Seismic-response-controlled structure with active mass driver system. Part 2: Verification

An Active Mass Driver (AMD) system is proposed to suppress actively the response of a building to irregular external excitations such as earthquakes and typhoons.¹ This system has been introduced to an actual ten-storey office building constructed in Tokyo in August, 1989. The proposed analytical methods utilize circuits of the system and mechanical characteristics to understand the real control effect of the system. Simulation analyses are also performed to verify the analytical model and the control effect during observed earthquakes.     

Numerical and experimental models on the formation mechanism of collapse basins during the Green Tuff orogenesis of Japan

The geological model about volcanism of the Green Tuff geosyncline deduced from the field observations consists of the following processes:

1. Dome-shaped uplift with a mean diameter of 30 km.
2. Collapse of the central part of the domes forming basins with a mean diameter of 10 km.
3. Volcanic activity inside the collapse basins. It is considered that these consecutive processes resulted from the magmatic uplift from a deep part of the crust.

In finite element analyses performed as plane strain problems, earth's crust is assumed to be an elasto-plastic homogeneous layer and to undergo sinusoidal vertical displacement at the base of the layer due to an ascending magma reservoir. These analyses reveal that the diameter of the dome is proportional to the depth of the magma reservoir rather than to its size. The magma reservoir is estimated at 12 ～ 24 km in depth. Scale model experiments using powdered material were performed in order to reproduce a collapse basin. These three-dimensional models are reduced to a scale of 1:200,000 th of the natural size. The results of experiments show that radial and concentric cracks are produced on top of the dome and a central part encircled by concentric cracks collapses to form a basin. The boundary of the collapsed portion forms a steep cliff with a height of about 2 mm. This is equivalent to 400 m in natural size and is nearly similar to field observations.     

Eigen oscillation of a fluid sphere and source mechanism of harmonic volcanic tremor

The eigen oscillation of a fluid sphere embedded in an infinite elastic medium is analyzed to understand the source mechanism of volcanic tremor that vibrates nearly monotonically and attenuates slowly. The dimensionless eigen frequencies of the sinusoidal oscillation are calculated in a complex form with the attenuation factor in its imaginary part for various combinations of the three parameters: the contrasts of P-wave velocity, density and rigidity between the fluid and the country rock. Eigen oscillations consist of a high attenuation mode with a rapidly decaying pulsive wave at a low frequency and infinite number of regular modes with slowly decaying vibrations. For regular modes, the frequency of oscillation obtained from the real part of an eigen value is distributed in approximately regular intervals while the attenuation factor from the imaginary part is almost constant independent of the mode. Each eigen frequency of regular and high attenuation modes is degenerated with two independent eigen functions describing different distributions of displacement, velocity and stress. The theory is applied to harmonic volcanic tremor observed at Kusatsu-Shirane Volcano, central Japan. Observed spectral peaks of the tremor are explained by the eigen frequencies and attenuation factors of several lowest regular modes if the spherical fluid oscillator has a radius of dozens of meters and a P-wave velocity of about several hundred meters per second.     

A laboratory experiment on the role of grass for infiltration and runoff processes

Two lysimeters with the same dimensions were provided, and filled with the same loam clay. On the soil surface of one lysimeter, grass was planted to compare the hydrologic response of the grassed lysimeter with that of the other bare soil lysimeter.

About half of the runoff from the bare soil lysimeter occurred as overland flow, the rest being groundwater flow. Overland flow scarcely occurred from the grassed lysimeter. Grass roots that developed deep into the soil layer play an important role in increasing the infiltration rate as well as in drying the soil uniformly throughout the soil layer by evapotranspiration, preparing for high infiltration and large rainwater storage for the subsequent rainfall event. Accordingly, the total loss by evapotranspiration from the grassed soil amounts to almost twice that from the bare soil.

For an evaporation- and evapotranspiration-prohibited experiment, the recession characteristics from a saturation state showed similar features for the bare and grassed soils, indicating the same microstructure of high moisture reservability for both soils.

The well-developed grass root system reformed the soil structure considerably to produce the seemingly contradicting characteristics of high moisture conductivity and high moisture reservability; i.e. a high infiltration rate and prolonged groundwater discharge.

Finally, the importance of the initial soil moisture in the rainfall-runoff process, rainfall loss and runoff ratio is stressed.     

Tectonic evolution of Northeastern Siberia and adjacent regions

Previous models for the tectonic evolution of northeastern Siberia have proposed the existence of a Kolyma plate composed of the Kolyma and Omolon massifs of presumed Precambrian age. Lithologic similarities between the Siberian platform and the Cherskiy Mountains and the presence of oceanic and island arc type deposits in the Kolyma-Indigirka interfluve suggest that no such plate exists. The eastern margin of the Siberian plate is suggested to lie along a line between the Ulakhan Sis Range, the Alazeya uplift and the Arga Tas Range; the Cherskiy Mountains and the Verkhoyansk fold belt are parts of the Siberian plate. The Paleozoic deposits of the Omolon massif are unlike those found in the Cherskiys or Siberia. Paleomagnetic data from the Omolon massif are discordant from data from Siberia. It is suggested that the Omolon massif represents a microplate which accreted onto Siberia in the Jurassic. Ophiolites in central Chukotka are of the same emplacement age as in the western Brooks Range and may have been emplaced at the initiation of the rotation of Arctic Alaska. Geometric and limited stratigraphic data suggest that the East Siberian Sea may be floored by oceanic crust left by an incomplete closure between Arctic Alaska, Siberia and Omolon. The tectonic position of the Prikolymsk massif remains ambiguous.     

Slow subduction of old lithosphere in the lesser antilles

The Lesser Antilles subduction zone is an extreme case of the subduction of old (~ 90 m.y.) lithosphere at a slow (~ 2 cm/y) convergence rate. Focal mechanisms of the largest earthquakes in the area have been obtained using body and surface wave data. During the time period (1950–1978) studied the subduction seismicity appears to represent primarily intraplate rather than interplate deformation. All three large (magnitude seven) earthquakes were from intraplate normal faults; no large thrust faulting earthquakes and few small ones occurred. These observations suggest that the plate boundary is largely decoupled, that subduction is at least partially aseismic, and that the downgoing slab is in a state of extension.     

Variation of precipitation in Langtang Valley,Himalayas

The variation of the δ 18O in precipitation and the relationship with precipitation amountat Kyangjin Base House and Yala Glacier Camp in Langtang Valley, Nepal Himalayas were ana-lyzed. The variations of the δ 18O with precipitation had great scatter, and the correlations betweenthe δ18O and precipitation changed with time on the synoptic scale. On the seasonal scale, therewas marked amount effect at Kyangjin Base House. However, the δ18O-precipitation gradient wassmaller than that on the synoptic scale. Because of the maintenance of the basic equilibrium be-tween stable isotopic compositions in atmospheric vapor and precipitation, the evaporation en-richment was light during the rainy season. Therefore, the variation of stable isotopic compositionsin precipitation was independent on the sampling intervals. Simulations show that the rainfall inLangtang Valley was not the outcome of the initial condensation of ocean vapor that originatedfrom low latitudes. The stable isotopic compositions in precipitation were greatly depleted due tothe strong rainout of the vapor from oceans as the vapor was raised over the Himalayas     

Subaqueous eruption and emplacement of the Shinjima Pumice, Shinjima (Moeshima) Island, Kagoshima Bay, SW Japan

The Shinjima Pumice is a fines-depleted pumice lapilli tuff emplaced several thousands years ago at about 100–140 m below sea level. This 40-m-thick deposit comprises many poorly defined flow units, which are 1–10 m thick, diffusely stratified and showing upward-coarsening of pumice clasts with a sharp to transitional base. Parallel to wavy diffuse stratifications are commonly represented by alignment of pumice clasts, especially in the lower half of the flow units. Pumice clasts of block to coarse-lapilli size commonly have thermal-contraction cracks best developed on the surfaces, demonstrating that they were hot but cooled down to the ambient temperatures prior to their emplacement. These features are suggestive of the direct origin of the Shinjima Pumice from subaqueous eruptions. A theoretical consideration on the behavior of subaqueous eruption plumes and hot and cold pumice clasts suggests that subaqueous eruption plumes commonly collapse by turbulent mixing with the ambient water and are transformed into water-logged mass flows.     

Horizontal distribution of particulate matter and its characterization using phosphorus as an indicator in surface coastal water,Harima-Nada,the Seto Inland Sea,Japan

To study the horizontal distribution and characterization of suspended particulate matter (SPM) in the coastal environment, the distribution and seasonal variation of particulate phosphorus (PP), divided as organic (POP) and inorganic (PIP) fractions with POC, PON, and Chl.a, in 4 seasons throughout Harima-Nada of the Seto Inland Sea, Japan, were investigated. A high concentration of PIP was observed not only in the northern coastal area, which received much freshwater discharge, but also in the channel parts. In winter, when Δσt was low, the difference between surface and bottom σt, resuspension of sinking particles or/and surface sediment which has a high PIP/PP ratio seems to have occurred, and then a high concentration of PIP in surface water was observed. In spring and summer, PIP/PP ratios (0.24) of surface waters were close to those of phytoplankton (0.1–0.2). On the other hand, PIP/PP ratios in autumn and winter, when Δσt was low, were similar to those of surface sediment in Harima-Nada (0.32–0.39). SPM in Harima-Nada showed higher PP contents than other coastal environments, and its mean value of 9.10 mg P/g was close to the phytoplankton P content of 9.75 mg P/g. This suggests that SPM in Harima-Nada was strongly affected by phytoplankton. The C/N ratio in Harima-Nada in spring was high (11), although high Chl.a and low PIP/PP were observed, possibly resulting from preferential decomposition of nitrogen in comparison with carbon. We consider that the PIP/PP ratio is a good indicator to characterize SPM and to predict its origin as well as the C/N.     

Total organic carbon and nitrogen contents in surface sediments of Harima Nada,eastern Seto Inland Sea,Japan: a comparison under two different trophic states

Phytoplankton abundance in the surface waters of Harima Nada, eastern Seto Inland Sea of Japan, decreased after around the late 1970s following a reduction of terrestrial phosphorous inputs, and relatively low levels persisted thereafter. This symptom of de-eutrophication in recent decades might have reduced organic matter storage in surficial sediments. To test this possibility, we examined total organic carbon (TOC) and nitrogen (TN) contents of Harima Nada sediments in 2011, and compared the results to past data from more eutrophic states. In 2011, the TOC and TN varied horizontally within the basin, in relation to physicochemical properties (water and mud content) and phytoplankton remnants (pheo-pigments) of the sediments. These horizontal patterns were fundamentally similar to those of the early 1980s (1982–1983), although discrepancies between the periods were observed in some areas. Differences of mean TOC and TN in the 0–2 cm layer at 63 stations from 1982–1983 to 2011 were consequently <10 %. Hence, the recent de-eutrophication in Harima Nada was associated with little overall change of TOC and TN storage in the sediment. In Harima Nada, changes in water clarity and particle size composition of the sediment seem to have occurred during recent decades. The aforementioned paradoxical phenomenon may therefore be explained by factors that are affected by these environmental changes. Such factors include the ratio of export flux to the seafloor to phytoplankton production, phytoplankton biomass and production in the entire euphotic zone, and accumulation rate of surface sediment.