A series of large earthquakes occurred off east coast of Honshu, Japan, during the period of October to November 1989. Mechanisms were studied for the foreshock (M=6.5), the mainshock (M=7.1) and the two aftershocks (M=5.2 and 5.3) using the STS‐I broadband records at station INU about 700km away from the epicenters. The lowpass filtered seismograms of these earthquakes show mutually very similar waveforms, in good agreement with those calculated for a known mechanism by an extended reflectivity method. The mainshock is 10 times as large as the foreshock and 2000 times as large as the aftershocks in terms of seismic moment. Assuming that the mainshock (or foreshock) is a collection of some elementary sources at a focal point and that aftershock seismograms represent the responses to such elementary sources, the broadband seismogram of the mainshock (or foreshock) for the first 60s was deconvolved with that of an aftershock to obtain the moment rate function. The moment rate functions of the mainshock and foreshock are both triangle‐like with durations of 14 and 9s respectively. Their rising slopes are mutually very different, a greater slope is associated with the greater shock. This and the similar results for other pairs of earthquakes suggest that the rising slope of moment rate function is causaly related to the eventual size of seismic rupture.
This paper uses data from the Capital Area Seismograph Network in North China (NC), currently the largest regional seismograph network in China to investigate the compressional stress orientations from the polarizations of faster shear waves (PFS), a parameter of shear wave velocity anisotropy. Data are restricted to earthquakes deeper than 5 km to reduce the influence of the heterogeneous uppermost few kilometres of the crust. The results show that the regional compressional stress direction is ENE-WSW to nearly EW in NC, which agrees with seismic mechanisms and drilling and GPS data. Predominantly, PFS are also nearly EW in NC and are influenced by faults, geology and tectonics. PFS orientations at stations on seismically active faults are consistent with the strike of strike-slip faults and indicate large seismically active faults breaking, or nearly breaking, the free surface. PFS orientations are influenced by both regional compressional stress and by nearby faults. It is an effective way to study detailed spatial distribution of crustal compressional stress by shear wave anisotropy with dense seismograph network. Shear wave splitting may also be used to indicate hitherto unknown faults.
We obtained phase velocities of fundamental Rayleigh waves in the northwestern Pacific ocean and the northern Philippine sea from seismograms retrieved from a long‐term broad‐band ocean bottom seismometer (LT‐OBS) array deployed in 1999–2000 and from those recorded in Japan, Guam and Ponape islands. This array observation was one of the first attempts to record continuous seismograms on the seafloor using broad‐band seismographs on a long‐term basis. The geographical distribution of the LT‐OBS stations and the surrounding land stations enabled us to determine pure‐path phase velocities in a frequency range from 0.01 to 0.05 Hz across the Mariana trough, the Parece Vela basin, the Shikoku basin and the Minami Daito basin, which were found to be systematically lower than those of the Pacific seafloor at comparable ages. This result, along with other geophysical and geological observations in the Philippine sea, may be explained by an uppermost mantle that is relatively Fe rich.
The dynamics and evolution of the Earth's mantle are considered to be influenced by several complexities of the physical processes. In order to understand the mantle convection in the Earth, a numerical simulation is a very effective tool. Therefore, we have been improving the models which take into account of such difficult aspects as the large variation of viscosity and the existence of phase transitions. On the other hand, models of the internal structure and evolution of the mantle have been proposed based on the seismological and geological observations. The seismic tomography reveals the large scale flow of mantle convection, while the geologic data suggest that the mode and activity of the mantle convection vary episodically. Our goal is to construct the models of the mantle convection which are consistent with these observational results. In this fiscal year, our effort was focused on the improvement of the model involving the phase transition around the 660 km depth. We discuss the implication for the dynamics and evolution of the Earth's mantle.
'%3e%3ccircle r='20' fill='%23008'/%3e%3ccircle r='17.5' fill='%23fff'/%3e%3ccircle r='3.5' fill='%23008'/%3e%3cg id='d'%3e%3cg id='c'%3e%3cg id='b'%3e%3cg id='a' fill='%23008'%3e%3ccircle r='.875' transform='rotate(7.5 -8.75 133.5)'/%3e%3cpath d='M0 17.5.6 7 0 2l-.6 5L0 17.5z'/%3e%3c/g%3e%3cuse xlink:href='%23a' transform='rotate(15)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(30)'/%3e%3c/g%3e%3cuse xlink:href='%23c' transform='rotate(60)'/%3e%3c/g%3e%3cuse xlink:href='%23d' transform='rotate(120)'/%3e%3cuse xlink:href='%23d' transform='rotate(-120)'/%3e%3c/g%3e%3c/svg%3e)
