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1978年以来,美国加州马默斯湖(Mammoth Lakes)地区沿Sierra Nevada山前沿发生了一系列中强地震。本文分析和计算了14次较大地震(M_L≥5.0)的宽频带地震记录的P波S波频谱以及其比值(APTS)。在此基础上划分出五种类型的地震。提出了三种数学模型对APTS作了解释。它们是(1)在震源附近有一个较宽广的地震波衰减区。(2)存在狭窄的入侵岩脉、岩脉本身具有高衰减、低波速的特征。(3)震源为传播的断层位错。用(1)和(2)可以很好地解释由传播路径效应造成的APTS的差异,而不是因震源机制的不同所致。特别是Jamestown台(位于马默斯湖以西约140公里)记录的地震图,其APTS在2.8赫芝处有一个明显的低值,它与代表岩脉的平板滤波器模型相适应。结果表明,在整个宽约3—4公里的区域内。当P波和S波通过5个左右平均宽为50米的垂直岩脉传播时。在观测的APTS中将产生极小值。不过仅从Jamestown的频谱记录,尚不能严格区分模型(1)和(2);不论是那种情况,频谱与双力偶和补偿线性向量偶极震源机制都是一致的。
Since 1978, a series of moderate earthquakes have taken place along the forefront of Sierra Nevada in the Mammoth Lakes, California, USA. In this paper, we analyzed and calculated the P-wave S-wave spectrum and its ratio (APTS) of 14 large earthquakes (M_L≥5.0) for broadband seismic records. Based on this, five types of earthquakes are divided. Three mathematical models are proposed to explain APTS. They are (1) a broader area of seismic attenuation near the source. (2) There is a narrow invaded dyke, which itself has the characteristics of high attenuation and low wave velocity. (3) Disastrous fault propagated by the source. Using (1) and (2) can explain the difference of APTS caused by the propagation path effect well, not due to the different source mechanism. In particular, the seismogram recorded at the Jamestown station (about 140 km west of Mammoth Lakes) shows a clear low APTS at 2.8 Hz that is compatible with the flat-panel filter model representing dykes. The results show that the entire width of about 3-4 km area. When P waves and S waves pass through about 5 vertical veins with an average width of 50 meters. A minimum will result in the observed APTS. However, it is not yet possible to strictly distinguish models (1) and (2) from Jamestown spectrum records only; in either case, the spectrum is consistent with the dual-dipole and compensating linear vector dipole sources.