We investigated the characteristics of the seismicity changes prior to the 2000M=6.8 Nemuro Peninsula earthquake by using the earthquake catalog of Japan Meteorological Agency (JMA). The JMA catalog is complete for all the events with M≥3.6 during January, 1984-March, 2000. The temporal variation of the number of earthquakes in the epicentral zone decreased significantly during 1995-1996. We applied another statistical method, called the RTL algorithm, to the same JMA earthquake catalog and found that a seismic quiescence started in 1995 and lasted till the end of 1996. A clear anomalous quiescence zone appeared around the epicenter in 1996. The maximum dimension of this quiescence zone is over 200km, several times larger than the rupture length of the main shock. These consistent results, which were obtained from different approaches, indicated that the seismic quiescence that occurred 3 years before the main shock reflected the preparation of the earthquake. Therefore, investigation of seismicity changes prior to the Nemuro Peninsula earthquake may provide useful information for understanding the physical nature of earthquake preparation.
This paper reports the activity in the development of the observation system of seafloor crustal deformation for understanding mechanism of interplate earthquakes at subduction plate boundaries around Japan. Currently, the method for the seafloor geodesy is under development in order to overcome several difficulties. The position of the sea-bottom transponders at the vicinity of the Nankai trough and the Japan Trench is observed using a GPS/Acoustic measurement system. Early results from processed data show a scatter of 4cm standard deviation for repeated horizontal positioning from drifting survey vessel. This paper describes two techniques developed to realize precise acoustic ranging, automatically detecting and correcting the Doppler-shift of acoustic propagation signal due to ship's drifting and attitude motion, and theoretically forming a pulse of one wavelength in cross correlation between transmission and reception of long-coded signals.
An aluminum fluoride, AlF3, forms during HF digestion of felsic rock samples, for trace element and isotope geochemistry, which use a Teflon bomb at high temperature and pressure. The AlF3 incorporates trace elements (Rb, Sr, Y, Cs, Ba, REE, Pb, Th, and U), and can not be decomposed by conventional methods such as evaporation with HClO4. The production of this AlF3 results in lower yields and poor accuracy in analyses of these trace elements by ICP-MS. The formation of AlF3 is controlled by the chemical composition of the rock samples, in particular, AlF3 does not form during decomposition of mafic samples with relatively high (Mg+Ca)/Al ratios. We have developed a new method to suppress the AlF3 formation, in which excess Mg is added to the sample prior to acid digestion in the bomb. This new method makes it possible to accurately determine the trace element compositions of higher-Al rock samples with lower concentrations of Mg and Ca (e.g. rhyolite and granite). In the trace element analyses by TIMS with isotope dilution techniques (ID-TIMS), AlF3 formation hinders the achievement of isotope equilibrium, resulting in erroneous results. However, the Mg-addition method removed this problem by suppression of AlF3 formation.