Variation of seismic source parameters and stress drops within a descending slab and its implications in plate mechanics

A least-squares searching technique has been developed to estimate the source dimensions of intermediate and deep focus earthquakes using azimuthal variations of body wave pulse-widths. With this method and also amplitude data, modes of rupture propagation, seismic moments, and stress drops of 17 intermediate and deep focus earthquakes in the Tonga-Kermadec region have been determined in order to investigate variations in source properties and the state of stress within the descending slab there. Three different modes of rupture; unilateral, bilateral, and circular faults, are compared and tested against observations. Results indicate that the unilateral fault is the best model for most of the earthquakes studied. Stress drops of the 17 events vary within a very large range, from 20 bar to about 4.6 kbar, and change significantly with depth. The magnitude of stress drops for depths between 220 and 430 km is very much higher than at shallower depths. This change in stress drop magnitude at about 220 km-depth seems to reflect a change in material properties both in the mantle and within the slab. Two regions of high stress drop are observed at depths of about 360 and 640 km. A relative minimum of stress drop is found at about 450–560 km where the earthquake frequency is particularly high. Earthquakes at the northern end to the Tonga arc, where the Benioff zone is laterally bent, show systematically higher stress drops than other events at comparable depths, but away from the bend. Also, events in regions of low seismicity appear to have higher stress drops than those in regions of high seismicity. The upper bound of seismic efficiency is found to decrease with depth, implying an increase of frictional force with depth at the earthquake source.     

High Ash Content in Net-Plankton Samples from Shallow Coastal Water: Possible Source of Error in Dry Weight Measurement of Zooplankton Biomass

Annual examination of net-plankton biomass in dry weight, ash-free dry weight, organic carbon, and nitrogen weight at the Manazuru harbor, central Japan revealed that net-plankton samples from shallow coastal water contained considerable amount of ash (53±13% of dry weight) which would be caused by contamination of inorganic materials from re-suspendion of sediments, terrestrial runoff and chain-forming diatoms. Therefore, in coastal water, dry weight is inadequate fro determination of zooplankton biomass in volving the possibility of over-estimation. Practical estimation of net-plankton biomass in shallow coastal waters, ash-free dry weight, organic carbon, and/or nitrogen are more adequate.     

Geological relationship between Anyui Metamorphic Complex and Samarka terrane,Far East Russia

The Anyui Metamorphic Complex (AMC) of Cretaceous age is composed of metachert, schist, gneiss, migmatite and ultramafic rocks, and forms a dome structure within the northernmost part of the Jurassic accretionary complex of the Samarka terrane. The two adjacent geological units are bounded by a fault, but the gradual changes of grain size and crystallinity index of quartz in chert and metachert of the Samarka terrane and the AMC, together with the gradual lithological change, indicate that at least parts of the AMC are metamorphic equivalents of the Samarka rocks. Radiolarian fossils from siliceous mudstone of the Samarka terrane indicates Tithonian age (uppermost Jurassic), and hence, form a slightly later accretion. This signifies that the accretionary complex in the study area is one of the youngest tectonostratigraphic units of the Samarka terrane. The relationship between the Samarka terrane and AMC, as well as their ages and lithologies, are similar to those of the Tamba–Mino–Ashio terrane and Ryoke Metamorphic Complex in southwest Japan. In both areas the lower (younger) part of the Jurassic accretionary complexes were intruded and metamorphosed by Late Cretaceous granitic magma. Crustal development of the Pacific‐type orogen has been achieved by the cycle of: (i) accretion of oceanic materials and turbidites derived from the continent; and (ii) granitic intrusion by the next subduction and accretion events, accompanied by formation of high T/P metamorphic complexes.     

Earthquakes produce carbon dioxide in crustal faults

Fourier transform infrared (FTIR) microanalysis of pseudotachylytes (i.e. friction-induced melts produced by seismic slip) from the Nojima fault (Japan) reveals that earthquakes almost instantaneously expel 99 wt.% of the wall rock CO₂ content. Carbon is exsolved because it is supersaturated in the friction melts. By extrapolation to a crustal-scale fault rupture, large events such as the M7.2 Kobe earthquake (1995) may yield a total production of 1.8 to 3.4 × 10³ tons CO₂ within a few seconds. This extraordinary release of CO₂ can cause a flash fluid pressure increase in the fault plane, and therefore enhance earthquake slip or trigger aftershocks; it may also explain the anomalous discharge of carbon monitored in nearby fault springs after large earthquakes. Because carbon saturation in silicate melts is pressure-dependent, FTIR can be used as a new tool to constrain the maximum depth of pseudotachylyte formation in exhumed faults.     

Color-change processes of a plinian pumice and experimental constraints of color-change kinetics in air of an obsidian

Colors of plinian pumices were measured by spectrocolorimetry, and their quantitative color parameters in the L*a*b* color space were determined. A series of heating experiments of obsidian was conducted to simulate the color-change processes of rhyolitic glasses. In these experiments, following three stages of color-change processes were observed. Stage I showed a rapid b* (yellowishness) increase associated with fast dehydration controlled by water diffusivity (D _water). In stage II, a* (reddishness) increase was accompanied by Fe²⁺ decrease. Both a* increase and Fe²⁺ decrease can be simulated by a diffusion model. Obtained diffusivity D _oxidation were about two orders of magnitude smaller than D _water . The a*-value increase after the oxidation in stage III appeared to be quasi-linear with time, indicating the zeroth order reaction corresponding to the formation of hematite-like structures in rhyolitic glasses. The diffusion-limited a* increase model in stage II was applied to a natural plinian pumice fall unit to evaluate time periods of color-change processes through oxidation by air of fragmented rhyolitic materials.     

Formative conditions and sedimentary structures of sandy 3D antidunes: an application of the gravel step‐pool model to fine‐grained sand in an experimental flume

Antidunes and their sedimentary structures can be useful in reconstructing paleo‐hydraulic conditions, especially for large discharge events. However, three‐dimensional (3D) antidunes in sand‐sized sediments have not yet been studied extensively, as compared to either two‐dimensional (2D) antidunes or antidunes in gravel‐sized sediments. In this study, we estimated formative conditions of gravel step‐pool morphologies and applied them to the formation of 3D antidunes over a sand bed. Formative conditions are expressed in terms of a relationship between the water discharge per unit width and the bed slope. Flume experiments demonstrated that 3D mound‐like antidune configurations and their associated internal sedimentary structures could be preserved. Internal sedimentary structures were characterized by shallow lens‐like structures whose bases were erosional. Although gently‐dipping concave‐upward lamination was dominant, convex‐upward lamination was occasionally observed. The dimensions of lenticular lamina‐sets can be used to estimate antidune geometry. Thus if 3D antidunes can be interpreted in the stratigraphic record, it is possible to estimate the paleo‐hydraulic parameters such as water discharge and bed slope more precisely than previously. Copyright © 2010 John Wiley & Sons, Ltd.     

Velocity distributions of high-velocity ejecta from regolith targets

We measured the velocity distributions of impact ejecta with velocities higher than ∼100 m s⁻¹ (high-velocity ejecta) for impacts at variable impact angle α into unconsolidated targets of small soda-lime glass spheres. Polycarbonate projectiles with mass of 0.49 g were accelerated to ∼250 m s⁻¹ by a single-stage light-gas gun. The impact ejecta are detected by thin aluminum foils placed around the targets. We analyzed the holes on the aluminum foils to derive the total number and volume of ejecta that penetrated the aluminum foils. Using the minimum velocity of the ejecta for penetration, determined experimentally, the velocity distributions of the high-velocity ejecta were obtained at α=15°, 30°, 45°, 60°, and 90°. The velocity distribution of the high-velocity ejecta is shown to depend on impact angle. The quantity of the high-velocity ejecta for vertical impact (α=90°) is considerably lower than derived from a power-law relation for the velocity distribution on the low-velocity ejecta (less than 10 m s⁻¹). On the other hand, in oblique impacts, the quantity of the high-velocity ejecta increases with decreasing impact angle, and becomes comparable to those derived from the power-law relation. We attempt to scale the high-velocity ejecta for oblique impacts to a new scaling law, in which the velocity distribution is scaled by the cube of projectile radius (scaled volume) and a horizontal component of impactor velocity (scaled ejection velocity), respectively. The high-velocity ejecta data shows a good correlation between the scaled volume and the scaled ejection velocity.     

Spatial and temporal evolution of arc volcanism in the northeast Honshu and Izu‐Bonin Arcs: Evidence of small‐scale convection under the island arc?

Abstract   Arc volcanism of the past 10 my in the northeast Honshu and Izu-Bonin Arcs shows several notable features. In the northeast Honshu Arc, the spatial distribution of volcanism exhibits several clusters elongated nearly perpendicular to the arc and the possible migration of volcanism from the back-arc side to the volcanic front side, at least, during the past 5 my. The pattern of clusters seems to have flip-flopped around 5 Ma. In the Izu-Bonin Arc, there are a series of across-arc seamount chains, in which volcanic activity occurred from ca 17 Ma to ca 3 Ma, similar to the clusters of the northeast Honshu Arc, although the recent active rifting occurs almost parallel to the arc. On the basis of studies of numerical modeling, these features might be explained, at least qualitatively, by the small-scale convection under the island arc. Several inferences can be made from our modeling results for the tectonics of the Izu-Bonin Arc. The angle of dip of subducting plate in the Izu-Bonin Arc might have increased. This can explain the disappearance of volcanism along the seamount chains and the recent along-arc volcanism with narrow rifting. The trend of seamount chains, which is oblique to the arc, might not be their intrinsic feature but rather a result of the lateral movement of the back-arc region after their formation. These inferences can be tested by the future detailed morphological and chronological studies of the Izu-Bonin Arc.