Study on seismic wave velocity changes in the crust associated with the 1980 Izu-Hanto-Toho-Oki Earthquake

1979-1986年在日本关东和东海地区进行了探测地震波速度随时间变化的实验。分析实验中观测到的旅行时，我们已经发现，由海洋潮汐产生的应力变化和构造应力积累引起了地壳速度的时间变化。1980年，伊豆-汉东-东浩-冲之地震发生在实验区内。据推测，与地震有关的应力变化引起了震源周围地壳中地震波的速度变化。从观测到的走时中剔除了海潮引起的速度变化和地壳长期应力积累的影响后，我们探测到了地震波速度的时间变化。我们还利用观测到的与潮汐、构造应力和地震有关的速度变化，研究了地壳中地震波应力敏感性的空间分布。我们利用观测到的速度变化和相应的沿波路径的应力变化来计算速度对应力的表观灵敏度。我们还汇编了许多其他研究人员报告的表观应力敏感性。观测到的视应力敏感度随基线长度迅速减小，这意味着应力敏感度应随深度急剧减小。据估计，地表的应力敏感性约为1(1/兆帕)，每隔几公里就会下降一个数量级。

An experiment to detect temporal velocity changes of seismic waves was conducted in 1979-1986 in Kanto and Tokai regions, Japan. Analyzing the travel times observed in the experiment, we have already found that stress variation generated by ocean tides and the tectonic stress accumulation gives rise to the temporal velocity changes in the crust. The Izu-Hanto-Toho-Oki earthquake took place in 1980 within the experiment area. It is supposed that the stress changes associated with the earthquake caused the velocity changes of seismic waves in the crust around the source. We detect the temporal variation of seismic wave velocities, after removing effects of the velocity changes caused by ocean tides and a secular stress accumulation in the crust from the observed travel times. We also investigate the spatial distribution of the stress sensitivity of seismic waves in the crust using the observed velocity changes related the tides, the tectonic stress, and the earthquake. We calculate apparent sensitivities of the velocity to the stress using the observed velocity changes and the relevant stress changes along the wave paths. We compile also apparent stress sensitivities reported by many other researchers. The observed apparent stress sensitivity rapidly decreases with the base line length, which means that the stress sensitivity should sharply decreases with depth. The stress sensitivity is estimated to be about 1 (1/MPa) at the surface and decrease by one order of magnitude every few kilometers.