bluuebunny/crossref_metadata_2025 huggingface dataset:
Crustal structure of the ocean-island arc transition at the mid Izu-Ogasawara (Bonin) arc margin
Authors: Nishizawa Azusa , Kaneda Kentaro , Nakanishi Ayako , Takahashi Narumi , Kodaira Shuichi
Published: August 2014

Wide-angle refraction experiments were conducted to reveal the crustal structure at the transition between the intra-oceanic island arc crust of the mid Izu-Ogasawara (Bonin) arc and the backarc oceanic crust of the Shikoku Basin. The island arc crust consists of an upper crust about 5 km thick with a P-wave velocity <6.0 km/s, a middle crust 5 km thick with a P-wave velocity of 6.0–6.3 km/s, and a lower crust 10 km thick with a P-wave velocity of 6.8–7.2 km/s. The total crustal thickness is about 20 km. The thickness thins to approximately 6 km over a distance of 30 km at the western margin of the Izu-Ogasawara arc (IOA). These features are very similar to those of the northern IOA, which indicates that the crustal structure is relatively constant within 200 km at the northern and mid IOA. The Kinan Escarpment, a 500-km-long fault with a maximum offset of 800 m, characterizes the transition zone between the IOA and Shikoku Basin. The seismic crustal model indicates that the escarpment is a fault which tears the whole oceanic crust along the western margin of the IOA. However, no significant differences exist in the crustal structure on either side of the escarpment, and the Kinan Escarpment seems to be a zone of the structural weakness from its birth.
Determination of the 30-year creep trend on the Ismetpaşa segment of the North Anatolian Fault using an old geodetic network
Authors: Kutoglu H. S. , Akcin H.
Published: August 2014

The Ismetpaşa segment of the North Anatolian Fault was ruptured during both the 1944 (M w = 7.2) Gerede and 1951 (M w = 6.9) Kurşunlu earthquakes. The field studies carried out in the aftermath of these two major earthquakes showed that the Ismetpaşa segment had experienced a creep movement. To monitor the surface creep, a geodetic network with six control points was established on the segment. This network was observed three times—in 1972, 1982 and 1992. Based on our evaluations of those observations, the creep on the segment was geodetically determined to be 1.02 cm/year (1972–1982) and 0.93 cm/year (1982–1992) respectively. In 1999, the North Anatolian Fault experienced two major shocks—the M w = 7.4 Gölcük and M w = 7.2 Düzce earthquakes—both on the western part of the Ismetpaşa fault. Using the global positioning system, our surveying team observed the network one more time in 2002 to assess whether these earthquakes affected the creep of the Ismetpaşa segment, or not. The evaluation of the observations revealed a creep of 0.78 cm/year for the period 1992–2002. This result reveals that the creep of the segment has decreased in a linear fashion between 1972 and 2002 and that it had not been triggered by the Gölcük and Düzce earthquakes.
Structural pattern at the northwestern sector of the Tepic-Zacoalco rift and tectonic implications for the Jalisco block, western Mexico
Authors: Urrutia-Fucugauchi Jaime , González-Morán Tomás
Published: August 2014

Analysis of the aeromagnetic anomalies over the northwestern sector of the Tepic-Zacoalco rift documents a NE-SW pattern of lineaments that are perpendicular to the inferred NW-SE boundary between the Jalisco block and the Sierra Madre Occidental. The boundary lies within the central sector of the Tepic-Zacoalco rift immediately north of the Ceboruco and Tepetiltic stratovolcanoes and extends up to the San Juan stratovolcano, where it intersects the NE-SW magnetic anomaly lineament that runs toward the Pacific coast (which intersects two volcanic centers). This N35°E lineament separates the central rift zone of low amplitude mainly negative anomalies (except those positive anomalies over the stratovolcanoes) from the zone to the north and west characterized by high amplitude positive long wavelength anomalies. The NE-SW lineament is parallel to the western sector of the Ameca graben and the offshore Bahia de Banderas graben and to the structural features of the Punta Mita peninsula at the Pacific coast, and thus seems to form part of a regional NE-SW pattern oblique to the proposed westward or northwestward motion of the Jalisco block. The orientation of this regional structural pattern at the northern end of the Tepic-Zacoalco rift seems consistent with proposed dominant SW-directed extension along the rift during the Pliocene and Quaternary, rather than with NW-SE lateral strike-slip faulting. The orthogonal pattern that characterizes the northernmost boundary of the Tepic-Zacoalco rift is oblique to the pattern observed in the Grande de Santiago river (which conforms the northern limit of the rift) and for the central-eastern sectors of the Ameca graben (south of the rift). This spatial arrangement of major lineaments and structural elements points to a complex tectonic history for the region that includes the rifting of the Gulf of California and margin deformation due to plate convergence and kinematic re-organization events, and which may have resulted in distinct episodes of left-lateral, right-lateral and normal faulting in Neogene times, as proposed in recent studies.
Magnetostratigraphy of the Cretaceous/Tertiary boundary and early Paleocene sedimentary sequence from the Chicxulub Impact Crater
Authors: Rebolledo-Vieyra Mario , Urrutia-Fucugauchi Jaime
Published: August 2014

We report on the magnetostratigraphy of the Chicxulub crater impact breccias and first 15 meters of the Paleocene sedimentary sequence recovered in three boreholes of the UNAM Scientific Drilling Program. Three geomagnetic polarity zones are documented in the impact breccias and sedimentary sequence, which span from chron 29R to 28N. For the 15 m interval they represent ∼2.5 Ma, which yields low apparent sedimentary rates for boreholes UNAM-5 (110 km from the center of the crater) and UNAM-7 (127 km from the center of the crater). The carbonate sedimentary sequence can be associated to a shallow basin depositional environment. In these boreholes the thickness between the 29R and the 29N chrons is just 0.5 m, suggesting that during the 100 ka from the K/T boundary to the polarity transition sediments were not deposited or eroded. Within borehole UNAM-6 (152 km from the center of the crater) it appears that sediments containing chron 29N are missing, the lack of the upper breccias, the long duration of a reversal event within the base of the sequence and low apparent sedimentary rate of 3.3 m/Ma, suggests a hiatus within the impact breccias and the basal Paleocene sedimentary sequence. Magnetic susceptibility logs confirm absence of the upper breccias at UNAM-6 borehole. Magnetic susceptibility values increase towards the base of the sequence, suggesting that basement and melt clasts were subjected to a low temperature hydrothermal alteration.
Hydrocarbon-induced magnetic contrasts in some Venezuelan and Colombian oil wells
Authors: Costanzo-Alvarez Vincenzo , Aldana Milagrosa , Díaz Marisel , Bayona Germán , Ayala Carolina
Published: August 2014

In this work we present a review of our current research on the subject of near-surface hydrocarbon-induced magnetic contrasts. Samples are drill cuttings from number of Venezuelan and Colombian oil fields, and some soils from an oil prospective area. Results from rock magnetic, electronic paramagnetic resonance (EPR), extractable organic matter (EOM) and scanning electron microscopy (SEM) are discussed in light of new experiments. Two types of magnetic susceptibility (MS) anomalies have been recognized in oil well samples (i.e. A and B-like). A-like anomalies are probably associated to a reducing environment caused by the underlying reservoirs, whereas B-like anomalies might reflect lithological contrasts. Samples from A-like anomalous levels reveal the restrictive presence of authigenic spherical aggregates of submicronic Fe-rich magnetic crystals. Peaks of organic matter free radical concentration (OMFRC) and EOM are also observed at depth levels that lie close to those with A-like anomalies. Hysteresis ratios J
rs/Js and H
cr/Hc could be used as additional criteria to recognize A-like magnetic contrasts. In soil samples, MS and OMFRC anomalies coincide with peaks of ethane concentration and an area underneath where source rock has reached appropriate conditions of pressure and temperature to generate hydrocarbons (Cretaceous kitchen). Different types of crude oils and subsurface structures could have also a direct effect on the observed amplitude of A-like anomalies.
Review of magnetism and heavy metal pollution studies of various environments in Argentina
Authors: Chaparro Marcos A. E. , Gogorza Claudia S. G. , Chaparro Mauro A. E. , Irurzun María A. , Sinito Ana M.
Published: August 2014

In recent years, the number of studies of pollution and magnetic proxies for environmental pollution in developed countries, especially European and North American countries, has gradually increased from the 80’s to date. Despite this trend that shows a positive interest in taking care of the environment and researching into the environmental impact of several human activities, pollution studies in Latin American countries have been reducing in number. Moreover, studies of magnetic proxies for pollution are scarce; in particular, studies of this nature has been carried out in Argentina over the past few years by Chaparro and coworkers. Studies of magnetic enhancement in soils due to the burning mechanism are discussed and the results of burnt soils affected by fires of different nature and natural soils are compared, taking into account their magnetic carriers. Nevertheless, this article deals mainly with the first studies of magnetic proxies for pollution conducted in a province in Argentina. Soils, lagoon and stream sediments from three areas were studied. These areas comprise La Plata, Chascomús, and Tandil districts. The influence of pollution was investigated in Tandil and La Plata, revealing magnetic enhancement and the presence of pollutants only in Tandil soils. On the other hand, stream and lagoon sediments were studied in La Plata and Chascomús. Magnetic carriers and the contents of some heavy metals were identified and investigated in both areas separately. Magnetic parameters show distinctive points and wide areas affected by pollution. Furthermore, this magnetic inference is supported by high contents of heavy metals, especially lead and zinc. Finally, a new statistical study of multiple correlation analysis concerning data from La Plata and Chascomús areas was tried in order to investigate the existence of a linear relation between sets of several magnetic parameters and several chemical variables.
Present-day crustal motion in northeast China determined from GPS measurements
Authors: Meng Guojie , Shen Xuhui , Wu Jicang , Rogozhin Eugene A.
Published: August 2014

GPS measurements carried out between 1997 and 2005 at 37 stations distributed throughout northeast China place indicate the presence of constraints on the present-day crustal motion in this region. Velocity vectors relative to Eurasia are very small, only 1.58 mm/yr on average, indicating that the region is generally stable. With the exception of five stations that show significant motion at the 95% confidence level, the insignificant motion of most stations precludes a clear association of their velocities with the rotation of a separate rigid microplate, suggesting that northeast China belongs to Eurasia plate. The velocities of several stations in the southwestern corner of the studied region consistently point to the southeast. This is most probably a distant effect of the eastward motion of northern Tibet. Small velocities of the stations of around the time of the 1999 M

W
= 7.1 and 2002 M

W
= 7.3 deep-focus earthquakes, which occurred in Wangqing county, suggest that these earthquakes did not cause wide-spread deformations to the shallow crust.
GPS observations of post-storm TEC enhancements at low latitudes
Authors: Kutiev Ivan , Otsuka Yuichi , Saito Akinori , Watanabe Shigeto
Published: August 2014

In a previous work (J. Geophys. Res., 110(A01308), 1–11, 2005), the authors developed an original approach to the processing of total electron content (TEC) data obtained by GPS signals from the Japan receiver network. This approach includes removing the diurnal and seasonal variation carried by 27-day medians and the solar rotation periodicity. The relative deviations of TEC from the median—from all measured locations at a given hour—were then approximated by a regression line along the main prolongation of the Japan islands, between latitudes 24° and 45°N. The two variables of the regression line, the average value at the center and the slope were obtained as a time series, and their behavior during geomagnetic storms in the period 2000–2002 were analyzed. One interesting result was the observed enhancement of TEC at the end of the recovery phase of the storms. The slope variations clearly showed that this enhancement started from the south and was interpreted as a poleward expansion of equatorial crest. In the present paper we further analyze this post-storm phenomenon, adding foF2 data from Japanese Kokubunji and Okinawa ionosondes. We also show the latitude extension of the poleward expansion by using lat/UT contour plots. The results confirm that most of the post-storm TEC enhancements are part of the equatorial crest region which extends poleward during nighttime. In some cases, the enhanced TEC structures develop by separating from the crest region. Daytime TEC enhancements were also observed. Their structures are not confined to the equatorial crests region, but occupy the whole latitude range considered in this study. TEC post-storm enhancements were generally found to be in agreement with foF2 variations.
The influence of pore fluids on seismic wave velocities under high temperature and high pressure conditions: Development of a new technique with gas apparatus at AIST, Japan
Authors: Kitamura Keigo , Masuda Koji , Takahashi Miki , Nishizawa Osamu
Published: August 2014

In order to study the effects of different pore fluids on elastic-wave velocities, we developed a new system for measuring elastic wave velocities (V

p
and V

s
) under high-pressure conditions up to 200 MPa and high-temperature conditions up to 200°C using a gas-medium high-pressure and high-temperature apparatus. During the V

p
and V

s
measurements, we can also control the pore pressures from outside the rock samples. We measured V

p
and V

s
of the Berea sandstone samples under three different pore-fluid conditions including dry (without pore-fluids), in the presence of pore water, and in the presence of pore gas. Here we describe our new measurement technique and establish the capacity of the gas apparatus and measurement technique and their potentialities for the solution of geological problems.
Strong ground motions recorded by a near-source seismographic array during the 16 August 2005 Miyagi-Ken-Oki, JAPAN, earthquake (Mw 7.2)
Authors: Nakahara Hisashi , Sawazaki Kaoru , Takagi Nobumasa , Nishimura Takeshi , Sato Haruo , Fujiwara Hiroyuki
Published: August 2014

An earthquake of Mw 7.2 took place on August 16, 2005 at a plate boundary between the Pacific plate and the North American plate off the coast of Miyagi Prefecture, Northeast Japan. During the Miyagi-Ken-Oki event, we succeeded in recording strong ground motions at six stations in a seismograhic array with an epicentral distance of about 70 km, where we have been operating seven strong-motion seismometers in an aperture of about 500 m since April 2004. The predominant period of the ground motion was shorter than 0.3 s. The peak ground acceleration exceeded 1.7 g at a station where non-linear site response may have occurred during the mainshock. The short-period strong ground motions show a large spatial variation, with up to a ten-fold difference in amplitude even within the array. However, there is a similarity between waveforms registered at different stations for periods longer than 0.4 s. Therefore, the difference in the ground motions may be mainly attributed to the difference in the shallow structure just beneath the stations.
Nonlinear site response from the 2003 and 2005 Miyagi-Oki earthquakes
Authors: Tsuda Kenichi , Steidl Jamison
Published: August 2014

We estimate nonlinear site response by comparing site response estimates from the 16 August 2005 Mj=7.2 and 26 May 2003 Mj=7.0 Miyagi-Oki earthquakes with site response estimates from aftershocks of the 2003 event. Site response is solved by a spectral inversion technique to separate source, path, and site components. The constraint motion in the inversion is a regional attenuation model derived from fitting the spectra of data recorded at borehole KiK-net stations in the region and a theoretical source spectrum for each event determined using the same borehole stations. Site response is calculated at the surface of the KiK-net and K-NET stations. In general, the average aftershock site response is larger than for the two mainshocks, especially at a higher frequency. When comparing site response with input ground motion level, the predominant frequency and the site response values tend to decrease as the level of input ground motion increases.
Precise aftershock distribution of the 2005West Off Fukuoka Prefecture Earthquake (Mj=7.0) using a dense onshore and offshore seismic network
Authors: Uehira Kenji , Yamada Tomoaki , Shinohara Masanao , Nakahigashi Kazuo , Miyamachi Hiroki , Iio Yoshihisa , Okada Tomomi , Takahashi Hiroaki , Matsuwo Norimichi , Uchida Kazunari , Kanazawa Toshihiko , Shimizu Hiroshi
Published: August 2014

The 2005 West Off Fukuoka Prefecture Earthquake (Mj=7.0) occurred on March 20, 2005 in the northern part of Kyushu, Japan. To study the aftershock activity, we deployed eleven pop-up type ocean bottom seismometers (OBSs), sixteen locally recorded temporary stations, and eight telemetered temporary stations in and around the epicenter region. We combined data from these stations and permanent stations located around the aftershock area, and determined the hypocenter of the mainshock and aftershocks. The mainshock was in the northwestern central part of the aftershock region, at a depth of 9.5 km. The mainshock was on a left-lateral strike-slip fault. Aftershocks were located in a depth range of 1–16 km and laterally extend for about 25 km in a NW-SE direction. We found that the aftershocks fell into four groups. This might be due to the heterogeneous structure in the source region. In the group that includes the mainshock, we estimated two fault planes bordering on the depth of the mainshock. There are 10-degree differences in both strike and dip angles between the lower and upper planes. From the aftershock distribution and the focal mechanisms, the rupture first propagated downward, and then propagated upward.
Spatial distribution of static stress drops for aftershocks of the 2005 West Off Fukuoka Prefecture earthquake
Authors: Iio Yoshihisa , Katao Hiroshi , Ueno Tomotake , Enescu Bogdan , Hirano Norio , Okada Tomomi , Uchida Naoki , Matsumoto Satoshi , Matsushima Takeshi , Uehira Kenji , Shimizu Hiroshi
Published: August 2014

We investigated the spatial distribution of static stress drops of the aftershocks of the 2005 West Off Fukuoka Prefecture earthquake, with the aim of assessing the possibility that another earthquake will occur on the SE extension of the earthquake fault. The waveforms from six temporary online telemetry stations installed in and around the aftershock region were measured. Small stress drops were estimated for the aftershocks that occurred relatively distant from the SE and NE ends of the earthquake fault. Conversely, the aftershocks that occurred around the SE end of aftershock region are characterized by large stress drops. These results imply the possibility of a stress concentration at the SE edge of the main shock fault.
Tsunami generated by the 2004 Kushiro-oki earthquake
Authors: Tanioka Yuichiro , Katsumata Kei
Published: August 2014

The 2004 Kushiro-oki earthquake generated a small tsunami that was observed at two tide gauge stations located on the Pacific coast of Hokkaido Province. Analysis of the tsunami waveforms shows that the slip amount of the fault was 2.1 m. The seismic moment was calculated to be 3.1×1019 Nm, which is consistent with the results of previous seismological studies. Tsunami simulation results indicate that a small first wave at Urakawa is caused by large shallow water off Cape Erimo. The tsunami generated from the source area off Kushiro circumvents the shallow area off the cape, propagates through the deep sea, and arrives at Urakawa as a small first wave. Larger tsunamis are propagated through the shallow region slowly and arrive at Urakawa as a later tsunami. These results suggest that a tsunami from a future large Nemuro-oki earthquake will also arrive at the west coast of Hidaka with a small first wave and large later phases.
Dynamical modeling of trench retreat driven by the slab interaction with the mantle transition zone
Authors: Tagawa Michio , Nakakuki Tomoeki , Tajima Fumiko
Published: August 2014

We present the 2-D self-consistent dynamical model of interactions of a subducting slab with the 410-km and 660-km phase boundaries to further our understanding of the relation between the slab stagnation/penetration and the trench migration. Our model takes into account freely-movable plate boundaries and the difference between tensional and compressional yield strengths in the lithosphere. For the case in which the tensional strength is weaker than the compressional one, the negative buoyancy of the subducting slab produces extension of the overriding lithosphere and, accordingly, the trench retreats. Interactions with the 410-km and 660-km phasetransition boundaries further promote the trench retreat, and the dip angle of the slab is substantially decreased. This enhances the resistance of the 660-km phase boundary against the slab penetration. Slab weakening caused by the grain-size reduction in the transition zone may result in a horizontally-lying slab and trench retreat.
Diagenetic effect on magnetic properties of marine core sediments from the southern Okhotsk Sea
Authors: Kawamura Noriko , Oda Hirokuni , Ikehara Ken , Yamazaki Toshitsugu , Shioi Kumi , Taga Shunsuke , Hatakeyama Shinsyo , Torii Masayuki
Published: August 2014

It is well known that magnetic records of marine sediments are affected to a certain degree by early diagenesis, the extent and mode of which are controlled by depositional environments. In order to investigate variations in the effects of early diagenesis, we have analyzed 11 gravity cores estimated to be Holocene in age that constitute a depth transect off the southern coast of the Okhotsk Sea. The rock magnetic parameters have peaks produced by the presence of volcanic ashes, basaltic granules and/or relatively coarser-grained layers. The concentration-dependent parameters display characteristic trends of a rapid decrease within the homogeneous silty clay zone in the middle parts of the cores, assuming that the total amount of magnetic minerals decreases through early diagenesis at this interval. This decrease is found in all of the cores from the transect (from both near-shore and offshore sites). In near-shore sites where larger volumes of organic materials are supplied compared to offshore sites, the depth of the decrease in magnetic minerals is relatively shallow. These results imply that the presence of organic materials accelerates the pyritization through the bacterial reduction of ferric ion, thereby causing the decreases in magnetic minerals to become shallower.
Long-term trends in geomagnetic daily variation
Authors: Macmillan Susan , Droujinina Anna
Published: August 2014

Long-term changes in the magnetic environment of the Earth are of interest to those studying space weather and climate change, particularly in the upper atmosphere. In this paper we examine long-term changes in daily variation as derived from hourly mean values from 14 geomagnetic observatories around the world. Their time series date back to the beginning of the 20th century. We find that there are similar features in all the records, with peaks in the amplitudes of the daily variation occurring in the 1950s and 1980s, and a small upward trend of 1.3 nT/century corresponding to an increase of over 10%. The extrema coincide with those seen in solar irradiance proxy data, in particular the F10.7 flux density dataset which starts in 1947.
Monitoring global traveling ionospheric disturbances using the worldwide GPS network during the October 2003 storms
Authors: Wang Min , Ding Feng , Wan Weixing , Ning Baiqi , Zhao Biqiang
Published: August 2014

The global traveling ionospheric disturbances (TIDs) during the drastic magnetic storms of October 29–31, 2003 were analyzed using the Global Position System (GPS) total electron content (TEC) data observed in the Asian-Australian, European and North American sectors. We collected the most comprehensive set of the TEC data from more than 900 GPS stations on the International GNSS Services (IGS) website and introduce here a strategy that combines polynomial fitting and multi-channel maximum entropy spectral analysis to obtain TID parameters. The results of our study are summarized as follows: (1) large-scale TIDs (LSTIDs) and medium-scale TIDs (MSTIDs) were detected in all three sectors after the sudden commencement (SC) of the magnetic storm, and their features showed longitudinal and latitudinal dependences. The duration of TIDs was longer at higher latitudes than at middle latitudes, with a maximum of about 16 h. The TEC variation amplitude of LSTIDs was larger in the North American sector than in the two other sectors. At the lower latitudes, the ionospheric perturbations were more complicated, and their duration and amplitude were relatively longer and larger. (2) The periods and phase speeds of TIDs were different in these three sectors. In Europe, the TIDs propagated southward; in North America and Asia, the TIDs propagated southwestward; in the near-equator region, the disturbances propagated with the azimuth (the angle of the propagation direction of the LSTIDs measured clockwise from due north with 0°) of 210° showing the influence of Coriolis force; in the Southern Hemisphere, the LSTIDs propagated conjugatedly northwestward. Both the southwestward and northeastward propagating LSTIDs are found in the equator region. These results mean that the Coriolis effect cannot be ignored for the wave propagation of LSTIDs and that the propagation direction is correlated with polar magnetic activity.
Finite element analysis of crustal deformation in the Ou Backbone Range, northeastern Japan, with non-linear visco-elasticity and plasticity: effects of non-uniform thermal structure
Authors: Shibazaki Bunichiro , Garatani Kazuteru , Okuda Hiroshi
Published: August 2014

A finite element analysis with non-linear visco-elasticity and plasticity was carried out with the aim of constructing a model of the slip and deformation processes in the deeper parts of the seismogenic zones of inland earthquakes. Our finite element code is based on the GeoFEM parallel finite element code and was developed using plug-ins to adopt several non-linear functions. We consider the effects of geothermal structures in the crust in a compressional tectonic setting to model the deformation and faulting that occur around the Ou Backbone Range in northeastern Japan. We set an area of high geothermal gradient in the center of the model. The numerical results show that shortening deformation due to non-linear viscous flow occurs in the high-temperature area in the lower part of the crust, which results in shear faulting in the upper part of the crust. In the case where the crust comprises two layers—the upper crust (quartz diorite) and the lower crust (wet diabase)—a weak viscous zone appears in the lower part of the upper crust and a strong viscous or plastic zone appears at the upper part of the lower crust. Our numerical results are able to explain the deformation and faulting that occur around the Ou Backbone Range in northeastern Japan.
Magnitude determination using duration of high frequency energy radiation and displacement amplitude: application to tsunami earthquakes
Authors: Hara Tatsuhiko
Published: August 2014

Recently, we developed a new method to determine earthquake magnitudes using durations of high frequency energy radiation and the maximum displacement amplitudes, which can be measured from processing of first arriving P-waves. In the present study, we applied this method to the 1992 Nicaragua, 1994 Java, 1996 Peru, and 2006 Java earthquakes which have been suggested to be “tsunami earthquakes.” Our magnitude estimates for these earthquakes are consistent with the moment magnitudes in the Global CMT catalog, which demonstrates that our method is applicable to tsunami earthquakes. The analyzed tsunami earthquakes are characterized in our method as those with longer source durations and smaller displacement amplitudes.
A numerical study on magnetic polarity transition in an MHD dynamo model
Authors: Takahashi Futoshi , Matsushima Masaki , Honkura Yoshimori
Published: August 2014

Magnetic polarity transitions in a Takahashi-Matsushima-Honkura dynamo model are analyzed. Distinctive differences in behavior of the axisymmetric poloidal magnetic field are found among a polarity reversal and excursions, including short polarity events. At the beginning of magnetic polarity transitions, the magnetic field with the reversed polarity is generated by anti-cyclonic convection columns deep within the outer core. In the case of excursion, it is soon advected by the radial flow toward a shallow interior of the core, and the transition can be detected at the core surface. However, the same process retrieves the original polarity from the deep interior, and the reversed field eventually vanishes. In the case of polarity reversal, on the other hand, the reversed polarity field is persistently generated deep within the core. It is then advected toward a shallow interior of the core, while the generation process of the reversed field occurs successively. The reversed polarity field near the core surface is collected by the downwelling flow associated with convection columns, as is the case for the original polarity field. The polarity reversal is completed by the advection process, the duration of which is consistent with the flow speed in the core.
Paleomagnetism of Unzen volcano: A volcanic record (Senbongi excursion) of the Iceland Basin event and the Brunhes VGP distribution for Japan
Authors: Shibuya Hidetoshi , Iwasaki Yuki , Tanaka Hidefumi , Hoshizumi Hideo
Published: August 2014

A paleomagnetic study was carried out on volcanic rocks from Unzen volcano: samples were collected from a total of 69 sites with 19 sites in pyroclastic flows and 50 sites in lava flows. Ages for the flows were determined either by K-Ar methods or detailed field surveys, and indicate that all of the flows were deposited during the Brunhes chron. After demagnetization 10 pyroclastic and 48 lava flows had stable site-mean directions. One lava flow in the Senbongi area with a K-Ar age of 197±17 ka had an intermediate virtual geomagnetic pole (VGP) at 8.3°N, 21.6°E. The age and VGP position apparently correlate with the Iceland Basin event, suggesting that it is a record of the event on volcanic rocks. Our study added 53 Brunhes-aged VGPs to the overall collection for Japan, increasing it by 40% to a total of 175 VGPs. After excluding all site-mean directions having α95 > 10° and VGP latitude < 50°, the remaining 148 VGPs have a mean pole at 89.7°N, 40.9°E (A
95 = 2.2°) showing no significant deviation from the geographic pole. The angular standard deviation (ASD) was calculated as 15.2° ± 1.2° (N = 148), which is compatible with paleosecular variation models from the literature. However, the data set was found to deviate from a Fisher distribution. The actual meaning of the ASD value after removing the intermediate VGPs needs to be reconsidered.
Geomagnetic 100-ky variation extracted from paleointensity records of the equatorial and North Pacific sediments
Authors: Yokoyama Yukiko , Yamazaki Toshitsugu , Oda Hirokuni
Published: August 2014

We have analyzed three sets of paleointensity series varying in duration from 300 to 1600 ka to confirm the existence of paleosecular variation on a 100-ky time scale. The data series are from sediment cores in the equatorial and North Pacific regions. We transformed the three time series into wavelet space and found characteristic variations in a 100-ky time scale. We also calculated wavelet correlation between the series of the paleointensity and rock magnetic parameters of each core to assess the efffect of rock magnetic properties on relative paleointensity. The wavelet correlation coefficients of the three cores are small, so the change in the paleointensity in each core is concluded to be independent of magnetic properties. In order to confirm the cause of the paleointensity variation, we calculated the wavelet correlation among the three cores. The relative intensity variations of the three cores are significantly correlated, while rock magnetic parameters are not, indicating that there are differences in rock magnetic variations among the cores, although they appear to occur on similar scales. On the other hand, all sets of relative paleointensity have synchronous variations. Such paleointensity variations observed over a wide area are considered to originate from changes in the Earth’s magnetic field.
Early Miocene magnetostratigraphy and a new palaeomagnetic pole position from New Zealand
Authors: Turner Gillian M. , Michalk Daniel M. , Morgans Hugh E. G. , Walbrecker Jan O.
Published: August 2014

We report palaeomagnetic results from a 26.5 m sequence of early Miocene sediments in NW Nelson, New Zealand. Analysis of the strong, stable characteristic component of natural remanent magnetization has yielded an important record of the part of Chron 5 from ca. 18.5 to 16.5 Ma, including positive identification of the cryptochron or “tiny wiggle” C5Dr-1, and a pole position ([{«mbda _k} = {78.4^ circ }]; [{☎i _p} = {283.0^ circ }]; dp = 2.2°; dm = 2.8°) for NW Nelson that is indistinguishable from the contemporaneous published pole for the Australian Plate. We infer that this portion of NW Nelson has undergone negligible rotation with respect to the main part of the Australian Plate over the past 17.5 Myr.
Sulfur abundance of asteroid 25143 Itokawa observed by X-ray fluorescence spectrometer onboard Hayabusa
Authors: Arai Takehiko , Okada Tatsuaki , Yamamoto Yukio , Ogawa Kazunori , Shirai Kei , Kato Manabu
Published: August 2014

The Japanese Hayabusa spacecraft successfully carried out in situ observations of S-class asteroid 25143 Itokawa, including the surface major elemental analysis with the X-ray fluorescence spectrometer (XRSHayabusa). Our previous results for the X-ray experiments (Okada et al., 2006a) indicated that major elemental ratios of Mg/Si and Al/Si on the surface of Itokawa resemble ordinary LL- or L-chondrites more than any other meteorite analogues. In the NEAR Shoemaker observations of S-class asteroid 433 Eros, the results of X-ray fluorescence observations indicated the depletion of sulfur, probably reflecting impact-induced volatilization, photoor ion-induced sputtering at the surface, or the loss of FeS-rich materials due to partial melting. Here, we determined the elemental abundance of sulfur (S) on the surface of Itokawa, in addition to that of Mg, Al, and Si, and its regional variation using XRS-Hayabusa observations. In particular, we carefully corrected the fluctuation of solar X-rays, variation of surface geometry, and sensor response function in this analysis, and thus we believe that the results are more accurate than those of our previous report. In this study, the upper and lower limits for Mg/Si, Al/Si, and S/Si overlap those of meteorite analogues for ordinary chondrites or primitive achondrites. In terms of the major elemental composition, Itokawa is best classified as a ordinary chondrite or a primitive achondrite. Our models do not include the mineral mixing effects. With the effects, the abundance of sulfur is expected to be 30% lower than our results. Hence, we conclude that the abundance of sulfur on the surface of Itokawa is almost equal to or even lower than the average abundance in ordinary chondrites. Although the abundances for Mg and Si are globally homogeneous, best-fit or upper limits of mass fraction for Al and S vary in local areas. There is a negative correlation (−0.92) for Al/Si vs. S/Si in ten facets. In particular, the area with the lowest sulfur, accompanied with enriched aluminum, is found in Arcoona, close to a cratered area. Therefore, aluminum enrichment and sulfur depletion features may support events of partial melting on the parent body of Itokawa or aluminum-rich material impacts on the surface of Itokawa. In some areas, Itokawa has a brighter geometric albedo and color variation. Little altered, fresh material may be exposed in these portions of the surface. The sulfur abundance on the surface appears to vary between little and highly altered areas by space weathering. Thus, the sulfur regional variation in our result may reflect the heterogeneity of a surface altered by space weathering.
The 2004 Las Campanas/Lowell Observatory Itokawa campaign: I. Simultaneous visible and near-infrared photometry of the Hayabusa mission target
Authors: Thomas-Osip J. E. , Lederer S. M. , Osip D. J. , Vilas F. , Domingue D. , Jarvis K. , Leeds S. L.
Published: August 2014

In 2004, Asteroid 25143 Itokawa made its final close approach to the Earth prior to its encounter with the Japanese spacecraft Hayabusa. This apparition was superb with Itokawa reaching magnitude 12 (two magnitudes brighter than the 2001 apparition and the brightest since its discovery in 1998) and covering a large range of observable solar phase angles. An extensive visible and near-infrared observing campaign of Itokawa was undertaken at Las Campanas and Lowell Observatories to obtain full rotational coverage and cover the largest possible range of solar phase angles (4–129°). Unresolved global color mapping over the complete light curve (best fit synodic period of 12.118 hr) shows no sign of rotational color variability with upper limits of a few percent across the full U-thru-K spectrum. These combined multi-wavelength (UBVRIJHK) rotational light curves allow for the concrete deconvolution of shape from albedo variation in the rotational models and as required for Hapke modeling presented in Paper II (Lederer et al., this issue), permits the removal of the rotational light curve effects from the solar phase curve. Furthermore, these derived solar phase curves can be fit with the IAU H,G magnitude system (Bowell et al., 1989) thus allowing the calculation of geometric albedos (p
v = 0.23 ± 0.02) as well as an estimate of the asteroid’s elongated shape (a/b = 1.9 ± 0.1) via the amplitudephase relationtionship (Zappalà et al., 1990). Results derived from the extensive ground-based campaigns are compared and contrasted with the much higher spatial resolution in situ measurements made by the Hayabusa spacecraft. The ‘ground-truth’ provided by the Hayabusa mission results shed light on the inferences that can begin to be made for the general asteroid population.
Tsunami generated by the 2007 Noto Hanto earthquake
Authors: Namegaya Yuichi , Satake Kenji
Published: August 2014

Tsunami numerical simulations of the 2007 Noto Hanto earthquake are carried out for three models based on the coastal vertical movements, the GPS observations, and the strong-motion data. The three fault models produce similar tsunami waveforms at five tide gauge stations around the source. Comparisons with the observed tsunami waveforms indicate that the fault model based on the coastal movements shows the best agreement but that a slightly smaller fault area with a larger slip amount would make the agreement better. At three tide gauge stations in Toyama Bay, unusual oscillatory waves were recorded before the expected tsunami arrival. The spectral analysis of the observed records indicates that they represent characteristic oscillation of Toyama Bay, and modal calculations show that they can be reproduced by the water displacement in the mouth of Nanao Bay generated by a possible extension of the mainshock fault toward the southeast or a possible landslide source at a steep part of ocean bottom.
Rock magnetic and geochemical analyses of surface sediment characteristics in deep ocean environments: A case study across the Ryukyu Trench
Authors: Kawamura Noriko , Kawamura Kiichiro , Ishikawa Naoto
Published: August 2014

Magnetic minerals in marine sediments are often dissolved or formed with burial depth, thereby masking the primary natural remanent magnetization and paleoclimate signals. In order to clarify the present sedimentary environment and the progressive changes with burial depth in the magnetic properties, we studied seven cores collected from the Ryukyu Trench, southwest Japan. Magnetic properties, organic geochemistry, and interstitial water chemistry of seven cores are described. Bottom water conditions at the landward slope, trench floor, and seaward slope are relatively suboxic, anoxic, and oxic, respectively. The grain size of the sediments become gradually finer with the distance from Okinawa Island and finer with increasing water depth. The magnetic carriers in the sediments are predominantly magnetite and maghemized magnetite, with minor amounts of hematite. In the topmost sediments from the landward slope, magnetic minerals are diluted by terrigenous materials and microfossils. The downcore variations in magnetic properties and geochemical data provided evidence for the dissolution of fine-grained magnetite with burial depth under an anoxic condition.
Lunar cratering chronology: Statistical fluctuation of crater production frequency and its effect on age determination
Authors: Morota Tomokatsu , Haruyama Jun’ichi , Honda Chikatoshi , Yokota Yasuhiro , Ohtake Makiko , Ogawa Yoshiko , Matsunaga Tsuneo
Published: August 2014

The crater frequency is often used to determine ages of planetary surfaces. In this study, we evaluate the statistical fluctuation of the crater frequency and its effect on age determination using a Monte Carlo simulation prior to the SELENE (KAGUYA) / Terrain Camera acquiring extensive high-resolution images over the entire Moon. We found that the ages estimated between 2.5 and 3.5 Gyr tend to be too young when there are not enough craters. Age determination for Eratosthenian units is least accurate. The errors defined with 1σ are 0.7 Gyr for units of 100 km2, 0.5 Gyr for units of 1000 km2, and 0.4 Gyr for units of 2500 km2. The maximum error exceeds 20% and is comparable with the error due to the cratering asymmetry.
Complexities of gamma-ray line intensities from the lunar surface
Authors: Yamashita Naoyuki , Hasebe Nobuyuki , Miyachi Takashi , Kobayashi Masanori , Okudaira Osamu , Kobayashi Shingo , Ishizaki Takeshi , Sakurai Kunitomo , Miyajima Mitsuhiro , Reedy Robert C. , d’Uston Claude , Maurice Sylvestre , Gasnault Olivier
Published: August 2014

Assuming different abundances of the Apollo lunar soil samples and the average spectrum of galactic cosmic ray protons, energy spectra of neutrons and gamma rays and emission rates of gamma-ray lines from major elements have been estimated by using the reviewed Monte Carlo simulation library Geant4 and nuclear data. Previously, such libraries were not able to reproduce gamma-ray lines properly for the planetary application. Results clearly show that the emission rate of gamma rays heavily depends not only on the chemical abundance but also neutron flux within the lunar subsurface. While the intensities of gamma-ray lines are mostly proportional to elemental abundances, the intensity per unit elemental abundance can vary. Such a complex correlation is attributed to the change in neutron flux within the lunar subsurface and petrological restriction of elemental variation.
Instrumentation and observation target of the Lunar Radar Sounder (LRS) experiment on-board the SELENE spacecraft
Authors: Ono Takayuki , Kumamoto Atsushi , Yamaguchi Yasushi , Yamaji Atsushi , Kobayashi Takao , Kasahara Yoshiya , Oya Hiroshi
Published: August 2014

The Lunar Radar Sounder (LRS) on-board the SELENE lunar orbiter is currently being equipped to provide the data of subsurface stratification and tectonic features in the shallow part (several km deep) of the lunar crust, by using an FM/CW radar technique in HF (∼5 MHz) frequency range. Knowledge of the subsurface structure is crucial to better understanding, not only of the geologic history of the Moon, but also of the Moon’s regional and global thermal history of the Moon and of the origin of the Earth-Moon system. In addition to the subsurface radar experiment, LRS will provide the spectrum of plasma waves and solar and planetary radio waves in a wide frequency range from 10 Hz to 30 MHz. This paper provides the basic function parameter of the LRS system based on the final function test and proposes observation targets and data analysis that will provide important information leading to a greater understanding of the tectonics and thermal history of the Moon.
Equivalent source mapping of the lunar crustal magnetic field using ABIC
Authors: Toyoshima M. , Shibuya H. , Matsushima M. , Shimizu H. , Tsunakawa H.
Published: August 2014

An objective scheme is presented for estimating the lunar crustal magnetic field from the LMAG (Lunar MAGnetometer) data of the SELENE (“KAGUYA”) spacecraft. Our scheme improves the equivalent source method in three respects. The first improvement is that the source calculation is performed simultaneously with detrending. The second is that a great number of magnetic charges (magnetic monopoles) are used as the equivalent sources. The third is that the distribution of the magnetic charges is detremined by the damped least squares method, and the optimum smoothness is determined objectively by minimizing Akaike’s Bayesian Information Criterion (ABIC). For testing the scheme, we apply it to the Lunar Prospector magnetometer data in the region centered at the Reiner Gamma magnetic anomaly. The magnetic field map at an altitude of 20 km is stably drawn from datasets for different altitudes (18 km and 34 km). The ABIC minimizing criterion successfully controls the smoothness due to the numerical damping and extracts as much information as possible from the given data. This scheme will help produce a coherent lunar magnetic anomaly map by integrating the observations from various altitudes of the SELENE and previous missions.
Simulation analysis of differential phase delay estimation by same beam VLBI method
Authors: Kikuchi Fuyuhiko , Liu Qinghui , Matsumoto Koji , Hanada Hideo , Kawano Nobuyuki
Published: August 2014

The same beam VLBI method (SBV) is newly applied to the multi-frequency VLBI method in the VRAD mission of SELENE (KAGUYA). By simultaneously observing two nearby spacecraft with one antenna, the error sources of VLBI measurement common in two propagation paths can be almost canceled out. In this paper, error estimation and simulation analysis are carried out for a feasibility study to apply the SBV method to the VRAD mission. Differential phase delay can be estimated without cycle ambiguity even if tropospheric fluctuation is large and/or traveling ionospheric disturbance occurs. The sensitivity of the differential phase delay with respect to the average elevation angle and the elongation of two spacecraft is also investigated. Moreover, a method is developed for estimating differential phase delay in switching VLBI observations using the cycle ambiguity derived from SBV observations. This method can be performed in more than 90% of the VRAD mission’s total paths. Precise positioning with SBV contributes to accurate estimation of the low degree coefficients of lunar gravity fields by more than one order of magnitude than previous results.