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在1989年近一个月内南加州橡树岭发生了一组约50个小震(M≈1.5)的震群。由于在地震图上的到时信号弱及其突然性,因此用一般的到时分析方法,这些事件的定位精度受到了限制。然而,由于在各个单台上记录到的地震波形的相似性,用波形互相关技术可以得到精确的相对的事件位置。重新定位后的事件形成了一个小丛集,约1km跨度,18km深,沿着一个向北西倾35°角的平面排列。估计定位的标准误差一般小于50m。序列的时间演化过程表明,地震活动逐渐离开它的起始点向外迁移。有3个续发性事件发生在几个月以后,它们沿着同一平面排列,但从主震群向南东方向偏移了约500m。由于可得到的台站记录少,发射角的范围受到限制,以及在许多地震图上初至波信号弱,因此对这些事件很难得到可靠的断层面解。把每一个台上不同事件的记录叠加起来,大大地降低了到时前的噪声水平,有助于分辨平均P波初动。对这些初动资料的分析表明,震源机制解的滑动面与由地震活动分布确定的平面不一致。地震活动排列面可能是锡米断层的进一步延伸,在此情况下,只有断层倾角变浅,才能在18km深处与观测到的35°倾角相当。
In the nearly one month in 1989, a group of about 50 small earthquakes (M≈1.5) swarm occurred at Oak Ridge, Southern California. Due to the weak arrival and suddenness of the arrival on the seismogram, the positioning accuracy of these events is limited by the usual time-of-arrival analysis. However, due to the similarities of seismic waveforms recorded on individual stations, the exact relative position of the events can be obtained using the waveform cross-correlation technique. The relocated event formed a small cluster of about 1km span, 18km deep, arranged in a plane that angles 35 ° northwest. Estimated standard error of positioning is generally less than 50m. The temporal evolution of the sequence shows that the seismic activity gradually migrates outward from its starting point. Three consecutive events occurred a few months after they were aligned along the same plane, but offset by about 500 m in the easterly direction from the mainshock swarm. Due to the small number of stations available, the limited range of launch angles and weak first-arrival signals on many seismograms, it is difficult to obtain reliable fault surface solutions for these events. Stacking the records of different events on each stage greatly reduces the noise level before arrival and helps to distinguish the average P wave initial move. Analysis of these initial data shows that the focal plane solution does not coincide with the plane determined by the distribution of seismic activity. Seismic activity planes may be further extensions of the Shimian fault, in which case only the fault dip shallower than the observed 35 ° dip at a depth of 18 km.