On the preseismic deformation changes prior to the Lushan MS7.0 earthquake
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摘要: 应用芦山MS7.0地震震中附近跨断层及连续形变观测资料， 分析了芦山地震前不同阶段地形变变化的特点， 讨论了震中附近区域异常时空演化过程. 结果表明: ① 自2013年1月起， 沿鲜水河断裂带一些跨断层基线观测到显著的加速转折变化, 沿安宁河、 则木河断裂带个别场地的跨断层水准基线， 2010年以来出现的巨幅异常等是突出的场兆变化； 沿龙门山断裂带一些水准观测在汶川MS8.0地震后持续的调整变化具有近震源区变形特征. ② 鲜水河、 龙门山和安宁河3条主要断裂围成的三叉口地区， 地倾斜、 应变、 重力及断层水准和蠕变观测临震前均未有显著的异常变化， GPS水平、 垂直位移年速率最小， 该地区是形变变化或形变异常分布的“空区”. ③ 在对近场与远场多种连续形变数据通过傅里叶变换提取年周期成分后发现， 临震前2—3年近震源区域的地倾斜、 重力年变化幅度不是增大， 而是减小. 芦山MS7.0地震前观测到的形变前兆现象特征与汶川MS8.0地震等震前的前兆现象较为接近. 因此， 芦山地震前近震源区及外围形变异常分布特征不是个别的现象.Abstract: This paper analyzes and discusses the spatio-temporal evolutionary characteristics of deformation prior to the Lushan MS7.0 earthquake, based on the data of cross fault and continuous deformation measurements near the epicenter of the earthquake. The results show that: ① the cross fault baseline length has been significant acceleration-turning at some cross-fault sites along the Xianshuihe fault since January of 2013, and some anomalies of leveling and baseline lengths recorded at individual sites along Anninghe and Zemuhe faults are the field precursors. Small adjustment of most of the leveling cross the Longmenshan fault presented the characteristic of source precursors. ② In the cross connected area of the Xianshuihe, Longmenshan and Anninghe faults, no significant anomalies were recorded by means of continuous deformation (tilt, strain, gravity), fault leveling, and creep observation. Moreover, the GPS horizontal and vertical velocities nearby the source are smaller than the outer region, so there is a concept of Weak Area, where no significant changes in deformation or no anomalies. ③ The annual amplitude of tilt and gravity in the near-field, extracted by the Fourier transform, is reduced 2—3 years prior to the event, which is our newly understanding about the earthquake precursors. The results mentioned above indicate that the precursory phenomena before the Lushan MS7.0 earthquake, observed by deformation measurements, are similar to those before Wenchuan MS8.0 and so on, so it is not individual.
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Keywords:
- Lushan earthquake /
- crustal deformation /
- near and far fields /
- deformation weak area
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图 1 芦山MS7.0地震前沿鲜水河断裂带跨断层基线变化. LAB和LAC分别表示AB边和AC边的边长
Figure 1. The changes in cross fault baseline lengths along Xianshuihe fault prior to the Lushan earthquake, where LAB and LAC are the length of AB and AC lines, respectively
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图 3 芦山MS7.0地震前沿龙门山断裂带跨断层水准变化图中HXX1 , H3—4, H2—1分别表示XX1边、 3—4边和 2—1边测线高程变化
Figure 3. The changes in cross fault leveling along the Longmenshan fault before the Lushan earthquake，where HXX1 , H3—4, and H2—1 denote the leveling of XX1, 3—4, and 2—1 lines, respectively
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图 4 芦山MS7.0地震前沿安宁河、 则木河断裂带跨断层水准、 基线变化HAB, HAC, H1—3分别表示相应测线的水准变化， LAB表示AB测线长度
Figure 4. The changes in cross fault leveling and baseline lengths along the Anninghe and Zemuhe faults before the Lushan earthquake. HAB, HAC, H1—3 separately represent corresponding leveling changes for the lines AB, AC, and 1—3, and LAB is the length of AB line
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图 5 芦山MS7.0地震前雅安台地倾斜M2波潮汐因子(a)、 相位(b)及相应的误差
Figure 5. The changes of tilt tidal factor (a) and phase (b) at Ya′an station before the Lushan earthquake where the bars denote the monthly errors
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图 6 芦山MS7.0地震前雅安台倾斜矢量模(a)及年周期变化(b)
Figure 6. The tilt vector modulus (a) and the annual period changes (b) at Ya′an station before the Lushan earthquake
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图 7 芦山MS7.0地震前姑咱台重力(a)及年周期成分变化(b)
Figure 7. The changes in gravity (a) and the annual periodic component (b) at Guzan station before Lushan earthquake
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图 8 远场异常与地震相关性判别的时间依赖准则
Figure 8. Sketch map of the time dependent criterion for constructing the correlativity between far-filed deformation anomalies and earthquakes
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图 9 芦山MS7.0地震前的形变异常分布及形变异常“空区”红色矩形表示芦山地震前形变变化较大或异常显著的场地，虚线椭圆表示形变异常“空区”，红色实心圆表示2012年以来研究区域内发生的MS5.5以上地震， 黑色三角形、 矩形分别表示连续形变观测台站和断层观测场地， 红色线、 绿色线分别表示断层及河流分布
Figure 9. The deformation anomaly distribution showing the deformation weak area (denoted by dashed ellipse) Red rectangles denote the sites with significant deformation changes or anomalies, the red circles denote the MS≥5.5 earthquakes since 2012, respectively; black triangles and rectangles denote continuous deformation observation stations and cross fault observation sites, the red and blue lines show the faults and rivers
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