Focal Depth and Mechanism of Intraplate Earthquakes in the Qinghai-Tibet Plateau
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摘要: 青藏高原板内地震以浅源地震为主, 下地壳基本上没有地震, 地震震源多集中在15~40 km的深度范围, 主要在中地壳内, 呈似层状弥散分布.其中30~33 km深度是一个优势层, 与壳内分层有关.总体上青藏高原南、北部的震源面略呈相向倾斜特征.70~100 km深度区间出现了比较集中的震级较小的地震, 可能与壳幔过渡带的拆离作用有关.高原内部的正断层系与板内地震密切相关, 是板内浅源地震的主控构造.总之, 青藏高原地震震源沿着活动的上地壳脆性层与软弱层之间的脆-韧性过渡带分布.这些板内地震活动属于大陆动力学过程, 与板块碰撞和板块俯冲无关.初步认为青藏高原浅层到深层多震层的成因分别是韧性基底与脆性盖层、韧性下地壳与脆性上地壳、韧性下地壳与脆性上地幔的韧-脆性转换、拆离和解耦的产物.Abstract: The Qinghai-Tibet plateau intraplate earthquakes are mainly shallow earthquakes, and basically no earthquakes happens in the lower crust. Earthquakes mostly distribute in the range of 15-40 km in depth, basically within the middle crust and concentrate at the depth of 30-33 km as an advantage layer related to the crustal stratification. On the South and North of the plateau, the focal surfaces slightly dip to the center. Some earthquakes concentrate within the range of 70-100 km depth, which may be associated with the detachment between crust and mantle. The intraplate earthquakes in the central Qinghai-Tibet are closely related to the normal faults which are dominant earthquake faults. In short, the Qinghai-Tibet plateau seismic activity distributes along the brittle-ductile transition zone between the brittle upper crust layer and the ductile lower crust. Intraplate seismic activity is a continental dynamic process, irrelevant to plate collision and subduction. The seismogenic layers from shallow to deep are separately the result of ductile-brittle transition, detachment and decoupling between basement and cover; ductile lower crust and brittle upper crust, and ductile lower crust and brittle upper mantle.
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Key words:
- the Qinghai-Tibet plateau /
- intraplate earthquake /
- focal depth /
- seismogenic layer
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图 1 青藏高原1970年以来(M > 5.0) 地震震中分布(CENC目录数据)
Fig. 1. Earthquake distributions in Qinghai-Tibet plateau (M > 5.0, since 1970, data source: CENC)
图 2 青藏高原现代5.0级以上地震震源深度一频度(%)直方图
a.CENC目录数据; b.ISN目录数据
Fig. 2. Histograms of earthquake focal depths in Qinghai-Tibet plateau (M>5.0, since 1970)
图 3 青藏高原1997—2007年5.0级以上地震震源深度-纬度(a)和深度-经度(b)分布剖面(CENC目录数据)
Fig. 3. Focal depth profile along latitudes from (a) 25° to 40'N and (b)75° to 110'E(M>5.0, since 1970, data source: CENC)
图 4 青藏高原地震深源深度-频度(%) 分布图(CENC目录数据)
a. 0~10级; b. 5.0级以上
Fig. 4. Prequencies of earthquake focal depths in Qinghai-Tibet plateau (since 1970, data source: CENC)
图 5 青藏高原多震层与低速、低阻层、Moho面对应关系(图中地壳结构源于崔作舟等, 1992)
Fig. 5. Low-velocity & resistivity layer and Frequencies of earthquake focal depths in Qinghai-Tibet plateau
图 6 青藏高原地壳结构与多震层对应关系(图中地壳结构数据源于李秋生等, 2004)
Fig. 6. Variation of the Moho as crossing the plateau from north to south and Frequencies of earthquake focal depths
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