High frequency seismic observation at the borehole which reaches an earthquake fault

在1984年日本长野县西部大地震的地震断层处的钻孔上安装了三组自测仪。该观测系统的频率响应高达1khz，动态范围为96db，记录了大量的微震震图。研究发现，微震纵波的起始时间比一般地震台站和观测系统所探测到的要早得多。结果表明，微地震的p波的初始上升是相当平稳的，而不是时间上的斜坡。这一事实与以前发表的许多理论所预测的不同。推导出在破裂生长初期微震的震源过程是相当缓慢的。这个缓慢的过程可能包含了断裂生长的关键。

Three components of selsmometers were installed at the borehole which reached the earthquake fault of the 1984 Western Nagano Prefecture, Japan Eartqhauke. A lot of seismograms from microearthqaukes were recorded the observation system which had a frequency response up to 1 kHz and a wide dynamic range of 96 dB.It is found that the onset of P-waves from microeartqhaukes arrives much earlier than that detected with a usual seismic station and observation system.It is made clear that the initial rise of P-waves of microearthquakes is fairly smooth and that it does not act as a ramp in time. This fact is different from that predicted by a lot of theories which had been previously published.It is deduced that the source process of microeathquakes is fairly slow at the beginning of rupture growth. It is possible that this slow process includes the key for the rupture growth.