Dynamic nucleation process of shallow earthquake faulting in a fault zone

Two distinct phases are commonly observed at the initial part of seismograms of large shallow earthquakes: low-frequency and low-amplitude waves following the onset of a P wave ( P ₁) are interrupted by the arrival of the second impulsive phase P₂ enriched with high-frequency components. This observation suggests that a large shallow earthquake involves two qualitatively different stages of rupture at its nucleation.
We propose a theoretical model that can naturally explain the above nucleation behaviour. The model is 2-D and the deformation is assumed to be anti-plane. A key clement in our model is the assumption of a zone in which numbers of pre-existing cracks are densely distributed; this cracked zone is a model for the fault zone. Dynamic crack growth nucleated in such a zone is intensely affected by the crack interactions, which exert two conflicting effects: one tends to accelerate the crack growth, and the other tends to decelerate it. The accelerating and decelerating effects are generally ascribable to coplanar and non-coplanar crack interactions, respectively. We rigorously treat the multiple interactions among the cracks, using the boundary integral equation method (BIEM), and assume the critical stress fracture criterion for the analysis of spontaneous crack propagation.
Our analysis shows that a dynamic rupture nucleated in the cracked zone begins to grow slowly due to the relative predominance of non-coplanar interactions. This process radiates the P₁ phase. If the crack continues to grow, coalescence with adjacent coplanar cracks occurs after a short time. Then, coplanar interactions suddenly begin to prevail and crack growth is accelerated; the P₂ phase is emitted in this process. It is interpreted that the two distinct phases appear in the process of the transition from non-coplanar to coplanar interaction predominance.     

Geological age of the Yokawa Formation of the Kobe Group (Japan) on the basis of terrestrial mammalian fossils

Abstract   The age of the Yokawa Formation of the Cenozoic Kobe Group distributed in Hyogo Prefecture of western Japan based on mammalian fossils is discussed. Two fossil dental specimens of terrestrial mammals discovered from the lowest part of the Yokawa Formation in the Sanda area are described. These two fossils described here are: (i) a right mandibular fragment with p2–m3 of Bothriodon sandaensis sp. nov. (selenodont anthracotheriid artiodactyl), which appears to be the most primitive among the species of the genus; and (ii) right m1–m3 of cf. Hyrachyus sp. (primitive rhinocerotoid perissodactyl). In the lower part of the Yokawa Formation, Zaisanamynodon (amynodontid perissodactyl) was previously reported. The morphology ('evolutionary stage') of B. sandaensis is indicative of the latest Middle to Late Eocene, that of cf. Hyrachyus sp. is indicative of the Early to Middle Eocene, and Zaisanamynodon is indicative of the Late Middle to Late Eocene. Therefore, the fossil mammals of the Yokawa Formation indicate an latest Middle Eocene ( ca . 38 Ma) correlation for the lower part of the formation, as a working hypothesis. Although the resolution of the geological age based on these mammalian fossils is relatively low compared to that based on marine index fossils, this result is concordant with the recent radiometric correlation of the lower part of the Yokawa Formation in the Sanda area.     

Decadal Variability of Subsurface Temperature in the Central North Pacific

Decadal variability of subsurface temperature in the North Pacific has been investigated. Two dominant regions were found; the central subarctic region (CSa) and the north-eastern subtropical region (NESt). In CSa, cooling (warming) of wintertime subsurface temperature corresponds to the large (small) temperature gradient and southward (northward) shift of subsurface temperature front, associated with the increase (decrease) of positive wind stress curl and the southward (northward) shift of curl τ zero line with 2 years delay. It is suggested that the relocation of subtropical-subarctic boundary plays an important role. In NESt, importance of heat flux through the sea surface and heat divergence in the Ekman layer is also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Earthquake rupture complexity due to dynamic nucleation and interaction of subsidiary faults

We numerically study the dynamic interaction of propagating cracks. It is assumed that propagating cracks can nucleate and drive subsidiary cracks because of shear strain enhancement near the propagating crack tips. The critical strain fracture criterion is assumed in the analysis. Intense interaction is expected to occur among the cracks. All the cracks are assumed to be parallel and antiplane strain deformation is assumed in the computation.In the interaction of two non-coplanar cracks, a strain shadow is formed in the neighborhood of each crack because of the strain release by the introduction of the crack. The growth of each crack is accelerated when the propagating tips of each crack are outside of the strain shadow of the other crack. In general, the crack tips enter the strain shadow, and the crack tips decelerate. The calculation shows that only one of the two cracks can continue to grow, and the other's growth is decelerated and arrested. If we can assume that the suite of cracks interact in a pairwise manner only, then this may suggest that only a limited number of cracks can continue to grow during the final stage of the rupture process. Hence the crack interaction causes complexity in dynamic earthquake faulting. The concepts of barrier and asperity have been employed by many researchers for the interpretation of complex seismic wave data. However, the physical realities of such concepts are obscure. Our calculations show that dynamic crack interactions can produce barriers and asperities in some cases; the crack tip deceleration or arrest due to the interactions among non-coplanar cracks can be interpreted as being due to a barrier. The dynamic coalescence among the coplanar cracks can be regarded as an asperity.Umeda found a localized area that strongly radiates high-frequency seismic waves in the epicentral areas of some large shallow earthquakes. He defined this as an earthquake bright spot. Our analysis implies that only a limited number of cracks continue to grow when many interactive cracks nucleate, and that all other cracks stop extending soon after nucleation. Hence, if the nucleation and termination of several cracks occur in a localized area, it will be observed seismologically as an earthquake bright spot. This is because it is theoretically known that the sudden termination of crack growth and dynamic crack coalescence efficiently emits high-frequency elastic waves.     

Fluorine contents of some hydrous minerals derived from upper mantle and lower crust

Fluorine contents have been determined in about forty samples of amphibole, mica and apatite in alkali basalt and kimberlite and their incorporated xenoliths. They show a wide variation ranging from 15,000 to 100 ppm, corresponding to about 40 to 0.2 per cent substitution of F for OH in hydroxyl site of hydrous minerals. Fluorine abundances in these minerals reflect those of their host magmas or rocks; Itinome-gata xenoliths are the lowest and South African kimberlites and their xenoliths are the highest. F/OH and also. D/H (Kuroda et al. 1975) ratios in coexisting phlogopite-potassic richterite from peridotite and mica nodules are thoughts to have formed under no simple equlibrium conditions.     

High-reliability zircon separation for hunting the oldest material on Earth: An automatic zircon separator with image-processing/microtweezers-manipulating system and double-step dating

Despite the recent development in radiometric dating of numerous zircons by LA-ICPMS, mineral separation still remains a major obstacle, particularly in the search for the oldest material on Earth. To improve the efficiency in zircon separation by an order of magnitude, we have designed/developed a new machine-an automatic zircon separator(AZS). This is designed particularly for automatic pick-up of100 μm-sized zircon grains out of a heavy mineral fraction after conventional separation procedures. The AZS operates in three modes:(1) image processing to choose targeted individual zircon grains out of all heavy minerals spread on a tray,(2) automatic capturing of the individual zircon grains with microtweezers, and(3) placing them one-by-one in a coordinated alignment on a receiving tray. The automatic capturing was designed/created for continuous mineral selecting without human presence for many hours. This software also enables the registration of each separated zircon grain for dating, by recording digital photo-image, optical(color) indices, and coordinates on a receiving tray. We developed two new approaches for the dating; i.e.(1) direct dating of zircons selected by LA-ICPMS without conventional resin-mounting/polishing,(2) high speed U-Pb dating, combined with conventional sample preparation procedures using the new equipment with multiple-ion counting detectors(LA-MIC-ICPMS).With the first approach, Pb-Pb ages obtained from the surface of a mineral were crosschecked with the interior of the same grain after resin-mounting/polishing. With the second approach, the amount of time required for dating one zircon grain is ca. 20 s, and a sample throughput of 150 grains per hour can be achieved with sufficient precision(ca. 0.5%).We tested the practical efficiency of the AZS, by analyzing an Archean Jack Hills conglomerate in Western Australia with the known oldest(4.3 Ga) zircon on Earth. Preliminary results are positive; we were able to obtain more than 194 zircons that are over 4.0 Ga out of ca. 3800 checked grains, and 9 grains were over 4300 Ma with the oldest at 4371 ± 7 Ma. This separation system by AZS, combined with the new approaches, guarantees much higher yield in the hunt for old zircons.     

Organic micropollutants in marine plastics debris from the open ocean and remote and urban beaches

To understand the spatial variation in concentrations and compositions of organic micropollutants in marine plastic debris and their sources, we analyzed plastic fragments (∼10 mm) from the open ocean and from remote and urban beaches. Polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), dichloro-diphenyl-trichloroethane and its metabolites (DDTs), polybrominated diphenyl ethers (PBDEs), alkylphenols and bisphenol A were detected in the fragments at concentrations from 1 to 10,000 ng/g. Concentrations showed large piece-to-piece variability. Hydrophobic organic compounds such as PCBs and PAHs were sorbed from seawater to the plastic fragments. PCBs are most probably derived from legacy pollution. PAHs showed a petrogenic signature, suggesting the sorption of PAHs from oil slicks. Nonylphenol, bisphenol A, and PBDEs came mainly from additives and were detected at high concentrations in some fragments both from remote and urban beaches and the open ocean.     

Simulation of seismicity due to fluid migration in a fault zone

Spatio-temporal variation of rupture activity is modelled assuming fluid migration in a narrow, porous fault zone formed along a vertical strike-slip fault in a semi-infinite elastic medium. The principle of the effective stress coupled to the Coulomb failure criterion introduces mechanical coupling between fault slip and the pore fluid. The fluid is assumed to flow out of a localized high-pressure fluid compartment in the fault at the onset of earthquake rupture. The duration of the earthquake sequence is assumed to be much shorter than the recurrence period of characteristic events on the fault. Both an earthquake swarm and a foreshock–main-shock sequence can be simulated by changing the relative magnitudes of the initial tectonic stress, pore fluid pressure, fracture strength and so on. When an inhomogeneity is introduced into the spatial distribution of fracture strength, high complexity is observed in the spatio-temporal variation of rupture activity. For example, the time interval between two successive events is highly irregular, and a relatively long quiescence of activity is sometimes observed in a foreshock–main-shock sequence. The quiescence is caused by the temporary arresting of rupture extension, due to an encounter with fault segments having locally high strengths. The frequency–magnitude statistics of intermediate-size events obey the Gutenberg–Richter relation. The calculations show the temporal variation of the b value during some foreshock sequences, and the degree of the change seems to depend on the statistical distribution of the fracture strength.     

Holocene paleolimnological changes in Lake Skallen Oike in the Syowa Station area of Antarctica inferred from organic components in a sediment core (Sk4C-02)

Antarctic climate changes influence environmental changes at both regional and local scales. Here we report Holocene paleolimnological changes in lake sediment core Sk4C-02 (length 378.0 cm) from Lake Skallen Oike in the Soya Kaigan region of East Antarctica inferred from analyses of sedimentary facies, a range of organic components, isotope ratios of organic carbon and nitrogen, and carbon-14 dating by Tandetron accelerator mass spectrometry. The sediment core was composed of clayish mud (378.0–152.5 cm) overlain by organic sediments (152.5 cm-surface). The age of the surface and the core bottom were 150 (AD1950-1640) and ca. 7,030 ± 73 calibrated years before present (cal BP), respectively, and the mean sedimentation rate was estimated to be 0.55 mm/year. Multi-proxy analyses revealed that the principal environmental change in the core is a transition from marine to lacustrine environments which occurred at a depth of 152.5 cm (ca. 3,590 cal BP). This was caused by relative sea level change brought about by ongoing retreat of glaciers during the mid-Holocene warming of Antarctica, and ongoing isostatic uplift which outpaced changes in global (eustatic) sea level. The mean isostatic uplift rate was calculated to be 2.8 mm/year. The coastal marine period (378.0–152.5 cm, ca. 7,030–3,590 cal BP) was characterized by low biological production with the predominance of diatoms. During the transition period from marine to freshwater conditions (152.5-approximately 135 cm, ca. 3,590–3,290 cal BP) the lake was stratified with marine water overlain by freshwater, with a chemocline and an anoxic (sulfidic) layer in the bottom of the photic zone. Green sulfur bacteria and Cryptophyta were the major photosynthetic organisms. The Cryptophyta appeared to be tolerant of the moderate salinity and stratified water conditions. The lacustrine period (approximately 135 cm-surface, ca. 3,290 cal BP-present) was characterized by high biological production by green algae (e.g. Comarium clepsydra and Oedegonium spp.) with some contributions from cyanobacteria and diatoms. Biological production during this period was 8.7 times higher than during the coastal marine period.     

Dynamic FE simulation of four‐story steel frame modeled by solid elements and its validation using results of full‐scale shake‐table test

Dynamic finite element analyses of a four‐story steel building frame modeled as a fine mesh of solid elements are performed using E‐Simulator, which is a parallel finite element analysis software package for precisely simulating collapse behaviors of civil and building structures. E‐Simulator is under development at the National Research Institute for Earth Science and Disaster Prevention (NIED), Japan. A full‐scale shake‐table test for a four‐story frame was conducted using E‐Defense at NIED, which is the largest shaking table in the world. A mesh of the entire structure of a four‐story frame with approximately 19 million degrees of freedom is constructed using solid elements. The density of the mesh is determined by referring to the results of elastic–plastic buckling analyses of a column of the frame using meshes of different densities. Therefore, the analysis model of the frame is well verified. Seismic response analyses under 60, 100, and 115% excitations of the JR Takatori record of the 1995 Hyogoken‐Nanbu earthquake are performed. Note that the simulation does not reproduce the collapse under the 100% excitation of the Takatori record in the E‐Defense test. Therefore, simulations for the 115% case are also performed. The results obtained by E‐Simulator are compared with those obtained by the E‐Defense full‐scale test in order to validate the results obtained by E‐Simulator. The shear forces and interstory drift angles of the first story obtained by the simulation and the test are in good agreement. Both the response of the entire frame and the local deformation as a result of elastic–plastic buckling are simulated simultaneously using E‐Simulator. Copyright © 2014 John Wiley & Sons, Ltd.