青藏高原东北缘基于小震震源机制解的区域应力场特征研究

关键词：地震预测；矩张量反演；震源机制解；主压应力；平均应力场；
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青藏高原东北缘基于小震震源机e fundamental causation of an earthquake occurrence is action of the stress field. Therefore, studying on the regional stress fields which come from the small and intermediate earthquakes is quite important effect to know the course of an earthquake occurrence. Exploring the epicenter and looking after the target of earthquake forecast, as a goal, geophysicists and seismologists are all along hardworking. In this paper, we use the data of ML≥3.0 earthquakes which have occurred since 2001 in northeast margin of Qinghai-Tibet Plateau, recorded by11broad frequency stations of GanSu Province seismic digital station network. We have calculated focal mechanism solutions of 242 ML≥3.0 earthquakes by moment tensor inversion. We have statistically analyzed the earthquakes types and characteristics of the stress fields in northeast edge of Qinghai-Tibet Plateau using the results which we have calculated. We traverse characteristics of the regional stress fields in 4 and 8 areas which depended on the subarea activity characteristics of northeast edge of Qinghai-Tibet Plateau. As a case Minle earthquake 2003, we analyze the local stress fields before and after Minle earthquake and discuss its generant mechanism. Through contrast analysis on the inversion results and existing results of 4 strong earthquakes, we investigate the characteristics of the strong earthquakes . Summing up the analysis and study from above, we get following conclusions: 1. Statistical analysis on the P ,B ,T axes of focal parameters of ML≥3.0 earthquakes, we get the characteristics of the stress fields in northeast margin of Qinghai-Tibet Plateau: in the middle part of Qilanshan , the preponderant orientation of the principal compressive stress P axis is in NE ;in middle east part, its orientation changes from NE to approximately EW and circumrotates some angles in the same direction as hour hand in interspace. In the southeast region of GanSu, the preponderant orientation of the principal compressive stress P axis is in NWW which is consistent to the way of the faults. In the whole, the distribution of the principal compressive stress has obvious divisional characteristics, from west to east, regional principal compressive stress direction changes from NNE-NE-NEE to EW presenting sector distribution. 2. Analysis on the elevation of P,T axes in the focal mechanism, we get that the elevation
Key words: Earthquake Prediction; Inversion of Seismic Moment Tensor; Focal Mechanism; the Principal Stress; mean stress field;
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中国地震局兰州地震研究所硕士学位论文
β of P axis less than 30º is 82.23％, the elevation γ of T axis less than 45º is 68.59％, the type of earthquakes gives priority to strike-slip fault; when the elevation γ of T axis more than 45º happens oblique fault is 31.41％. Based on the results we get that the properties of faults in the investigative area are mostly strike-slip fault, concurrently reverse fault and normal fault . 3. Analysis on the mean stress field we get that in the Qilanshan earthquake belt, the elevation of σ1 and σ3 axes are close to horizontal direction, the stress present in horizon and the principal axis is in NNE and NE. In southeast region of GanSu, the azimuth and elevation are 123º、153º、109º，9º、34º、15º , separately , NW or near NS , the stress show itself in horizon . 4. We think that the direction of the principal compressive stress axes appears changeable process by phases before the main shock, the middle part of Qilianshan belt where the main shock occurred was in stress-accumulation state. The most pressure stress axis rotated some angles before and after MinLe earthquake in its zone. With the development of seismic digital station network and improvement of monitor ability about some areas, study on the focal mechanism solutions in location is important scientific meaning for earthquake prediction.
中国地震局兰州地震研究所 硕士学位论文 青藏高原东北缘基于小震震源机制解的区域应力场特征研究 姓名：张辉 申请学位级别：硕士 专业：固体地球物理 指导教师：刘小凤;张元生 20070712
中国地震局兰州地震研究所硕士学位论文
摘
要
地震发生的根本原因就是应力场的作用，因此，对中小地震形成的区域应力场的研究 对地震发生过程的认识有着十分重要的作用。探索震源、寻求地震预测指标一直是地球物 理学家和地震学家孜孜以求的目标之一。 本文利用甘肃省数字地震台网中的 11 个宽频带台记录到的青藏高原东北缘自 2001 年 以来发生的 ML≥3.0 级地震资料，采用矩张量反演的方法计算了该区域 242 个 ML≥3.0 级 地震震源机制解，利用所得结果统计分析青藏高原东北缘的地震类型及应力场特征。根据 研究区内活动断裂的分区特征，把研究区分为四个区域及八个小区域，详细研究了区域应 力场特征。以 2003 年民乐地震作为震例，分析了民乐地震前后震源区应力场情况及探讨了 它的发震机制。通过对 4 次中强震的反演结果与已有结果的对比分析，研究了中强震的发 震特征。 综上研究，得出如下结论： 1. 由 ML≥3.0 级地震的震源机制解中 P、B、T 轴统计分析得到青藏高原东北缘构造应 力场的整体特征：在祁连山中段，主压应力 P 轴优势方向主要在 NE 向；中东段，P 轴优势 方向从 NE 变化到近东西向， 从空间上看， P 轴方向沿顺时针旋转了一定角度； 甘东南地区， P 轴优势方向为 NWW 向，与该区域的断层走向基本一致。整体来看水平最大主应力分布具有 明显的分区特征，自西向东，区域主压应力方向由 NNE-NE-NEE 近 EW 向，呈扇形分布。 2. 通过对震源机制解中 P、T 轴仰角统计分析得到：P 轴仰角 β 小于 30ｏ占 82.23％， T 轴仰角 γ 小于 45ｏ占 68.59％，地震类型以走滑为主；T 轴仰角 γ 大于 45ｏ时发生倾滑逆 断层的运动， 占 31.41％； 整体上形成了以走滑为主， 兼有逆冲型和少量正断层的运动特征。 3. 分析平均应力场得出：祁连山地震带的σ1、σ3 轴近水平向，区内的应力以水平作 用为主，主压应力轴方向为北北东和北东向。甘东南地区的主压应力轴方位角分别为 123ｏ、 153ｏ、109ｏ，仰角分别为 9ｏ、34ｏ、15ｏ，呈北西向或近南北向，显示为近水平力作用。 4. 民乐地震前主压应力轴方向出现了阶段性变化过程，在主震发生的祁连山中段地区 则处于应力积累状态，震源区最大压应力轴震前震后发生了一定角度的旋转。 随着甘肃省数字地震台网的不断完善，对某些区域检测能力的提高，研究特定区域发 生地震的震源机制解，对地震预测工作将具有十分重要的科学意义。