Osaka Group Research Articles

Dip angles of active faults from the surface to the seismogenic zone inferred from a 2D numerical analysis of visco-elasto-plastic models: a case study for the Osaka Plain

The geometries (i.e., dip angles) of active faults from the surface to the seismogenic zone are the most important factors used to evaluate earthquake ground motion, which is crucial for seismic hazard assessments in urban areas. In Osaka, a metropolitan city in Japan, there are several active faults (e.g., the Uemachi and Ikoma faults), which are inferred from the topography, the attitude of active faults in surface trenches, the seismic reflection profile at shallow depths (less than 2 km), and the three-dimensional distribution of the Quaternary sedimentary layers. The Uemachi and Ikoma faults are N–S-striking fault systems with total lengths of 42 km and 38 km, respectively, with the former being located ~ 12 km west of the latter; however, the geometries of each of the active faults within the seismogenic zone are not clear. In this study, to examine the geometries of the Uemachi and Ikoma faults from the surface to the seismogenic zone, we analyze the development of the geological structures of sedimentary layers based on numerical simulations of a two-dimensional visco-elasto-plastic body under a horizontal compressive stress field, including preexisting high-strained weak zones (i.e., faults) and surface sedimentation processes, and evaluate the relationship between the observed geological structures of the Quaternary sediments (i.e., the Osaka Group) in the Osaka Plain and the model results. As a result, we propose geometries of the Uemachi and Ikoma faults from the surface to the seismogenic zone. When the friction coefficient of the faults is ~ 0.5, the dip angles of the Uemachi and Ikoma faults near the surface are ~ 30°–40° and the Uemachi fault has a downward convex curve at the bottom of the seismogenic zone, but does not converge to the Ikoma fault. Based on the analysis in this study, the dip angle of the Uemachi fault zone is estimated to be approximately 30°–40°, which is lower than that estimated in the previous studies. If the active fault has a low angle, the width of the fault plane is long, and thus the estimated seismic moment will be large.

Discovery of an orthoquartzite gravel from the Osaka Group in the southern Nara basin and its geological significance

Discovery of an orthoquartzite gravel from the Osaka Group in the southern Nara basin and its geological significance

A fossil aphid gall from the middle Pleistocene sediment in Hyogo Prefecture, western Japan

AbstractWe described the external morphology of a fossil insect gall collected from the middle Pleistocene sediment in the Osaka Group of Nishinomiya City, Hyogo Prefecture, western Japan. The fossil was well preserved without major compression and was identified as a gall induced by an aphid species of the genus Nipponaphis (Nipponaphidini: Hormaphidinae: Aphididae), based on its external morphology. This is the first reliable fossil record of an Aphididae gall. This fossil provides evidence that the intimate aphid–plant association can be traced back to the middle Pleistocene, approximately 0.4 million years ago.

The First Cetacean Record from the Osaka Group (Middle Pleistocene, Quaternary) in Osaka, Japan

Abstract. A new partial skeleton consisting of a left mandible and five caudal vertebrae, OMNH-QV 282 from the Osaka Group (Middle Pleistocene, about 0.3 million years ago) of Osaka City is reported as the first cetacean record from the group. The skeleton is identified as Balaenopteridae gen. et sp. indet. based on the combination of mandibular characters, such as having a small mandibular foramen, reflected neck in dorsal view and lack of a satellite process of the mandible. OMNH-QV 282 expands diversity for the local fauna, and also adds evidence for the existence of large-sized balaenopterids from a poorly known epoch, the Middle Pleistocene.

131. 火山ガラスの組成からみた大阪層群ピンク,アズキ火山灰層の起源

131. 火山ガラスの組成からみた大阪層群ピンク,アズキ火山灰層の起源

大阪層群の研究と"地球科学"( 地球科学200号を記念して,200号記念号)

大阪層群の研究と"地球科学"( 地球科学200号を記念して,200号記念号)

枚方丘陵の大阪層群の花粉分析 : -とくに海成・淡水成層と気候変化について-近畿地方の新期新生代層の研究IV

枚方丘陵の大阪層群の花粉分析 : -とくに海成・淡水成層と気候変化について-近畿地方の新期新生代層の研究IV

大阪層群中の「不整合」について

大阪層群中の「不整合」について

火山ガラスの主成分・微量成分元素組成に基づくテフラ層の岩石学的特徴 : 大阪層群と琵琶湖高島沖コアに挟まるテフラ層の例

火山ガラスの主成分・微量成分元素組成に基づくテフラ層の岩石学的特徴 : 大阪層群と琵琶湖高島沖コアに挟まるテフラ層の例

Re-identification of Shishimuta-Pink tephra samples from the Japanese Islands based on simultaneous major- and trace-element analyses of volcanic glasses

The Shishimuta-Pink (Ss-Pnk) tephra erupted from the Shishimuta caldera on Kyushu Island, Japan, at 1 Ma, and it is distributed extensively over the Japanese Islands. We quantitatively analyzed 58 elements in volcanic glass shards of this tephra, which sampled at 9 points, using laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS). The nine samples have been correlated as Ss-Pnk tephra in previous studies, based on various geologic and petrographic properties. The element patterns of the tephra samples are similar, except for that of the Kuratama–Rindo O7 sample obtained in the Kazusa Group at the Boso Peninsula, located ∼900 km from the source volcano. The results of the Tukey–Kramer multiple comparison test suggest that 62%–71% of the concentrations of the 58 elements in the Kuratama–Rindo O7 sample differ significantly from the element concentrations in the other eight Ss-Pnk samples, whereas only 2%–31% of the element concentrations of the eight Ss-Pnk samples vary significantly. The element pattern of the sample of the Osaka Group Pink (OGPK), which is treated as being representative of the Ss-Pnk tephra, can be clearly distinguished from five other widespread marker tephras erupted from Kyushu Island. These results strongly suggest that the multi-element analysis of volcanic glass shards from tephras can be used as a basis for the identification and correlation of widespread Quaternary tephras.

P1-23 大阪層群ピンク火山灰の堆積ユニット(ポスターセッション)

P1-23 大阪層群ピンク火山灰の堆積ユニット(ポスターセッション)

B39 中部九州の今市火砕流堆積物の給源カルデラの推定, 及び大阪層群アズキ火山灰と今市火砕流堆積物との対比

B39 中部九州の今市火砕流堆積物の給源カルデラの推定, 及び大阪層群アズキ火山灰と今市火砕流堆積物との対比

Brief sea‐level fall event and centennial to millennial sea‐level variations during Marine Isotope Stage 19 in Osaka Bay, Japan

ABSTRACTDetailed sea‐level variation was investigated for Marine Isotope Stage (MIS) 19, based on diatom and grain size analyses of a marine sequence in a core of the Osaka Group. Diatom sea‐level proxies represent precession‐related signals correlated with highstands MIS 19.3 and 19.1, and lowstand MIS 19.2. Astronomical tuning shows the marine sequence has a uniform accumulation rate of about 60 cm ka−1. A rapid sea‐level fall event was found in the earliest MIS 19, demonstrated by several independent sea‐level proxies of diatom and grain size. This event began with a rapid sea‐level drop, followed by a gradual recovery, at about 783–782 ka. A maximum abundance of pelagic diatom taxa at a core depth of 402.20 m evidently shows the highest sea‐level peak in MIS 19, supported by many other proxies. Based on the diatom data, sea‐level change across MIS 19.1 is characterized by centennial to millennial fluctuations. The sea‐level fall event began just after the onset of a cooling event previously reported from the same core. Observations of a comparable sea‐level fall signal in many deep‐sea core records suggest the event is global.

A pathway to laser fusion energy in Japan

High-density compression of DT to one thousand times its liquid density is the critical path of inertial fusion and was demonstrated in Japan and US in late 1980's. The Osaka group has achieved high-density compression that meets one of the critical requirements for thermonuclear ignition and burn. Although the compression densities were well reproduced by computer simulations, the neutron yields were much lower than the simulation predictions by three orders of magnitudes, suggesting catastrophic collapse of a hot spark, from which thermonuclear reactions are triggered. In order to overcome this difficulty the international ICF community has adopted two approaches: one is to generate a larger hot spark than the mixed layer with MJ-Class lasers, such as NIF and LMJ. The other approach is to externally heat the compressed fuel. The second approach is the fast ignition. After the proof-of-concept experiment in 2002, we started the Fast Ignition Realization Experiment (FIREX) project to complete the world most powerful high-energy peta-watt laser “LFEX” as a heating laser.

大阪層群下部下半部の堆積速度と堆積環境の復元

大阪層群下部下半部の堆積速度と堆積環境の復元

Timing of the Matuyama–Brunhes geomagnetic reversal: Decoupled thermal maximum and sea-level highstand during Marine Isotope Stage 19

Recent high-resolution magnetic and microfossil data from a long core penetrating the Osaka Group, Japan are astronomically tuned and reassessed to reveal detailed stratigraphic features across the Matuyama–Brunhes (MB) polarity transition during Marine Isotope Stage (MIS) 19. The sediments have a uniform and high average accumulation rate of 63 cm/ky. Astronomical tuning based on six calibration points, including a clear MIS 19.2 sea-level lowstand, dates the main MB boundary (MBB) to 777.6 ka, the sea-level peak of the MIS 19.3 highstand to 780.7 ka, and the climatic thermal maximum (TM) to 776.4 ka. The MB transition began with a short-lived normal polarity episode that occurred before the highstand at MIS 19.3, and terminated with multiple rapid reversals that occurred between highstand MIS 19.3 and lowstand 19.2. The TM just postdates the termination of the MB transition, and occurs about 4000 years after the sea-level peak. The delayed TM is also observed at Gesher Benot Ya'aqov, Israel and on the Mediterranean coast, and possibly at Lake Baikal. In high-resolution records from Chinese loess–paleosols and deep-sea sediments, the rapid reversal zone of the MBB has a very similar climatostratigraphic character. These magneto-climatostratigraphic features in MIS 19 are useful for correlating between terrestrial and marine sediments. The delay in warming relative to the sea-level highstand seems to have been due to a cloud-albedo effect induced by an increase in galactic cosmic ray flux during an extremely low magnetic field intensity interval.

Diatom analysis of the Osaka Group at Nigawa, Nishinomiya, Hyogo Prefecture, western Japan

Diatom analysis of the Osaka Group at Nigawa, Nishinomiya, Hyogo Prefecture, western Japan

Determination of U–Pb Ages for Young Zircons using Laser Ablation‐ICP‐Mass Spectrometry Coupled with an Ion Detection Attenuator Device

We have developed new analytical procedures to measure precise and accurate 238U–206Pb and 235U–207Pb ages for young (~ 1 Ma) zircons using laser ablation‐ICP‐mass spectrometry. For young zircons, both careful correction for the background counts and analysis of very small Pb/U ratios (i.e., 206Pb/238U < 0.00016 and 207Pb/235U < 0.0001 for 1 Ma zircons) are highly desired. For the correction of the background, the contribution of the background signal intensities for the analytes, especially for the residual signal intensities for 206Pb and 207Pb, was defined through laser ablation of synthesised zircons (ablation blank) containing negligible Pb. The measured signal intensities for 202Hg, 206Pb and 207Pb signals obtained by the ablation blank were slightly higher than those obtained by data acquisition without laser ablation (gas blank). For the wider dynamic range measurements on Pb/U ratios, an attenuator device for the ion detection system was employed to extend the capability to monitor high‐intensity signals (i.e., > 3 Mcps). Through the attenuator device, the ion currents were reduced to 1/450 of the signal intensity without the attenuator. Because the switching time for the attenuator was shorter than 1 ms, signal intensities for only specific isotopes could be reduced. With attenuation of the 238U signal, counting statistics on 206Pb and 207Pb isotopes could be improved and counting loss on the 238U signal could be minimised. To demonstrate the reliability of this new analytical technique, 238U–206Pb and 235U–207Pb ages for three young zircon samples (collected from Osaka Group Pink Volcanic Ash, Kirigamine and Bishop Tuff) were measured. The data presented here demonstrate clearly that the present technique could become a major analytical tool for in situ U–Pb age determination of young zircons (~ 1 Ma).

Variants in melanogenesis‐related genes associate with skin cancer risk among Japanese populations

Human skin color is known to be associated with the risk of cutaneous cancer. Some reports indicated that pigmentation-related gene variants were associated with cutaneous cancer risk in Caucasian populations, but there are no similar reports in East Asian populations. This study aimed to evaluate the association between pigmentation-related genes and the risk of skin cancer in Japanese populations. We studied the associations between 12 variants of four pigmentation-related genes and melanin index variations in 198 Japanese patients with skin cancer and compared these findings to those of 500 Japanese controls by using multiple logistic regression analysis. Furthermore, we analyzed an independent sample of 107 Japanese patients with skin cancer. A non-synonymous variant, H615R in the oculocutaneous albinism 2 gene (OCA2), was associated with the risk of malignant melanoma in the Yamagata group (odds ratio [OR], 0.38; 95% confidence interval [CI], 0.17-0.86; P=0.020). Another non-synonymous variant, A481T in OCA2, was associated with the risk of squamous cell carcinoma and actinic keratosis in the Osaka group (OR, 3.16; 95% CI, 1.41-7.04; P=0.005). In malignant melanoma cases, the minor allele in OCA2 H615R might have induced the development of lesions in sun-exposed skin (OR, 26.32; 95% CI, 1.96-333; P=0.014). Our results suggest that some OCA2 variants are definite risk factors for the onset of cutaneous cancer in Japanese populations.

Tephrostratigraphy of Lower Part of the Osaka Group, at the Senpoku Hill, Southern part of the Osaka Plain.

Tephrostratigraphy of Lower Part of the Osaka Group, at the Senpoku Hill, Southern part of the Osaka Plain.