The rupture zone of Cascadia great earthquakes from current deformation and the thermal regime
The rupture zone of Cascadia great earthquakes from current deformation and the thermal regime
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从当前变形和热状态看卡斯卡迪亚大地震的破裂带
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发表时间：
1995
期刊：
影响因子：
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通讯作者：
Kelin Wang
中科院分区：
文献类型：
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作者：
R. Hyndman;Kelin Wang
从不列颠哥伦比亚省南部到加利福尼亚北部的北美西部沿海城市，地震危险的一个重要但鲜为人知的部分来自卡斯卡迪亚俯冲带的大逆冲地震。虽然在历史记录中没有这样的事件，但有充分的地质证据表明它们在过去发生过。从(1)当前变形数据的位错模拟得到的锁断带和(2)热状态出发，估计了整个卡斯卡迪亚边缘俯冲逆冲断层上发震带或地震破裂带的下倾陆向界限，认为断层地震行为的下倾界限受温度控制。测地资料包括10条水准线、沿岸6个地点的潮汐计、1条高精度重力线、7个水平应变阵列和一个连续记录的全球定位系统（GPS）网络。目前，大部分沿海地区以每年几毫米的速度抬升，内陆地区抬升减少，沿海地区抬升速度约为0.1克/年（即在100克朗的距离上抬升10毫米/年）。目前的震间隆升与沿海隐伏盐沼及其他古地震活动性资料推断的大地震同震沉降一致。被认为是积累弹性应变的锁定带的模型宽度平均为60krn完全锁定，加上60krn过渡（90km完全锁定，没有过渡，会产生类似的震间变形）。它在华盛顿北部的奥林匹克半岛较宽，在俄勒冈中部到加利福尼亚北部较窄。与许多其他俯冲带相比，这种异常狭窄的俯冲范围是与年轻的海洋板块和进入地壳的厚层绝缘沉积物相关的高温的结果。根据大地测量数据模拟的锁止带的变化与数值热模型估计的断层下倾温度沿边缘的变化相吻合，完全锁止的孕震带的最高温度为350℃，过渡带为450℃。俯冲逆冲断层上的温度以及孕震带的下倾程度取决于5个局部俯冲参数：(1)俯冲板块的年龄，(2)板块收敛速度，(3)入地壳绝缘沉积物的厚度，(4)逆冲断层的倾角剖面，(5)上覆物质的热性质。在卡斯卡迪亚内陆100-200公里的大型城市，震源带向陆地的边界几乎没有延伸到海岸以下，这限制了大俯冲地震的地面运动。狭窄的宽度也限制了地震的大小，但远超过8级的事件仍然有可能发生；最大值取决于沿边缘的长度。如果整个卡斯卡迪亚边缘地震带在一次地震中失效，经验断层面积与震级关系给出的地震强度可达9级。
An important but poorly -known part of the earthquake hazard at near-coastal cities of western North America from southern British Columbia to northern California is from great thrust earthquakes on the Cascadia subduction zone. Although there have been no such events in the historical record, there is good geological evidence that they have occurred in the past. The downdip landward limit of the seismogenic or seismic rupture zone on the subduction thrust fault has been estimated for the whole Cascadia margin from (1) the locked zone from dislocation modeling of current deformation data, and (2) the thermal regime, taking the downdip limit of seismic behavior on the fault to be controlled by temperature. The geodetic data include ten leveling lines, tide gauges at six locations along the coast, one high precision gravity line, seven horizontal strain arrays, and a continuously recording Global Positioning System (GPS) network. There is present uplitfor most of the coast at a rate of a few millimeters per year, decreasing inland, and shortening across the coastal region at about 0.1 gstrain/yr (i.e., 10 mm/yr over a distance of 100 krn). The present interseismic uplift is consistent with the great earthquake coseismic subsidence inferred from buried coastal salt marshes and other paleoseismicity data. The modeled width of the locked zone that is taken to be accumulating elastic strain averages 60 krn fully locked, plus 60 krn transition (90 km fully locked with no transition gives similar interseismic deformation). It is wider off the Olympic Peninsula of northern Washington and narrower off central Oregon to northern California. This unusually narrow downdip extent compared to many other subduction zones is a consequence of high temperatures associated with the young oceanic plate and the thick blanket of insulating sediments on the incoming crust. The variations in the modeled locked zone from geodetic data correspond well to variations along the margin of downdip temperatures on the fault as estimated from nmnerical thermal models, taking the maximran temperature for the fully locked seismogenic zone to be 350oC with a transition zone to 450oC. The temperatures on the subduction thrust fault and thus the downdip extent of the seismogenic zone depend on five local subduction parameters: (1) the age of the subducting plate, (2) the plate convergence rate, (3) the thickness of insulating sediments on the incoming crust, (4) the dip angle profile of fle fault, and (5) the themal properties of the overlying material. The landward limit to the seismogenic zone, extending little if at all beneath the coast, limits the ground motion from great subduction earthquakes at the larger Cascadia cities that lie 100-200 km inland. The narrow width also limits the earthquake size but events of magnitude well over 8 are still possible; the maximum depends on the along-margin length. If the whole Cascadia margin seismogenic zone fails in a single event, empirical fault area versus magnitude relations give earthquakes as large as M w =9.