Crustal deformation associated with glacial fluctuations in the eastern Chugach Mountains, Alaska
Crustal deformation associated with glacial fluctuations in the eastern Chugach Mountains, Alaska
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DOI：
10.1029/1999jb900433
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发表时间：
2000-04
影响因子：
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通讯作者：
J. Sauber;G. Plafker;B. Molnia;M. A. Bryant
中科院分区：
文献类型：
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作者：
J. Sauber;G. Plafker;B. Molnia;M. A. Bryant
我们估计了阿拉斯加中南部固体地球在不同时间和空间尺度上对两次主要冰川波动的响应变化，并评估了它们对该地区应力状态和持续构造变形的影响。在最近(1993-1995年)白令冰川激增期间，发生了一次从巴格利冰场到白令冰川末端地区的大规模冰转移。我们从全球定位系统(GPS)的测量中估计了在冰川涌动附近的地点由于冰块重新分配而造成的固体地球的弹性位移。我们可以通过将大约14千米的冰体积从涌浪库区转移到终点区来解释这些位移。我们检查了涌浪发生前、期间和之后的背景地震活动(ML≥2.5)。我们发现，在涌浪期间，水库地区发生的小震(ML≤4.0)增加，这可能是对冰量减少的反应。这表明，在这个以逆冲为主的断层环境中，垂直应力σ3的小幅下降可能足以调节小规模浅层地震的发生。在本世纪，阿拉斯加南部沿海冰川的体积一直在减少。基于我们对马拉斯皮纳和白令之间沿海冰川范围和厚度变化的汇编，我们计算了由于地球对年度变薄的粘弹性反应和过去100年的累积退缩造成的地表位移。消融区年平均减薄速率为1~6m/a，区域抬升为1~12 mm/a。对于我们的参考模型，粘性为5×10~(19)帕的S，在≈40-200公里之间，由于上个世纪的退缩，在冰湾附近的海岸消融带内，总的粘弹性响应可能高达几米。在0~10千米范围内，σv的最大降幅为≈1.0 Mpa，这与近期地震(≈2~10 Mpa)的应力下降有关，但与估计的构造应力大小关系不大。因此，像圣埃利亚斯(1979，MS=7.2)这样的地震的发生可能已经提前了；然而，大部分持续的应力积累将主要是由于构造力。
The changes of the solid Earth in south central Alaska in response to two major glacial fluctuations on different temporal and spatial scales have been estimated and we evaluated their influence on the stress state and ongoing tectonic deformation of the region. During the recent (1993–1995) Bering Glacier surge, a large transfer of ice from the Bagley Ice Field to the Bering Glacier terminus region occurred. We estimated the elastic displacement of the solid Earth due to ice mass redistribution from Global Positioning System (GPS) measurements at sites near the surging glacier. We can account for these displacements by transfer of an ice volume of about 14 km3 from the surge reservoir area to the terminus region. We examined the background seismicity (ML ≥ 2.5) before, during, and after the surge. We found that the occurrence of small earthquakes (ML ≤ 4.0) in the surge reservoir region increased during the surge time interval possibly in response to a decrease in ice mass. This suggests that a small decrease in the vertical stress, σ3, could be enough to modulate the occurrence of small, shallow earthquakes in this dominantly thrust fault setting. During this century the southern Alaska coastal glaciers have been undergoing an overall decrease in volume. Based on our compilation of changes in the extent and thickness of the coastal glaciers between the Malaspina and Bering, we calculated surface displacements due to the Earth's viscoelastic response to annual thinning and to the cumulative retreat over the last 100 years. The uplift of the region due to an average annual thinning rate of 1–6 m/yr in the ablation region is 1–12 mm/yr. For our reference model with a viscosity of 5×1019 Pa s for depths between ≈ 40 and 200 km the total viscoelastic response due to the retreat over the last century may be as much as a couple of meters within the coastal ablation zone near Icy Bay. The maximum decrease in σv between 0 and 10 km was ≈ 1.0 MPa, which is significant in relation to the stress drops in recent earthquakes (≈ 2 to 10 MPa) but small in relation to the estimated tectonic stress magnitude. Therefore the occurrence of an earthquake such as the St. Elias (1979, MS = 7.2) may have been advanced in time; however, most of the ongoing stress accumulation would be primarily due to tectonic forces.