Reyners Martin, Eberhart-Phillips Donna, Stuart Graham
GNS Science, PO Box 30 368, Lower Hutt 5040, New Zealand.
Nature. 2007 Apr 26;446(7139):1075-8. doi: 10.1038/nature05743.
The occurrence of earthquakes in the lower crust near continental rifts has long been puzzling, as the lower crust is generally thought to be too hot for brittle failure to occur. Such anomalous events have usually been explained in terms of the lower crust being cooler than normal. But if the lower crust is indeed cold enough to produce earthquakes, then the uppermost mantle beneath it should also be cold enough, and yet uppermost mantle earthquakes are not observed. Numerous lower-crustal earthquakes occur near the southwestern termination of the Taupo Volcanic Zone (TVZ), an active continental rift in New Zealand. Here we present three-dimensional tomographic imaging of seismic velocities and seismic attenuation in this region using data from a dense seismograph deployment. We find that crustal earthquakes accurately relocated with our three-dimensional seismic velocity model form a continuous band along the rift, deepening from mostly less than 10 km in the central TVZ to depths of 30-40 km in the lower crust, 30 km southwest of the termination of the volcanic zone. These earthquakes often occur in swarms, suggesting fluid movement in critically loaded fault zones. Seismic velocities within the band are also consistent with the presence of fluids, and the deepening seismicity parallels the boundary between high seismic attenuation (interpreted as partial melt) within the central TVZ and low seismic attenuation in the crust to the southwest. This linking of upper and lower-crustal seismicity and crustal structure allows us to propose a common explanation for all the seismicity, involving the weakening of faults on the periphery of an otherwise dry, mafic crust by hot fluids, including those exsolved from underlying melt. Such fluids may generally be an important driver of lower-crustal seismicity near continental rifts.
大陆裂谷附近下地壳发生地震的现象长期以来一直令人困惑，因为一般认为下地壳温度过高，不会发生脆性破裂。这类异常事件通常被解释为下地壳比正常情况更冷。但如果下地壳确实冷到足以产生地震，那么其下方的上地幔顶部也应该足够冷，然而却未观测到上地幔顶部发生地震。在新西兰一个活跃的大陆裂谷——陶波火山带（TVZ）的西南端附近，发生了大量下地壳地震。在此，我们利用密集地震仪部署所获得的数据，展示了该区域地震波速度和地震衰减的三维层析成像。我们发现，利用我们的三维地震波速度模型精确重新定位的地壳地震，沿着裂谷形成了一个连续带，从陶波火山带中部大多小于10千米的深度，向火山带末端西南方向下地壳30 - 40千米的深度延伸。这些地震常常成群发生，表明在临界加载的断层带中有流体运动。该带内的地震波速度也与流体的存在相一致，而且地震活动的加深与陶波火山带中部高地震衰减（解释为部分熔融）和西南部地壳低地震衰减之间的边界平行。上地壳和下地壳地震活动与地壳结构的这种联系，使我们能够对所有地震活动提出一个共同的解释，即包括从下方熔体中析出的那些热流体，使原本干燥的镁铁质地壳周边的断层弱化。这类流体通常可能是大陆裂谷附近下地壳地震活动的一个重要驱动因素。
