<p>地震的发生与地壳区域应力的累积以及活动板块的相对运动密切相关，研究地震构造应力场时空变化有助于我们深刻认识构造形变与地震破裂的关系，对防震减灾工作具有重要的科学意义和实用价值。本文利用GNSS同震位移数据反演Ridgecrest M_w6.4左旋和M_w7.1右旋两次地震的滑动分布。将2014年1月-2019年10月的震源机制解数据按其时空分布划分为三个时段和五个子区域，计算应力场的时空变化。基于太平洋板块和北美板块相对运动的大构造背景，解释加州区域共轭地震频发且具有共性的原因。本文主要的研究工作以及最终成果如下：</p> <p>（1）滑动分布结果的准确性对计算应力降 和偏应力 具有影响。由于InSAR数据重访周期长，很难区分2019年7月4日M_w6.4和7月6日M_w7.1两次地震各自引起的地表形变，使用其反演的滑动分布模型会使 和 偏大。GNSS数据能够捕获Ridgecrest两次地震之间的破裂部分，使得两次破裂的形变得以分解。本文基于GNSS数据分别反演两次地震的同震滑动分布，两次地震并非纯走滑地震，均带有少量倾滑分量。M_w6.4前震导致断层F2、F3破裂，断层F2发生左旋走滑，最大滑动量~0.7 m；断层F3发生右旋走滑，最大滑动量~0.3 m。M_w7.1主震导致断层F1、F2 和F3破裂，断层F1、F3均表现为右旋走滑，最大滑动量分别为~2.2 m和~3.1 m；断层F2几乎没有滑动。</p> <p>（2）2019年Ridgecrest地震区域最大水平应力(SHmax)方向具有先逆时针旋转再顺时针旋转的特征。M_w6.4前震发生后，SHmax方向逆时针旋转，前震震中区域由3.8&deg;变为2.8&deg;，主震震中区域由4.0&deg;变为3.2&deg;，在误差允许范围内仅逆时针旋转了~1&deg;，表明M_w6.4前震并未造成其震源区应力方向的显著变化。M_w7.1主震发生后，SHmax方向顺时针旋转，前震震中区域由2.8&deg;变为5.8&deg;，顺时针旋转了3.0&deg;，应力变化较为明显。但主震震中区域平均失配角 值高达53&deg;，判定该区域为非均质应力场，计算该区域偏应力 ~4.5 MPa，应力降 ~4.3 MPa，发现两者几乎相等，表明震后非均质应力场区域的应力接近完全释放。应力变化较大区域与滑动分布较大区域相对应。主震后由于较大余震造成的应力转移以及应力完全释放等因素的影响，使得主应力轴的方位角或倾角发生改变，但不同深度的应力仍以走滑状态为主，受应力异质性影响较小。地震半年前，b值开始显著变化，震后其波动趋于稳定。</p> <p>（3）太平洋板块与北美板块相对运动速率差构成顺时针旋转趋势，使加州地区处于相对运动形成的右旋剪切带中。由于两板块交界处有弧状构造，向NE-SW方向挤压，造成该区域应力累积和分布不均匀，其累积能量主要受NW-SE向右旋和NE-SW向左旋的共轭断裂来共同调节。从而解释了加州区域共轭地震频发且前震或主震或余震均产生这种&ldquo;NW-SE向右旋和NE-SW向左旋&rdquo;现象的原因以及相应的应力方向变化。</p>

<p>The occurrence of earthquakes is closely related to the accumulation of stress in the crustal region and the relative motion of active plates. Studying the spatiotemporal changes of the seismic tectonic stress field helps us to deeply understand the relationship between tectonic deformation and earthquake rupture, which is of great scientific significance and practicality value for earthquake prevention and disaster mitigation. In this paper, the GNSS coseismic displacement data are used to invert the slip distribution of the Ridgecrest M_w 6.4 sinistral and M_w 7.1 dextral earthquakes. The focal mechanism data from January 2014 to October 2019 were divided into three periods and five subareas according to their spatiotemporal distribution, and the spatiotemporal variations of stress field were calculated. Based on the tectonic background of the relative movement of the Pacific plate and the North American plate, the common reasons for the frequent occurrence of conjugate earthquakes in California are explained. The main research work and final results of this paper are as follows:</p> <p>(1) The accuracy of the slip distribution results has an influence on the calculation of the stress drop &nbsp;and the deviatoric stress . Due to the long revisit period of InSAR data, it is difficult to distinguish the surface deformation caused by the two earthquakes on July 4, 2019, M_w 6.4 and July 6, 2019, M_w 7.1, respectively, and the slip distribution model inversion by InSAR data will make &nbsp;and &nbsp;larger. Because the GNSS data can capture the part of rupture between the Ridgecrest two earthquakes, allowing the two ruptures were decomposed. Based on GNSS data, this paper inverts the coseismic slip distribution of the two earthquakes respectively. The two earthquakes are not purely strike-slip, and both have minor dip-slip components. The M_w 6.4 foreshock caused the rupture of faults F2 and F3, and fault F2 exhibited sinistral strike-slip with a maximum slip of ~0.7 m; fault F3 exhibited dextral strike-slip with a maximum slip of ~0.3 m. The M_w 7.1 mainshock caused the rupture of faults F1, F2, and F3, and faults F1 and F3 exhibited dextral strike-slip with a maximum slip of ~2.2 m and ~3.1 m, respectively; fault F2 has almost no slip.</p> <p>(2) The maximum horizontal stress (SHmax) direction in the Ridgecrest earthquake region in 2019 has the characteristics of first counterclockwise and then clockwise rotation. After the M_w 6.4 foreshock, the SHmax direction rotated counterclockwise, and the foreshock epicenter area changed from 3.8&deg; to 2.8&deg;, and the mainshock epicenter area changed from 4.0&deg; to 3.2&deg;, which only rotated counterclockwise by ~1&deg; within the error tolerance, indicating that the M_w 6.4 foreshock did not cause substantial changes in the stress direction in its source area. After the M_w 7.1 mainshock, the SHmax direction rotated clockwise, and the foreshock epicenter region changed from 2.8&deg; to 5.8&deg;, rotating clockwise by 3.0&deg;, and the stress change was more significant. However, the average misfit angle &nbsp;in the epicenter area of the mainshock is as high as 53&deg;, and this area is determined to be a heterogeneous stress field. Calculating the deviatoric stress &nbsp;~4.5 MPa and the stress drop &nbsp;~4.3 MPa in this area, it is found that they are almost equal, indicating that the post-earthquake heterogeneous stress field region had experienced almost complete stress release. The area with large stress changes corresponds to the area with large slip distribution. After the mainshock, due to the influence of the stress transfer caused by the larger aftershock and the complete release of stress, the trend or plunge angle of the main stress axis is changed, but the stresses at different depths are still dominated by the strike-slip state, which is less affected by the stress heterogeneity. Six months before the earthquake, the b-value began to change significantly, and its fluctuation tended to be stable after the earthquake.</p> <p>(3) The difference in relative motion rates between the Pacific plate and the North American plate constitutes a clockwise rotational trend, which puts the California region in a dextral shear zone formed by relative motion. Due to the arc-shaped structure at the junction of the two plates, the stress accumulation and distribution are uneven in this region, and the accumulated stress is mainly jointly regulated by the NW&ndash;SE dextral and NE&ndash;SW sinistral conjugate fracture zone. The frequent occurrence of conjugate earthquakes in the California region and the reason for this phenomenon of &quot;NW-SE dextral and NE-SW sinistral&quot; in foreshocks, mainshocks and aftershocks, as well as the corresponding stress direction changes were explained.</p>

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