bluuebunny/crossref_metadata_2025 huggingface dataset:
Source process of the 1923 Kanto earthquake inferred from historical geodetic, teleseismic, and strong motion data
Authors: Kobayashi Reiji , Koketsu Kazuki
Published: August 2014

The historical geodetic and teleseismic data are jointly inverted for the rupture process of the 1923 Kanto earthquake together with the strong motion data recently discovered. The resultant slip distribution shows two asperities (areas of large slips) on the fault plane with a total seismic moment of 1.1 ⊙ 1021 Nm (Mw8.0). The asperity in the shallow central portion of the fault is larger with maximum slip of approximately 10 m than the one in the hypocentral region. The recovered slip functions indicate that the initial slip was abrupt in the hypocentral asperity, though it was gradual in the larger asperity. The major discrepancies between the observations and synthetics are attributed to volcanic activities and the 3-D structure of the Kanto Basin. We compare the slip and aftershock distributions and also discuss the relation of the asperities to silent earthquakes.
Tsunami due to the 2004 September 5th off the Kii peninsula earthquake, Japan, recorded by a new GPS buoy
Authors: Kato Teruyuki , Terada Yukihiro , Ito Keiji , Hattori Ryuji , Abe Takenori , Miyake Toshihide , Koshimura Shun’ichi , Nagai Toshihiko
Published: August 2014

A tsunami due to the 2004 M7.4 September 5th earthquake off of the Kii peninsula, Japan, was recorded at the GPS buoy newly designed and established in April 2004, about 13 km off of the Muroto Promontory, southwestern Japan. The tsunami arrived at the buoy about 8 minutes before its arrival to the nearest tide gauge station at the tip of the Muroto Promontory. The predicted tsunami record based on preliminary source model shows excellent agreement (rms 2.7 cm) with the observed GPS record. This demonstrates that GPS buoy observations can be used not only for a tsunami warning system, but also for precise determination of the tsunami source.
The 2004 sequence of triggered earthquakes off the Kii peninsula, Japan
Authors: Park Sun-Cheon , Mori Jim
Published: August 2014

We examine the spatial and temporal relationships of the sequence of strong earthquakes that occurred off the Kii Peninsula, Japan, on 5 September 2004. The first event (Mj 7.1) occurred at 10:07:08 (UTC) on a northward dipping plane within the subducting Philippine Sea plate. From 10:16 to 14:47 the seismicity shows a group of earthquakes (Mj 3.2 to 4.8) 35 km to the east which are regarded as foreshocks to the second large earthquake. At 14:57:17, a Mw 6.1 strike-slip event occurred on a northwest trending plane. Some 14 seconds later, a large (Mj 7.4) thrust earthquake started 4.2 km southeast of the initial epicenter of the second earthquake. This largest earthquake is thought to have occurred on a southward dipping plane with the strike in an east-southeastly direction. Using the geometry of faults determined in this study, calculations of the Coulomb failure function show that simple static stress changes do not provide a good explanation for the triggering of the subsequent earthquakes.
The September 5, 2004 off the Kii Peninsula earthquakes as a composition of bending and collision
Authors: Seno Tetsuzo
Published: August 2014

The 2004 off the Kii Peninsula earthquakes ruptured the subducting Philippine Sea plate near the axis of the Nankai Trough. A series of major events (the fore- and mainshocks with Mw>7) showed very peculiar rupture characteristics. The depth extent of the foreshock is around 12–23 km below the sea surface; in contrast, that of the mainshock is around 7–17 km, shallower and closer to the upper surface of the plate than the foreshock. The focal mechanism of the foreshock is almost pure reverse faulting, with a P-axis directing roughly N-S, that is rotated clockwise by 20–25° from the trough normal. In contrast, the mainshock involved a significant component of strike-slip faulting, with a similar P-axis. The OBS survey revealed a double layer of the aftershock activity and the upper NW trending layer contains aftershocks with strike-slip focal mechanisms. These features indicate that the compressional and tensional stress domains due to bending are significantly modified by tectonic stress disturbance and produced the fore- and mainshock ruptures deviated from simple reverse and normal faulting. I show that a supposed N-S compressional stress, which might be produced by the collision south of the Izu Peninsula, can rotate the principal stress axes and change the stress type, which are consistent with the P-axes of these events and the strike-slip component for the mainshock. The complex rupture mode of the mainshock might have been produced by superposition of the depth-dependent tensional bending stress in the upper half of the lithosphere and the collisional stress. This collisional stress should be comparable to the bending stresses in magnitude, which is much larger than the stress perturbation caused by nearby interplate locking.
Slow slip in the focal region of the anticipated Tokai earthquake following the seismo-volcanic event in the northern Izu Islands in 2000
Authors: Kobayashi Akio , Yoshida Akio , Yamamoto Takeyasu , Takayama Hiromi
Published: August 2014

Transient crustal deformation occurred in the regions of Kanto and Tokai during the seismo-volcanic event in the northern Izu Islands in 2000. In our investigation of the observed deformation, we constructed an optimum-source model of the event between Miyake and Kozu Islands. We then made an inversion analysis of the differences between the observed displacement field and the calculated displacement field from the optimum model, assuming that the differences were caused by the changes in the interplate coupling beneath the Tokai region. From the inversion analysis of data for each of three-month periods, May to August, June to September, and July to October, we found decreased interplate coupling in the early stages of the 2000 event. In the first stage, either a slow slip or a temporary suspension of the plate subduction occurred in the focal region of the anticipated Tokai earthquake. The area then extended to the west and, finally, a slow slip exceeded the secular convergence velocity on the plate interface near Lake Hamana in the fall of 2000. We believe this ongoing slow slip began in August or early September 2000.
Ground motion and rupture process of the 2004 Mid Niigata Prefecture earthquake obtained from strong motion data of K-NET and KiK-net
Authors: Honda R. , Aoi S. , Morikawa N. , Sekiguchi H. , Kunugi T. , Fujiwara H.
Published: August 2014

The 2004 Mid Niigata Prefecture earthquake (37.289°N, 138.870°E, 13.1 km, MJMA 6.8; JMA), also known as the 2004 Niigata Prefecture Chuetsu earthquake, was a thrust type earthquake that occurred on October 23, 2004 at 17:56 (JST). Strong ground motions of PGA 800-1700 cm/s2 and PGV 60-130 cm/s were observed at stations located immediately above the source region. We deduced the rupture process of this earthquake with a multi-time-window linear waveform inversion procedure. We used near-fault strong ground motion data observed at nine K-NET and KiK-net stations within 50 km from the epicenter. In order to obtain appropriate Green’s functions for the waveform inversion, we constructed two velocity structure models for stations on the hanging wall and one structure model for stations on the footwall. The estimated total slip distribution contains three asperities: (a) around the hypocenter, (b) in the upper-middle section of the fault plane, and (c) southwest of the hypocenter. The maximum slip is 3.8 m at the hypocenter and the total seismic moment is 1.2 × 1019 Nm, which corresponds to Mw=6.7. The moment rate functions in asperities (a) and (c) have a short rise time, while those in asperity (b) have a longer rise time.
Structural interpretation of the Tuzgolu and Haymana Basins, Central Anatolia, Turkey, using seismic, gravity and aeromagnetic data
Authors: Aydemir Attila , Ates Abdullah
Published: August 2014

The Tuzgolu Basin is the largest basin in Central Anatolia. It is connected to the Haymana Basin by a small channel-shaped basin called the Tersakan Basin. The study area has smooth topography with a stable young sedimentary cover and limited outcrop zones, but displays a complex geology with intense active tectonism and poor seismic quality beneath the surface. In the study reported here an attempt was made to determine the subsurface structure of the basement by three-dimensional modelling of gravity data. The seismic and magnetic data were also integrated with the available geological data to check the modelling results. The modelling results suggest that the deeper parts of the Haymana Basin are located to the east of Haymana city centre, at a maximum depth of 8 km, with the exception of the depression to the north, which is located 25 km northwest of Bala city centre at a maximum depth of 10 km. The channel-shaped Tersakan Basin (connecting the Haymana and Tuzgolu Basins) is located west of Lake Tuzgolu, and the deepest part of this basin is approximately 5.5 km. The Tuzgolu Basin has three major depressions: (1) the Southern Aksaray Depression, which is covered by younger volcanic rocks, is the largest but in general the least explored area; (2) the Eregli Depression; (3) the Sultanhani Depression. Almost all of these depressions have an average depth of 8 km; however, some of the deeper segments are at a depth of approximately 12–13 km. Our results suggest the possibility of hydrocarbon potential in these basins.
Aeromagnetic data analysis for the identification of concealed uranium deposits: a case history from Singhbhum uranium province, India
Authors: Anand S. P. , Rajaram Mita
Published: August 2014

Aeromagnetic data over a part of the Singhbhum uranium province, India, within the framework of ‘Operation Hard Rock’ have been analyzed. The aeromagnetic anomaly map, its analytic signal amplitude, the Euler solutions and apparent susceptibility map helped in identifying the nature and depth of the magnetic sources in the study region. The Singhbhum Shear Zone is clearly delineated. The location of the mined uranium deposits coincide with the shallow magnetic sources. The present study also identified distinctive magnetic sources between the Dalma Volcanics and the Chotanagpur Granitic Gneissic Complex that suggest the presence of a sub-surface shear. The magnetic sources in this newly identified shear zone lie at a depth of 200 m and depict a similar magnetic signature and susceptibility as those of the Singhbhum Shear Zone where uranium is being mined. This shear could be the subsurface contact between high-grade metamorphic rocks (amphibolite facies) to the north and the greenschist facies low-grade metamorphics to the south. This shear zone, also characterized by radiometric anomalies, possibly indicates a zone of concealed uranium deposit that can be explored in the future. The present analysis shows the importance of aeromagnetic surveys and its utility in exploration for concealed mineral deposits.
Magnetostratigraphy of the lower Cretaceous strata in Tlayúa Quarry, Tepexi de Rodriguez, State of Puebla, Mexico
Authors: Benammi Mouloud , Alvarado-Ortega Jesus , Urrutia-Fucugauchi Jaime
Published: August 2014

A stratigraphic sequence of magnetic polarity reversals consisting of nine magnetozones is recognized in Early Cretaceous sedimentary strata in the Tlayúa Quarry of Tepexi de Rodriguez, state of Puebla. Combined with biostratigraphic age assignments using ammonites that apparently belonging to the Albian, the geomagnetic polarity sequence can be correlated with the middle part of chron C34n (C34n.1n-C34n.2n chrons, with an age of 100–105 Myr), the Cretaceous long normal interval. The magnetic properties and characteristic remanence of magnetization are dominated by soft and hard coercivity magnetic minerals. Rock-magnetic properties and a positive reversal test suggest that remanence is primary. The section displays both reverse and normal polarities with mean directions: D=344.9°, I=32.4°, k=21, a95=4.2° and D=149.4°, I=−36.6°, k=17, a95=8.7°, respectively. Comparison with the North American apparent polar wander path indicates tectonic stability of the region since the Cretaceous.
Low-latitude paleosecular variation and the time-averaged field during the late Pliocene and Quaternary—Paleomagnetic study of the Michoacan-Guanajuato volcanic field, Central Mexico
Authors: Conte-Fasano G. , Urrutia-Fucugauchi J. , Goguitchaichvili A. , Morales-Contreras J.
Published: August 2014

We report paleomagnetic, rock magnetic, and paleointensity studies for 24 volcanic lava flows from the Michoacan-Guanajuato volcanic field (MGVF), erupted between 2.27 Ma to present according to available radiometric ages and historic records. The MGVF located in the central-western sector of the Plio-Quaternary Trans-Mexican volcanic belt is mainly composed by cinder cones and shield volcanoes. Rock magnetic experiments show remanence is carried in most cases by Ti-poor titanomagnetites, resulting from oxy-exsolution of original titanomagnetites during flow cooling. Unblocking temperature spectra and high coercivities point to “small” pseudo-single domain grains for the titanomagnetites. Single component, linear vector plots are obtained after alternating field and thermal demagnetization. Seven flows yield reverse polarity magnetization while sixteen flows are normally magnetized. The overall mean paleodirection obtained (with 14 flows normal and 6 flows reverse) is I=28.4°, D=357.9°, k=21, α95=7.3°, with a paleomagnetic pole position ofPlat=85.7°,Plong= 104.5°, K=27, A95=6.4°. The paleodirection is undistinguishable from expected Plio-Quaternary paleodirections derived from reference poles for the North American polar wander curve, and previously reported paleodirections for central Mexico. Paleointensity experiments give high quality results for only twelve samples from two flows with mean values of 7.3 and 8.1 × 1022Am2, which are close to present geomagnetic field intensity. The combination of new MGVF directional results with currently available paleomagnetic data from central Mexico yield angular dispersion estimates ofSF= 15.4 withSU= 19.6 andSL= 12.7, which are in agreement with the latitude-dependent PSV model of McFaddenet al.(1988, 1991) for the last 5 Ma, and show no significant inclination anomaly.
Archaeomagnetic investigation of oriented pre-Columbian lime-plasters from Teotihuacan, Mesoamerica
Authors: Soler-Arechalde A. M. , Sánchez F. , Rodriguez M. , Caballero-Miranda C. , Goguitchaishvili A. , Urrutia-Fucugauchi J. , Manzanilla L. , Tarling D. H.
Published: August 2014

Results of an archaeomagnetic study of two excavation field seasons at the Xalla and Teopancazco residential areas of the ancient Prehispanic city of Teotihuacan, Central Mexico are reported. One-hundred and fifty three oriented samples of lime-plasters from the two archaeological sites were collected for the study. NRM directions are reasonably well grouped. Alternating field demagnetization shows single or two-component magnetizations. Rock magnetic measurements point to fine-grained titanomagnetites with pseudo-single domain behaviour. Characteristic site mean directions from both sites are correlated to the available palaeosecular variation curve for Mesoamerica. The mean directions obtained from Xalla site point to average dates of 550 AD±25 years that matches with the documented ‘Big Fire’ of Teotihuacan (AD 575). Two consecutive construction levels at Teopancazco were estimated as AD 250–350 and AD 350–425 respectively.
Earthquake probability based on multidisciplinary observations with correlations
Authors: Imoto Masajiro
Published: August 2014

A number of researchers have formulated earthquake probabilities based on precursory anomalies of multidisciplinary observations in which the underlying assumption is that the occurrence of one precursory anomaly is independent from those of other kinds of anomalies. Observations were classified into two groups, those events followed by an earthquake and those that were not, and the ratio of observed precursors in both groups was taken into consideration. In the present report, recent advances in statistical seismology are considered within the framework of these earthquake probabilities, and the formulations are extended to cases in which precursory anomalies are observed as continuous measurements. The effects originating from mutual correlations between two precursory anomalies are also considered. It is assumed that observed values of each discipline follow a normal distribution, either as a group of observations followed by an earthquake (conditional density distribution) or as a group of observations not followed by an earthquake (background density distribution). Special attention is given to the case in which two kinds of observations are correlated in the conditional density distribution but not in the background density distribution. The results obtained are compared with cases in which the observations are independent of each other in both distributions. The geometrical mean of the probability gain is greater in the correlated case than in the independent case and becomes infinitely large when the absolute value of the correlation coefficient approaches one. This finding enables a wider application of earthquake probability than has been previously possible based on multidisciplinary observations.
Kinematics of the East Anatolian Fault Zone between Turkoglu (Kahramanmaras) and Celikhan (Adiyaman), eastern Turkey
Authors: Yilmaz Huseyin , Over Semir , Ozden Suha
Published: August 2014

In this study we determined the stress regime acting along the East Anatolian Fault Zone between Turkoglu (Kahramanmaras) and Celikhan (Adiyaman), from the Neocene to present-day, based on the inversion of striations measured on faults and on the focal mechanisms of earthquakes having magnitudes greater than 5.0. The inversions yield a strike-slip stress regime with a reverse component (i.e., transpression) operative in the Neocene to present with a consistent N-to NW-trending σ1 axis 156 ± 11° and an E- to NE-trending σ3 axis, 67 ± 9°σ3, producing left-lateral motion along the East Anatolian Fault Zone. The inversions of focal mechanisms yield a strike-slip stress deviator characterized by an approximately N-S (N89°W)-trending σ1 and an approximately E-W (N89°E)-trending σ3 axis. Both the kinematic analysis and structural observations indicate that the stress regime operating in the study area has had a transpressional character, giving rise to the Mio-Pliocene compressive structures (reverse faults, thrusts and folds) observed in the study area. Field observations allow estimation of a Pliocene age for the strike-slip East Anatolian Fault Zone.
Assessing potential seismic activity in Vrancea, Romania, using a stress-release model
Authors: Imoto Masajiro , Hurukawa Nobuo
Published: August 2014

Over the last 500 years, an average of five intermediate-depth earthquakes of Mw = 7 and larger have occurred during each 100-year period in Vrancea, Central Romania. We have therefore attempted to assess the long-term seismic hazards in Vrancea using a stress-release (SR) model which models the elastic rebound theory in a stochastic process. The hypocenter catalog, ROMPLUS, provided by the National Institute for Earth Physics (NIEP), was adopted for the study. We only used data on earthquakes located in Vrancea and applied the SR-model to data sets of earthquakes with a magnitude of 7.0 and larger for three different time periods. Renewal models, such as the Brownian and Weibull model, were also applied to the same data set, but these did not perform as well as the SR-model. The SR-model can assess future earthquake probability and has identified that the probability of an earthquake occurring in Vrancea in a 5-year period exceeds 40% by the end of this decade.
Investigation of the earthquake sequence off Miyagi prefecture with historical seismograms
Authors: Kanamori Hiroo , Miyazawa Masatoshi , Mori Jim
Published: August 2014

We investigated the old seismograms of several earthquakes which occurred offshore of Miyagi prefecture in 1933 (June 18, 21:37 UT), 1936 (Nov. 2, 20:45 UT), 1937 (July 26, 19:56 UT), 1978 (June 12, 08:14 UT), and 2005 (Aug. 16, 02:46 UT). A characteristic earthquake model for the sequence of the 1936, 1978, and several other earlier events is the basis of the recent forecast by the Earthquake Research Committee (2003) that “The probability of the occurrence of another similar earthquake in the next 30 years is 99%”. To assess the validity of the characteristic earthquake model, we compared the waveforms, size and other characteristics of these earthquakes recorded at Pasadena, DeBilt, Abuyama, Aso, Weston, Strasbourg, and Christchurch. We conclude that (1) The 1978 event is 3 to 4.5 times larger (in seismic moment) than the 2005 event; (2) The 1936 and the 2005 events are about the same size and are fairly close in location; (3) The 1937 event is smaller than the 1936 event, and is significantly deeper, possibly as deep as 90 km. In contrast, the 1933 event is significantly shallower than the 1936 event. The differences between these events are too large to justify the use of a simple characteristic earthquake model for the probabilistic forecast. The seismic slip rate in this area and along the adjacent subduction zone to the south is about 1/4 of the plate convergence rate, which has an important implication for the long-term seismic hazard in this area.The 2005 West Off Fukuoka Prefecture earthquake caused serious damage to and on Genkai Island as well as to downtown Fukuoka City. There were no strong motion instruments on the island, therefore no one knows how the strong ground motion occurred during the mainshock. The ground motion simulation on Genkai Island is very important to our understanding of earthquake damage at the near-source region. We have conducted an aftershock observation on the island in order to verify site amplification due to steep topography and to record aftershocks for reproducing ground motion during the mainshock by the empirical Green’s function method. The observed records of aftershocks show small variations in the input motions in the island, indicating that the amplification due to the topography seems to be small below 2 Hz. We first estimated the strong motion generation area for the mainshock using the observation records at stations surrounding the source region. We then carried out broadband ground motion simulation on Genkai Island by using the aftershock records as empirical Green’s functions. The simulated ground velocities exceed 1 m/s with a dominant period of 1–2 s due to the forward rupture directivity, and the instrumental seismic intensity reaches 6.6.
Preparatory process reflected in seismicity-pattern change preceding the M=7 earthquakes off Miyagi prefecture, Japan
Authors: Matsumura Shozo
Published: August 2014

Two M=7 interplate earthquakes recently occurred off Miyagi prefecture, northeastern Japan. These belong to a new sequence of earthquake series. The occurrence of this sequence was forewarned by the government, based primarily on a statistical approach utilizing historical records. Such a process, however, does not account for the present on-going status in the assessment. During the stage when stress approaches a critical level, there is a possibility of stress redistribution caused by a quasi-static slip, which will be reflected in a temporal change in seismic activity. Delineating a spatial map of the seismic-activity rate changes in the seismogenic zone off Miyagi prefecture, we found that there was an activation of microseismicity in the rupture zones several months prior to both M=7 earthquakes. This was interpreted as evidence of a preparatory process preceding each M=7 earthquake, during which a quasi-static slip progressed, driving stress redistribution and resulting in stress concentration on asperities of these earthquakes.
Shear wave polarization anisotropy in and around the focal region of the 2005 West off Fukuoka Prefecture earthquake
Authors: Watanabe Atsushi , Matsumoto Satoshi , Matsushima Takeshi , Uehira Kenji , Matsuwo Norimichi , Shimizu Hiroshi
Published: August 2014

Crustal shear wave polarization anisotropy is caused by the alignment of vertical microcracks. Leading shear wave polarization directions (LSPDs) are presumed to be consistent with the maximum horizontal compressional axis in many cases. We analyzed shear wave polarization anisotropy in and around the focal region of the 2005 West off Fukuoka Prefecture earthquake. Almost all of the LSPDs are oriented in the E-W direction, which is consistent with the maximum horizontal compressional axis inferred from the mechanism of the main shock. These E-to W-oriented LSPDs are caused by the alignment of stress-induced microcracks. Crack densities at most stations are estimated to be 0.02. Little spacial stress variation around focal region is suspected
Empirical Green’s function simulation of broadband ground motions on Genkai Island during the 2005 West Off Fukuoka Prefecture earthquake
Authors: Miyake Hiroe , Tanaka Yasuhisa , Sakaue Minoru , Koketsu Kazuki , Ishigaki Yuzo
Published: August 2014

The 2005 West Off Fukuoka Prefecture earthquake caused serious damage to and on Genkai Island as well as to downtown Fukuoka City. There were no strong motion instruments on the island, therefore no one knows how the strong ground motion occurred during the mainshock. The ground motion simulation on Genkai Island is very important to our understanding of earthquake damage at the near-source region. We have conducted an aftershock observation on the island in order to verify site amplification due to steep topography and to record aftershocks for reproducing ground motion during the mainshock by the empirical Green’s function method. The observed records of aftershocks show small variations in the input motions in the island, indicating that the amplification due to the topography seems to be small below 2 Hz. We first estimated the strong motion generation area for the mainshock using the observation records at stations surrounding the source region. We then carried out broadband ground motion simulation on Genkai Island by using the aftershock records as empirical Green’s functions. The simulated ground velocities exceed 1 m/s with a dominant period of 1–2 s due to the forward rupture directivity, and the instrumental seismic intensity reaches 6.6.
Electrostatic quasi-monochromatic waves in the downstream region of the Earth’s bow shock based on Geotail observations
Authors: Shin Koichi , Kojima Hirotsugu , Matsumoto Hiroshi , Mukai Toshifumi
Published: August 2014

Geotail plasma wave observations show the existence of intense electrostatic quasi-monochromatic (EQM) waves in the downstream region of the Earth’s bow shock. They oscillate parallel to the ambient magnetic field and appear at frequencies between the electron plasma and ion plasma frequencies. Although these waves have been believed to be Doppler-shifted ion acoustic waves, the typical plasma parameters observed in the downstream region do not support the generation conditions for ion acoustic waves. In this paper, the existence of cold electron beam-like components accompanying EQM waves is considered based on waveform and statistical analyses. Linear dispersion analyses using realistic plasma parameters revealed that the cold electron beams cause destabilization of electron acoustic waves at frequencies consistent with those of observed EQM waves. The results of observations and linear analyses suggest that EQM waves are generated by the destabilization of the electron acoustic mode.
Tectonic history of Europa: Coupling between internal evolution and surface stresses
Authors: Kimura Jun , Yamagishi Yasuko , Kurita Kei
Published: August 2014

A stress history in the ice shell of Europa is presented. Europa’s surface is ubiquitous in extensional tectonic features such as banded terrains. These surface features suggest that the surface may have been fractured and extended due to tensional stress, and various origins for such stresses have been proposed. We have focused on the solidification of the liquid water layer and the accompanying volume change as one of the dominant sources for such stresses. To estimate the stress state in the ice shell, we first performed numerical simulations of the thermal history. Based on the resulting structural evolution, we calculated stresses in the viscoelastic ice shell due to the solidification of the liquid layer. Europa’s liquid layer solidifies slowly and may partially survive at present, and its solidification induces sufficient tensional stress to drive extensional tectonic activity. Consequently, we propose the tectonic scenario that the volume change due to phase change develops the basic amplitude of the stress, while tidal forces work as a trigger to fracture the surface.
Variations in the total electron content near the crest of the equatorial ionization anomaly during the November 2004 geomagnetic storm
Authors: Dashora Nirvikar , Pandey R.
Published: August 2014

Space weather studies using a Global Positioning System (GPS) receiver are underway near the crest of the equatorial ionization anomaly (EIA), at Udaipur (MLAT 16.0°N), India. These studies are probably, the first of its kind in the Indian zone near the crest of the EIA. Results for the geomagnetic storm that occurred during November 2004 are presented. Drastic variations in the total electron content (TEC) have been observed as a result of this storm. It is inferred that the reduced levels of TEC following the storm may be attributed to the weakened equatorial plasma fountain due to the disturbance dynamo fields.
First results from the Penn State Allsky Imager at the Arecibo Observatory
Authors: Seker Ilgin , Mathews John D. , Wiig Johannes , Gutierrez Paloma F. , Friedman Jonathan S. , Tepley Craig A.
Published: August 2014

The Penn State Allsky imager (PSASI), a user-owned-public-access (UOPA) instrument installed at Arecibo Observatory (AO: 18.3°N, 66.75°W; altitude: 350 m a.s.l.; L = 1.43 at 300 km; dip angle: 46°; geomagnetic coordinates: 29°N, 5.5°E), is a CCD-based high-resolution allsky optical imager that has been collecting ionospheric airglow data at night since May 2003. The computer controlled six-position filter wheel is equipped with three filters at 630 nm (red), 557.7 nm (green), and 777.4 nm (near-IR), respectively, which correspond to ionosphere-related oxygen emissions. The imager data, taken for more than 3.5 years now, is being used to study various ionospheric processes, such as mapped equatorial spread-F plumes, E-region gravity waves, among other, in conjunction with the AO incoherent scatter radar (ISR), mesosphere and lower thermosphere (MLT) metals lidar, and other instruments, including microbarographs. Data availability and quality as well as specific airglow events on both small/large time/spatial scales are examined, categorized, and made freely available at a data-server website. Our goal here is to briefly review the airglow science enabled by allsky imaging at AO, to describe the instrument and the data-collection methodology, and to present some of the significant results, including airglow events that correspond to ISR results.
Relocation of a seafloor transponder—Sustaining the GPS-Acoustic technique
Authors: Gagnon K. L. , Chadwell C. D.
Published: August 2014

Rigid seafloor arrays of three to four precision acoustic transponders have been repeatedly positioned with the GPS-Acoustic technique to measure horizontal plate motion. In the event that one transponder becomes inactive, a replacement transponder must be precisely located relative to the existing array. Here we present a technique to determine the geodetic azimuth and baseline between the inactive and replacement transponders. We include three examples of relocations between 2002 and 2003 on the Juan de Fuca plate and near the Peru-Chile trench, which add ±16–29 mm uncertainty to the GPS-Acoustic estimated position. A simulation of optimal network geometry shows that the relocation’s contribution to uncertainty can be as low as ±10 mm.
Gravity wave momentum flux in the upper mesosphere derived from OH airglow imaging measurements
Authors: Suzuki Shin , Shiokawa Kazuo , Otsuka Yuichi , Ogawa Tadahiko , Kubota Minoru , Tsutsumi Masaki , Nakamura Takuji , Fritts David C.
Published: August 2014

We report procedures to identify small-scale (20–100 km) atmospheric gravity waves from OH airglow images to estimate momentum fluxes carried by the waves. We also deduce contamination of background continuum emission in OH image, by comparing a simultaneous observation of OH lines measured by the Spectral Airglow Temperature Imager (SATI). We applied the procedures to a one-night dataset obtained at Shigaraki, Japan (34.9°N, 136.1°E) on November 19, 1999. The background wind, which is essential for deriving the intrinsic parameters of gravity waves, was measured by the Middle and Upper Atmosphere (MU) radar. Contamination of background continuum emission with the OH filter was deduced to be 30%. From these procedures, we found that the gravity waves identified in the OH images were mainly propagating southward or southeastward with horizontal wavelengths of 60–90 km and apparent phase speeds of 40–80 m/s. The estimated momentum fluxes on this night was 1–15 m2 s−2, with an average of 4.9 m2 s−2.
Diurnal variation of atmospheric Maxwell current over the low-latitude continental station, Tirunelveli, India (8.7°N, 77.8°E)
Authors: Panneerselvam C. , Nair K. U. , Selvaraj C. , Jeeva K. , Anil Kumar C. P. , Gurubaran S.
Published: August 2014

Observations of atmospheric Maxwell current from the low-latitude continental station, Tirunelveli (8.7°N, 77.8°E), over a period of 8 years are presented in this study. The horizontal long-wire antenna is used as a sensor for picking up charges from the atmosphere. The objectives of the present work have been to understand the antenna system in response to different meteorological conditions and on fairweather days. We define a fairweather day as a day when there is no snowfall/rainfall at the measuring site, when there are high clouds less than 3 octas throughout the day, and when the wind speed is less than 10 m s−1. A sunrise effect is observed in the measured Maxwell current during fairweather days in all seasons. The measured current exhibits an increase that commences about 30 min before sunrise and lasts for nearly 1.5 h. The sunrise effect is found to be inhibited on days when there is pre-dawn convection and during the presence of rain, haze, or cloud cover. Barring the sunrise effect, the rest of the variations on fairweather day appear to follow a trend typical of the Carnegie curve. On clear cloudless conditions, reasonably good data are obtained during equinox and winter months but not in summer when strong winds associated with the onset of the southwest monsoon make the measurement of Maxwell current difficult and limited data are available.
Spectroscopic measurements of Si-O recombination process in laser-induced quartz vapor plumes
Authors: Fuyuki Masanori , Sugita Seiji , Kadono Toshihiko , Hasegawa Sunao , Matsui Takafumi
Published: August 2014

The thermal dissociation of SiO2 in impact-induced vapor is very important because it controls the redox state of the vapor. However, the thermal dissociation of SiO2 and its recombination are not well understood. The present study investigates experimentally the kinetics of the Si-O recombination process at high temperatures. Laser-induced quartz vapor is observed by means of time-resolved spectroscopy, and the atomic lines of Si and molecular bands of SiO are measured in order to estimate the temperature and the column densities of Si and SiO. The results of these experiments show that Si and O recombine as the vapor cools from 5000 to 3000 K. A comparison of the observed chemical composition and equilibrium calculations suggests that the recombination reaction between Si and O proceeds rather efficiently under a condition that differs significantly from the thermo-chemical equilibrium. This result is explained well if the rate constant of the Si-O recombination process does not depend strongly on the temperature; the activation energy is very small. These results suggest that the Si-O recombination process may not be approximated by a frequently-used ‘freeze-out’ model.
Variation in crustal structure along the Kyushu-Palau Ridge at 15–21°N on the Philippine Sea plate based on seismic refraction profiles
Authors: Nishizawa Azusa , Kaneda Kentaro , Katagiri Yasutaka , Kasahara Junzo
Published: August 2014

We acquired coincident wide-angle and multi-channel seismic reflection data along four profiles perpendicular to the Kyushu-Palau Ridge (KPR) between 15°N and 20°N on the Philippine Sea plate. The crustal thickness beneath the KPR, which is a remnant arc created in the Late Eocene, varies along the strike from 8 to approximately 20 km and is always thicker than the adjacent oceanic crust of the West Philippine Basin to the west and the Parece Vela Basin to the east. The thickest crust among the four profiles, which is primarily due to a thickening of the lower crust, is found where the KPR adjoins Oki-no-Tori-Shima Island. There is no clear evidence of the thick (>5 km) middle crustal layer with a P-wave velocity of 6.0–6.5 km/s that has been inferred beneath the conjugate rifted counterpart of the Izu-Ogasawara(Bonin)-Mariana Island-arc. Our results suggest that the crust of the KPR at 15–21°N represents a less mature island arc crust relative to that further north along the ridge where a mid-crustal layer of 6 km/s has been reported.
Stability of VLBI, SLR, DORIS, and GPS positioning
Authors: Feissel-Vernier M. , de Viron O. , Le Bail K.
Published: August 2014

The residual signal in VLBI, SLR, DORIS and GPS station motion, after a linear trend and seasonal components have been removed, is analysed to investigate site-specific and technique-specific error spectra. The study concentrates on 60 sites with dense observation history by two or more space geodetic techniques. Statistical methods include the Allan variance analysis and the three-cornered hat algorithm. The stability of time-series is defined by two parameters, namely the Allan deviation for a one-year sampling time (noise level) and the slope of the Allan variance graph with its spectral interpretation (noise type). The site-specific noise level is found to be in the range 0.5–3.5 mm in either horizontal direction and 1–4.5 mm in height for most sites. The distribution of site-specific noise type includes both white noise and flicker noise. White noise is predominant in the East direction. Both types of noise are found in the North direction, with no particular geographical clustering. In the Up direction, the Northern hemisphere sites seem to be split in two large geographical sectors characterised either by white noise or by flicker noise signatures. Technique-specific noise characteristics are estimated in several ways, leading to a white noise diagnostic for VLBI and SLR in all three local directions. DORIS has also white noise in the horizontal directions, whereas GPS has a flicker noise spectrum. The vertical noise spectrum is indecisive for both DORIS and GPS. The three-dimensional noise levels for the one-year sampling time are 1.7 mm for VLBI, 2.5 mm for SLR, 5.2 mm for DORIS, and 4.1 mm for GPS. For GPS, the long-term analysis homogeneity has a strong influence. In the case of a test solution reanalysed in a fully consistent way, the noise level drops to the VLBI level in horizontal and to the SLR level in vertical. The three-dimensional noise level for a one-year sampling time decreases to 1.8 mm. In addition, the percentage of stations with flicker noise drops to only about 20% of the network.
True polar wander associated with continental drift on a hypothetical Earth
Authors: Nakada Masao
Published: August 2014

Long-term true polar wander of the Earth (TPW) has generally been discussed by taking into account con-vective processes in the mantle such as downgoing slabs and upwelling plumes. Here I examined a relationship between continental drift and TPW on a hypothetical Earth with no such convective processes in the mantle. I evaluated temporal changes in moments of inertia owing to continental drift during a period of ~250 Ma based on a paleogeographic reconstruction, in which I estimated the lateral density heterogeneities by factoring in the observed mean land elevation of continents and average age of the oceanic lithosphere. The predictions for a viscoelastic Earth model with plausible viscosity models indicate that the long-term TPW might have been affected by continental drift throughout Cenozoic and Mesozoic times, which has wholly proceeded by maintaining isostasy at a certain depth, as well as convective processes in the mantle.
Electrostatic electron cyclotron harmonic waves observed by the Akebono satellite near the equatorial region of the plasmasphere
Authors: Shinbori A. , Ono T. , Iizima M. , Kumamoto A. , Shirai S. , Hanaoka A. , Okamoto K. , Ohashi M. , Oya H.
Published: August 2014

Analysis of the plasma wave observation data provided by the plasma waves and sounder experiment (PWS) on board the Akebono satellite frequently reveals the presence of electrostatic electron cyclotron harmonic (ESCH) waves in the low-latitude region (MLAT < 45°) of the plasmasphere within an altitude range from about 3000 km to the apogee of the satellite (initial apogee was 10,500 km). Even at moderate or low geomagnetic activity, intense ESCH waves often appear near the equatorial region of the plasmasphere above the upper hybrid resonance (UHR) frequency at the lowest harmonic number branch of the f
Qn ESCH waves. We identified these plasma waves as the equatorial plasmasphere f
Qn waves (EP-f
Qn). The spectra of the EP-f
Qn waves are characterized by a narrow band structure and by a strong nature, with a wave intensity that ranges from 3.46 × 10-8 to 3.31 × 10-4 V/m. The maximum intensity is nearly coincident with the upper limit of the PWS receiver in the low-gain mode. Statistical analysis results reveal that the EP-f
Qn waves are observable in all the local time sectors; however, the occurrence probability shows a clear enhancement in the early morning sector of 01–03 MLT in the plasmasphere. The EP-f
Qn wave activities are suppressed within a period of strong magnetic disturbances as well as solar minimum phase. The linear dispersion relation analysis using a two-component plasma model reveals that supra-thermal plasma with the energy of about 750 eV and with a large temperature anisotropy (A = T-perp/T-parallel–1 > 40) must be present in order to realize an appearance of a positive growth rate at the observed frequency and propagation angle of the ESCH waves. Since the hot plasma with such a high anisotropy has not been detected, the validity of the present two-component plasma model remains an open question. The occurrence feature of the ESCH waves showed that there is a constant activation or a constant flow-in of free energy to generate the strong plasma instability of ESCH waves near the post-midnight sector of the plasmasphere. The existence of ESCH waves revealed that the nature of the plasmaspheric plasma is more turbulent and active than has been believed.
Geomagnetic constraints on stratification at the top of Earth’s core
Authors: Gubbins David
Published: August 2014

The geodynamo requires that at least part of the Earth’s liquid core be convecting so vigorously as to mix it thoroughly to uniform composition and adiabatic temperature. It is possible, however, that part of liquid core is stably stratified, either thermally because of a low temperature gradient or compositionally because light material has separated out. The top of the outer core is the most likely site for stability because the adiabat is steepest there and because light material will rise to the surface. Here I show that part of the observed secular variation, that associated with flux expulsion in the southern hemisphere, can only be caused by fluid upwelling in the electromagnetic boundary layer at the top of the core or by very strong poloidal field gradients at the top of the core. Any stratified layer is limited to roughly the uppermost 100 km if flux expulsion is the mechanism; if the layer is any thicker the region in the southern hemisphere where reverse flux patches are growing must also be the site of very strong field gradients caused by very strong near-surface shear flows.
The use of the ‘Kono perpendicular applied field method’ in microwave palaeointensity experiments
Authors: Hill Mimi J. , Shaw John
Published: August 2014

For a complete understanding of the Earth’s magnetic field behaviour it is necessary to have information regarding variations in its strength as well as directional changes. In order to reduce the lengthy experimental time for palaeointensity experiments and potentially reduce the possibility of mineralogical alteration Kono suggested a single heating perpendicular applied field variant of the most widely used double heating palaeointensity method (the Thellier method). Whilst in the past this method has not been generally used, due mainly to technical difficulties, it is ideal for use with the microwave palaeointensity technique. Reproducing the amount of microwave energy absorbed by a sample is not as simple as reproducing the temperature in an oven so a palaeointensity method that only has a single application of microwave power at each step is an advantage. One of the uncertainties in the original Kono method lies in getting the natural remanent magnetisation of the sample aligned perpendicular to the applied laboratory field. With the microwave systems this uncertainty has been essentially removed. The microwave variant of the perpendicular applied field method which incorporates checks for perpendicularity, as well as the usual checks for alteration and non ideal behaviour is routinely applied as it is quick and easy to use, avoids any potential power reproducibility issues and provides a comparison with results obtained through other methods.
Further K-Ar dating and paleomagnetic study of the Auckland geomagnetic excursions
Authors: Mochizuki Nobutatsu , Tsunakawa Hideo , Shibuya Hidetoshi , Tagami Takahiro , Ozawa Ayako , Smith Ian E. M.
Published: August 2014

Three different excursional paleomagnetic directions were reported from eight volcanoes of the Auckland volcanic field in New Zealand: north-down (ND) directions obtained from five volcanoes, west-up (WU) from two volcanoes, and south-up (SU) from one volcano. K-Ar ages have been reported for two of these volcanoes: 27±5 (1σ) ka for the Wiri volcano of the ND group and 55±5 ka for the Hampton Park volcano of the WU group. In the present study, we have carried out further K-Ar age determinations on three other volcanoes and obtained reliable ages for two of them: 30±5 ka for the Puketutu volcano of the ND group and 50±6 ka for the McLennan Hills volcano of the SU group. The age of Puketutu agrees well with that of Wiri, and these two ages give a weighted mean age of 29±3 (1σ) ka for the ND group. The age of the ND group is distinguishable from those of the SU and WU groups at the 2σ level, confirming that excursions occurred at two different times separated by a few tens of thousands of years. The age of the SU group is indistinguishable from that of the WU group, and a weighted mean age of 53±4 ka can be calculated for this combined group (SU-WU group). The age of the ND group and that of the SU-WU group are distinguishable from the latest age estimate of the Laschamp excursion. Overall, these age data from volcanic rocks show that at least three excursions occurred between approximately 30 and 60 ka. These three excursions are likely to be confined in the weak dipole interval of 20–70 ka, and all of these excursions yield particularly low virtual dipole moments (VDMs) of 2×1022 A m2 or less. Since it is suggested that the larger virtual geomagnetic pole (VGP) deviations from the geographic pole are related to the lower VDMs, the excursional fields possibly have resulted from a significantly reduced dipole field and comparable non-dipole components.