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在时间间隔超过20年的两份全球地震目录里,选取所记录的浅源M≥6和M≥7主震以及M≥5的前震,计算了前震的发生率。当把它们各自的震级差范围作了归一化后,观测到的总比率与以前世界范围和区域范围的研究相似。把观测到的世界范围的比率同根据加利福尼亚小的和中等余震图象的普通地震成丛发生模型进行了比较。把余震模型推广到大主震前的中等前震的情况。总的来说,观察到的世界范围的前震比率超过了推广的加利福尼亚普通模型的2倍。发现由地震的震源机制和区域构造所划分的地震子序列之间,前震比率有很大的不同,逆断层地震的前震比率高于世界范围的平均值,而走滑断层地震的前震比率低于世界范围的平均值。在逆断层地震之中,大多数位于浅源消减带的地震有高的前震比率,而少数位于大陆的逆断层带地震有低的比率。这些差别或许能解释为什么以前的地质调查发现在加利福尼亚(特别是南加利福尼亚)发生的逆断层地震中有低的前震比率,而世界范围的观测则表明情况相反。加利福尼亚大部分地区缺乏活动的消减带,且其南部有造山逆断层区域,故明显反映出以大陆逆断层为典型的低比率,而世界范围观测的主要是浅源消减带的地震,它们是多前震的。假设这是真的,那么加利福尼亚普通模型或许大大地低估了卡斯凯迪亚地区潜在(M≥7)前震之后发生特大(M≥8)地震的条件概率。在哈佛大学目录里,在已识别的前震一主震对中,震级差与观测范围的均匀分布一致。
In the two global seismograms with the interval of more than 20 years, the recorded precursors of shallow mains M≥6 and M≥7 and the foreshock of M≥5 were selected to calculate the incidence of foreshocks. After normalizing their respective magnitude ranges, the observed overall ratios are similar to those of previous world-wide and regional-scale studies. The observed world-wide ratio was compared with a general earthquake-clump model based on small and medium aftershocks images of California. Extend the aftershock model to the case of a moderate pre-earthquake before the main shock. In general, the world-wide observed foreshock ratio has more than double the generalized California model. It is found that there is a great difference in the foreshock ratios between the seismic sub-sequences divided by the focal mechanism of the earthquake and the regional structure. The foreshock ratios of the reverse-fault earthquakes are higher than the world average values, while the pre-seismic ratios of the strike-slip fault earthquakes are lower than World average. Among the reverse-fault earthquakes, most of the earthquakes located in the shallow source-depleted zone have a high ratio of the front earthquakes, whereas a few of the reverse-zone earthquakes located in the mainland have a low rate. These differences may explain why previous geological surveys found low pre-earthquake ratios in reverse-thrust earthquakes that occurred in California (especially Southern California), whereas world-wide observations indicate the opposite. The absence of active depleted zones in most parts of California and the presence of overthrust zones in the southern part of the area clearly reflect the low ratio typical of continental thrust faults, whereas the world-wide observed mainly shallow source depleted zone earthquakes are more Before the earthquake. Assuming this is true, the common California model may greatly underestimate the conditional probability of a very large (M ≥ 8) earthquake following a potential (M ≥ 7) foreshock in the Cascadia region. In the Harvard Catalog, in the identified foreshock-mainshock pair, the magnitude difference is consistent with the uniform distribution of the observed range.