bluuebunny/crossref_metadata_2025 huggingface dataset:
Generation mechanism of Z-mode waves in the equatorial plasmasphere
Authors: Nishimura Y. , Ono T. , Iizima M. , Shinbori A. , Kumamoto A.
Published: August 2014

In order to clarify the generation mechanism of Z-mode waves observed in the equatorial plasmasphere, the growth rate of Z-mode electromagnetic waves has been calculated under the higher-order cyclotron interaction process. Z-mode waves can interact with some tens of keV electrons with large pitch angles even in the dense cold background, and the amplitude is consistent with the Akebono plasma wave measurements. UHR and whistler mode waves are also excited by the same electron distribution, and this is also consistent with observations. The origin of these energetic electrons are identified as the ring current electrons injected into the plasmasphere by the intense large-scale electric field during geomagnetic storms, accelerated perpendicular to the ambient magnetic field and confined around the geomagnetic equator conserving the first and second adiabatic invariants. Since the intensity of Z-mode and UHR waves is associated with the development and decay of the ring current, ring current particles are most possible candidate for the free energy source of these waves.
Mapping absolute migration of global mid-ocean ridges since 80 Ma to Present
Authors: Masalu Desiderius C. P.
Published: August 2014

We have computed and mapped the absolute migration of global mid-ocean ridges in the past 80 m.y. and found that ridges have migrated substantially during that period. Presently, the faster-migrating ridges are Pacific-Antarctic, Central Indian, Southeast Indian, Juan de Fuca, Pacific-Nazca, Antarctic-Nazca and Australia- Antarctic ridges which migrate between 3.3 and 5.5 cm/yr. The slower-migrating ridges are Mid-Atlantic and Southwest Indian ridges which migrate between 0.3 and 2.0 cm/yr. Comparing these results with mantle tomography and geochemistry suggests that slower-migrating ridges have deeper depth of origin than faster-migrating ridges, implying a correlation between migration velocity and depth of origin of ridges. The reconstructed Southwestern Indian ridge (near 44°E) between 60 Ma and Present, lies atop reconstructed Central Indian ridge between 80 and 50 Ma, and the present-day Antarctic-Nazca or (Chile) ridge lies atop the reconstructed East Pacific Rise at 70 Ma. Furthermore, the South Mid Atlantic Ridge and the East Pacific Rise near 10?S appear to have been stationary relative to the mantle for the last 80 m.y. These observations suggest that different portions of the mantle have undergone different recycling history, and may explain the origin of mantle heterogeneities.
Gravity basement of the Guinsaugon landslide along the Philippine Fault Zone
Authors: Makino Masahiko , Mandanas Allan A. , Catane Sandra G.
Published: August 2014

A gravity survey was conducted in Guinsaugon, St. Bernard, Southern Leyte, Philippines, to determine the subsurface structure of the Leyte segment of the Philippine Fault Zone (PFZ), where a massive landslide killed 1119 villagers on 17 February 2006. The landslide started on top of a 780-m-high ridge linked with the PFZ. The build-up of pore pressure in the slope due to saturation and possible earthquake-triggered ground shaking along the PFZ, may have initiated the landslide. A two-dimensional structure using homogeneous density models was applied to interpret the gravity anomalies in the Guinsaugon landslide. The most suitable model depicts a deep, narrow graben filled with low-density sediments. The graben is 1.5 km wide and 2 km deep and has a density contrast of -0.4 g/cm3. The western boundary of the graben is steep and coincides with the terminus (southern) Leyte segment of the PFZ. The existence of a deep-seated seismically active fault at the foot of the steep slopes and the soft sediments in the narrow graben are important factors in understanding the trigger and failure mechanisms of the landslide and in the assessment of hazards and risks due to landslides and earthquakes.
Effect of elastic inhomogeneity on the surface displacements in the northeastern Japan: Based on three-dimensional numerical modeling
Authors: Sato Kachishige , Minagawa Naoya , Hyodo Mamoru , Baba Toshitaka , Hori Takane , Kaneda Yoshiyuki
Published: August 2014

In geodetic inversions such as estimation of coseismic slip and/or afterslip distribution on faults, the displacements on the surface calculated under an assumption of homogeneous elastic half space have been mostly used as the Green’s functions (GF’s). However, this seems not adequate for better estimations of such slip distribution, because the subsurface structures are more or less inhomogeneous, especially those in and around Japan where the structure must be much complicated. In this study, to examine how much the inhomogeneous subsurface structure affects on the surface displacements, we conduct some 3-D finite element calculations with a grid for the region of 1400 km (EW) × 1200 km (NS) × 200 km (depth) including the Tohoku and Hokkaido, northeastern Japan. Assuming homogeneous and inhomogeneous elastic models with various values for the Young’s modulus and Poisson’s ratio, we calculated the surface displacements due to a dip-slip type dislocation of 1 m on many cell-like subfaults assumed on the interface between the Pacific and land side plates. Comparing the results, we find a large discrepancy in the surface displacements between the homogeneous and inhomogeneous elastic models and less dependency of the surface displacements on the Poisson’s ratio. The discrepancy is found to be more than 20% and can be as large as ~40% in some cases. Such a large discrepancy indicates that the surface displacements calculated for inhomogeneous elastic medium with realistic subsurface structure, unlike as in usual cases, should be used as the GF’s for better geodetic inversions.
Sporadic aurora from Spain
Authors: Vaquero J. M. , Trigo R. M. , Gallego M. C.
Published: August 2014

Some authors have shown recently that “sporadic auroras” have occurred at relatively low latitude when the geomagnetic activity is quiet or moderate. Here we present and analyze a case of “sporadic aurora” observed from Spain on the 29 December 1845 and compare its low geomagnetic activity with other three aurorae also observed in Spain in mid-19th century.
Tectonomagnetic study in the eastern part of Hokkaido, NE Japan (II): Magnetic fields related with the 2003 Tokachi-oki earthquake and the 2004 Kushiro-oki earthquake
Authors: Nishida Yasunori , Utsugi Mitsuru , Mogi Toru
Published: August 2014

We examined short-term geomagnetic changes related with the 2003 Tokachi-oki earthquake (M 8.0) and the 2004 Kushiro-oki earthquake (M 7.1) in Hokkaido, Japan. However, we could not find the precursory and co-seismic signals above several nT at a magnetic station whose epicentral distances were about 120 km and 50 km, respectively. Model calculations showed that co-seismic piezomagnetic fields did not amount to 1 nT at the station in both cases when we assumed the relevant fault parameters, in-situ Curie temperature depth, subsurface magnetic structure and stress sensitivity of rocks. Therefore, it may be reasonable that we could not detect the piezomagnetic signals at the station. We also made model calculations to forecast the piezomagnetic amplitudes caused by M 7.9 and M 8.5 earthquakes which have been expected to occur along the southern Kurile trench in the future. The model calculations reveal the piezomagnetic fields up to about −4 nT and −7 nT are expected in the eastern part of the Hokkaido island for the M 7.9 and M 8.5 earthquakes respectively, encouraging magnetic observations hereafter.
Quality control of Ebro magnetic observatory using momentary values
Authors: Curto J. J. , Marsal S.
Published: August 2014

Intercomparison of momentary values from observatories across Europe can be used as a test of reliability for a particular magnetic station in this area, as well as the whole network. The method presented by Voppel at the IAGA Assembly of Grenoble (1975) and developed by Schulz and Voppel at the IAGA Assembly of Edinburgh (1981) is based on simultaneous measurements taken at 02:00 UT, which coincides with the period least disturbed by Sq associated currents on central European longitudes. A selected list of ten least disturbed days per month is provided by the Niemegk (initially Wingst) Geomagnetic Observatory which gathers the corresponding momentary values from the collaborating institutions. This method can be applied to detect fluctuations or jumps in geomagnetic standards. Independent techniques, like linear regression and axial intercept of the standard deviation of the mutual differences of monthly mean values, have been applied to the magnetic elements of Ebro Observatory (EBR) for the period 1997–2001. These tools give results in good agreement amongst them, and most of the coefficients are similar to those obtained for the most significant observatories of the network. No jumps or trends in data are observed, indicating excellent performance of EBR.
A new technique of radiation thermometry using a consumer digital camcorder: Observations of red glow at Aso volcano, Japan
Authors: Saito Takeshi , Sakai Satoshi , Iizawa Isao , Suda Eriko , Umetani Kazuhiro , Kaneko Katsuya , Furukawa Yoshitsugu , Ohkura Takahiro
Published: August 2014

We newly developed a technique of radiation thermometry using a Sony’s consumer digital camcorder. Our system is not only convenience and cost effective but with a better performance than previous infrared thermometers, particularly in the place like a crater of volcano where is abundant in gas. This is because our system uses the submicron wavelength band, in which radiation is less influenced by absorption of gas than in the thermal infrared wavelength (>3 μm). We carried out observations of red glow at Aso volcano and succeeded in measuring the temperature of about 800°C, which is much more acceptable than previously reported values of 200–400°C. When we measure the temperature of about 300–700°C and 600–900°C in the place where is abundant in gas, using the camcorder with the near-infrared and with the visible wavelength mode is better than the thermal infrared region, respectively.
Evidence for geomagnetic jerks in comprehensive models
Authors: Chambodut Aude , Mandea Mioara
Published: August 2014

The rate of secular variation occasionally undergoes a sudden, sharp change, called a geomagnetic jerk. Such jerks have been detected in geomagnetic time series, centered—over the last four decades—around 1971, 1980, 1991, and 1999; others have been inferred from historical records. The geomagnetic jerks represent a reorganization of the secular variation, implying an internal origin, as established through spherical harmonic and wavelet analysis. However, some characteristics of jerks are not well understood. Here we estimate the occurrence dates for geomagnetic jerks, as they can be detected from a global geomagnetic model. This choice makes the present study novel, for two reasons. First, utilizing the comprehensive modelling approach allows for the use of a secular variation signal free of time-varying external fields and their corresponding induced counterpart, and observatory biases. Second, the model utilizes satellite data when available, in addition to observatory data. Indeed, POGO (1967 to 1971), MAGSAT (1979 to 1980), Ørsted (1999 to present time) and CHAMP (2000 to present time) satellite measurements help to separate the different magnetic sources. In this study the CM4 comprehensive model is used for a global search of geomagnetic jerks and their occurrence dates. Our first result indicates that found geomagnetic jerks might not have been worldwide in occurrence. Moreover, the obtained dates suggest that jerks detected in the CM4 model over the last four decades occurred not simultaneously but at slightly different times around 1971, 1980 and 1991.
Receiver function images of the central Chugoku region in the Japanese islands using Hi-net data
Authors: Ramesh D. S. , Kawakatsu H. , Watada S. , Yuan X.
Published: August 2014

Crustal configuration of the central Chugoku region with disposition of the Philippine Sea Plate (PHS) in this area are investigated through the receiver function approach using short-period Hi-net data. Images of the upper mantle discontinuities are also obtained. Restituted short-period receiver functions bring out discernible variations in average composition of the crust and its thickness in the study region. TheVp/Vsvalues in the study area are generally high, reaching values in excess of 1.85 at a few places. The central part of the study region showing the highestVp/Vsvalues is coincidentally a subregion of least seismicity, possibly bestowed with special subsurface structure. Migrated receiver function images, bothPsandPpsimages, unambiguously trace the NW subducting PHS taking a steeper plunge in the northwest part of the Chugoku region reaching depths of 70 km from its low dip disposition in the southeast. An excellent correlation of the subducting PHS with the hypocenters is also seen. We demonstrate that short-period data after restitution and application of appropriate low pass filters can indeed detect presence of the global 410-km and 660-km discontinuities and map their disposition reasonably well. Our migrated receiver functions image the deflections in the 410-km and 660-km discontinuities in an anti-correlated fashion on expected lines of Clapeyron slope predictions induced by subduction of the Pacific plate (PAC) beneath Japanese islands, though PAC itself is feebly traced but shows good correlation with slab seismicity.
Estimation of the source model for the foreshock of the 2004 off the Kii peninsula earthquakes and strong ground motion simulation of the hypothetical Tonankai earthquake using the empirical Green’s function method
Authors: Suzuki Wataru , Iwata Tomotaka , Asano Kimiyuki , Yamada Nobuyuki
Published: August 2014

We estimated the source model for the foreshock of the 2004 off the Kii peninsula earthquakes by empirical Green’s function modeling. The size and the rise time of the strong motion generation area (SMGA) were estimated to be 30 × 15 km, and 0.9 sec, respectively. The stress drop of the SMGA was calculated to be 8.3 MPa. This model could reproduce long-period ground motions following the direct S-wave observed in the Osaka basin well. Using the derived source parameters, we simulated strong motions of the hypothetical Tonankai earthquake. Distribution of the seismic intensity derived here is similar to that obtained by the previous report. We could predict long-period ground motions which last for a long duration at the basin sites.
Spatial distribution of centroid moment tensor solutions for the 2004 off Kii peninsula earthquakes
Authors: Ito Yoshihiro , Matsumoto Takumi , Kimura Hisanori , Matsubayashi Hirotoshi , Obara Kazushige , Sekiguchi Shoji
Published: August 2014

Two large intraplate earthquakes (MW 7.0 and 7.3) occurred off the southeastern Kii Peninsula on September 5, 2004. We determined the centroid moment tensors of these earthquakes and their aftershocks. Earthquakes can be divided into two categories: (1) the reverse-fault type with a range in focal depth from 5 km to 20 km; and (2) earthquakes with a near-vertical nodal plane and a focal depth shallower than 5 km. Earthquakes of type (1) are distributed inside the Philippine Sea plate beneath the Nankai trough, while normal-fault events predominate in the other regions along the trough. The distribution of type (2) events shows two clear lineaments with strikes of NNW-SSE to the north and NE-SW to the west of the focal area. These lineament strikes coincide approximately with strikes at the nodal planes of such events.
Characteristics of the surface rupture associated with the 2004 Mid Niigata Prefecture earthquake, central Japan and their seismotectonic implications
Authors: Maruyama Tadashi , Fusejima Yuichiro , Yoshioka Toshikazu , Awata Yasuo , Matsu’ura Tabito
Published: August 2014

The 2004 Mid Niigata Prefecture earthquake sequence produced a nearly 1-km-long, N-S to NNW-SSE-striking, west-side-up surface rupture along a previously unmapped fault at Obiro, Uonuma City, eastern margin of the epicentral region. The maximum vertical displacement is 20 cm. The topographic and geometric features of the surface rupture are indicative of reverse faulting with dip to the west, which is consistent with focal mechanism solutions and aftershock distribution. Along the major active faults and folds that form the framework of landforms in the epicentral region, systematic surface ruptures were not found. Exceptionally small rupture dimension and offset amount of the surface rupture compared with those of the modeled fault suggest that the fault slip appears to have been accommodated by both internal deformation in thick Neogene-Quaternary deposits and growth of the folds and blind thrusts around the epicentral region.
Aftershock distribution of the 2004 Mid Niigata Prefecture Earthquake derived from a combined analysis of temporary online observations and permanent observations
Authors: Shibutani Takuo , Iio Yoshihisa , Matsumoto Satoshi , Katao Hiroshi , Matsushima Takeshi , Ohmi Shiro , Takeuchi Fumiaki , Uehira Kenji , Nishigami Kin’ya , Enescu Bogdan , Hirose Issei , Kano Yasuyuki , Kohno Yuhki , Korenaga Masahiro , Mamada Yutaka , Miyazawa Masatoshi , Tatsumi Ken’ichi , Ueno Tomotake , Wada Hiroo , Yukutake Yohei
Published: August 2014

The 2004 Mid Niigata Prefecture Earthquake (Mj = 6.8) occurred on 23 October 2004 in the northeastern part of the Niigata-Kobe Tectonic Zone where large contraction rates were observed. The mainshock was followed by an anomalously intense aftershock activity that included nine Mj ≥5.5 aftershocks. We deployed three temporary online seismic stations in the aftershock area from 27 October, combined data from the temporary stations with those from permanent stations located around the aftershock area, and determined the hypocenters of the mainshock and aftershocks with a joint hypocenter determination (JHD) technique. The resulting aftershock distribution showed that major events such as the mainshock, the largest aftershock (Mj = 6.5), the aftershock on 27 October (Mj = 6.1), etc. occurred on different fault planes that were located nearly parallel or perpendicular to each other. This might be due to heterogeneous structure in the source region. The strain energy was considered to have been enough accumulated on the individual fault planes. These features are probably a cause of the anomalous intensity of the aftershock activity.
Imaging of S-wave reflectors in and around the hypocentral area of the 2004 mid Niigata Prefecture Earthquake (M6.8)
Authors: Matsumoto Satoshi , Iio Yoshihisa , Matsushima Takeshi , Uehira Kenji , Shibutani Takuo
Published: August 2014

An S-wave reflector is considered to relate to the existence of liquid in the seismogenic zone of the crust, which plays an important role in understanding the mechanism of earthquakes. We studied a distribution of S-wave reflectors in and around the hypocentral zone of the 2004 mid Niigata Prefecture Earthquake (M6.8). The earthquake was followed by several aftershocks that were greater than M6. Moreover, the aftershocks were not only located on the fault plane of the main shock but also on conjugate fault planes and on a parallel plane to that of the main shock. In order to discuss the relationship between this complex activity and the crustal heterogeneities, we analyzed the seismograms observed at the seismic stations in this region. Normal moveout processing (NMO) was applied to the data of the aftershock. Several S-wave reflectors could be identified from the NMO sections for every station. In particular, relatively strong S-wave reflectors exist in the lower crust at a depth of approximately 20-25 km in the middle part of the aftershock region. Additionally, reflectors were found beneath the fault planes of the main shock and the largest aftershock. This suggests a possibility of the correlation of the crustal heterogeneities to the occurrence of an earthquake.
FDM computation of seismic wavefield for an axisymmetric earth with a moment tensor point source
Authors: Toyokuni Genti , Takenaka Hiroshi
Published: August 2014

Axisymmetric modeling has been playing an important role in global seismic waveform modeling since it can correctly model geometrical spreading effects in 3-D within computational resources comparable to 2-D modeling. However, in the previous investigations on axisymmetric modeling, seismic sources were restricted to axisymmetric sources such as an explosive source. In this paper, we propose implementation of an arbitrary moment tensor point source to axisymmetric modeling using the finite-difference method (FDM). The validity and efficiency of this technique are demonstrated by comparing synthetic seismograms with analytical solutions for a homogeneous earth model, as well as with the DSM synthetics for a spherically symmetric earth. We also show a numerical example with a subducting slab structure.
Three-dimensional P and S wave velocity structures beneath the Hokkaido corner, Japan-Kurile arc-arc junction
Authors: Katsumata Kei , Wada Naoto , Kasahara Minoru
Published: August 2014

We applied an inverse method developed by Zhao et al. (J. Geophys. Res., 97, 19909–19928, 1992) to 42,834 P and 18,263 S wave arrival time data observed at 152 seismographic stations for 1143 local earthquakes at depths between 0 and 200 km in order to estimate three-dimensional P and S wave velocity structures beneath the Hokkaido corner, Japan-Kurile arc-arc junction. High- and low-velocity zones were clearly imaged in the Hidaka Mountain Range at depths shallower than 35 km. The low-velocity anomalies of P and S waves were found to be distributed in the mantle wedge at depths between 35 and 100 km beneath the volcanic front, as also observed in the Tohoku region. Another low-velocity zone was found to exist in the fore-arc region at depths of 50–70 km above the plate boundary; this zone was not detected in Tohoku, suggesting that the dehydration process in the fore-arc region is different from that in the Tohoku region.
A search for a correlation between time change in transfer functions and seismic energy release in northern Taiwan
Authors: Chen Kuang-Jung , Chiu Bonbbon , Lin Cheng-Horng
Published: August 2014

The geomagnetic data of the Lunping Geomagnetic Observatory from 1988 to 2000 are utilized for computing daily geomagnetic transfer functions. The method of analysis is based on the power spectrum analysis developed by Everett and Hyndman. Monthly means are statistically obtained from these daily values of transfer functions. In order to compare these time changes with seismic activity (seismicity), the earthquakes occurring within 150 km from Lunping, with a magnitude M
L greater than 4.0, are located. The related energy releases by those earthquakes, summed month by month, are correlated to the transfer functions. After removing the seasonal effect, we find that the time changes of magnitude of A
u and B
u (real parts of the transfer function) for frequencies of 2, 3, 4, and 6 cycles/hour seem to be strongly related to the energy release within the whole study period. Two significant precursors were found from the A and B values, 40 and 20 months before high seismicities, respectively.
Deviation of linear relation between streaming potential and pore fluid pressure difference in granular material at relatively high Reynolds numbers
Authors: Watanabe Tohru , Katagishi Yoshiko
Published: August 2014

We conducted streaming potential measurements on the packing of glass beads, and investigated the deviation of streaming potential from the Helmholtz-Smoluchowski (H-S) equation. The H-S equation was originally derived on the assumption of laminar flows. Studies using a capillary have shown that the H-S equation is valid for turbulent flows in so far as the viscous sublayer is thicker than the electrical double layer and the entrance effect is negligible. Although the streaming potential in porous media has been reported to deviate from the H-S equation for turbulent flows, its mechanism is still poorly understood. We measured the fluid flux and the streaming potential as a function of the pore fluid pressure difference. The fluid flux begins to deviate from Darcy’s law at Reynolds number >3, and the streaming potential begins to deviate from the linear relation at larger Reynolds numbers. When the flow is fast, the fluid inertia separates the boundary layer from the solid surface and induces the counter flows. The fluid in the counter-flow region is separated from the circulating fluid, and ions there cannot contribute to the convection current. We think that this results in a lower streaming potential than expected from the H-S equation.
Two electrical conductors beneath Kusatsu-Shirane volcano, Japan, imaged by audiomagnetotellurics, and their implications for the hydrothermal system
Authors: Nurhasan  , Ogawa Yasuo , Ujihara Naoto , Tank S.Bulent , Honkura Yoshimori , Onizawa Shin’ya , Mori Takehiko , Makino Masahiko
Published: August 2014

Kusatsu-Shirane volcano, Japan, is known for its active phreatic eruptions. We have investigated its hydrothermal system by conducting audio-magnetotelluric soundings at 22 stations along a profile that extends across the volcano. The final two-dimensional model is characterized by two conductors. One is a 300- to 1000-m-thick conductor of 1–10 Ωm, which is located on the eastern slope and covered with 200-m-thick resistive layers of Kusatsu-Shirane lava and pyroclastics. This conductor indicates the presence of a Montmorillonite-rich layer of Pliocene volcanic rocks that may function both as an impermeable floor for the shallow fluid path from the peak to the hot springs to the east and as an impermeable cap for the deeper fluid path from the summit region to the foot of the volcano. The second conductor is found at a depth of 1–2 km from the surface, at the peak of the volcano, and its resistivity is as low as 1 Ωm or less. This low resistivity can be explained by fluids containing high concentrations of chloride and sulfate which were supplied from the magmatic gases. Micro-earthquakes cluster above this conductor, and the cut-off of the earthquakes corresponds to the top of the conductor. This conductor infers the presence of the fluid reservoir, and the upward release of these fluids from the reservoir through the conduit presumably triggers the micro-earthquakes at the peak area of the volcano. Crustal deformation modeling using GPS and leveling data of the past 10 years revealed that the center of the deflation coincides with the top of the second conductor, indicating that the fluid reservoir itself can be hosting the deformation.
Lava identification by paleomagnetism: a case study and some problems surrounding the 1631 eruption of Mount Vesuvius, Italy
Authors: Conte-Fasano Gennaro , Urrutia-Fucugauchi Jaime , Goguitchaichvili Avto , Incoronato Alberto , Tiano Pasquale
Published: August 2014

Detailed rock magnetic, paleomagnetic and absolute paleointensity studies of lava flows from the disputed 1631 Mount Vesuvius eruption are reported. The magnetic carrier consists of pseudo-single domain state Ti-poor titanomagnetites. Characteristic magnetization directions determined from detailed stepwise alternating field and thermal demagnetizations provide four new well-defined flow unit mean directions, with α95 ranging from 0.7° to 2.6°. Paleodirections for 11 lava flows from 24-four flows studied previously appear to be related to the 1631 eruption, as indicated by their correlation to the early 17th century segment of the Italian paleosecular variation reference curve. This provides new evidence supporting the conclusion that the 1631 episode was an explosive-effusive eruption. The paleointensity results obtained from this study are the first to be published for Mount Vesuvius, with virtual dipole moments of 9.24±1.8 × 1022 and 13.5±0.4 × 1022 Am2 higher than the present-day geomagnetic field strength.
Mantle wedge deformation by subducting and rotating slab and its possible implication
Authors: Honda S. , Orihashi Y. , Mibe K. , Motoki A. , Sumino H. , Haller M. J.
Published: August 2014

We have constructed a simple model of the deformation of the mantle wedge caused by the subducting and rotating slab based on corner flow model. By applying this model to the geological settings of the Somuncura plateau volcanic region, northern Patagonia, which is located far from the volcanic front, we constrain the mechanical aspect of the hypothesis that the volcanisms of the Somuncura region are triggered by the dehydration-induced melting of the up-warped transition zone which may contain more water than other regions do. Assuming that the water concentrates in the transition zone under the Somuncura plateau, we find that the above scenario may be possible, if the speed of the subducting slab is less than ∼2 cm/yr or the dip angle changes significantly (∼50 degrees within ∼8 Myr).
Tectonic implications of post-folding Permian magnetizations in the Carapacha Basin, La Pampa province, Argentina
Authors: Tomezzoli Renata N. , Melchor Ricardo N. , MacDonald William D.
Published: August 2014

Paleomagnetic results from Permian clastic and igneous rocks of the Carapacha Basin of the Gondwáides orogenic zone of central Argentina are mainly consistent with results reported previously from the same zone further east, e.g., in the Sierra de la Ventana. Three lithologic entities were analysed: the lower member and upper member of the Carapacha Formation, and an andesite intrusive into the upper member. The directions of their characteristic remanences are similar, differ significantly from the present field direction, and are post-folding. The in situ magnetization directions are moreover consistent with directions expected for late Permian poles of the APWP for South America. The in situ pole for the lower Carapacha Formation is 70°S, 049°E, A95=11° (San Roberto pole); the pole for the upper Carapacha, combined with similar directions from the intrusive, is 64°S, 005°E, A95=5° (Río Curacó pole). These magnetizations imply that the Carapacha Formation, with a minimum age of early Late Permian (about 260 Ma.), was deformed before the end of the Permian. Structural evidence, as well as paleomagnetic, IRM, and AMS experimental results, support the interpretations. The paleogeographic implications of these results are interpreted as a significant counterclockwise movement of Gondwana between the early and the late Permian.
A new Late Permian paleomagnetic pole for stable South America: The Independencia group, eastern Paraguay
Authors: Rapalini Augusto E. , Fazzito Sabrina , Orué Delio
Published: August 2014

The Late Permian segment of the South American apparent polar wander path (APWP) is poorly defined and further paleomagnetic efforts on suitable rocks of such age from stable areas of South America are needed. A study was carried out on the Late Permian Independencia Group, which is discontinuously exposed along Eastern Paraguay and consists of several hundred meters of flat lying continental white, pink and reddish sandstones and siltstones, deposited on the western margin of the Paraná basin. Thirteen sites (83 samples) were collected on the top unit of the Group (the latest Permian Tacuary Fm) from outcrops distributed over 1000 square kms. Detailed AF and thermal demagnetization permitted the isolation of within-site consistent characteristic remanence in eleven sites, being generally hematite the main carrier of the remanence. Excluding one outlier, a paleomagnetic pole was computed by averaging 10 virtual geomagnetic poles of mixed polarities (6 reversed, 4 normal), yielding a position at: 80.7°S, 7.0°E, A95: 6.6°. The new pole may be considered key for a better definition of the South American APWP by filling a gap between a relatively reliable Early Permian and a poorly defined Early-Middle Triassic groups of poles.
Paleomagnetism of the late Cenozoic basalts from northern Patagonia
Authors: Mena Mabel , Ré Guillermo H. , Haller Miguel J. , Singer Silvia E. , Vilas Juan F.
Published: August 2014

Late Cenozoic volcanic rocks outcrop in the northern Patagonia Extrandina. Lava flows, characterized as olivine and alkaline basalts, belong to intraplate volcanism. We report paleomagnetic and rock-magnetic studies carried out on Late Cenozoic basalts belonging to the Cráter, Mojón and Moreniyeu Formations. The paleomagnetic sampling comprised 75 sites in lava flows and dikes from the Cráter Formation, three sites in a lava flow from the Mojón Formation and three sites in a lava flow from the Moreniyeu Formation. Alternating field (AF) and thermal detailed demagnetization techniques were used. Most of the samples have a viscous component. The AF procedure was more effective than thermal demagnetization in destroying viscous components and in defining the characteristic remanent magnetizations. Demagnetization curves and rock-magnetic studies suggest that the main remanence carrier is Ti-poor magnetite. Radiometric K-Ar ages were performed on these basalts. The radiometric ages are 0.8±0.1 Ma from outcrops located at Cerro Fermín and 1.9±0.4 Ma from outcrops at Cerro Negro, both at the Cráter Formation. These ages suggest an early-middle Pleistocene age for the lava flows from Cerro Fermín, and a late Pliocene to early Pleistocene age for the Cerro Negro lava flows. Based on the magnetic polarity temporal scale, the Cerro Fermín lava flows have registered the beginning of the Brunhes Chron, while the Cerro Negro basalts could have been extruded during the Olduvai Subchron. The K-Ar radiometric age of the Moreniyeu Formation (1.6±0.2 Ma) suggests an early Pleistocene age for this lava flow. The reverse polarity of its virtual geomagnetic poles (VGPs) is in agreement with the predominant one during the Matuyama Chron and suggests that the Moreniyeu Formation constitutes another volcanic event clearly separate from those of the Cráter Formation. The K-Ar radiometric age of the Mojón Formation (3.3±0.4 Ma) locates it in the middle Pliocene. The VGP polarity would be correlated with some reverse subchron located in Gauss Chron or with the end of the Gilbert Chron. The petrographical and geochemical similarities between the studied basalt and the Somuncura plateau basalts (late Oligocene-early Miocene, located northern and eastern of the study area), together with the time lapsed among between the Mojón and Cráter basalt extrusion suggest the presence in the area of a temporarily extensive thermal anomalies.
Paleomagnetism of the latest Cretaceous-Paleocene intrusive suite of the Mezcala district, southern Mexico
Authors: Molina Garza Roberto S. , Valdivia Luis Alva
Published: August 2014

Paleomagnetic data from three plutons of the latest Cretaceous to early Tertiary Mezcala intrusive suite, of the Mezcala mining district in northern Guerrero State, in southern Mexico, are characterized by well-defined near univectorial magnetizations of both normal and reverse polarities. The magnetizations reside primarily in a ferromagnetic cubic phase (magnetite or maghemite) and partly in hematite. Within-site dispersion is relatively small, but between-site dispersion is not. The overall mean for 9 selected sites is of D=318.5° and I=47.1° (k=18.5; α95=12.3°), and is discordant with respect to the reference direction for North America. The discordance may be explained by tilt down-to-the-northeast of the structural block that contains the intrusive suite of about 40° about a NW-SE trending axis, or perhaps more likely and based on additional geological evidence, by a combination of ∼20° of counterclockwise rotation about a vertical axis and a smaller amount of tilt. Vertical-axis-rotation is supported by paleomagnetic data from nearby localities, whereas a small magnitude of tilt is supported by geological field relations. This result is consistent with the hypothesis that Laramide structures of the Guerrero-Morelos platform were reactivated by a younger deformation event, and that there is a deformation event superimposed that involves lateral slip. The contrasting orientation of NW-SE trending structures within the Mezcala mining district affecting the plutons, and N-S trending Laramide structures, suggests that the discordant paleomagnetic directions are best explained by a combination of tilt and rotation.
Relative paleointensity of the geomagnetic field over the last 21,000 years BP from sediment cores, Lake El Trébol (Patagonia, Argentina)
Authors: Gogorza C. S. G. , Irurzun M. A. , Chaparro M. A. E. , Lirio J. M. , Nunez H. , Bercoff P. G. , Sinito A. M.
Published: August 2014

Four cores from Lake El Trébol (Patagonia, Argentina) have been used to estimate regional geomagnetic paleointensity. The rock magnetic studies indicate that the magnetic mineralogy of the clay-rich sediments is dominated by pseudo-single domain magnetite in a range of grain sizes and concentration which are suitable for paleointensity studies. The remanent magnetisation at 20 mT (NRM20 mT) has been normalised using the an-hysteric remanent magnetisation at 20 mT (ARM20 mT), the saturation of the isothermal remanent magnetisation at 20 mT (SIRM20 mT) and the low field magnetic susceptibility (k). Coherence function analysis indicates that the normalised records are free of environmental influences. Our paleointensity (NRM20 mT/ARM20 mT) versus age curve shows good agreement with published record from another lake in Argentina and with records from other parts of the world, suggesting that, in suitable sediments, paleointensity of the geomagnetic field can give a globally coherent, dominantly dipolar signal.
Paleomagnetism of the Pleistocene Tequila Volcanic Field (Western Mexico)
Authors: Ceja Maria Rodríguez , Goguitchaichvili Avto , Calvo-Rathert Manuel , Morales-Contreras Juan , Alva-Valdivia Luis , Elguera José Rosas , Fucugauchi Jaime Urrutia , Granados Hugo Delgado
Published: August 2014

This paper presents new paleomagnetic results from 24 independent cooling units in Tequila area (western Trans-Mexican Volcanic Belt). These units were recently dated by means of state-of-the-art 40Ar-39Ar method (Lewis-Kenedy et al., 2005) and span from 1130 to 150 ka. The characteristic paleodirections are successfully isolated for 20 cooling units. The mean paleodirection, discarding intermediate polarity sites, is I = 29.6°, D = 359.2°, k = 26, α95 = 7.1°, n = 17, which corresponds to the mean paleomagnetic pole position P
lat = 85.8°, Plong = 84.3°, K = 27.5, A95 = 6.9°. These directions are practically undistinguishable from the expected Plestocene paleodirections, as derived from reference poles for the North American polar wander curve and in agreement with previously reported directions from western Trans-Mexican Volcanic Belt. This suggests that no major tectonic deformation occurred in studied area since early-middle Plestocene to present. The paleosecular variation is estimated trough the study of the scatter of virtual geomagnetic poles giving S
F = 15.4 with S
U = 19.9 and S
L = 12.5 (upper and lower limits respectively). These values are consistent with those predicted by the latitude-dependent variation model of McFadden et al. (1991) for the last 5 Myr. The interesting feature of the paleomagnetic record obtained here is the occurrence of an intermediate polarity at 671 ± 13 ka which may correspond the worldwide observed Delta excursion at about 680–690 ka. This gives the volcanic evidence of this event. Two independent lava flows dated as 362 ±13 and 354 ±5 ka respectively, yield transitional paleodirections as well, probably corresponding to the Levantine excursion.
Correlation between paleosol-soil magnetic signal and climate
Authors: Orgeira M. J. , Compagnucci R. H.
Published: August 2014

A comparison of the index of potential water storage (PWS) with the magnetic properties of soils in Russia and of Argentina and China paleosols suggests the existence of one or two climatic thresholds that affect the formation, preservation or depletion of ferrimagnetic minerals. Soils characterized by a positive PWS are wetted during an important part of the year, creating an appropriate environment that favors the depletion of ferrimagnetic minerals due to mainly reductive dissolution. Such soils are characterized by a depletion of detrital ferrimagnetic minerals, as observed in Argentinean soils and paleosols. On the other hand, a negative PWS prevents highly reducing conditions in the soil, and the detrital ferrimagnetic minerals are preserved. The environmental conditions of these soils allow the formation of (superparamagnetic) pedogenic minerals, together with the preservation of lithogenic minerals. These conditions produce a net magnetic enhancement of the soil, as observed in Russia and China. The second threshold in PWS could be at a positive value. Above this second threshold, the water content of the soil during practically the whole year could allow the formation of (superparamagnetic) pedogenic minerals. This threshold could explain differences in the magnetic enhancement or depletion of Argentina soils characterized by a positive PWS.
A geomagnetic total intensity anomaly originated from lightning-induced isothermal remanent magnetization: case of the Yatsugatake Magnetic Observatory, central Japan
Authors: Shimizu Hisayoshi , Koyama Takao , Koyama Shigeru , Utada Hisashi
Published: August 2014

Magnetic total intensity anomaly maps of the Yatsugatake Magnetic Observatory, central Japan, in 1975, 1982 and 2005 are examined. The magnetic environment seems to be stable during this period except for the appearance of an anomalous thin patch-pair in the middle part of the observatory sometime between 1975 and 1982. The peak-to-peak amplitude of the patch-pair is about 80 nT at 2 m above ground level. The magnetization induced by the horizontal electric current in the ground due to the lightning which hit the observatory area in July 1981 is a probable cause for the patch-pair. Modeling confirmed that the characteristics of the patch-pair are well reproduced by model magnetizations that represent lightning-induced isothermal remanent magnetization acquired from horizontal lightning electric current in the ground. The required maximum magnetization to explain the peak-to-peak amplitude of the patch-pair is estimated to be in the range of 10 to 40 A/m.
Micro-tilt changes preceding summit explosions at Semeru volcano, Indonesia
Authors: Nishi Kiyoshi , Hendrasto Muhamad , Mulyana Iyan , Rosadi Umar , Purbawinata Mas Atje
Published: August 2014

A two-axis tiltmeter installed at the summit of Semeru volcano, Indonesia, reveals two different modes of deformation, short-term and longer-term. The short-term variations are inflations precursory to the occurrence of summit explosions. The time intervals between the explosions range from several minutes to several tens of minutes, and the maximum precursory tilt change observed is about 0.1–0.2 μrad. This change in tilt is interpreted to reflect a shallow pressurization source, just beneath the active crater. The longer-term tilt changes are displayed over a period of several days to weeks, and a steady inflation during a period of nearly 2 weeks preceded the occurrence of pyroclastic flows on December 22, 2005. Both short-term and longer-term changes are confined to the radial tilt component, pointing toward the active crater and conduit. To explain both of these tilt changes we propose relatively shallow conduit pressurization, although at somewhat deeper levels in the case of longer-term deformation. The causative mechanisms involve gas exsolution, microlite crystallization, and rheological stiffening, which results in greater flow resistance and dynamic pressure in the upper conduit, thus causing the edifice tilts observed. The gas pore pressure in the magma builds until the threshold required to trigger explosive fragmentation is reached.
Atmospheric stability at 90 km, 78°N, 16°E
Authors: Hall C. M. , Aso T. , Tsutsumi M.
Published: August 2014

We employ observations obtained from a meteor wind radar to derive ambipolar diffusion coefficients, neutral temperatures, temperature gradients and, subsequently, Brunt-Väisälä frequencies at an altitude of 90 km over Svalbard (78°N, 16°E). The derived values showed a good agreement with independent measurements at each step of the analysis. Current atmospheric models are based on sparse data obtained at such high latitude, so these results represent a viable alternative for incorporating in subsequent studies of atmospheric dynamics, particularly if the derived monthly variabilities are included. The Brunt-Väisälä frequencies are then combined with wind shear measurements to estimate horizontally averaged gradient Richardson Numbers (Ri). We find Ri to be consistently larger in summer than winter due to wind shears being similarly larger in winter and augmented by the inverse seasonal variation in Brunt-Väisälä frequency. These seasonal variations result in Ri indicative of dynamic stability in summer and instability in winter. The variabilities in wind shear and Brunt-Väisälä frequency are then included to—albeit more qualitatively—illustrate the distribution between stability and static and dynamic instabilities as a function of season, using a novel portrayal pioneered by Zink and Vincent (J. Geophys. Res., 109, doi:10.1029/2003JD003992, 2004). The resulting picture is discussed in the framework of current conceptions of distribution of turbulent energy dissipation with height and season and of current opinion of the mesopause structure at 78°N.
Change of strain rate and seismicity in the Chubu district, central Japan, associated with a Tokai slow event
Authors: Kobayashi Tomokazu , Hashimoto Manabu
Published: August 2014

We have evaluated temporal variation in strain rates in the Chubu and Kinki districts, central Japan, between 1997 and 2004 using continuous GPS data of GEONET. We found that strain rates throughout the southern part of Chubu district, including the Yoro fault and Nobi fault zone, have significantly deviated from the secular crustal deformation pattern since at least 2001. The results of an inversion procedure show that the strain rate change can be well explained by a slow slip beneath the Tokai area, suggesting that the Tokai slow event has significantly affected the crustal deformation not only in the Tokai area but also in an area located more than 100 km from the main slip area of the event. We also found that the seismicity rate has clearly decreased by approximately 50% around the Yoro fault since the latter half of 2000. The δCFF inferred from our Tokai slow slip model agrees with the observed reduction in seismicity. These results were interpreted as indicating that the seismicity, which has been decreasing for several years, is controlled by the reduction in the shear stressing-rate due to the Tokai slow slip.
An efficient boundary integral equation method applicable to the analysis of non-planar fault dynamics
Authors: Ando Ryosuke , Kame Nobuki , Yamashita Teruo
Published: August 2014

We develop a novel and efficient boundary integral equation method based on the spatio-temporal formulation for the two-dimensional dynamic and quasistatic analyses of an earthquake fault in a single scheme. A major advantage of this method is its applicability to the analysis of non-planar faults with the same degree of accuracy as to that of planar faults. Calculation time and memory requirement are reduced through the employment of asymptotic representations of the integration kernels appearing in the convolution integral. Asymptotic kernels are factorized into terms dependent on space or time alone, resulting in efficient numerical computations. In addition, the dependence on time is found to vanish in the asymptotic kernels far behind the S-wave front, which also contributes to the time-saving efficiency of the calculations. We show that, in a dynamic analysis, if a 3% error is allowed for the slip rate, computation time and memory requirement are reduced by 25% and 45%, respectively, in an in-plane fault case, and by 60% and 75%, respectively, in an anti-plane fault case. This method can be employed as a powerful numerical tool in simulating an entire earthquake cycle consisting of both quasi-static and dynamic processes with a more realistic non-planar geometry of faults.