SEISMOLOGY AND GEOLOGY ›› 2015, Vol. 37 ›› Issue (1): 53-67.DOI: 10.3969/j.issn.0253-4967.2015.05

• CONTENTS • Previous Articles     Next Articles

SIMULATION STUDY OF THE INFLUENCE OF MEDIUM VISCOSITY ON AFTERSHOCK ACTIVITIES

QU Jun-hao1,2, JIANG Hai-kun3, SONG Jin3, LI Jin4   

  1. 1 Institute of Geology, China Earthquake Administration, Beijing 100029, China;
    2 Earthquake Administration of Shandong Province, Jinan 250014, China;
    3 China Earthquake Networks Center, Beijing 100045, China;
    4 Earthquake Administration of Xinjiang Uygur Autonomous Region, Urumqi 830011, China
  • Received:2013-11-27 Revised:2014-02-10 Published:2015-05-15

介质黏滞性质对余震活动影响的数值模拟

曲均浩1,2, 蒋海昆3, 宋金3, 李金4   

  1. 1 中国地震局地质研究所, 北京 100029;
    2 山东省地震局, 济南 250014;
    3 中国地震台网中心, 北京 100045;
    4 新疆维吾尔自治区地震局, 乌鲁木齐 830011
  • 通讯作者: 蒋海昆,男,研究员,E-mail:jianghaikun@sohu.com
  • 作者简介:曲均浩,男,1981年生,工程师,中国地震局地质研究所在读博士研究生,主要从事数字地震学应用及地震序列研究,E-mail:gisqjh@126.com。
  • 基金资助:

    山东省自然科学基金(ZR2014DQ019)和山东省地震局青年基金(JJ1308Y)共同资助

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Abstract:

After a large earthquake, more seismic activities are observed in the focal region and its adjacent area. The obvious increased earthquakes are called the aftershocks. Generally speaking, aftershock sequence gradually weakens and sometimes has ups and downs. The time when the aftershock activity begins to be confused with background seismic activity is known as the aftershock activity duration. Aftershock sequence is one of the enduring research fields in seismology. Aftershocks accord with two important statistical relationships, one is the G-R relationship describing the relation between the magnitude and frequency, the other is the modified Omori formula describing the characteristics of aftershock decay with time. On this basis, a number of studies from different angles explain the mechanism of aftershock activity. From the perspective of the medium heterogeneity, it is universally accepted that aftershock is a result of further rupture of residual asperities. From the perspective of stress, these models, e.g. rate-state dependence, subcritical crack growth, creep or afterslip and so on, think that the fault stress change caused by mainshock is the main cause for aftershock. But other researchers, by studying real aftershock observations, think that the fault stress change caused by mainshock is not the main cause or has very weak control over the aftershocks. Pore pressure diffusion caused by mainshock fault slip is also considered as an important incentive for aftershocks. There is a relationship between the frequency of aftershocks and pore pressure changes. Dry rock pressurized in physical experiment can produce acoustic emission sequence similar to mainshock-aftershock sequence type earthquake. Though fluid plays an important role in aftershock activities, it is not the essential element for aftershock. Overall, there is no single model which can fully explain the phenomenon of aftershock activity.
Assuming the rupture of the residual asperities inside the mainshock rupture plane randomly leads to the aftershocks, the size of the residual asperities conforms to fractal distribution, and the rupture or instability strength of the residual asperities accords with the lognormal distribution. Taking the postseismic stress relaxation as the mechanical load, the loading stress attenuates according to negative exponential law. Taking the Coulomb failure as the judgment criterion of the instability, combining the mechanical interactions among the residual asperities, the artificial aftershock sequence, including occurring time, location and magnitude, is simulated under different conditions. The agreement between output and the actual statistical characteristics of aftershock activities is detected by G-R relationship and modified Omori formula as a basis for further adjustments to the model parameters. On this basis, the influences of the medium viscosity properties on aftershock activities have been discussed.
The results show that viscosity coefficient of rheological properties of the lower part of the lithosphere has an important effect on the duration of aftershock activity. The viscosity coefficient of the lower part of the lithosphere controls the duration of the aftershock activity, the lower the viscosity coefficient, the sooner the stress relaxation of the lower lithosphere, and the faster the loading rate to the upper part of the lithosphere, the shorter the duration of the aftershock activity. On the contrary, the higher the viscosity coefficient, the slower the loading rate to the upper part of the lithosphere, and the longer the duration of the aftershock activity. This simulation conclusion is consistent with the observed result. The viscosity coefficient as one of the important lithosphere physical parameters controls the decay rate of aftershock activity. Under this model conditions, p value, the decay rate of modified Omori law, changes with the viscosity coefficients in a negative exponential function. The relationship that the viscosity coefficient is lower and the decay of aftershock sequence is faster provides a reference for the study of the main influence factors of aftershock decay. The relationship corresponds to the observation that the decay rate of the aftershock sequence shows a good positive correlation. The b value of the G-R relationship of aftershock sequence characterizes the ratio relationship of large to small earthquakes. The modeling studies suggest that the G-R relationship of the aftershock sequence is irrelevant with the viscosity coefficient, but mainly controlled by the size distribution of the residual asperities. In another word, it is mostly correlative to the heterogeneity of tectonics and medium.

Key words: mainshock rupture plane, aftershock activity, residual asperities, stress relaxation, viscosity coefficient

摘要：

假定主震破裂面上残余凹凸体的破裂导致余震, 残余凹凸体尺度符合分形分布, 在破裂面上随机分布, 残余凹凸体破裂/失稳强度符合对数正态分布;以岩石圈下部粘弹介质震后应力松弛作为导致残余凹凸体破裂的动力输入, 加载应力按负指数形式衰减;以库仑破裂准则作为失稳判据, 考虑残余凹凸体破裂对附近区域的应力加载, 在不同条件下模拟生成包含时、空、强三要素的人工 "余震"序列。通过G-R关系及修改的大森公式, 检测模拟输出与实际余震活动统计特征的吻合程度, 以此作为进一步调整模型参数的依据。在此基础上重点讨论介质黏滞性质对余震活动的影响。结果显示: 表征岩石物理特性的黏滞系数对余震活动持续时间、衰减快慢起重要的控制作用, 黏滞系数与衰减系数之间显示负指数的相关关系。余震序列G-R关系与黏滞系数关系不大, 主要受控于残余凹凸体的尺度分布, 即与构造不均匀性关系更为密切。

关键词: 主震破裂面, 余震活动, 残余凹凸体, 应力松弛, 黏滞系数

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