深源地震：橄榄石→尖晶石相变诱发断层  

DEEP EARTHQUAKES:FAULTING TRIGGERED BY THE OLIVINE→SPINEL TRANSFORMATION

作　　者：Harry W. Green,Ⅱ(Institute of Geophysics and Planetary Physics and Department of Earth Sciences,University of California,Riverstde,CA 92521,USA) 

出　　处：《地学前缘》1995年第1期19-25,共7页Earth Science Frontiers

摘　　要：７０年代以来，人们已经知道在地球几百公里处有深源地震存在，在这个深处压力和温度不可能使岩石产生脆性剪切断裂。在３００ｋｍ之下，地震出现频率随深度增加而迅速减少，但是在６００ｋｍ处又再次增加，在７００ｋｍ处深源地震停止。五年之前，我们发现这种深源地震与一种新的塑性不稳定性有关。在地球４００ｋｍ处，当上地幔最主要矿物橄榄石（Ｍｇ，Ｆｅ）＿２ＳｉＯ＿４发生相变时会导致一种新的断层类型产生。我们首先利用锗橄榄石Ｍｇ＿２ＧｅＯ＿４，（即晶体结构与镁铁橄榄石相同，而化学成分有所不同，它可以在比较低压力条件下发生相变）进行模拟实验，借以证明在上地幔很高压力条件下橄榄石也会发生相变。这种新的机制叫自组织类的脆性断裂（ｓｅｌｆ─ｏｒｇａｎｉｚｉｎｇｌｉｋｅｂｒｉｔｌｅｆａｉｌｕｒｅ），其特征是：在断层生长过程中会释发地震能量；在由很细尖晶石颗粒组成的“超塑性层”中会发生断层滑移（即所谓显微反向断裂，ｍｉｃｒｏａｎｔｉｃｒａｃｋ）。由这种机制形成的断层只在放热反应过程中产生，它可以用来解释为什么深源地震停止于７００ｋｍ，因为在这个深度，橄榄石到尖晶石相变为下地幔高密度矿物的吸热反应。深源地震的地震学证据与这种新机制是一致的。For 70 years,it has been known that earthquakes occur to depths of hundreds ofkilometers in the earth ,where the pressure and temperature make conventional brittle shearfracture impossible. Earthquake frequency of occurrence decreases rapidly with depth for about300 km,but then rises again,only to peak at about 600 km and then drop to zero by 700 km.The deeper population now appears to be attributable to a new instability that we discoveredfive years ago. The new faulting process is triggered when a phase transformation takes place inthe mineral olivine,(Mg,Fe)_2SiO_4 ,the most abundant mineral in the uppermost 400 km of theearth.We discovered this new faulting mechanism and have extensively investigated it in thechemical system Mg_2GeO_4(which undergoes the olivine→ spinel transformation at pressuresmuch lower than does silicate olivine)and confirmed it in olivine of mantle composition at veryhigh pressure. The new mechanism is self-organizing like brittle failure,it emits seismic energyduring fauIt growth only,and fault slip takes place in a “ superplastic”layer of extremely fine-grained spinel that is manufactured in microanticracks. Faulting by this mechanism is possibleonly during exothermic reactions, thereby providing an explanation for why earthquakes stop at700 km where an endothermic phase change takes place from the spinel form of olivine to thestill-denser minerals of the lower mantle. Seismological evidence from deep earthquakes is con-sistent with this new mechanism.

关 键 词：深源地震 断层 橄榄石 尖晶石 相变 地幔 

分 类 号：P315.2[天文地球—地震学]