bluuebunny/crossref_metadata_2025 huggingface dataset:
Paleomagnetism of the Eastern Alkaline Province (Mexico): contribution to the time-averaged field global database and geomagnetic instability time scale
Authors: Goguitchaichvili Avto , Petronille Marie , Henry Bernard , Valdivia Luis Alva , Morales Juan , Urrutia-Fucugauchi Jaime
Published: August 2014

We report a detailed paleomagnetic and rock-magnetic study of 19 independent lava flows belonging to the Eastern Alkaline Province (EAP) in Mexico. In total, 162 oriented samples were collected in four areas (Sierra de Tantima-Alamo, Tlanchinol, Chiconquiaco-Palma Sola and Poza Rica). All sites analyzed in this study were previously dated by means of theunspikedK-Ar geochronological method (Ferrariet al., J. Volcanol. Geotherm. Res.,146, 284–306, 2005) and span from 14.6 to 1.5 Ma. Rock-magnetic experiments point to simple magnetic mineralogy. In most cases, the remanence is carried by Ti-poor titanomagnetite of pseudosingle-domain grain size. In a few cases, Ti-rich titanomagnetites are responsible for the magnetization. The characteristic paleodirections are successfully isolated for all of the studied units. The mean paleodirection, discarding two intermediate polarity sites, isD=359.5°,I=32.9°,n=17,k=30, α95=6.7°. This direction is practically undistinguishable from the expected Mio-Pliocene paleodirections, as derived from reference poles for the North American polar wander curve, and is in agreement with the previously reported directions from the western Trans-Mexican Volcanic Belt. This suggests that no major tectonic rotation occurred in studied area since the middle Miocene to Present. The paleosecular variation is estimated through the study of the scatter of virtual geomagnetic poles, givingSF=12.7 withSU=16.5 andSL=10.3 (upper and lower limits, respectively). These values are consistent with those predicted by the latitude-dependent variation model of McFaddenet al.(Geophys. Res.,93, 11583–11588, 1991) for the last 5 Myr. Eleven sites yielded reverse magnetic polarity, six are normally magnetized and two lava flows provided apparently intermediate paleodirections. An interesting feature of the paleomagnetic record obtained from EAP volcanics is that two independent lava flows, dated as 2.04±0.04 and 1.97±0.04 Ma, respectively, both yield normal paleodirections during the reverse Matuyama period. These sites were apparently formed during the Réunion geomagnetic event worldwide observed.
A relative paleointensity record of the geomagnetic field since 1.6 Ma from the North Pacific
Authors: Yamazaki Toshitsugu , Kanamatsu Toshiya
Published: August 2014

A paleomagnetic study was conducted on a sediment core KR0310-PC1 taken from the central North Pacific in order to obtain a relative paleointensity record in the Matuyama chron from this region. The core reached to about 1.6 Ma. The age control is based on the correlation of the S ratio (S
−0.1T) variations with a global oxygen-isotope stack. Isothermal remanent magnetization (IRM) was used as the normalizer of the relative paleointensity estimation; anhysteretic remanent magnetization (ARM) was not adopted because ARM is sensitive to magnetostatic interaction among magnetic particles, which is evidenced in these sediments by an inverse correlation between the ratio of ARM to saturation IRM (SIRM) and SIRM without significant magnetic grain-size changes. For the last 350 kyrs, the record of core NGC65, which was obtained at practically the same site as KR0310-PC1 and covers the Brunhes chron (Yamazaki, 1999), was incorporated because the upper part of KR0310-PC1 was physically disturbed. In the record of NGC65/KR0310-PC1, the average paleointensity in the late Matuyama chron is not lower than that during the Brunhes chron, which does not support the conclusion of Valet et al. (2005) based on their Sint-2000 stack. A spectral analysis on the NGC65/KR0310-PC1 paleointensity record shows a power at the ∼100 kyr eccentricity period. The relative paleointensity and magnetic properties of NGC65/KR0310-PC1 were compared with those of MD982185 from the western equatorial Pacific (Yamazaki and Oda, 2002, 2005). The two sites belong to different oceanographic regimes. Coherent variations in the relative paleointensity despite incoherent changes in the magnetic properties suggest that rock-magnetic contamination to the relative paleointensity is small, if any, and the ∼100 kyr period in the relative paleointensity records would reflect the geomagnetic field behavior.
High-resolution evidence for dynamic transitional geomagnetic field behaviour from a Miocene reversal, McMurdo Sound, Ross Sea, Antarctica
Authors: Roberts Andrew P. , Bakrania Anisch , Florindo Fabio , Rowan Christopher J. , Fielding Christopher R. , Powell Ross D.
Published: August 2014

We report a high-resolution record of a Miocene polarity transition (probably the Chron C6r-C6n transition) from glacimarine sediments in McMurdo Sound, Ross Sea, Antarctica, which is the first transition record reported from high southern latitudes. The transition is recorded in two parallel cores through a 10.7 m stratigraphic thickness. The sediments are interpreted as having been deposited in a marine environment under the influence of floating ice or seaward of a glacier terminus from which a large sediment load was delivered to the drill site. The core was recovered using rotary drilling, which precludes azimuthal orientation of the core and determination of a vector record of the field during the transition. However, constraints on transitional field behaviour are provided by the exceptional resolution of this record. Large-scale paleomagnetic inclination fluctuations in the two cores can be independently correlated with each other using magnetic susceptibility data, which suggests that the sediments are reliable recorders of geomagnetic field variations. Agreement between the two parallel transition records provides evidence for highly dynamic field behaviour, as suggested by numerous large-scale inclination changes (∼90?) throughout the transition. These large-scale changes occur across stratigraphically narrow intervals, which is consistent with the suggestion of rapid field changes during transitions. In one intact portion of the core, where there is no apparent relative core rotation between samples, declinations and inclinations are consistent with the presence of a stable cluster of virtual geomagnetic poles within the transition (although the possibility that this cluster represents a rapid depositional event cannot be precluded). These observations are consistent with those from other high-resolution records and provide a rare detailed view of transitional field behaviour compared to most sedimentary records, which are not as thick and which appear to have been smoothed by sedimentary remanence acquisition processes.
Early and middle Matuyama geomagnetic excursions recorded in the Chinese loess-paleosol sediments
Authors: Yang Tianshui , Hyodo Masayuki , Yang Zhenyu , Ding Lin , Fu Jianli , Mishima Toshiaki
Published: August 2014

A detailed paleomagnetic and rock-magnetic investigation on the early and middle Matuyama loess-paleosol sediments has been carried out at the Baoji section, Shaanxi province, southern Chinese Loess Plateau. Our new magnetostatigraphy revises the position of the lower Olduvai boundary from L27 to S26. Seven shortlived geomagnetic excursions, tentatively named as E1, E2, E3, E4, E5, E6, and E7, have been recognized in the L13, S22, L26, L27, S29, and upper and middle parts of L32, respectively. Results of the anisotropy of low-field magnetic susceptibility (AMS) show that the studied loess-paleosol sediments retain the primary sedimentary fabric. Rock magnetic experiments reveal that the sediments from the excursional and polarity transitional intervals have the same magnetic characteristics as those from the surrounding normal and reversed polarity intervals. Assuming a constant accumulation rate between polarity boundaries, these seven excursions are estimated to be at about 1.11 Ma (E1), 1.58 Ma (E2), 1.92 Ma (E3), 2.11 Ma (E4), 2.25 Ma (E5), 2.35 Ma (E6), and 2.42 Ma (E7) Ma. The E1 and E2 in the middle Matuyama Chron can be correlated with the Punaruu and Stage 54 (Gilsa) excursions, respectively. The E4, E5, and E7 in the early Matuyama Chron can be correlated with the Réunion II, Réunion I, and cryptochron C2r.2r-1 (X-subchron), respectively. The E3 in the lower Olduvai subchron and E6 in the early Matuyama Chron have no comparable events. At present they can only be correlated with the anomalous directions observed in the Osaka Bay core (Biswas et al., 1999). Therefore, further investigations are necessary to support their global occurrences. The present result together with the two late Matuyama excursions dated at about 0.89 Ma and 0.92 Ma (Yang et al., 2004) show that the Baoji section yields at least nine Matuyama excursions which, along with the results of the study, suggests that eight excursions occur at 0.9–2.2 Ma (Channell et al., 2002), thereby providing evidence that the short-lived geomagnetic excursions may also be a basic characteristic of the geomagnetic field during the Matuyama Chron.
Magnetizations of the seamounts in the Izu-Ogasawara arc with special reference to the origin of their normal polarity biases
Authors: Ueda Yoshio
Published: August 2014

Magnetizations of the seamounts in the Izu-Ogasawara arc are calculated using correlation analyses of magnetic anomalies and topographic data. The calculated results of the seamounts in the Sitito-Iozima ridge (present volcanic front) show normal magnetizations with a mean value of 5.10±1.38 A/m. The results also show that the majority of the seamounts in the Nisi-Sitito ridge, which are Pliocene and Middle Miocene in origin, are magnetized in a normal magnetic field direction with a mean value of 2.74±1.07 A/m. Seamounts in the Kyushu-Palau ridge, which are Oligocene in origin, also show predominantly normal polarities, with a mean of 2.67±0.71 A/m. For such a polarity bias to be explained by induced magnetization components (IM), including viscous remanent magnetization (VRM), the intensity of IM should be comparable to the mean of 2.74 A/m. However, no significant differences in the standard deviations of magnetization intensities are recognized between the seamounts in the Nisi-Sitito Ridge and those in the Sitito-Iozima Ridge, contrary to expectations if the normal polarity bias is IM in origin. Three-dimensional (3-D) multi-block model analyses are also applied to 25 seamounts having magnetization directions considerably different from those of the present magnetic field to estimate the normally and reversely magnetized volumes of the seamounts. The results show that 88% (22/25) of the seamounts have relatively greater volumes of normal magnetization compared to reverse magnetization. The IM component estimated from the 3-D multi-block model is a maximum of 0.66 A/m, which is too small to explain the observed normal polarity bias. A possible alternative explanation for the observed normal polarity bias may be enhanced volcanic activity during normal magnetic periods, although this is difficult to justify theoretically at the present time.
Size-frequency statistics of boulders on global surface of asteroid 25143 Itokawa
Authors: Michikami Tatsuhiro , Nakamura Akiko M. , Hirata Naru , Gaskell Robert W. , Nakamura Ryosuke , Honda Takayuki , Honda Chikatoshi , Hiraoka Kensuke , Saito Jun , Demura Hirohide , Ishiguro Masateru , Miyamoto Hideaki
Published: August 2014

The surface of asteroid 25143 Itokawa is covered with numerous boulders although gravity is very small compared with that of other asteroids previously observed from spacecraft. Here we report the size-frequency statistics of boulders on the entire surface of Itokawa based on high-resolution images (1 pixel ≈0.4 m) obtained by the Hayabusa spacecraft. There are 373 boulders larger than 5 m in mean horizontal dimension on the entire surface—0.393 km2—and the number density is nearly 103/km2. The cumulative boulder size distribution on the entire surface has a power-index of −3.1 ± 0.1. For the east and west sides and the head and body portions of Itokawa, the power-index of the size distributions and the number densities of boulders of these areas are thought to be similar from the statistical point of view. A global mapping of boulders shows that there is no apparent correlation in the locations of boulders and craters. The ratio of the total volume of the boulders to the total excavated volume of the craters on Itokawa is ≈25% when only craters larger than 50 m in mean diameter are considered, and this ratio is extremely larger than that on Eros and the Moon, respectively. The origin of boulders on the surface of Itokawa was examined quantitatively by calculating the number of boulders and the size of the largest boulder using a model based on impact cratering experiments. The result indicated that the boulders on the surface of Itokawa cannot solely be the product of craters. Our results suggest that the boulders originated from the disruption of the larger parent body of Itokawa, as has been described in previous papers (Fujiwata et al., Science, 312, 1330–1334, 2006; Saito et al., Science, 312, 1341–1344, 2006).
Source model of the 2007 Noto-Hanto earthquake (Mw 6.7) for estimating broad-band strong ground motion
Authors: Kurahashi Susumu , Masaki Kazuaki , Irikura Kojiro
Published: August 2014

A source model for estimating broad-band ground motions from the 2007 Noto-Hanto earthquake (Mw6.7) is estimated from a comparison of the observed records of the mainshock and synthesized motions based on the characterized asperity model using the empirical Green’s function method. The observed records of aftershocks used as the empirical Green’s functions are carefully selected to have almost the same radiation characteristics and source distance as the asperities of the mainshock. The best-fit source model consists of two asperities of different size. A large one is located just above the hypocenter, with an area of 6.3×6.3 km2and stress drop of about 26 MPa. A smaller one is located north-east of the large one, with an area of 3.6×3.6 km2and stress drop of about 10 MPa. The stress drop of the large one is about twofold higher than the average values of inland crustal earthquakes so far estimated, while that of smaller one is almost average. We found that the remarkable directivity pulses from the source model struck the northern part of the Noto peninsula, causing heavy damage in some towns there.
Analysis of the tsunami generated by the 2007 Noto Hanto earthquake
Authors: Tanioka Yuichiro
Published: August 2014

The 2007 Noto Hanto earthquake generated a small tsunami that was recorded at several tide gauge stations along the coast of the Japan Sea. The most important feature of this tsunami is that two waveforms recorded at the Wajima and Noto tide gauge stations, which are located 30 km apart, showed very different later phases—the large later phases recorded at Noto were not observed at Wajima. Numerical simulation of the tsunami indicated that the difference was caused by the shallow water bathymetry around the Noto peninsula. The large tsunami that was amplified at a few tens of kilometers off the north coast of the Noto peninsula propagated towards the Noto tide gauge station, but not towards the Wajima station. This study indicates that the propagation of a tsunami caused by a shallow earthquake beneath a coastal area is significantly affected by the local bathymetry. A comparison of the observed and computed tsunami waveforms indicated that the slip amount of the fault was 0.8 m. The seismic moment of the Noto Hanto earthquake was calculated to be 0.94 × 1019 N m (M
w 6.6).
Postseismic displacements following the 2007 Noto peninsula earthquake detected by dense GPS observation
Authors: Hashimoto Manabu , Takahashi Hiroaki , Doke Ryosuke , Kasahara Minoru , Takeuchi Akira , Onoue Kenusuke , Hoso Yoshinobu , Fukushima Yo , Nakamura Kajuro , Ohya Fumio , Honda Ryo , Ichiyanagi Masayoshi , Yamaguchi Teruhiro , Maeda Takahiro , Hiramatsu Yoshihiro
Published: August 2014

We have been conducting dense GPS observation in and around the epicentral region of the 2007 Noto peninsula earthquake since March 25, 2007, in order to detect postseismic displacements. Continuous observation has been underway at 12 sites to fill the gap of GEONET. Preliminary analysis of data up to early May shows that initial postseismic displacement rapidly decayed within 20 days after the occurrence of the mainshock. Horizontal displacements do not exceed 20 mm even at sites above the aftershock zone for this period. We also found a maximum uplift of about 20 mm there. Inversion of postseismic displacements with the variable slip model suggests a nearly right-lateral afterslip of less than 5 cm on the shallow portion of the source fault. Fitting a theoretical function to a time series of coordinate changes also suggests that the observed postseismic displacements might have been generated by afterslip.
Exploring the feasibility of on-site earthquake early warning using close-in records of the 2007 Noto Hanto earthquake
Authors: Wu Yih-Min , Kanamori Hiroo
Published: August 2014

In view of the remarkable success of the Japan Meteorological Agency earthquake early warning system (developed jointly with the National Research Institute for Earth Science and Disaster Prevention) during the 2007 Noto Hanto earthquake, we explore the use of an on-site early warning method with the hope of eventually enhancing the existing system for more robust and faster warning. We determined two early warning parametersτcandPdusing the K-NET and KiK-net data for the 2007 Noto Hanto earthquake as well as 20 large earthquakes in Japan. An extended method suggests a possibility of using the initial displacement amplitude data for faster warning. For the two nearby sites at the epicentral distances of 7 and 19 km of the 2007 Noto Hanto earthquake, an early warning can be ideally issued at 2.27 and 5.46 s, respectively, before the peak ground motion velocity occurs at the site.
Temporal variation of sound speed in ocean: a comparison between GPS/acoustic and in situ measurements
Authors: Kido Motoyuki , Osada Yukihito , Fujimoto Hiromi
Published: August 2014

The GPS/acoustic technique applied to seafloor geodesy intrinsically measures integrated sound speed along a trajectory of an acoustic signal as well as the position of a seafloor transponder array. We present here a generalized expression of sound speed variation in terms of a traveltime residual normalized to the vertical component. With this expression, residual traveltimes to any seafloor transponders will have a same value regardless of their depths and slant angles. This is valid even for the case having horizontal gradient in sound speed structure; the gradient affects only on positioning of a transponder array and not on the estimate of sound speed just beneath the observation point. We monitored temporal variation of this quantity through a GPS/acoustic survey and compared it with in situ expendable bathythermograph (XBT) measurements periodically carried out during the survey. We found that the relative change of the two independent measurements are in good agreement within 5% of the typical amplitude of temporal variation.
Laboratory experiments of particle size effect in X-ray fluorescence and implications to remote X-ray spectrometry of lunar regolith surface
Authors: Maruyama Yoko , Ogawa Kazunori , Okada Tatsuaki , Kato Manabu
Published: August 2014

We performed laboratory experiments to investigate the effect of particle size in X-ray fluorescence (XRF) with respect to interpreting remote X-ray spectrometry. To simulate microscopic roughness of the uppermost surface of lunar regolith, we used powdery specimens of crashed rocks ranging 25 to 500 μm in size and flat rock plates for comparison. Our results show that XRF intensities from powdery specimens decrease relative to those from flat plates by up to 50%, especially for larger particle size and at increasing source-to-surface-to-detector angle (phase angle). Corrections should be required for elemental analysis by XRF spectrometry in the SELENE ‘ Kaguya’ and other planetary orbiter missions.
Plasma wave observation using waveform capture in the Lunar Radar Sounder on board the SELENE spacecraft
Authors: Kasahara Yoshiya , Goto Yoshitaka , Hashimoto Kozo , Imachi Tomohiko , Kumamoto Atsushi , Ono Takayuki , Matsumoto Hiroshi
Published: August 2014

The waveform capture (WFC) instrument is one of the subsystems of the Lunar Radar Sounder (LRS) on board the SELENE spacecraft. By taking advantage of a moon orbiter, the WFC is expected to measure plasma waves and radio emissions that are generated around the moon and/or that originated from the sun and from the earth and other planets. It is a high-performance and multifunctional software receiver in which most functions are realized by the onboard software implemented in a digital signal processor (DSP). The WFC consists of a fast-sweep frequency analyzer (WFC-H) covering the frequency range from 1 kHz to 1 MHz and a waveform receiver (WFC-L) in the frequency range from 10 Hz to 100 kHz. By introducing the hybrid IC called PDC in the WFC-H, we created a spectral analyzer with a very high time and frequency resolution. In addition, new techniques such as digital filtering, automatic filter selection, and data compression are implemented for data processing of the WFC-L to extract the important data adequately under the severe restriction of total amount of telemetry data. Because of the flexibility of the instruments, various kinds of observation modes can be achieved, and we expect the WFC to generate many interesting data.
Telescope of extreme ultraviolet (TEX) onboard SELENE: science from the Moon
Authors: Yoshikawa I. , Yamazaki A. , Murakami G. , Yoshioka K. , Kameda S. , Ezawa F. , Toyota T. , Miyake W. , Taguchi M. , Kikuchi M. , Nakamura M.
Published: August 2014

The Upper Atmosphere and Plasma Imager (UPI) is to be launched in 2007 and sent to the Moon. From the lunar orbit, two telescopes are to be directed towards the Earth. The Moon has no atmosphere, which results in there being no active emission near the spacecraft; consequently, we will have a high-quality image of the near-Earth environment. As the Moon orbits the Earth once a month, the Earth will also be observed from many different directions. This is called a “science from the Moon”. The two telescopes are mounted on a two-axis gimbal system, the Telescope of Extreme ultraviolet (TEX) and Telescope of Visible light (TVIS). TEX detects the O II (83.4 nm) and He II (30.4 nm) emissions scattered by ionized oxygen and helium, respectively. The targets of extreme-ultraviolet (EUV) imaging are the polar ionosphere, the polar wind, and the plasmasphere and inner magnetosphere. The maximum spatial and time resolutions are 0.09 Re and 1 min, respectively.
Southern Ocean mass variation studies using GRACE and satellite altimetry
Authors: Kuo Chung-Yen , Shum C. K. , Guo Jun-yi , Yi Yuchan , Braun Alexander , Fukumori Ichiro , Matsumoto Koji , Sato Tadahiro , Shibuya Kazuo
Published: August 2014

The Southern Ocean is a major link between the world oceans via complicated processes associated with the melting and accumulation of the vast Antarctic ice sheets and the surrounding sea ice. The Southern Ocean sea level is poorly observed except from recent near-polar orbiting space geodetic satellites. In this study, the Southern Ocean mass variations at the seasonal scale are compared using three independent data sets: (1) the Gravity Recovery And Climate Recovery Experiment (GRACE) observed ocean bottom pressure (OBP), (2) steric-corrected satellite altimetry (ENVISAT) and, (3) the Estimating the Circulation and Climate of the Ocean (ECCO) model OBP data. The height difference between sea level derived from altimetry and steric sea level contains the vertical displacement of the Earth surface due to elastic loading. Here we provide a formulation of this loading term which has not been considered previously in other studies and demonstrate that it is not negligible, especially for regional studies. In this study, we first conduct a global comparison using steric-corrected JASON-1 altimetry with GRACE to validate our technique and to compare with recent studies. The global ocean mass variation comparison shows excellent agreement with high correlation (∼0.81) and with discrepancies at 3–5 mm RMS. However, the discrepancies in the Southern Ocean are much larger at 12–17 mm RMS. The mis-modeling of geocenter variations and the second degree zonal harmonic (J
2) degrade the accuracy of GRACE-derived mass variations, and the choice of ocean temperature data sets and neglecting the loading correction on altimetry affect the OBP comparisons between GRACE and altimetry. This study indicates that the satellite observations (GRACE and ENVISAT) are capable of providing an improved constraint of oceanic mass variations in the Southern Ocean.
Analysis of gravity field variations derived from Superconducting Gravimeter recordings, the GRACE satellite and hydrological models at selected European sites
Authors: Neumeyer J. , Barthelmes F. , Kroner C. , Petrovic S. , Schmidt R. , Virtanen H. , Wilmes H.
Published: August 2014

If we restrict the spatial resolution to a half-wavelength of about 1500 km and the temporal resolution to 1 month, GRACE-derived temporal gravity variations can be resolved within the μgal (10-8 m/s2) range. A comparison with ground gravity measurements from selected Superconducting Gravimeter (SG) stations forming the Global Geodynamics Project (GGP) provides an independent validation. For this study, five European SGstations were selected that both cover a large test field and allow closely located SG-stations to be studied. Prior to this comparison, GRACE and SG data sets have to be reduced for the same known gravity effects due to Earth and ocean tides, pole tide, and atmosphere. After these reductions, the remaining part can be mainly attributed to mass changes in terrestrial water storage. For this reason, gravity variations derived from global hydrological models are included in the comparison of SG and GRACE results. Conversely, the hydrology models can be checked by gravity variations determined from GRACE and SG observations. For most of the SG locations investigated here, the comparison based primarily on computed correlations shows quite a good agreement among the gravity variation derived from the three different kinds of data sets: SG, GRACE, and hydrology models. The variations in SG gravity (point measurements) prove to be representative for a large area within the μgal accuracy range, if local gravity effects are removed correctly. Additionally, a methodology for an analysis of dominant common features based on the EOF-technique is proposed and illustrated. The first principal component shows strong periodicity, and the search for arbitrary periods confirms a strong common annual component, which reduces the total signal content considerably. The first eigenvector reveals common features and differences between distinct SG stations. Discrepancies between SG, GRACE, and hydrology models at individual SG stations, detected by both methods, may provide valuable hints for further investigations of respective data series.
Mapping the upper mantle discontinuities beneath China with teleseismic receiver functions
Authors: Shen Xuzhang , Zhou Huilan , Kawakatsu Hitoshi
Published: August 2014

We used teleseismic body waves recorded at stations of the Chinese Center of Digital Seismic Network to map the upper mantle discontinuities beneath continental China. The CRUST2.0 and anS-tomography model beneath each station were combined with the one-dimensional tracing method to convert time series of radial receiver functions to depth series. Clear signatures corresponding to the 410- and 660-km discontinuities (‘410’ and ‘660’) are visible at almost all of the stations. The averageSvelocity contrast of ‘410’ beneath the study area is close to the global average, but that for ‘660’ is smaller than the global average. The average depth of ‘410’ is 413 km, and the peak-to-peak topography is about 36 km, with regional depressions that correlate with the Datong quaternary volcano in northern China. The ‘660’ topography exhibits a peak-to-peak variation of about 43 km, and its average depth is 669 km; the depressions of the ‘660’ in northeastern, southeastern and northern China are well correlated with the past subduction around the Pacific Ocean and Philippine Sea. The width of the transition zone is also increased in the region with the deeper ‘660’. Our results would appear to indicate that there may be a low-velocity layer below a depth of approximately 600 km that may be the accumulated garnetite layer of an ancient crust above the ‘660’.
A study of VLF wave propagation characteristics in the earth-ionosphere waveguide
Authors: Indira Devi M. , Khan I. , Madhusudhana Rao D. N.
Published: August 2014

In the light of newly found applications for very low frequency (VLF) (3–30 kHz) measurements in the prediction of earthquakes and the detection of lightning discharges and gamma ray bursts, there has been a revival of interest in the study of VLF propagation in the earth-ionosphere waveguide. The propagation characteristics of VLF radiowaves in the earth-ionosphere waveguide critically depend upon the lower ionospheric ionization, which determines the conductivity profile of the upper boundary of the waveguide. In this context, it is potentially worthwhile to revisit the waveguide mode analysis to compute propagation parameters of long-distance VLF transmissions while taking into account different approximations to the ionospheric conductivity. We have carried out a waveguide mode analysis of 16-kHz VLF waves traveling great distances, assuming three different models for conductivity. The computational results show that at heights of less than 70 km, the change in the phase of the VLF waves due to changes in phase velocity is smaller when the ionosphere is sharply bound and assumed to have finite conductivity rather than infinite conductivity. The effect of a non-sharp or diffuse boundary at the top of the earth-ionosphere waveguide is found to cause a lowering of the height of reflection.
Development of an automatic procedure to estimate the reflection height of tweek atmospherics
Authors: Ohya Hiroyo , Shiokawa Kazuo , Miyoshi Yoshizumi
Published: August 2014

This paper presents an automated procedure to estimate apparent reflection heighth(from the cutoff frequency for the first waveguide mode,fc), horizontal propagation distanced, and propagation timeTgof tweek atmospherics. Tweek data recorded at the Kagoshima Observatory (31.48°N, 130.72°E), Japan, were used to evaluate the procedure by comparing the results estimated by the automatic method to those read manually by an operator. The two types of results showed differences (automatic–manual) of +0.58 km, −9.9 Hz, and +3058.9 km for meanh,fc, andd, respectively. The difference inh(fc) was less than the resolution of the fast Fourier transform used to obtain the tweek spectra. These comparisons indicate that the automatic estimation procedure of tweek parameters developed in this paper performs well and is a useful tool for studying long-term height variations of the ionosphericDand lowerEregions using very low frequency (VLF) and extremely low frequency (ELF) records observed in Japan over the past 30 years.
Spatiotemporal distribution of very-low frequency earthquakes in Tokachi-oki near the junction of the Kuril and Japan trenches revealed by using array signal processing
Authors: Asano Youichi , Obara Kazushige , Ito Yoshihiro
Published: August 2014

We have developed a signal-processing method for array data recorded at densely and widely distributed tiltmeter stations in Japan and analyzed the data recorded during the last 5 years. Filtered seismograms were analyzed to detect coherent seismic waves and estimate the epicenters. In order to identify very-low frequency earthquakes (VLFEs) with only low-frequency energies only, we selected only events not listed in the earthquake catalogue provided by the Japan Meteorological Agency from all of the detected events. The epicenters of the obtained VLFEs were estimated to be distributed not only along the Nankai Trough, where the events occurred on the reverse faults in the well-developed accretionary prism, but also in Tokachi-oki without accretion. This result suggests that even in such a region, VLFEs can occur on a shallower portion of the plate boundary than the seismogenic region of ordinary earthquakes and/or in a deformed area of the overriding plate along the subduction zone without accretion.
A precise hypocenter determination method using network correlation coefficients and its application to deep low-frequency earthquakes
Authors: Ohta Kazuaki , Ide Satoshi
Published: August 2014

A knowledge of the precise locations of deep low-frequency earthquakes (LFEs) along subduction zones is essential to be able to constrain the spatial extent of various slow earthquakes and the underlying physical processes. We have developed a hypocenter determination method that utilizes the summed cross-correlation coefficient over many stations, denoted a network correlation coefficient (NCC). The method consists of two parts: (1) an estimation of relative hypocenter locations for every pair of events by a grid search, and (2) a linear least squares inversion for self-consistent relative hypocenter locations for the initial centroid. We have applied this method to ten LFEs in the Tokai region, Japan. Statistically significant values of NCC indicate the relative locations for many pairs, which in turn determine the self-consistent locations. While the catalog depths are widely distributed, the relocated hypocenters fall within a 2-km depth range, which implies that LFEs in the Tokai region occur on the plate interface, similar to LFEs in western Shikoku.
Fault models of unusual tsunami in the 17th century along the Kuril trench
Authors: Satake K. , Nanayama F. , Yamaki S.
Published: August 2014

Geologic evidence has shown that unusual tsunami deposits are traced as high as 18 m above the current sea level or as far as 1–4 km inland from the shoreline on the Pacific coast of eastern Hokkaido, and that such unusual tsunamis have recurred at about 500 year interval with the most recent event in the 17th century. We computed coastal tsunami heights along the Hokkaido and Sanriku coasts and inundation at five coastal marshes in Hokkaido where the tsunami deposits were mapped. Three types of faults were tested: giant fault, tsunami earthquake and interplate earthquake models. The giant fault model, with the largest seismic moment, yields the lowest tsunami heights and smaller inundation than the distribution of tsunami deposits in Hokkaido, while the tsunami heights are largest in Sanriku. The tsunami earthquake model yields little inundation in Hokkaido and the smallest heights in Sanriku. The interplate earthquake model produces the largest tsunami heights and inundation in Hokkaido, reproducing the distribution of tsunami deposits on the Nemuro coast. The multi-segment interplate earthquake with variable slip (10 m on Tokachi and 5 m on Nemuro segment) can reproduce the distribution of tsunami deposits on the Tokachi coast as well, and considered as the best source model for the 17th century tsunami, although the Sanriku tsunami heights are more than 3 m, exceeding an inferred detection threshold of historical documents. The seismic moment is estimated as 8 × 1021N m (Mw8.5). Comparison with the recent 2003 Tokachi-oki earthquake indicates that the 17th century tsunami source was longer and located further offshore at shallower depth.
Shock equation of state of basalt
Authors: Sekine Toshimori , Kobayashi Takamichi , Nishio Mineyuki , Takahashi Eiichi
Published: August 2014

Detailed wave profiles for Kinosaki basalt at pressures up to 25 GPa are measured using a laser velocity interferometer in order to determine the dynamic properties. The results indicate a Hugoniot elastic limit of ∼2 GPa and a relationship between shock velocity (U
s) and particle velocity (U
p) approximated by U
s (km/s) = 3.5 + 1.3U
p (km/s) in the low-pressure plastic region (U
p below ∼4 km/s). These data are compared with the known data for rocks with basaltic compositions, and tensile strength of the basaltic rocks was found to be about one tenth of that of compression strength.
Does the 2007 Noto Hanto earthquake reveal a weakness in the Japanese national seismic hazard map that could be remedied with geological data?
Authors: Toda Shinji , Awata Yasuo
Published: August 2014

The Noto Hanto earthquake struck one of the lowest earthquake probability regions on the national seismic hazard map of Japan. To contribute to future updates of the hazard map, we examined the predictability of the 2007 earthquake on the basis of geological data that were available before it occurred. Sonic prospecting profiles of active faulting and the absence of an onshore fault could have limited the potential rupture length to 12–15 km, similar to the 2007 source. Empirical relationships between magnitude and fault length would have given usMj= 6.6–6.8 andMw= 6.3–6.4. The emergence of one marine terrace, which inclines to the south and reaches an altitude of approximately 50 m, can be dated to 120–130 ka and yields an uplift rate of approximately 0.4 mm/year.Mw-displacement empirical relationships and examples of recent blind fault events that have occurred at various locations around the world suggest that the conceivable maximum coseismic uplift of such shocks is 40–70 cm. Together with the uplift rate, we would have obtained an average recurrence interval of 1000–2000 years and, consequently, a 1.5–3.0% time-independent (Poisson) probability for 30 years. In addition, the significant inclination of the marine terraces—3.2 per mille (0.32%)—is better explained by the accumulation of frequent southward tilting as large as that of the 2007 type event with approximately 1600-year intervals, without any significant contributions from other seismic sources. We therefore conclude that the Noto Hanto earthquake source would have been better evaluated and identified if we had taken into account not only major active faults but also the active tectonics of moderate-size faults and their associated scale and rate.
Coastal deformation associated with the 2007 Noto Hanto earthquake, central Japan, estimated from uplifted and subsided intertidal organisms
Authors: Awata Yasuo , Toda Shinji , Kaneda Heitaro , Azuma Takashi , Horikawa Haruo , Shishikura Masanobu , Echigo Tomoo
Published: August 2014

The March 25, 2007 Noto Hanto earthquake (Mj= 6.9,Mw= 6.7) generated vertical crustal movement along the northwestern coast of the Noto Peninsula, central Japan. Soon after the event, we estimated the pattern and amount of coseismic coastal movement based on uplifted and subsided intertidal sessile organisms. Our observations reveal a broad 20-km-wide asymmetric zone of surficial deformation above and across the south-dipping source fault, with a steep north-facing frontal limb and a gentle south-facing back limb. The maximum coseismic uplift was approximately 40 cm at the crest of the zone of deformation. The result of forward modeling suggests that the top of the south-dipping source fault is buried at a depth of approximately 2 km, and that 1.2 m of slip on the fault provides the best fit to our surface observations. Our results demonstrate that traditional field investigations should be combined with modern instrumental observations such as GPS and InSAR to obtain the most effective and reliable spatio-temporal estimates of crustal movement associated with large earthquakes.
Rupture process of the 2007 Chuetsu-oki, Niigata, Japan, earthquake—Waveform inversion using empirical Green’s functions—
Authors: Nozu Atsushi
Published: August 2014

A waveform inversion was conducted to reveal the rupture process of the 2007 Chuetsu-oki, Niigata, Japan, earthquake using near-source strong ground motion records. Based on recent results on the re-location of the aftershocks, a southeast-dipping fault plane was assumed. Aftershock records were used as empirical Green’s functions to avoid uncertainty in subsurface structure in and around the source region for the calculation of Green’s functions. To ensure that the path and the site effects are shared between ground motions from the mainshock and those from the aftershocks, the mainshock fault plane was divided into three domains, each of which was allocated to one of the aftershocks used. Based on the results of the inversion, a major distinctive asperity was identified approximately 20 km southwest of the hypocenter, near Kashiwazaki City. A minor, rather obscure asperity was recognizable between the hypocenter and the major asperity. The rupture continued approximately 10 s. Agreement between the observed and synthetic ground motions is quite satisfactory, not only for the portions used for the inversion but also for the later phases. The rupture of the main asperity, which is close to Kashiwazaki City, may be a primary reason for the large amplitude ground motion and damage in Kashiwazaki City.
Rock magnetic and paleointensity results from Mesozoic baked contacts of Armenia
Authors: Shcherbakova V. V. , Perrin M. , Shcherbakov V. P. , Pavlov V. E. , Ayvaz’yan A. , Zhidkov G. V.
Published: August 2014

Samples were obtained from three baked contacts and one lava flow along the upper Turonian-lower Coniacian Tovuz section, two baked contacts along the upper Coniacian-lower Santonian Paravakar section in the northern part of Armenia, and three baked contacts along the Titonian-Valanginian Kafan section in southern Armenia. A total of 130 samples were studied. Updated mean paleomagnetic poles were calculated for the Upper Cretaceous Tovuz-Paravakar sections (65.6°N, 162.2°E, A95 = 4.3, paleolatitude = 27.0 ± 3.4°) and the Upper Jurassic-Lower Cretaceous Kafan section (61.7°N, 158.9°E, A95 = 4.8°, paleolatitude = 24.7 ± 3.8°). Paleointensity determinations could be estimated from two of the upper Cretaceous and three of the Upper Jurassic-Lower Cretaceous baked contacts, corresponding to a 30% success rate. The mean virtual dipole moments obtained were low (1.7-5.5 × 1022 A m2), which is in agreement with data published by Bol’shakov and Solodovnikov (1981a, 1983) for the same sections (3.0-4.4 × 1022 A m2). Our results support the hypothesis of the Mesozoic Dipole Low, even though the overall data are widely dispersed.
Thermal fluctuation fields in basalts
Authors: Muxworthy Adrian R. , Heslop David , Michalk Daniel M.
Published: August 2014

The thermal fluctuation field (Hf) is central to thermoremanent acquisition models, which are key to our understanding of the reliability of palaeomagnetic data, however,Hfis poorly quantified for natural systems. We reportHfdeterminations for a range of basalts, made by measuring rate-dependent hysteresis. The results for the basalts were found to be generally consistent within the space ofHfversus the coercive forceHC, i.e., the “Barbier plot”, which is characterized by the empirically derived relationship; logHf∝ 1.3 logHCobtained from measurements on a wide range of different magnetic materials. Although the basalts appear to occupy the correct position within the space of the Barbier plot, the relationship within the sample set, logHf∝ 0.54 logHC, is different to the Barbier relationship. This difference is attributed to the original Barbier relationship being derived from a wide range of different synthetic magnetic materials, and not for variations within one material type, as well as differences in methodology in determiningHf. We consider the relationship betweenHCand the activation volume,υact, which was found to beHC∝ υ ***** for our mineralogically homogeneous samples. This compares favourably with theoretical predictions, and with previous empirical estimates based on the Barbier plot, which defined the relationship asHC∝****.
Natural magnetite nanoparticles from an iron-ore deposit: size dependence on magnetic properties
Authors: Rivas-Sánchez M. L. , Alva-Valdivia L. M. , Arenas-Alatorre J. , Urrutia-Fucugauchi J. , Perrin M. , Goguitchaichvili A. , Ruiz-Sandoval M. , Molina M. A. Ramos
Published: August 2014

We report on the discovery of magnetite nanoparticles ranging in size from 2 to 14 nm in the mineralized zones of the Pe~na Colorada iron-ore deposit, southern Mexico. Micrometric scale magnetite was magnetically reduced and divided into distinct size ranges: 85-56 μm, 56-30 μm, 30-22 μm, 22-15 μm, 15-10 μm, 10-7 μm and 7-2 μm. Nanometric-scale magnetite in the size range 2-14 nm was identified. The magnetite was characterized by X-ray diffraction, transmitted and reflected light microscope, high-resolution transmission electron microscopy (TEM), high angle annular dark field, Mossbauer spectroscopy and its magnetic properties. Crystallographic identification of nanostructures was performed using high-resolution TEM. Characteristic changes were observed when the particles make the size transition from micro- to nanometric sizes, as follows: (1) frequency-dependent magnetic susceptibility percentage (χFD%) measurements show high values (13%) for the 2-14 nm fractions attributed to dominant fractions of superparamagnetic particles; (2) variations of χFD% < 4.5% in fractions of 56-0.2 μm occur in association with the presence of microparticles formed by magnetite aggregates of nanoparticles (< 15 nm) embedded in berthierine; (3) Mössbauer spectroscopy results identified a superparamagnetic fraction; (4) nanometric and 0.2-7 μm grain size magnetite particles require a magnetic field up to 152 mT to reach saturation during the isothermal remanent magnetization experiment; (5) coercivity and remanent magnetization of the magnetite increase when the particle size decreases, probably due to parallel coupling effects; (6) two-magnetic susceptibility versus temperature experiments of the same 2-14 nm sample show that the reversibility during the second heating is due to the formation of new magnetite nanoparticles and growth of those already present during the first heating process.
Palaeosecular variation for 0.1-21 Ka from the Okataina Volcanic Centre, New Zealand
Authors: Tanaka Hidefumi , Komuro Naoyuki , Turner Gillian M.
Published: August 2014

Studies of palaeodirections and palaeointensities were carried out on mainly rhyolitic lavas and pyroclastics from the Okataina Volcanic Centre, New Zealand, which has erupted during the past 32 kyr. Of the 17 sites studied, 14, spanning the period 0.1-21 Ka yielded good mean palaeodirections, while three carried unstable natural remanent magnetizations. Of 49 specimens from 7 sites, on which Thellier palaeointensity experiments were carried out, 21 specimens gave successful results, yielding 3 site mean palaeointensities for 1886 AD, 5 Ka, and 7.5 Ka. When the new palaeodirections, together with previously reported archaeo and volcanic data, were compared with sedimentary records from New Zealand and eastern Australia for the last 10 kyr, good agreement was obtained in inclination but discrepancies were observed in declination. The new 7.5 Ka and 5 Ka palaeointensities are moderately high and relatively low, respectively, and are concordant with the global trend. The mean palaeointensity obtained for 1886 AD is, 11% higher than the IGRF1900. The difference is scarcely significant, but might indicate a small bias toward high values. Although the sedimentary directional curves show excellent agreement with the prediction from CALS7K, the fit of the palaeointensity data to model values was relatively poor over the wider Pacific region. Further reliable palaeointensity data are needed to solve the discrepancy.