bluuebunny/crossref_metadata_2025 huggingface dataset:
Resistivity and density modelling in the 1938 Kutcharo earthquake source area along a large caldera boundary
Authors: Ichihara Hiroshi , Mogi Toru , Hase Hideaki , Watanabe Tomonori , Yamaya Yusuke
Published: August 2014

We present the crustal structure around the fault zone pertaining to the 1938 Kutcharo earthquake (M 6.0), northern Japan, to consider why large earthquakes have occurred around calderas. The study was based on gravity anomalies and magnetotelluric and direct-current (DC) electrical-resistivity survey data. The density structure obtained from gravity anomalies indicated that the fault plane corresponded to the main depression boundary of the Kutcharo caldera. The resistivity section, based on audio-frequency magnetotelluric surveys, indicated that the estimated fault plane was located along the boundary of resistivity blocks, which also corresponded to the depression boundary. A detailed resistivity section in the ruptured zone revealed by a DC electrical-resistivity survey showed a discontinuity of layers, implying cumulative fault displacements. These results indicate that the 1938 earthquake was an abrupt slip along the main depression boundary of the Kutcharo caldera. The most likely hypothesis pertains to fluid intrusion along the depression boundary. However, additional seismic and geodetic studies are required to identify other feasible earthquake mechanisms.
Waveform inversion for slip distribution of the 2006 Java tsunami earthquake by using 2.5D finite-difference Green’s function
Authors: Okamoto Taro , Takenaka Hiroshi
Published: August 2014

We first incorporate the large effect of near-source heterogeneity on teleseismic body waveforms in the inversion of the slip distribution of the 2006 Java tsunami earthquake. We incorporate the effect by computing the response of an assumed “2.5D” model structure of the Java trench by a 2.5D finite-difference method. Based on a simulation of inversion, we suggest that intense smearing is possible when we apply 1D Green’s functions in the analysis, and that it may obscure the slip pattern. In the inversion of real data, we confirm macroscopic features, such as a long duration (∼ 165 s) and a slow rupture velocity (∼ 1.25 km/s). The region of the initial rupture is found to be isolated from the eastern broad region in which we further identify a heterogeneous slip distribution. Most of these regions are likely to be at the sedimentary plate interface where the accreted sediment and the subducting plate are in contact. In particular, the nearly “isolated” feature of a shallow slip region suggests a possible faulting in the shallowest part of the sedimentary plate interface without being strongly enforced by the rupture propagated from the deeper part of the fault. Such heterogeneity suggests a highly variable frictional behavior at the sedimentary plate interface.
Seismic structure of the northern end of the Ryukyu Trench subduction zone, southeast of Kyushu, Japan
Authors: Nishizawa Azusa , Kaneda Kentaro , Oikawa Mitsuhiro
Published: August 2014

P-wave velocity models related to the Philippine Sea plate subduction have been obtained from seismic explorations at the northern end of the Nansei-Shoto (Ryukyu) Trench. Both multi-channel seismic reflection and ocean bottom seismographic records provide clear images of the rough seafloor topography and irregular P-wave velocity structure of the northwestern extension of the Kyushu-Palau Ridge and Amami Plateau below the landward trench slope. Low velocity (V

p
≤ 4–5 km/s) and thick (>7 km) materials above the plate boundary characterize the landward trench slope structure. An exception to this is found where the subducting plate contacts higher velocity (V

p
> 5 km/s) materials of the landward plate at the estimated position of the seismic asperity of 1968 Hyuganada earthquake (M
w 7.5).
DC railways and the magnetic fields they produce—the geomagnetic context
Authors: Lowes F. J.
Published: August 2014

DC electric railways produce magnetic fields, not only from the intended traction currents, but also from unintended earth-leakage currents; these fields, particularly those from the leakage currents, are becoming an increasing problem for geomagneticians. This paper introduces the relevant properties of DC-railway traction-power circuits, and the various ways in which earth-leakage currents are produced, and discusses models of how these leakage currents vary along the track and with train position. It describes the geometry of the resultant magnetic fields, and gives the formal algebra for calculating the magnetic field when these leakage currents are known, but also suggests some simple approximations that could be used when the current distribution is not known in detail. This paper also summarises previous relevant papers.
Stress field in the source region after the 2007 Mw 6.6 Niigataken Chuetsu-oki earthquake deduced from aftershock focal mechanisms: Implication for a pre-mainshock stress field
Authors: Imanishi Kazutoshi , Kuwahara Yasuto
Published: August 2014

Stress fields in the source region before and after the 2007Mw6.6 Niigataken Chuetsu-oki earthquake were investigated using aftershock focal mechanism solutions. We have attempted to determine aftershock focal mechanisms usingP-wave polarity data as well as body wave amplitudes because this earthquake occurred offshore, where observation station coverage is poor. This approach enabled us to obtain 76 well-determined aftershock focal mechanisms. Although the stress field in the studied area is known to be generally characterized by a reverse-faulting regime, the application of a stress tensor inversion method to the aftershock focal mechanisms revealed that the stress field spatially varied on a scale smaller than the fault dimension of the mainshock, with a mixture of strike-slip and reverse faulting regimes. The post-mainshock stress field estimated by the stress tensor inversion was compared with the theoretical stress field computed using an observed non-uniform slip distribution of the mainshock and variously assumed pre-shock stress fields. The results of this comparison suggest that the pre-mainshock principal stress in the WNW-ESE direction was dominant and that the magnitudes of the other two principal stresses were similar.
Spatial distribution of coda Q estimated from local earthquakes in Taiwan area
Authors: Chung Jen-Kuang , Chen Yen-Ling , Shin Tzay-Chyn
Published: August 2014

Attenuation of seismic wave energy and its spatial distributions in Taiwan area have been investigated using a large amount of short-period seismograms recorded from the local earthquakes with magnitude ranging from 2.5 to 4.5. The codaQvalues were estimated in the frequency range 1.5∼18 Hz, applying the single backscattering model proposed by Aki and Chouet in 1975. The estimated average frequency dependent codaQvalues within the crust give the relationship,QC= 93f0.75, while varying from 143±33 at 1.5 Hz to 904±164 at 18 Hz central frequencies. However, using the data from deeper earthquakes, the average codaQrelationship is given asQC= 122f0.71in related to the material property of the crust and upper lithosphere, in which the average codaQvalues vary from 180±67 at 1.5 Hz to 1000±144 at 18 Hz. The comparison of different lapse times starting at double the time of the primaryS-wave from the origin time and corresponding to the datasets with different focal depths, reveals that codaQvalues increase with the coda generating volume. As a whole, the regressive relationships of frequency dependent codaQfor Taiwan region approximately agree with those by previous investigators. In addition, the average codaQvalues for three subregions of Taiwan were calculated and correlated with the geology and tectonic features, respectively. The lowestQvalues were obtained in the region consisting of the Coastal Range and the Longitudinal Valley of eastern Taiwan, whilst the highestQfor the region underlain by the Peikang High beneath the western Coastal Plain. Such distinguishable difference has definitely explained that the seismic waves should exhibit lower decay rate in a stable region than within a seismoactive plate boundary.
Slip distribution of the 1973 Nemuro-oki earthquake estimated from the re-examined geodetic data
Authors: Nishimura Takuya
Published: August 2014

Geodetic data, including leveling, tide-gauge, triangulation/trilateration, and repeated EDM data, from eastern Hokkaido, Japan, were re-examined to clarify the crustal deformation associated with the 1973 Nemuro-oki earthquake. We inverted the geodetic data to estimate the slip distribution on the interface of the subducting Pacific plate. The estimated coseismic slip, potentially including afterslip, showed a patch of large slip (i.e., an asperity) near the epicenter of the mainshock. The moment magnitude of the Nemuro-oki earthquake was estimated to be 8.0 from the geodetic data, which is comparable to the 2003Mw= 8.0 Tokachi-oki earthquake. The estimated slip distribution suggests a 50 km-long gap in the coseismic slip between the 1973 Nemuro-oki and the 2003 Tokachi-oki earthquakes. The slip area of the 2004Mw= 7.0 Kushiro-oki earthquake, estimated from GPS data, was located at the northwestern edge of the Nemuro-oki earthquake, which implies that the area may have acted as a barrier during the Nemuro-oki earthquake. The postseismic deformation observed by leveling and tide-gauge measurements suggests that the afterslip of the Nemuro-oki earthquake occurred at least in a western and northern (i.e., deeper) extension of the asperity on the plate interface.
On estimation of earthquake magnitude in Earthquake EarlyWarning systems
Authors: Sokolov Vladimir , Wenzel Friedemann , Furumura Takashi
Published: August 2014

Determination of earthquake magnitude from the initial P-wave portion of ground motion is the key problems for Earthquake Early Warning (EEW). We analyzed performance of scaling relations between one of the frequently used early warning parameters, so-called characteristic period τC and moment magnitude M
W in respect of (a) characteristics of datasets accumulated in various regions (earthquake depth, characteristics of network) and (b) variation of initial conditions applying for determination of the parameter τC (length of Pwave window, number of used stations). The used data contain strong-motion records from 110 earthquakes (moment magnitude range 4.4–7.6) occurred in Japan and Taiwan. We show that, although the standard error of regression τC = f (M
W) in general becomes smaller with the increase of the length of P-wave window (PL) and the number (N) of averaged observations (stations), the uncertainty in estimation of MW given observed tC does not decrease further for PL > 3–4 seconds and N > 3–4 stations. The information about earthquake depth also plays an important role in reducing the uncertainty of magnitude estimations. The table of confidence limits for estimations of the moment magnitude MW based on observations of the characteristic period τC is provided.
Magnetohydrodynamic parametric instabilities of parallel propagating incoherent Alfvén waves
Authors: Nariyuki Yasuhiro , Hada Tohru
Published: August 2014

Numerical experiments for parametric instabilities of incoherent Alfvén waves in the context of one dimensional Hall-MHD equation sets are studied. The reason why the decay instability of incoherent waves can be explained in terms of that of the coherent wave is understood through analysis on nonlinearly driven finite amplitude density fluctuations. Numerical results suggest the importance of modulational instabilities of left-hand polarized Alfvén waves.
Processing the Bouguer anomaly map of Biga and the surrounding area by the cellular neural network: application to the southwestern Marmara region
Authors: Aydogan Davut
Published: August 2014

An image processing technique called the cellular neural network (CNN) approach is used in this study to locate geological features giving rise to gravity anomalies such as faults or the boundary of two geologic zones. CNN is a stochastic image processing technique based on template optimization using the neighborhood relationships of cells. These cells can be characterized by a functional block diagram that is typical of neural network theory. The functionality of CNN is described in its entirety by a number of small matrices (A, B and I) called the cloning template. CNN can also be considered to be a nonlinear convolution of these matrices. This template describes the strength of the nearest neighbor interconnections in the network. The recurrent perceptron learning algorithm (RPLA) is used in optimization of cloning template. The CNN and standard Canny algorithms were first tested on two sets of synthetic gravity data with the aim of checking the reliability of the proposed approach. The CNN method was compared with classical derivative techniques by applying the cross-correlation method (CC) to the same anomaly map as this latter approach can detect some features that are difficult to identify on the Bouguer anomaly maps. This approach was then applied to the Bouguer anomaly map of Biga and its surrounding area, in Turkey. Structural features in the area between Bandirma, Biga, Yenice and Gonen in the southwest Marmara region are investigated by applying the CNN and CC to the Bouguer anomaly map. Faults identified by these algorithms are generally in accordance with previously mapped surface faults. These examples show that the geologic boundaries can be detected from Bouguer anomaly maps using the cloning template approach. A visual evaluation of the outputs of the CNN and CC approaches is carried out, and the results are compared with each other. This approach provides quantitative solutions based on just a few assumptions, which makes the method more powerful than the classical methods.
Spatial variations in the frequency-magnitude distribution of earthquakes in the southwestern Okinawa Trough
Authors: Lin Jing-Yi , Sibuet Jean-Claude , Lee Chao-Shing , Hsu Shu-Kun , Klingelhoefer Frauke
Published: August 2014

The relations between the frequency of occurrence and the magnitude of earthquakes are established in the southern Okinawa Trough for 2823 relocated earthquakes recorded during a passive ocean bottom seismometer experiment. Three highb-values areas are identified: (1) for an area offshore of the Ilan Plain, south of the andesitic Kueishantao Island from a depth of 50 km to the surface, thereby confirming the subduction component of the island andesites; (2) for a body lying along the 123.3°E meridian at depths ranging from 0 to 50 km that may reflect the high temperature inflow rising up from a slab tear; (3) for a third cylindrical body about 15 km in diameter beneath the Cross Backarc Volcanic Trail, at depths ranging from 0 to 15 km. This anomaly might be related to the presence of a magma chamber at the base of the crust already evidenced by tomographic and geochemical results. The highb-values are generally linked to magmatic and geothermal activities, although most of the seismicity is linked to normal faulting processes in the southern Okinawa Trough.
Compressional and shear wave velocities of serpentinized peridotites up to 200 MPa
Authors: Watanabe Tohru , Kasami Hiroaki , Ohshima Shohei
Published: August 2014

Compressional and shear wave velocities of serpentinized peridotites were measured at room temperature and high confining pressures of up to 200 MPa. Rock samples were collected from the Hida outer belt, Central Japan, and classified into High-T (containing antigorite) and Low-T (containing lizardite and/or chrysotile) types. Antigorite is stable up to 600∼700°C, while lizardite and chrysotile are stable below 300°C. High-T type samples have distinctly higher velocities than their Low-T type counterparts with the same density. The High-T type with strong foliation shows significant velocity anisotropy, and the azimuthal anisotropy of the compressional wave velocity reaches 30%. These properties can be explained by the crystallographic structure of antigorite. Poisson’s ratio increases with serpentinization in both types. The High-T type shows a lower Poisson’s ratio than the Low-T type with the same density. The High-T type requires a higher degree of serpentinization than the Low-T type to give a certain value of Poisson’s ratio. Observations of high Poisson’s ratio have been interpreted using Low-T type properties. However, High-T type serpentinized peridotite is expected in warm subduction zones. The use of Low-T type properties will lead to a significant underestimation of serpentinization. For good interpretations, it is essential to use the properties of the appropriate type of serpentinized peridotite.
Contributions of the external field to the observatory annual means and a proposal for their corrections
Authors: Verbanac Giuli , Lühr Hermann , Rother Martin , Korte Monika , Mandea Mioara
Published: August 2014

In this study we separate, interpret and explain magnetospheric and ionospheric signals present in the observatory annual means. The data from 46 European geomagnetic observatories collected over 42 years (1960–2001) are used. To characterise the various field components, we use predictions from latest magnetic field models. The core field and its secular variation are described by the CM4 model, and the magnetospheric contributions are successfully removed by parameterising the POMME model with the Dst index. We regard the remaining signal as being caused by ionospheric currents. The annual averages of the Sq variation estimated by the CM4 model are subtracted from the residuals. A remaining variation in anti-phase with the magnetic activity index Ap finally can be removed with a function properly scaled by Ap and Dst. We offer an objective procedure to suppress the external field contributions in the annual means to an uncertainty level of ±2 nT. Except for the ionospheric currents, this could be achieved by applying recent magnetic field models, which shows that the quality of present day models is sufficient to correct observatory data for average external field contributions. Understanding the signal contained in the annual means is a prerequisite for obtaining reliable and physically meaningful results when such data are used in studies of the core field and its secular variation.
VHF radar studies of counter electrojet events during the northern winter solstice period of 1992
Authors: Manju G. , Viswanathan K. S.
Published: August 2014

A sequence of counter electrojet events during the winter solstice period of 1992 are studied along with control quiet day observations at the magnetic equatorial location of Trivandrum (8.5°N, 76.5°E; dip 0.5°N) using VHF (54.95 MHz) coherent backscatter radar observations. Some of the unusual features observed on these days are: (1) an unusual radar signal with reversed Doppler values corresponding to westward electric field, for a brief duration of 5 min at approximately 1330 h, at the higher altitudes 112–117 km on 21 January 1992, as against the usual observation of the signal at lower altitudes below 112 km during CEJ times; (2) type I plasma waves in the late afternoon around 1800 h (after the CEJ event) at altitudes above 101 km (on 21 and 23 January 1992) when the backscattered power is low, indicating the unusual tidal influences on the electric field behavior on certain CEJ days; (3) distortions in the height structure of the vertical polarization electric field, attributed to the modulation of the electrojet altitude structure by action of zonal neutral winds; (4) westward electric field, as seen by the radar, even while the ΔH values are positive.
GPS ionospheric tomography: A comparison with the IRI-2001 model over South Korea
Authors: Jin Shuanggen , Park Jong-Uk
Published: August 2014

The International Reference Ionosphere model 2001 (IRI-2001) is one of the most comprehensive empirical models of the ionosphere and has been widely used to estimate the electron density profiles in the altitude ranging from about 60 to 2000 km and the total electron content (TEC) at any given location, time and date, which reflect smooth-average global ionospheric behaviors. However, whether it provides normal actual estimations in the ionosphere over some regions should be tested with real observation data. In this paper, the three-dimensional ionospheric electron density profiles over South Korea in 2003 are obtained using the ionospheric tomography reconstruction technique with the permanent Korean GPS Network (KGN) data, and its validity is further verified by another independent ionosonde data. The GPS ionospheric reconstruction results are used to compare then results obtained with the IRI-2001 model in South Korea in terms of NmF2 and TEC. The monthly averaged diurnal values of these key parameters in January, April, July and October 2003 are considered to represent the winter, spring, summer and autumn seasons, respectively. Compared with the GPS reconstruction results, averaged monthly NmF2 medians from the IRI-2001 are overestimated in daytime and underestimated in nighttime for all seasons, but the deviation magnitudes in autumn and winter are smaller than in spring and summer. In addition, averaged monthly TEC medians from the IRI-2001 are overestimated in daytime in winter, but almost always underestimated in other seasons.
Collisional disruption of weakly sintered porous targets at low-impact velocities
Authors: Setoh Masato , Nakamura Akiko M. , Hirata Naru , Hiraoka Kensuke , Arakawa Masahiko
Published: August 2014

Porous structure is common in the asteroids and satellites of the outer planets. In order to study the relationship between the structure of small bodies and their thermal and collisional evolution, we performed impact disruption experiments on porous sintered targets using a light-gas gun at velocities ranging from 10 to 100 m/s. The sintered glass bead targets were prepared to have roughly the same porosity but with different compressive strengths, ranging over an order of magnitude, by controlling sintering duration and temperature. The results of the impact experiments show that the targets of higher compressive strength have higher impact strengths. However, compared to previous results on impact disruption of porous sintered targets with a collisional velocity of approximately 6 km/s, the values of impact strength in this study were found to be lower by an order of magnitude.
Stress due to the interseismic back slip and its relation with the focal mechanisms of earthquakes occurring in the Kuril and northeastern Japan arcs
Authors: Takeuchi M. , Sato T. , Shinbo T.
Published: August 2014

Assuming a uniform and inhomogeneous distribution of back slip, based on a realistic configuration of the plate boundary that was newly determined from the distribution of earthquakes, we have investigated if the stress due to the interseismic back slip can explain the focal mechanisms of earthquakes occurring in the Kuril and northeastern Japan arcs. The inhomogeneous back-slip model was taken from the distribution of slip deficit estimated from GPS data collected in recent years. We examined the change in the Coulomb Failure Function (ΔCFF) for several target faults to evaluate the fitness of the models to the observed focal mechanisms. The results of ΔCFF are clearly different in the two back-slip models near the down-dip end of the locked zone. Despite this difference, the models can generally explain the earthquake focal mechanisms in the overriding plate as well as those on the megathrust plate boundary. However, they cannot explain the focal mechanisms of intermediate-depth earthquakes in the double-planed deep seismic zone. Neither can they fully explain the focal mechanisms with the pressure axis parallel to the trench that are observed at the junction of the Kuril and northeastern Japan arcs. Other tectonic processes, such as the transcurrent movement of the forearc sliver along the Kuril arc, may be involved in producing the stress anomaly.
Relocation of large intermediate-depth earthquakes in the Vrancea region, Romania, since 1934 and a seismic gap
Authors: Hurukawa Nobuo , Popa Mihaela , Radulian Mircea
Published: August 2014

We relocatedM6 or larger intermediate-depth earthquakes in Vrancea, Romania that have occurred since 1934. To determine their absolute hypocenters accurately, we combined them with recent (from January 1996 to November 2003) smaller earthquakes recorded by the Romanian local network and then relocated all of them simultaneously using a modified joint hypocenter determination method. Using the data on recent small earthquakes we first obtained an estimate of the dimensions of the active seismic region of intermediate-depth earthquakes: length, 85 km; along a NE-SW direction; width, 20 km; depth 60-160 km. We then determined that the 1940 (M7.7), 1977 (M7.5) and 1990 (M6.9) major earthquakes were located near the NE edge of the seismic region, while the 1986 (M7.2) was located inside the seismic region. The focal depths of the 1940, 1977, 1986 and 1990 major earthquakes were calculated to be 124, 98, 135 and 84 km, respectively and their source areas to range from 60 to 140 km in depth. The only unbroken area is at a depth of 140–160 km, which we current consider to be a seismic gap. Based on the regularity of past large earthquakes, we propose that the nextM7 intermediate-depth earthquake can be expected to occur in Vrancea at a depth interval of 140–160 km sometime early in this century.
Coseismic and postseismic deformation related to the 2007 Chuetsu-oki, Niigata Earthquake
Authors: Ohta Yusaku , Miura Satoshi , Iinuma Takeshi , Tachibana Kenji , Matsushima Takeshi , Takahashi Hiroaki , Sagiya Takeshi , Ito Takeo , Miyazaki Shin’ichi , Doke Ryosuke , Takeuchi Akira , Miyao Kayo , Hirao Akihiko , Maeda Takahiro , Yamaguchi Teruhiro , Takada Masamitsu , Iwakuni Makiko , Ochi Tadafumi , Meilano Irwan , Hasegawa Akira
Published: August 2014

An intermediate-strength earthquake of magnitudeMj6.8 occurred on July 16, 2007, centered beneath the Japan Sea a few kilometers offshore of Niigata Prefecture in central Japan. We constructed a dense GPS network to investigate postseismic deformation after this event, choosing our GPS sites carefully so as to complement the nationwide GPS GEONET array. Coseismic displacements caused by the mainshock detected at some GEONET sites were used to estimate coseismic fault parameters. The results indicate that the geodetic data can be explained by a combination of two rectangular faults dipping northwest and southeast. Minor but definite postseismic deformation was detected largely in the southern part of the dense network. The time series of site coordinates can be characterized by a logarithmic decay function, and the estimated time constant seems to be almost similar in range to that of the 2004 Mid-Niigata Prefecture Earthquake. We also found a possible site instability at 960566 (Izumo-zaki, GEONET) caused by a small, local landslide associated with the mainshock and therefore concluded that the data obtained at this site should not be used for coseismic or postseismic analysis.
Determination of dip direction for the 2007 Chuetsu-oki earthquake from relocation of aftershocks using arrival times determined by cross-correlation
Authors: Mori Jim
Published: August 2014

Waveform correlations were used to redetermine P arrival times at 31 stations for a selected set of 321 aftershocks of the 2007 Chuetsu-oki earthquake. These data were used with several different relocation techniques, 1-D velocity structure, 3-D velocity structure using SIMULPS12, 1-D velocity structure using hypoDD, and 3-D velocity structure using tomoDD. The results from all of these methods show a consistent southeast dipping trend for the entire aftershock sequence. The same pattern is also seen for the time period before the largest aftershock. These results indicate that the mainshock fault dips to the southeast.
Seismic scaling relation of the 2007 Off Mid Niigata, Japan, earthquake (Mw 6.6) sequence in comparison with two other earthquake (Mw 6.6) sequences
Authors: Tajima Reiko , Tajima Fumiko
Published: August 2014

The relationship of seismic moment (M0) to corner frequency (fc) of the 2007 Off Mid Niigata prefecture (Chuetsu-oki) earthquake sequence is examined in comparison with that of two other earthquake sequences. TheM0-f0relation of this sequence (3.4 ≤Mw≤ 6.6) isM0∝fc−3.87, which deviates fromM0∝fc−3. The relatively large deviation fromfc−3may reflect the influence of fluids identified in tomographic studies around the source region of the tectonic strain concentration. Moreover, the lowerfcs of the small events (3.4 ≤Mw≤ 3.8) in this sequence are similar to those of off-main-fault events in the 2004 Mid Niigata prefecture earthquake sequence. The characteristics in the scaling relation may be interpreted in the context of the event locations, which are either on- or off-main faults, and may have implications for the seismogenic conditions. Key words: The 2007 Off Mid Niigata earthquake, seismic scaling, seismogenic conditions.
Seismic vertical array analysis of the velocity structure and phase decomposition during aftershocks of the 2007 Chuetsu-oki earthquake
Authors: Yoshida Kunikazu
Published: August 2014

A velocity profile derived from PS-logging was examined using the mainshock and aftershock records from KK5 and KSH seismic vertical arrays in the Kashiwazaki-Kariwa nuclear power plant, which is located on the focal region. A seismic phase decomposition analysis method was applied to aftershock vertical array records. Peak frequencies of spectral ratios of the aftershock records from the KSH array between various depths were slightly different from those of theoretical transfer functions. A comparison of observed and theoretical S-wave travel times at various depths indicated that travel times for the mainshock measured by picking S-wave arrival times were identical to the theoretical travel times calculated from the PS-logging profile, although travel times for the aftershocks were different from the theoretical travel times. The decomposition analysis of seismic vertical array records from KK5 array for a small aftershock shows that a clear wave packet of surface waves follows just after the direct S-wave arrival. In contrast, a decomposition analysis of the KSH array records show that a wave packet of surface waves are not clearly discernable.
Equatorial GPS ionospheric scintillations over Kototabang, Indonesia and their relation to atmospheric waves from below
Authors: Ogawa Tadahiko , Miyoshi Yasunobu , Otsuka Yuichi , Nakamura Takuji , Shiokawa Kazuo
Published: August 2014

Using Global Positioning System (GPS) satellites, we have been conducting equatorial ionospheric scintillation observations at Kototabang, Indonesia since January 2003. Scintillations caused by equatorial plasma bubbles appear between 2000 and 0100 LT in equinoctial months with a seasonal asymmetry, and their activity decreases with decreasing solar activity. A comparison between scintillation index (S
4) and Earth’s brightness temperature (T
bb) variations suggests that the scintillation activity can be related to tropospheric disturbances over the Indian Ocean to the west of Kototabang. To understand better the reasons of day-to-day variability of S
4, we analyze S
4, T
bband lower thermospheric neutral wind ("Equation missing") data. The results show that S
4 fluctuates with periods of about 2.5, 5, 8, 14 and 25 days, possibly due to atmospheric waves from below and that similar periods are also found in the T
bband "Equation missing" variations. Using a general circulation model, we made numerical simulations to determine the behavior of neutral wind in the equatorial thermosphere. The results indicate the following: (1) 2- to 20-day waves dissipate rapidly above about an altitude of 125 km, and 0.5- to 3-hour waves become predominant above 100 km, (2) zonal winds above 200 km altitude are, on the whole, eastward during sunset-sunrise, (3) zonal wind patterns due to short-period (1–4 h) atmospheric gravity waves (AGWs) above 120 km altitude change day by day, exhibit wavy structures with scale lengths of about 30–1000 km and, as a whole, move eastward at about 100−1 while changing patterns over time. These simulations suggest that the Rayleigh-Taylor instability responsible for plasma bubble generation can be seeded by AGWs with short periods of about 0.5–3 h, and that background conditions necessary for this instability are modulated by planetary-scale atmospheric waves propagating up to an altitude of about 120 km from below.
Relative effects of electric field and neutral wind on positive ionospheric storms
Authors: Balan N. , Alleyne H. , Otsuka Y. , Vijaya Lekshmi D. , Fejer B. G. , McCrea I.
Published: August 2014

The paper studies the relative importance of penetrating eastward electric field (PEEF) and direct effects of equatorward neutral wind in leading to positive ionospheric storms at low-mid latitudes using observations and modeling. The observations show strong positive ionospheric storms in total electron content (TEC) and peak electron density (N
max) at low-mid latitudes in Japan longitudes (≈125°E–145°E) during the first main phase (started at sunrise on 08 November) of a super double geomagnetic storm during 07–11 November 2004. The model results obtained using the Sheffield University Plasmashpere Ionosphere Model (SUPIM) show that the direct effects of storm-time equatorward neutral wind (that reduce poleward plasma flow and raise the ionosphere to high altitudes of reduced chemical loss) can be the main driver of positive ionospheric storms at low-mid latitudes except in N
max around the equator. The equatorward wind without PEEF can also result in stronger positive ionospheric storms than with PEEF. Though PEEF on its own is unlikely to cause positive ionospheric storms, it can lead to positive ionospheric storms in the presence of an equatorward wind.
Gravity waves in the equatorial thermosphere and their relation to lower atmospheric variability
Authors: Miyoshi Yasunobu , Fujiwara Hitoshi
Published: August 2014

Using a general circulation model that contains the region from the ground surface to the upper thermosphere, we have examined characteristics of gravity waves in the equatorial thermosphere. At an altitude of 150 km, the dominant periods of gravity waves for zonal wave number 20 (zonal wavelength λ
x
≈ 2000 km), 40 (λ
x
≈ 1000 km) and 80 (λ
x
≈ 500 km) are 3, 1.5 and 1 h, respectively. For individual zonal wave numbers, the corresponding dominant period becomes shorter at higher altitudes due to dissipation processes in the thermosphere, such as molecular viscosity and ion drag force, indicating that gravity waves with a larger horizontal phase velocity (larger vertical wavelength) can penetrate into the thermosphere. The longitudinal variation of gravity wave activity in the equatorial thermosphere and upward propagation of gravity waves from the lower atmosphere were also studied. The longitudinal distribution of gravity wave activity in the thermosphere is quite similar to that of gravity wave activity in the lower atmosphere and the cumulus convective activity in the tropical troposphere. Our results indicate that the strong energy flux due to gravity waves from the enhanced cumulus convective activity propagates upward into the upper thermosphere. The relation between the wind fluctuation associated with gravity waves and the ionospheric variation is discussed. Fluctuations of the neutral zonal wind with periods of 1–2 h are significant in the 200- to 300-km height region, and its amplitude sometimes exceeds 50 m s−1. These results suggest that upward propagating gravity waves can affect the ionospheric variation in the F-region.
Thickness variation of the sedimentary cover in the South Western Desert of Egypt as deduced from Bouguer gravity and drill-hole data using neural network method
Authors: Zaher Mohamed Abdel , Senosy M. M. , Youssef M. M. , Ehara Sachi
Published: August 2014

The Bouguer anomaly map of scale 1:500,000 and the lithological logs of more than 120 deep wells distributed in the Southern part of Western Desert of Egypt were used to determine the thickness of the sedimentary cover containing the main sandstone water formation. The predominant structures affecting both the basement rock and the sedimentary cover were also studied. Gravity stripping approach was applied to separate density anomalies within the sedimentary fill from the influence of deeper levels in the crystalline crust. The study indicated that the surface of the basement rock is highly rugged and mostly controlled by structures causing variation of the sedimentary cover thickness from location to other all over the area. Isopach maps were constructed based on the Artificial Neural Network (ANN) model which is considered a best method for that operation. The maximum thickness of sandstone formations is recorded at west Oweinat, southwest of Aswan, Dakhla oasis and west of Qena town. As this formation is the main water aquifer in the study area, therefore these locations are characterized by the presence of huge amount of ground water. Accordingly, these areas must be taking the priority in the programs of sustainable development in southern Egypt.
Location of the North Magnetic Pole in April 2007
Authors: Newitt L. R. , Chulliat A. , Orgeval J. -J.
Published: August 2014

Observations have been made at five locations in the vicinity of the North Magnetic Pole (NMP). These were used in four different analyses—virtual geomagnetic pole, simple polynomial, spherical cap harmonic, best fitting grid—to derive positions of the NMP. The average position at 2007.3 was 83.95°N, 120.72°W, with a positional uncertainty of 40 km. This position is only 27 km from the pole position given by the CHAOS magnetic model. The NMP continues to move in a northwesterly direction but its drift speed has stabilized at just over 50 km per year. The number of direct observations is insufficient to determine if the NMP has started to decelerate.
Assessment of radar interferometry performance for ground subsidence monitoring due to underground mining
Authors: Ng Alex Hay-Man , Chang Hsing-Chung , Ge Linlin , Rizos Chris , Omura Makoto
Published: August 2014

This paper describes the results from the recently launched SAR satellites for the purpose of subsidence monitoring over underground coal mine sites in the state of New South Wales, Australia, using differential interferometric synthetic aperture radar (DInSAR) technique. The quality of the mine subsidence monitoring results is mainly constrained by noise due to the spatial and temporal decorrelation between the interferometric pair and the phase discontinuities in the interferogram. This paper reports on the analysis of the impact of these two factors on the performance of DInSAR for monitoring ground deformation. Simulations were carried out prior to real data analyses. SAR data acquired using different operating frequencies, for example, X-, C- and L-band, from the TerraSAR-X, ERS-1/2, ENVISAT, JERS-1 and ALOS satellite missions, were examined. The simulation results showed that the new satellites ALOS, TerraSAR-X and COSMO-SkyMed perform much better than the satellites launched before 2006. ALOS and ENVISAT satellite SAR images with similar temporal coverage were searched for the test site. The ALOS PALSAR DInSAR results have been compared to DInSAR results obtained from ENVISAT ASAR data to investigate the performance of both satellites for ground subsidence monitoring. Strong phase discontinuities and decorrelation have been observed in almost all ENVISAT interferograms and hence it is not possible to generate the displacement maps without errors. However these problems are minimal in ALOS PALSAR interferograms due to its spatial resolution and longer wavelength. Hence ALOS PALSAR is preferred for ground subsidence monitoring in areas covered by vegetation and where there is a high rate ground deformation.
Effect of troposphere slant delays on regional double difference GPS processing
Authors: Nordman Maaria , Eresmaa Reima , Boehm Johannes , Poutanen Markku , Koivula Hannu , Järvinen Heikki
Published: August 2014

The demand for geodetic time series that are accurate and stable is increasing. One factor limiting their accuracy is troposphere refraction, which is hard to model and compute with sufficient resolution, both in time and space. We have studied the effect of numerical weather model (NWM)-derived troposphere slant delays and the most commonly used mapping functions, Niell and Vienna, on Global Positioning System (GPS) processing. Six months of data were calculated for a regional Finnish network, FinnRef, which consists of 13 stations, using Bernese v. 5.0 in double difference mode. The results showed that when site-specific troposphere zenith delays or gradients are not estimated, the use of NWM-based troposphere delays improved the repeatabilities of all three components of station positions (north, east and up) statistically significantly and up to 60%. The more realistic troposphere model also reduces the baseline length dependence of the solution. When site-specific troposphere delays and the horizontal gradients were estimated, there was no statistically significant improvement between the different solutions.
Characteristic activity and migration of episodic tremor and slow-slip events in central Japan
Authors: Obara Kazushige , Sekine Shutaro
Published: August 2014

Continuous seismic data and tilt measurements in the Aichi and Mie regions of central Japan indicate significant migration of episodic non-volcanic deep tremors and short-term slow-slip events on the subducting plate interface at the deeper extension of the mega-thrust earthquake seismogenic zone. These episodic tremor and slip (ETS) events occur in two concentrated regions on either side of Ise Bay at recurrence intervals of approximately every 6 months throughout 2004 and 2005, separated by a distinct tremor gap. A migrating tremor episode from the southern edge of the Mie region in January 2006, however, is found to have crossed aseismic Ise Bay along a continuous 200 km long strike of the subducting Philippine Sea Plate. Coincident with the migration of tremors, very low frequency earthquakes with a predominant frequency of 0.05 Hz and tilt changes lasting for a few days were detected. Inversion analysis of the tilt data indicates that at least five reverse faultings with an average slip length of 1 cm took place on the plate boundary, migrating from southwest to northeast, equivalent to a total moment release of Mw6.2. The tremor seismicity gap at Ise Bay is attributed to the presence of a small ridge in the Philippine Sea Plate that blocks most propagating ETS events. The 2006 episode propagated orthogonal to the majority of ETS event in this region, and was found not to be obstructed by the ridge structure.
Relations between the thermal properties and porosity of sediments in the eastern flank of the Juan de Fuca Ridge
Authors: Goto Shusaku , Matsubayashi Osamu
Published: August 2014

The empirical relations of the thermal properties (thermal conductivity, heat capacity, specific heat, and thermal diffusivity) to the porosity and mineral composition of clay and sandy sediments recovered in the eastern flank of the Juan de Fuca Ridge are examined using the observed thermal properties, index properties, and mineral composition of the sediments. Observed thermal conductivity-porosity relations are explained using the geometric mean model. The observed relations of heat capacity and specific heat, respectively, to porosity are given by the arithmetic mean formula. A new model for the sediment thermal diffusivity-porosity relation is proposed based on models of thermal conductivity and heat capacity. This model, expressed by the geometric mean model with a correction function for the porosity and heat capacities of grain sediment and pore-filling fluid, explains the observed thermal diffusivity-porosity relations. These thermal property models are applicable to thermal properties of other sediment lithology types and are useful as standard models for estimating the thermal properties of marine sediment.
Site-specific correlation of GPS height residuals with soil moisture variability using artificial neural networks
Authors: Ogaja Clement
Published: August 2014

Historical time series generated from GPS sites reveal significant seasonal variations in the vertical direction. It is well known that continental waters (soil moisture, snow, ground water) mass redistribution is one of the potential contributors to these observed seasonal variations although their actual loading effects on GPS results are least well understood. A number of hydrology model outputs exist with a fair degree of uncertainty. Studies of interrelations between anomalous vertical variations observed at geodetic sites and hydrology model outputs are useful, in particular, as the hydrology models continue to be refined. In this paper, artificial neural networks is proposed for correlating GPS height residuals with the soil moisture variability. Time series from eight sites of the global GPS network are used to correlate with the soil moisture information from the US National Oceanographic and Atmospheric Administration (NOAA) Climate Prediction Center’s land data assimilation system (CPC LDAS). The results show the feasibility of the neural interpretation in terms of the correlation coefficients (∼0.6) and root mean square errors (about 15% of residual range). Other geodetic time series can be used for the same purpose, such as from SLR, VLBI, and absolute gravity.
Aftershock distribution of the 26 December 2004 Sumatra-Andaman earthquake from ocean bottom seismographic observation
Authors: Araki Eiichiro , Shinohara Masanao , Obana Koichiro , Yamada Tomoaki , Kaneda Yoshiyuki , Kanazawa Toshihiko , Suyehiro Kiyoshi
Published: August 2014

We deployed an OBS network in February–March 2005 in the rupture area of the Sumatra Andaman earthquake on 26 December 2004. We placed 17 short-term OBSs and two long-term OBSs, and recovered OBSs after observation for 19–22 days. The hypocenter distribution from 10-day data of 17 OBS revealed the detailed structure of aftershock seismicity offshore of Sumatra Island. Aftershock seismicity associated with the subducting slab starts 40 km inward from the Sunda trench axis; it ceases at 50 km depth beneath the Aceh Basin, approximately 240 km inward from the trench axis. Aftershocks in 120–170 km from the trench axis consist of a surface with a dip of 10–12° dominated by a dip-extension type mechanism. Beyond the southwestern edge of the Aceh Basin, the aftershock activity becomes higher, and dominated by dip-slip type earthquakes, with a slightly increased dipping angle of 15–20°. Three along-arc bands of shallow seismicity were identified at 70 km inward from the Sumatra trench, 110 km inward from the trench, and in the south of the Aceh Basin. These locations correspond to steep topographic slopes in the accretionary prism, suggesting the present evolutional activity of the accretionary prism offshore Sumatra Island.