We have developed an imaging algorithm for subsurface faults on the basis of the steepest descent method in our previous paper. In this paper we introduce two new techniques, the multi-event stacking and multi-master stacking, to improve detectability for subsurface faults. The multi-event stacking is a technique using data from plural earthquakes with different incident angles. The multi-master stacking is a technique using data of a single earthquake, in which the images obtained by employing different master receiver are stacked. We examined the feasibility of our approach using several trial models and synthetic data with different signal to noise ratios, different pulse durations and different incident angles in the two dimensional SH cases. In the numerical experiments, we could recover the image of the subsurface fault for all the trial models used, which indicates our method has a potential for sensing real subsurface faults.

View full abstract

The natural sedimental deposits in basins show strong vertical heterogeneity in their material parameters. The aim of this paper is to investigate the effects of such vertical heterogeneity, especially vertical velocity gradient, inside basin on the seismic ground motion through the parametric study on the response of a two-dimensional semi-cylindrical sediment-filled basin to a vertical incidence of plane SV wave using the pseudospectral method. This numerical study has tried to find the effects caused by vertical velocity gradient through the use of synthetic seismograms, wavefield snapshots and surface amplitude distribution. Simulation results clearly demonstrate the detailed character of wave propagation phenomena in basins with vertical velocity gradient, which produces characteristic amplification pattern of the surface motion caused mainly by the generation of strong Rayleigh wave induced at the basin edge associated with large lateral velocity change across the basin edge. Amplification pattern at the surface strongly depends on both the vertical velocity gradient in the basin and the predominant frequency of the incident wave. Although similar phenomena on wave propagation and surface motion found in previous studies for homogeneous basin models have also been observed in this study, it has been found that the vertical velocity gradient enhances such phenomena. The results suggest that it is important to represent the vertical velocity profiles accurately when we construct a structural model for realistic modelling of ground motion.

View full abstract

As a part of the earthquake prediction project in the Tokai district, levelling observations are being carried out four times a year in the Omaezaki peninsula. To analyze this time dependent data, an adjustment method in which the velocity of vertical motions, the amplitude and phase of annual variation of each benchmark are assumed to be unknown is applied. The main results of the analysis are summarized as follows: (1) at reference time 1981.0, the average phase of benchmarks along the levelling route between Kakegawa city and Omaezaki is about 21 degree, (2) at BM140-1 (Kakegawa), relative to BM92305 (Omaezaki) the maximum amplitude of one-year period variation is about 7.55 mm, i.e., the difference between the top and bottom of wave movement may reach 15.1 mm, and (3) the subsidence rate on the levelling route from BM140-1 to BM92305 decreases gradually as the benchmark is farther from the fixed point BM92305 with the maximum subsiding rate of 7.7 mm/yr.
The estimated rates of vertical deformation (i.e., after removing the seasonal variation) are inverted to estimate interplate coupling between the subducting Philippine Sea plate and the overriding continental plate. This facilitates understanding of the strain accumulation process in this complicated region. The results indicate that there is a high coupled region extending from 7 to 27 km deep, the maximum back-slip rate in this region reaches 27 mm/yr, and the direction of the plate convergence at the Suruga Trough is N 54°W ± 5°.

View full abstract

Hiss-triggered chorus emissions recorded at low-latitude ground station Gulmarg (geomag. lat., 24°26N; geomag. long., 147°09E, L = 1.28) on March 8, 1986 are reported. From the detailed spectral analysis of these emissions, it is found that the chorus is hiss-triggered and each chorus element has tendency to originate from the hiss band. Based on the theory of Helliwell, and previous satellite VLF data, a possible generation mechanism of low-latitude hiss-triggered chorus emissions is proposed. An attempt has been made to find out the chorus source location and the length of resonance region from which the various parameters associated with generation mechanism are determined.

View full abstract

During a geomagnetic storm in early November 1993, NOAA satellite observations revealed that a population of energetic electrons appeared in the center of the outer radiation belt during the main phase of the storm. At the beginning of the main phase of the magnetic storm, the number of electrons with energies from 30 keV to 100 keV increased rapidly and contributed to build up of the ring current. At the end of the main phase the flux of electrons with energies greater than 300 keV increased significantly. Akebono satellite observations showed that the flux of electrons with energies ranging from 300 keV to 950 keV increased late of the storm main phase and that the flux of electrons with energies from 950 keV to 2.5 MeV increased during the storm recovery phase. The electron flux increase observed by both NOAA and Akebono took place first in the central part of the outer radiation belt (L-4) and propagated to higher L shells with a significant time delay. We think that the ring current electrons that appeared first and near L-4 during the main phase seeded the subsequent increase in the flux of MeV electrons in the entire outer radiation belt.

View full abstract

An ultraviolet imaging spectrometer (UVS) on board the PLANET-B (NOZOMI) spacecraft has been developed. The UVS instrument consists of a grating spectrometer (UVS-G), an absorption cell photometer (UVS-P) and an electronics unit (UVS-E). The UVS-G features a flat-field type spectrometer measuring emissions in the FUV and MUV range between 110 nm and 310 nm with a spectral resolution of 2-3 nm. The UVS-P is a photometer separately detecting hydrogen (H) and deuterium (D) Lyman α emissions by the absorption cell technique. They take images using the spin and orbital motion of the spacecraft. The major scientific objectives of the UVS experiment at Mars and the characteristics of the UVS are described. The MUV spectra of geocoronal and interplanetary Lyman α emissions and lunar images taken at wavelength of hydrogen Lyman α and the background at 170 nm are presented as representative examples of the UVS observations during the Earth orbiting phase and the Mars transfer phase.

View full abstract

Sr and Nd isotopes together with trace elements for ocean island basalts in the Polynesian region have been analyzed in order to investigate the origin of the HIMU and EM sources. Both whole rocks and cpx phenocrysts were analyzed for isotopic composition. Cpx samples from HIMU islands show quite uniform ⁸⁷Sr/⁸⁶Sr ratios (-0.70274), while leached and unleached whole rock samples show variable and higher ⁸⁷Sr/⁸⁶Sr than those of cpx samples. These results suggest that even leached whole rock samples have been affected by secondary contaminations of sea water. On the other hand, cpx preserves a pristine isotopic signature with minimal secondary effects.
Using only the cpx analyses, HIMU form a vertical linear trend in the Sr-Nd isotope diagram with small variation in ε_Nd (+3.3-+5.5) and constant ⁸⁷Sr/⁸⁶Sr. This trend is explained by a mixing of the HIMU endmember and the MORB source. Since ε_Nd of the HIMU end-member is constrained to be less than +3.3, the HIMU source should include former sediment added to oceanic crust. To explain the vertical nature of the mixing trend, the HIMU end-member should have similar Rb/Sr to the MORB source, or much lower Sr/Nd ratio than the MORB source, which favors a mixing model between extensively dehydrated oceanic crust and sediment as the HIMU source. The correlation between ε_Nd and trace element ratios such as Pb/Ta also supports the model.

View full abstract