THERMAL CONSTRAINTS ON THE ZONE OF MAJOR THRUST EARTHQUAKE FAILURE - THE CASCADIA SUBDUCTION ZONE
THERMAL CONSTRAINTS ON THE ZONE OF MAJOR THRUST EARTHQUAKE FAILURE - THE CASCADIA SUBDUCTION ZONE
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DOI：
10.1029/92jb02279
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发表时间：
1993-02-10
影响因子：
3.9
通讯作者：
WANG, K
中科院分区：
文献类型：
--
作者：
HYNDMAN, RD;WANG, K
沿卡斯卡迪亚边缘俯冲冲断带的孕震部分受热环境的制约。粘滑“锁定”区（地震可以形成核）可能受到350摄氏度左右温度的限制，而过渡稳定滑动区（同震位移可以扩展到450摄氏度左右的温度）可能受到限制。粘滑带的向海边界可能与100 ~ 150℃稳定滑粘土的脱水和变形锋区域高孔压的消散有关。在地表热流和详细的结构信息的约束下，特别是在温哥华岛南部，通过对三条剖面的热状态进行数值模拟，估计了推力上的温度。最适合热流数据的模型可以忽略剪切应变加热。喀斯喀迪亚俯冲边缘异常热，这是由于板块年龄非常年轻和进入板块的厚绝缘沉积层的结果；在形变前沿的海洋地壳顶部的温度约为250摄氏度。因此，模拟的粘滑孕震带被限制在大陆斜坡和外大陆架下方的一个狭窄地带，过渡带延伸到内大陆架。与南温哥华岛相比，奥林匹克半岛附近的孕震带更宽，因为逆冲倾角要浅得多，而且进入的板块略老。俄勒冈州的剖面具有中等宽度的带。一个重要的假设是，逆冲拆离位于下沉的海洋地壳的顶部，这一假设仅在温哥华岛南部海域得到充分证明。同样的建模技术表明，更典型的俯冲带，如智利中部，具有更古老的海洋岩石圈，具有热限制的孕震带，这些孕震带要宽得多，通常延伸到海岸以下。简单位错模型预测的水平和垂直地震间变形与邻近沿海地区验潮仪和大地测量观测到的速率相比较，为热结果对卡斯卡迪亚锁带位置的支持提供了依据。普遍的共识是，任何地震“锁止带”必须位于近海，即俯冲逆冲断层深度小于约20公里，并且接触在海洋地壳和沉积楔体之间，而不是在海洋地壳和大陆地壳之间。在不列颠哥伦比亚省西南部、华盛顿州和俄勒冈州，对近海区域的限制为最大震级以及俯冲大逆冲地震的地面运动和地震危险提供了重要的限制。
Constraints on the seismogenic portion of the subduction thrust zone along the Cascadia margin are provided by the thermal regime. The zone of stick-slip ''locked'' behavior where earthquakes can nucleate may be limited downdip by a temperature of about 350-degrees-C, and the transition stable sliding zone into which coseismic displacement can extend by a temperature of approximately 450-degrees-C. The seaward limit of the stick-slip zone may be associated with the dehydration of stable sliding clays at 100 to 150-degrees-C and dissipation of high pore pressures in the area of the deformation front. Temperatures on the thrust have been estimated by numerically modelling the thermal regimes along three profiles crossing the margin with constraints provided by surface heat flow and detailed structural information, particularly at southern Vancouver Island. The models that best fit the heat flow data have negligible shear strain heating. The Cascadia subduction margin is unusually hot as a consequence of the very young plate age and the thick insulating sediment section on the incoming plate; the temperature at the top of the oceanic crust at the deformation front is about 250-degrees-C. As a result, the modelled zone of stick-slip seismogenic behaviour is restricted to a narrow zone beneath the continental slope and outer shelf, with the transition zone extending to the inner shelf. The seismogenic zone is wider off the Olympic Peninsula compared to off southern Vancouver Island because of the much shallower thrust dip angle and the slightly older incoming plate. The profile off Oregon is found to have intermediate width zones. An important assumption, well justified only off southern Vancouver Island, is that the thrust detachment is located at the top of the downgoing oceanic crust. The same modelling technique shows that more typical subduction zones with older incoming oceanic lithosphere such as central Chile have thermally restricted seismogenic zones that are much wider, commonly extending well beneath the coast. Support for the position of the Cascadia locked zone from the thermal results is provided by a comparison of the horizontal and vertical interseismic deformation predicted by simple dislocation models with the observed rates from tide gauge and geodetic surveys on adjacent coastal regions. The general agreement indicates that any seismic ''locked zone'' must be located offshore where the subduction thrust fault is less than about 20 km deep and where the contact is between the oceanic crust and the accreted sedimentary wedge, not between the oceanic and continental crusts. The restriction to an offshore zone provides an important limit to the maximum magnitude and to the ground motion and seismic hazard from subduction megathrust earthquakes in southwestern British Columbia, Washington, and Oregon.