East African earthquakes below 20 km depth and their implications for crustal structure
East African earthquakes below 20 km depth and their implications for crustal structure
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东非20公里以下地震及其对地壳结构的影响
DOI：
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发表时间：
1995
期刊：
影响因子：
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通讯作者：
C. Langston
中科院分区：
文献类型：
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作者：
A. Nyblade;C. Langston
本文报道了用多种技术获得的东非8个地震的震源参数，包括（1）用长周期P波和SH波反演矩量和震源-时间函数，（2）用短周期地震图正演模拟初动极性和P波和pP波振幅，（3）用短周期记录确定pP-P和sP-P差分走时。这些地震的震源位于太古代和元古代岩石圈的24和34公里之间，除了一个断层面的解决方案表明正断层（主要是E-W扩展），与东非的区域应力状态一致。由于这些地震中有许多发生在地壳可能因裂谷作用而变薄的地区，因此很难确定它们的震源是在下地壳还是上地幔。然而，其中一些发生在远离裂谷结构的元古代地壳，可能是35-40公里厚或更厚。因此它们可能在下地壳内成核。强度剖面计算表明，为了解释孕震（即脆性）行为在足够的深度来解释东非的下地壳地震，下地壳必须不仅是由镁铁质岩石，如以前的研究人员所建议的，但也有更多的热量（-100%）必须来自上地壳比预测的地壳热源分布从1-在非洲东部和南部的元古代地形中观测到的热流和产热之间线性关系的三维解释。前寒武纪基性岩墙群在整个东非提供的证据，岩浆事件可能有dclived大量的镁铁质物质的下地壳在一个非常广泛的地区，从而解释了为什么在东非的下地壳可能是镁铁质远离火山裂谷。
SUMMARY We report source parameters for eight earthquakes in East Africa obtained using a number of techniques, including (1) inversion of long-pcriod P and SH waves for momcnt tcnsors and source-time functions, (2) forward modelling of first-motion polarities and P and pP amplitudes on short-period seismograms, and (3) determination of pP-P and sP-P differential traveltimes from short-period records. The foci of these earthquakes lie between depths of 24 and 34km in Archean and Proterozoic lithosphere, and all but one fault-plane solution indicates normal faulting (primarily E-W extension), consistent with the regional stress regime in East Africa. Because many of these earthquakes occurred in areas where the crust may have been thinned by rifting, it is difficult to ascertain whether or not their foci lie within the lower crust or upper mantle. Some of them, however, occurred away from rift structures in Proterozoic crust that is possibly 35-40 km thick or thicker. and thus they probably nucleated within the lower crust. Strength profile calculations suggest that in order to account for seismogenic (i.e. brittle) behaviour at sufficient depths to explain lower crustal earthquakes in East Africa, the lower crust must not only be composed of mafic lithologies, as suggested by previous investigators, but also that significantly more heat (-100 per cent) must come from the upper crust than predicted by the crustal heat source distribution obtained from a 1-D interpretation of the linear relationship between heat flow and heat production observed in Proterozoic terrains within eastern and southern Africa. Precambrian mafic dike swarms throughout East Africa provide evidence for magmatic events which could have dclivered large amounts of mafic material to the lower crust over a very broad area, thus explaining why the lower crust in East Africa might be mafic away from the volcanogenic rift valleys.