Optimum design and earthquake-response constrained design of elastic shear buildings

The necessary and sufficient conditions for global optimality and the closed form solution are derived for a problem of minimizing a weighted sum of storey stiffnesses of a shear building subject to a fundamental frequency constraint. A set of higher frequency formulae is derived for optimally designed shear buildings with equal masses. It is shown that, if the weight coefficients are regarded as parameters for adjustment, the distribution of the SRSS estimates of the maximum interstorey drifts can be adjusted so as to coincide with a specified one. The base shear–fundamental period formulae and the base shear–design drift formulae are derived semi-analytically for shear buildings with equal masses on the basis of the optimum design formulae and the SRSS estimates of the maximum interstorey drifts. Some numerical examples are presented in order to illustrate the validity of the proposed method of earthquake-response constrained design.     

Structure of air flow separation over wind wave crests

Air flow over wind waves generated in a wind-wave tunnel was visualized by numerous tiny suspended particles (zinc stearate), and instantaneous air flow fields over about one wavelength of wind waves were obtained. Air flow separation was detected over the wave crest in about a half of the samples. In such cases, the separation started near the crest about half of the time, with a vortex trapped over the convergence point of the surface flow which appeared at the leeward face of the crest. This structure was much different from a previously imagined picture in which the separation started at the convergence point. The high frequency of its occurrence suggested the stability of this structure. However, even when this structure was clearly seen, the structure behind the vortex to the next wave crest had various patterns. This variety seems to be related to an instability of the high-shear layer accompanied by separation. Other varieties were also seen, such as the occurrence of separation without the above mentioned structure, as well as the existence of non-separated air flow structures. These varieties seem to be related to the variability of individual wind wave crests. An analysis of correlation between the wave form and the air flow structure over it shows that there is a critical value of local gradient of wave form, above which the air flow always separates. This fact suggests a strong coupling between the air and the water, i.e., the local stress exerted on the water surface changes the nature of a wave crest, especially its form, and as a result, the air flow structure over it changes drastically.Decreased 21 November, 1981. Final draft of the paper prepared by Professor Yoshiaki Toba, Geophysical Institute, Tohoku University.     

Visualization of airflow separation over wind-wave crests under moderate wind

An intermittently-smoking smoke-wire was devised to visualize the airflow structure over individual crests of actual wind waves. The device was used under a moderate wind 6 m s^-1 (maximum speed in the vertical cross-section) at a fetch 3.8 m in a wind-wave tunnel. Airflow patterns with separation were clearly visualized over wind-wave crests which were not accompanied by wave breaking characterized by air entrainment. A classification of 41 samples of airflow structures showed that two distinct patterns (with and without separation) exist, with significant frequency of occurrence for each.     

Indicators of critical point behavior prior to rock failure inferred from pre-failure damage

To investigate possible indicators of critical point behavior prior to rock failure, the statistical properties of pre-failure damage were analyzed based on acoustic emission events (AE) recorded during the catastrophic fracture of typical rock samples under differential compression. AEs were monitored using a high-speed 32-channel waveform recording system. Time-dependent statistics, including the energy release rate, b-value of the magnitude–frequency distribution, fractal dimension and spatial correlation length (SCL) of the AE hypocenters were calculated for each data set. Each parameter is a function of the time-to-failure and thus can be treated as an indicator of the critical point. It is clear that the pre-failure damage evolution prior to catastrophic failures in several common rock-types is generally characterized by: 1) accelerated energy release, 2) a decrease in fractal dimension and SCL with a subsequent precursory increase, and 3) a decrease in b-value from  1.5 to  0.5 for hard rocks, and from  1.1 to  0.8 for soft rocks such S–C cataclasite. However, each parameter also reveals more complicated temporal evolution due to either the heterogeneity of the rock mass or the micro-mechanics of shear fracturing. This confirms the potential importance of integrated analysis of two or more parameters for successfully predicting the critical point. The decreasing b-value and increasing energy release may prove meaningful for intermediate-term prediction, while the precursory increase in fractal dimension and SCL may facilitate short-term prediction.     

Vertical heat transfer based on direct microstructure measurements in the ice-free Pacific-side Arctic Ocean: the role and impact of the Pacific water intrusion

This study quantifies diapycnal mixing and vertical heat transfer in the Pacific side of the Arctic Ocean, where sea-ice cover has disappeared between July and September in the last few decades. We conducted microstructure measurements in the open water region around the Canada Basin from late summer to fall in 2009 and 2010 using R/V Mirai. In the study domain, the dissipation rate of turbulent kinetic energy, ε, is typically as low level as O(10^?10) W kg^?1, resulting in vertical heat diffusivity of O(10^?7) m² s^?1, which is close to the molecular diffusivity of heat, suggesting comparatively little predominance of mechanical turbulent mixing. An exception is the case at the Barrow Canyon, where the strong baroclinic throughflow generates substantial vertical mixing, producing ε > O(10^?7) W kg^?1, because of the shear flow instability. Meanwhile, in the confluence region, where the warm/salty Pacific water and the cold/fresh Arctic basin water encounter, the micro-temperature profiles revealed a localized enhancement in vertical diffusivity of heat, reaching O(10^?5) m² s^?1 or greater. In this region, an intrusion of warm Pacific water creates a horizontally interleaved structure, where the double-diffusive mixing facilitates vertical heat transfer between the intruding Pacific water and the surrounding basin waters.     

Petrology and shock history of the first depleted-like poikilitic shergottite Asuka 12325

Asuka (A) 12325 is the first poikilitic shergottite having a depleted pattern in light rare earth elements (REE). Compared with known poikilitic shergottites, A 12325 has smaller but more abundant pyroxene oikocrysts with remarkable Fe-rich pigeonite rims, indicating that A 12325 cooled relatively faster at a shallower part of the crust. The redox condition (logfO₂ = IW + 0.6-IW + 1.7) and Fe-rich chemical compositions of each mineral in A 12325 are close to enriched shergottites. The intermediate shergottites could not form by a simple mixing between parent magmas of A 12325 and enriched shergottites. Although A 12325 contains various high-pressure minerals such as majorite and ringwoodite, plagioclase is only partly maskelynitized. Therefore, the maximum shock pressure may be within 17–22 GPa. Thermal conduction and ringwoodite growth calculation around a shock vein revealed that the shock dwell time of A 12325 is at least 40 ms. The weaker shock pressure and longer shock dwell time in A 12325 may be attained by an impact event similar to those of nakhlites and Northwest Africa (NWA) 8159. Such a weak shock ejection event may be as common on Mars as a severe shock event recorded in shergottites. Alteration of sulfide observed in A 12325 may imply the presence of magmatic fluid in its reservoir on Mars. A 12325 expands a chemical variety of Martian rocks and has a unique shock history among poikilitic shergottites while A 12325 also implies that poikilitic shergottites are common rocks on Mars regardless of their sources.     

Spinel inclusions in olivine and plagioclase crystals in a layered gabbro: a marker and a tracer for primary phenocrysts in a differentiating magma reservoir

The problem of whether cumulate rocks were formed by crystal settling or by in situ crystallization after magma emplacement is an important issue concerning the mechanisms of magmatic differentiation. However, it is hard to distinguish these two processes for plutonic rocks because the primary texture and chemical composition have generally been modified by postcumulus processes. To contribute this problem, we studied the distribution and compositions of Cr-spinel inclusions hosted in olivine and plagioclase in the Murotomisaki Gabbroic Intrusion (MGI), SW Japan. It is shown that the olivine-hosted inclusions are restricted to specific horizons where accumulation of olivine phenocrysts is thought to have occurred and that the compositional variations of the Cr-spinel are explained by a secondary compositional modification that probably took place after the magma emplacement. It is also shown that the Cr-spinel inclusions in a chilled margin have suffered the least compositional modification and nearly retains the primary composition. Those in the interior of the intrusion, on the contrary, have been significantly modified by re-equilibration with residual melt driven by cation diffusions through the host phases. Those in plagioclase have been less modified. It is shown that all the spinel inclusions had primarily the same and common composition at the time of magma emplacement. This implies that all the inclusion-bearing crystals, olivine and plagioclase, represent primary phenocrysts that had already existed in the emplaced magma. In this way, spinel inclusion in the MGI may be regarded to be a useful petrographic “marker” for identifying intratelluric phenocrysts and also as a “tracer” to trace the motion of the primary phenocrysts after the magma emplacement.