Quantitative mapping of precursory seismic quiescence before the 1989, M 7.1off-Sanriku earthquake, Japan
Quantitative mapping of precursory seismic quiescence before the 1989, M 7.1off-Sanriku earthquake, Japan
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1989 年日本三陆 7.1 级地震前前兆地震静止的定量测绘
DOI：
10.4401/ag-3765
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发表时间：
1999
影响因子：
1
通讯作者：
N. Umino
中科院分区：
文献类型：
--
作者：
M. Wyss;A. Hasegawa;S. Wiemer;N. Umino
三里库外海地震序列的第一次主震(1989年2月11日，7.1级；1992年7月18日，6.9级；1994年12月28日，7.5级)之前有一次持续2.5±1年的前兆地震平静，一直持续到这次主震。使用ZMAP计算机程序通过网格技术将这种静止的详细特性绘制为时间和空间的函数，并通过根据使用的数据集东北大学的微震目录生成合成目录来估计统计意义。这种前兆平静在统计上最显著的表现是随机发生的概率为0.1%，位于余震区的东部，1994年的余震也是在这一点上延续的。如果用一个垂直圆柱体来定义静止异常的尺度，它以整个孕震层的深度为中心，以最显著的静止点为中心，降幅为75%，则其半径为25±9公里。如果我们允许其他形状，如1989年简化的余震体积，或其他简单的几何图形，来定义减震率，我们发现维度为80×80公里。在以下范围内，静止异常的特征与自由参数的选择没有很强的依赖关系：100？活动数量？400个，2.0个？Mmin GB 3.0，1？时间窗口？三年。用我们的方法，由于1989年延长余震序列的干扰，不可能对1994年主震前的一段时间进行彻底的分析。然而，我们发现1994年余震区中心附近的地震几乎为零，持续了一年，直到那次主震。然而，这一静止期在重要性上只排在第46位，落后于通过数据集在时间和空间上分布的相同持续时间和相似维度的其他静止期。由于在一年这样的短周期内普遍存在接近零活动的周期，我们发现，在大多数地震目录中，小于约1.5年的停顿不能被定义为具有很高统计意义的停顿。在最近两年(1995.3-1997.3)的数据中，我们在现有的数据中没有看到本州东海岸在36.5°到42°N之间的大范围的高度平静。
The first main shock of the off-Sanriku earthquake sequence (02/11/89, M 7.1; 18/07/92, M 6.9; 28/12/94, M 7.5) was preceded by a precursory seismic quiescence lasting 2.5 ± 1 year and up to this main shock. The detailed properties of this quiescence were mapped as a function of time and space by a gridding technique using the ZMAP computer code, and the statistical significance was estimated by generating a synthetic catalog based on the microearthquake catalog of Tohoku University, which was the data set used. The statistically most significant expression of this precursory quiescence has a probability of 0.1% to have occurred at random and was located in the eastern part of the 1989 aftershock area, at a point to which the 1994 aftershocks extended also. If we define the dimensions of the quiescence anomaly by a vertical cylinder with the depth of the entire seismogenic layer, centered at the point of most significant quiescence and showing a rate decrease of 75%, then we find its radius is 25 ± 9 km. If we allow other shapes, such as the simplified aftershock volume of 1989, or other simple geometric figures, to define the rate decrease we find dimensions of 80 by 80 km. The characteristics of the quiescence anomaly do not depend strongly on the choice of free parameters within the following ranges: 100 ? number of events ? 400, 2.0 ? Mmin £ 3.0, 1 ? time window ? 3 years. With our method, a thorough analysis of the period before the 1994 main shock is not possible because of the interference of the extended aftershock sequence of 1989. Nevertheless, we identified a quiescence of nearly zero earthquakes located near the center of the 1994 aftershock area that lasted for one year up to that main shock. However, this quiescence period ranked only 46th in significance, behind other quiescences of equal duration and similar dimensions distributed in time and space through the data set. Because of the ubiquitous existence of periods of near zero activity during short periods like one year, we find that quiescences shorter than about 1.5 years cannot be defined with high statistical significance in most earthquake catalogs. In the last two years of the data (1995.3-1997.3) we see no extensive quiescence of high significance off the east coast of Honshu between 36.5° and 42°N in the currently available data.