Contemporary seismicity, faulting, and the state of stress in the Colorado Plateau
Contemporary seismicity, faulting, and the state of stress in the Colorado Plateau
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科罗拉多高原的当代地震活动、断层和应力状态
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发表时间：
1989
期刊：
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通讯作者：
J. Humphrey
中科院分区：
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作者：
I. Wong;J. Humphrey
根据过去30年的地震监测，科罗拉多高原当代地震活动的特点是小规模到中等震级，与以前的观点相反，地震分布广泛，出现率低到中等。在高原的一些地区观察到了地震的集中。科罗拉多高原地震最活跃的地区是东部的瓦萨奇高原-书崖，那里的煤矿开采引发了丰富的小规模地震活动。到目前为止观测到的最大地震，估计为里氏5-6级(L M)，通常发生在亚利桑那州北部。虽然很少有地震与科罗拉多高原的已知地质构造或构造特征有关，但地震活动似乎是先前存在的断裂重新激活的结果，没有表面表达，但有利于构造应力场。小至中等规模地震及其广泛分布与前寒武纪基底和上地壳高度断裂、中等差异构造应力水平相一致。高原的地震一般发生在从近地表到15-20公里深度的上地壳，尽管在低到正常热流区的下地壳和最上地幔都观察到了地震。后者表明，这些深度的温度足够低，因此仍有可能发生脆性破坏和地震。高原内部构造变形的主要方式是北西向北西向北西向断裂的正断层，在浅层的北西向或北东向平面上也有局部的走滑断裂。与以前认为高原受到东西向构造挤压相反，高原内部当代应力状态的特点是大致向东北方向伸展。高原的一个地区，东部的瓦萨奇高原和书崖，可能仍然以压应力为特征；然而，这些应力的性质还没有被很好地理解。
The contemporary seismicity of the Colorado Plateau based on seismic monitoring in the past 30 yr can be characterized as being of small to moderate magnitude, and contrary to earlier views, of a low to moderate rate of occurrence with earthquakes widely distributed. Concentrations of earthquakes have been observed in a few areas of the plateau. The most seismically active area of the Colorado Plateau is the eastern Wasatch Plateau-Book Cliffs, where abundant small-magnitude seismicity is induced by coal mining. The largest earthquakes observed to date, of estimated Richter magnitude (M L ) 5-6, have generally occurred in northern Arizona. Although very few earthquakes can be associated with known geologic structures or tectonic features in the Colorado Plateau, seismicity appears to be the result of the reactivation of pre-existing faults lacking surficial expression but favorably oriented to the tectonic stress field. The small to moderate size of the earthquakes and their widespread distribution are consistent with a highly faulted Precambrian basement and upper crust, and a moderate level of differential tectonic stress. Earthquakes in the plateau generally occur in the upper crust from the near-surface to a depth of 15-20 km, although events have been observed in both the lower crust and uppermost mantle in areas of low to normal heat flow. The latter suggests that temperatures are sufficiently low at these depths that brittle failure and hence earthquakes are still possible. The predominant mode of tectonic deformation within the plateau appears to be normal faulting on northwest- to north-northwest-striking faults with some localized occurrences of strike-slip faulting on north-west- or northeast-striking planes at shallow depths. The contemporary state of stress within the plateau is characterized by approximate northeast-trending extension in contrast to the previous belief that the plateau was being subjected to east-west tectonic compression. One area of the plateau, the eastern Wasatch Plateau and Book Cliffs, may still be characterized by compressive stresses; however, the nature of these stresses is not well understood.