Anomalous deformation of the Earth's bow shock in the lunar wake: Joint measurement by Chang’E-1 and SELENE

Because the solar wind (SW) flow is usually super-sonic, a fast-mode bow shock (BS) is formed in front of the Earth's magnetosphere, and the Moon crosses the BS at both dusk and dawn flanks. On the other hand, behind of the Moon along the SW flow forms a tenuous region called lunar wake, where the flow can be sub-Alfvénic (and thus sub-sonic) because of its low-density status. Here we report, with joint measurement by Chang’E-1 and SELENE, that the Earth's BS surface is drastically deformed in the lunar wake. Despite the quasi-perpendicular shock configuration encountered at dusk flank under the Parker-spiral magnetic field, no clear shock surface can be found in the lunar wake, while instead gradual transition of the magnetic field from the upstream to downstream value was observed for a several-minute interval. This finding suggests that the ‘magnetic ramp’ is highly broadened in the wake where a fast-mode shock is no longer maintained due to the highly reduced density. On the other hand, observations at the 100 km altitude on the dayside show that the fast-mode shock is maintained even when the width of the downstream region is smaller than a typical scale length of a perpendicular shock. Our results suggest that the Moon is not so large to eliminate the BS at 100 km altitude on the dayside, while the magnetic field associated with the shock structure is drastically affected in the lunar wake.     

Evaluation of slope stability by finite element method using observed displacement of landslide

To monitor land deformation in detail, we ran a large-scale field test in which an artificial landslide was induced by the application of a load to a natural slope. The measured landslide displacement was reproduced numerically through the use of finite element model analysis with a two-dimensional elasto-viscoplastic model. The analysis suggested that the strength of the sliding surface decreased as the landslide mass moved. We propose a simple method for estimating safety factors. The method involves back-calculation of shear strength parameters through reproduction of observed landslide displacements and calculating the ratio of driving force to resisting force acting on the sliding surface as modeled by joint elements. This ratio, the “stability index”, shows the same trend as safety factors calculated by a two-dimensional limit equilibrium method and a shear strength reduction method that use back-calculated shear strength parameters estimated from the limit equilibrium state. The results indicate that the stability index may be applicable to the assessment of slope stability.     

Effects of ocean particles on the upwelling radiance and polarized radiance in the atmosphere-ocean system

Based on a vector radiative transfer model of the atmosphere-ocean system, the influence of oceanic components on radiation processes, including polarization effects, was investigated in the wavelength region ranging from 0.380 to 0.865 μm. The components considered were phytoplankton, inorganic suspended material (sediment), and colored, dissolved organic matter. Due to their important roles in oceanic radiation processes, the sensitivity of the bidirectional reflectance to the rough ocean surface, represented by the wind velocity 10 m above the ocean surface, and aerosol, were taken into account. The results demonstrated that both radiance and polarized radiance just below the ocean surface were sensitive to the change of the concentrations of the considered components, while the dependence of polarized radiance on the observation geometry was more sensitive than radiance. Significant differences in the specular plane existed between the impacts of the phytoplankton and sediment on the degree of polarization just above the ocean surface at 670 nm. At the top of the atmosphere (TOA), polarization was relatively insensitive to changing concentrations of ocean particles at longer wavelengths. Furthermore, the radiance at the TOA in the solar plane was more sensitive to the aerosol optical thickness than wind velocity. In contrast, wind velocity strongly influenced the radiance at the TOA in the sun glint region, while the polarization degree showed less dependence in that region. Finally, a nonlinear optimal inversion method was proposed to simultaneously retrieve the aerosol and wind velocity using radiance measurement.     

Book review

Book Reviewed in this Article:
Observatory seismology Ed. Litehiser, J. J., University of California Press, 1990, x + 379 pp, ISBN 0520 065824, US$ 48.     

Degradation of common pharmaceuticals and personal care products in mixed solutions by advanced oxidation techniques

Widespread detection of pharmaceutical compounds in water environment has been a serious concern recently, while conventional sewage treatments are ineffective for their elimination. But, advanced oxidation techniques are very promising to remove varieties of organic contaminants in water. This research aims to elucidate oxidation potentials of sixteen commonly used pharmaceutical compounds in mixed solutions by seven advanced oxidation techniques in laboratory batch experiments. The removal profiles exhibited four distinct patterns: a) easily degradable by all seven techniques, b) not easily degradable by all seven techniques, c) easily degradable by ozone-based techniques, but not by ultraviolet radiation-based techniques and d) easily degradable by ultraviolet radiation-based techniques, but not by ozone-based techniques. Ozone-based techniques rather than ultraviolet radiation-based techniques were very powerful for simultaneous removal of the compounds efficiently. Moreover, ozonation combined with ultraviolet radiation was the most appropriate technique for simultaneous removal of the tested compounds efficiently. Increased ozone dissolution and decomposition with ozone-based techniques did not always enhance the compounds’ removal. Physicochemical properties of the compounds and solution pH also presumably played an important role on the removal which merits further attention.     

Source parameters of selected earthquakes on the central and western margin of Afar

Spectral analysis of 196 short-period and long-period vertical- and horizontal-component seismograms from ten earthquakes on the central and western margin of Afar is performed to determine source parameters and discuss their tectonic implications. For the earthquakes in the regions under study, the stress drop varies from 2 to 31 bar while the seismic moment varies from 2 × 10²⁴ to 154 × 10²⁴ dyn cm. In general, low stress-drop values are obtained indicating the presence of softer material (especially for central Afar) at a shallower depth. It is observed that there is an increase in stress drop with the increase in moment-magnitude which in turn is obtained from the calculated average seismic moment. Energy estimates show that the mode of energy release is different in the two regions indicating that different tectonic processes are involved in the two regions. The slip rate obtained for the Serdo area is of the order of 1.6 cm/yr and is in close agreement with the spreading rate already obtained for central Afar. Spreading rates obtained earlier and that of the present study show a low spreading rate for Afar and neighbouring regions as compared to those of the other regions of the world.     

Monitoring degradation of the 1995 Nojima earthquake fault scarps at Awaji Island, southwestern Japan

The Nojima fault on the northwestern coast of Awaji Island, south of Kobe, was reactivated during the January 17, 1995 Hyogoken-nanbu earthquake. This fault rupture was dominated by right-lateral offset (max. 1.7 m) along a high-angle reverse fault which has a maximum vertical displacement of 1.3 m on the southeastern side. We repeatedly measured seven profiles across the fault scarp in two areas (Hirabayashi to the northeast and Ogura to the southwest) for approximately 1 year following the earthquake. The original profile of the fault scarp was an overhanging scarp at Hirabayashi and Ogura, corresponding to the 70–80 ° dip of the fault plane. The fault scarp at Hirabayashi displaces Plio-Pleistocene siltstones of the Osaka Group and is overlain by a thin bed of unconsolidated gravel. The Ogura area is entirely underlain by the Osaka Group. Scarp degradation at Hirabayashi occurred by collapse of the gravel bed and proceeded more quickly than at Ogura, where fault scarp degradation proceeded mainly by exfoliation of the Osaka Group siltstones. The degradation occurred at a very fast rate until March at Hirabayashi, and until June or July at Ogura. Since then, the degradation has been very slow. Our data strongly indicate that the scarp profile was initially controlled mainly by the dip of the fault plane, and scarp degradation has been primarily controlled by lithological factors. The degradation of the Nojima earthquake fault scarp proceeded much more quickly than that of normal fault scarps in the western U.S.A., where many observations of the initial stages of scarp degradation have been carried out. The extremely rapid degradation of the Nojima fault scarp in weak late Neogene siltstones might, in combination with rapid cultural modification of the landscape, explain the paucity of geomorphic scarps along the numerous active faults in Japan. This observation may also have implications for tectonic geomorphology and paleoseismicity studies in other countries characterised by weak bedrock and moderate to high rainfall regimes.     

Spatial and temporal variations of seismicity in the Horn of Africa from 1960 to 1993

Spatial and temporal variations of seismic energy release and b -value are investigated in the Horn of Africa. The results indicate that the area around the Afar Depression and southern Sudan is at a higher stress level than the southern Red Sea and Gulf of Aden. The distribution of earthquakes in the vicinity of Afar shows a systematic pattern and suggests the existence of two microplates (blocks) centred about the Danakil and Aisha horsts.     

The 1992 Cairo earthquake: A case study of a small destructive event

On 12 October 1992, an earthquake, magnitude mb = 5.9 and M s = 5.2, hit the City of Cairo, Egypt. It was this century's largest earthquake in northern Egypt with related destruction in the City of Cairo, the Nile Valley and the Nile Delta areas. Our source parameter determinations show that the 1992 earthquake had a normal faulting mechanism, seismic moment M o = 5.2 × 1017 Nm, centroid depth of 23 km and a source time function duration of 3 seconds. The mechanism is compared with those corresponding to two other events that occurred in the northern Red Sea. The similarity between the mechanisms as well as the spatial distribution of the geological faults around Cairo suggest seismic activity along the extension of the stress field of the Red Sea rift system to the area around the City of Cairo. This situation affects the level of seismic hazard in the Cairo area. The 1992 earthquake belongs to an unusual class of relatively small, M w > 6.0, yet destructive earthquakes. The damage caused by these events is usually attributed to their shallow focal depth, 5 km, and to amplification of seismic waves in the local soil beneath the damaged structures. However, the Cairo earthquake deviates from other events of this class since the focal depth was determined to be 23 km. We calculated synthetic accelerograms for the 1992 earthquake with the loose sediments observed in the Nile Valley, and show that this enhanced the amplitude of the acceleration by a factor of two. However, the determined accelerations, about 0.5 m s-2, cannot alone explain the relatively large amount of damaged structures. Hence, a major cause to the destruction is likely the poor state of construction of the Cairo buildings.     

Hydrographic profiling across the East China sea shelf edge by an underwater sliding vehicle with CTD sensors

Temperature, salinity and density structures were observed on Sept. 23 and 24, 1986 at one vertical section across the East China Sea shelf edge by an advanced type of towed vehicle with CTD sensors which was developed by the Japan Marine Science and Technology Center. The vehicle was towed at a speed of 2.5 m s⁻¹ down to 150 m depth and at intervals of 170–500 m width. The observed profile was 50 km long on Sept. 23 and 70 km long on Sept. 24 along the cross-shelf section. An on-ship acoustic Doppler current profiler was simultaneously used to measure current velocities at depths of 20, 50 and 100 m.Interesting features were noticed. Firstly, there was a vertical displacement of pycnoclines at the lower edge of the surface mixed layer accompanied by vertical inversion of the salinity and temperature in the vicinity of the shelf edge. Pycnoclines were displaced upward by 12 m toward the outer edge on Sept. 23 and by 20 m on Sept. 24. On Sept. 23, the salinity inversion took place in a layer 20 m thick and 8 km wide, whereas the temperature inversion took place in a layer 8 m thick and 1.5 km wide. These vertical inversions were probably generated by vertical shear of tidal currents which was observed by the Doppler current profiler. These results throw light on understanding the vertical mixing process of stratified water on the continental shelf edge. Secondly, an intrusion of the shelf water into the Kuroshio water was observed along pycnoclines below the surface mixed layer 60 to 70 m deep in the Kuroshio region outer break. The measurement was successful in showing a horizontal mixing process of the shelf water and the Kuroshio water which could not be found out by standard CTD observations.