constant at this site. Observations made during different meteorological conditions, such as different wind speeds, humidities, and temperatures, are also discussed.
Effects of gravity data quality and spacing on the accuracy of the geoid in South Korea
Authors: Hong Chang-Ki , Kwon Jay Hyoun , Lee Bo Mi , Lee Jisun , Choi Yun Soo , Lee Suk-Bae
Published: August 2014

The effects of gravity data quality and spacing on the accuracy of the computed geoid are analyzed. The analysis is performed using simulated gravity data that accommodate the real gravity signal in South Korea. The reference geoid is generated using both simulated gravity data and digital terrain models (DTM), assuming that both data sets are errorless. By artificially controlling the gravity data quality and spacing, we are able to calculate and analyze the geoid errors. The results show that the current distribution of real gravity data in South Korea causes geoid errors, with the standard deviation being as much as 8 cm, and that these geoid errors are mainly caused by the distribution of gravity data rather than by the noise in the data. Areas showing large geoid errors are also clearly identified; these areas should be subjected to supplementary gravity surveying at data spacing smaller than 2 km to achieve a 5-cm level of geoid accuracy.
Crustal deformations associated with the great Sumatra-Andaman earthquake deduced from continuous GPS observation
Authors: Hashimoto Manabu , Choosakul Nithiwatthn , Hashizume Michio , Takemoto Shuzo , Takiguchi Hiroshi , Fukuda Yoichi , Fujimori Kunio
Published: August 2014

We analyzed continuous GPS data from more than 20 sites in Asia, Australia and islands in Indian Ocean in order to detect crustal deformations associated with the Sumatra-Andaman earthquake of December 26, 2004. Coseismic steps can be recognized at sites about 3,000 km away from the epicenter such as Kunming in south China, Quezon in Philippines, and Diego Garcia Island in central Indian Ocean. The largest displacement of about 26 cm is found at Phuket in Thailand about 600 km away from the epicenter, about twice as large as that at Sampari, the nearest site in northern Sumatra. These observations suggest that as large slip as 14 m occurred beneath the Nicobar Islands. Large postseismic displacements are observed at Phuket and Sampari after the mainshock, but the former is three times larger than the latter. This suggests that the spatial distribution of afterslip is different from the coseismic slip distribution. The temporal variation of postseismic displacements can be explained by a logarithmic function derived from rate-state dependent friction law with short characteristic time. The area where coseismic displacements from the Nias earthquake of March 28, 2005 are detected is much smaller than that from the December mainshock, but displacement at Sampari is larger than that during the mainshock. These displacements suggest less than 4 m slip on a shallow dipping thrust fault and resultant moment release is smaller than that estimated from seismological data. Finally, total moment released by afterslip amounts to 3.83 × 1022 Nm which is equivalent to Mw 8.99 for about five months, including the afterslip for the Nias earthquake.