Strain Partitioning in Northwest Venezuela: Potential for a Great Quake

委内瑞拉安第斯山脉的5公里高度是委内瑞拉西北部和向东移动的加勒比板块之间斜辐合的结果。地球上的斜向辐合主要表现为走滑断层上的剪切断裂和附近倾滑断层上的逆冲断裂。这种剪切运动和逆冲运动的分割在委内瑞拉西北部很明显。如果南美洲和加勒比板块之间19.6 mm/年的相对运动全部被安第斯山脉的逆冲断层和走滑断层吸收，则安第斯山脉两侧的逆冲断层将以11 mm/年的速度吸收辐合，而安第斯山脉的博肯断层系统将以15 mm/年的速度向右滑动。然而，横跨安第斯山脉的GPS测量显示，这里的右切变不超过12毫米/年，辐合速度最多为5毫米/年。因此，安第斯山脉和西北海岸之间每年约有3毫米的切变和5毫米的辐合仍未得到解释。我们认为，大部分缺失的辐合被委内瑞拉西北部和哥伦比亚东北部海岸附近相对缓慢的俯冲所吸收。地震反射剖面和微震地震活动性表明，加勒比海板块向东经10-26东南倾入地幔。现有的稀疏GPS数据表明，断层蠕变不是该俯冲带的重要机制。因此，大片（200公里× 100公里）被锁定，并可能在未来的大地震中破裂。被锁定的区域是不确定的，目前提议的一个主要目标是通过使用GPS方法量化安第斯山脉和北部海岸之间的地表应变场来估计其下降程度。研究人员认为，未来地震的震级可能超过8.5 Mw。其滑动的综合模型表明，这次潜在地震引发的海啸将导致哥伦比亚北部海岸和委内瑞拉西部海岸以及中美洲、牙买加和海地的南岸海岸上升1-10米。其影响的严重程度取决于下倾锁定破裂与海岸的接近程度，以及自上次地震以来储存的潜在滑动，我们知道这至少是500年，可能更长。这项工作的测量旨在确认和完善上述右旋和收敛速度，安第斯山脉的右旋和收敛速度高于全新世断层所显示的速度，并量化委内瑞拉俯冲带发生的大地震对加勒比社区造成的地震危害。研究人员认为，委内瑞拉西北部可能偶尔会发生大地震。在过去的500年里没有发生过一次大地震，这要么意味着这些推断出来的大地震每隔很长一段时间就会发生一次，要么可能是目前导致我们得出这个结论的稀疏测量结果可能有另一种解释。令人担忧的是，委内瑞拉西北部的一场大地震将伴随着一场大海啸，这将影响委内瑞拉和哥伦比亚的北部海岸以及加勒比海其他几个岛国。这里的大地震的影响实际上是全球性的：每年有200万游客访问牙买加，150万游客访问阿鲁巴，这两个岛屿都在这次破坏性海啸的直接路径上。在其他十几个将经历海啸的岛屿的海滩上，也可以发现类似密度的游客。马拉开波湾大地震和海啸导致的石油供应中断将对全球经济造成不利影响。两名研究生将参与这项研究：一名在委内瑞拉，另一名在科罗拉多州的博尔德。我们提出的测量是由于委内瑞拉、哥伦比亚、科罗拉多大学博尔德分校和联合国维和办事处的科学家之间的紧密联系而成为可能的，他们正在整个加勒比地区安装和维护一个新的GPS网络（COCONET）。

The 5 km elevations of the Venezuelan Andes are the result of oblique convergence between NW Venezuela and the eastward moving Caribbean plate. Oblique convergence in the Earth is typically manifest as shear faulting on strike-slip faults and by thrust faulting on nearby dip-slip faults. Such partitioning of shear and thrust motions is evident in NW Venezuela. Were all 19.6 mm/yr of relative motion between S. America and the Caribbean plate absorbed by the thrust and strike-slip faults of the Andes, the thrust faults flanking the Andes would absorb convergence at 11 mm/yr, and the Andean Bocon fault system would slip in a dextral sense at a rate of 15 mm/yr. GPS measurements across the Andes, however, reveal that dextral shear here does not exceed 12 mm/yr, with convergence of at most 5 mm/yr. Thus approximately 3 mm/year of shear and 5 mm/year of convergence remain unaccounted for between the Andes and NW coast. We contend that that much of the missing convergence is absorbed by relatively slow subduction near the coasts of NW Venezuela and NE Colombia. Seismic reflection profiling and microearthquake seismicity reveal that the Caribbean plate dips 10-26 SE into the mantle. Existing sparse GPS data suggest fault creep is not an important mechanism on this subduction zone. As a result a large patch (200 km x100 km) is locked and might rupture in a future large earthquake. The area that is locked is uncertain, and a primary objective of the current proposal is to estimate its downdip extent by quantifying the surface strain field between the Andes and the northern coast using GPS methods. The researchers believe that the magnitude of a future earthquake could exceed Mw=8.5. Synthetic models of its slip indicate that a tsunami generated by this potential earthquake would result in 1-10 m of coastal run-up on the northern shores of Colombia and western Venezuela, and on the south-facing shores of central America, Jamaica and Haiti. The severity of its effects depend on the proximity of the downdip locked rupture to the coast, and to the potential slip stored since the last earthquake, which we know to be at least 500 years, and possibly much longer. The work's measurements are designed to confirm and refine the dextral and convergence rates discussed above, which in the Andes are higher than indicated by Holocene faulting, and to quantify the seismic hazard posed to the Caribbean community by a great earthquake in the Venezuelan subduction zone.The researchers believe that a great earthquake may occasionally occur in NW Venezuela. That none have occurred in the past 500 years means either that these inferred great earthquakes recur at long intervals of time, or that possibly the sparse measurements that currently lead us to this conclusion, may have an alternative explanation. Of concern is that a great earthquake in NW Venezuela would be accompanied by a large tsunami, that would affect the noerthern coast of Venezuela and Colombia and several other island nations in the Caribbean. The effects of a great earthquake here are in fact global: two million tourists visit Jamaica and 1.5 million visit Aruba each year, both islands in the direct path of this damaging tsunami. Similar densities of tourists can be found on the beaches of a dozen other islands that will experience a tsunami surge. The interruption to oil supplies from the Gulf of Maracaibo following a great earthquake and tsunami will adversely subdue the global economy. Two graduate students will be associated with the research: one in Venezuela and the other in Boulder, Colorado. The measurements we propose are made possible by strong ties between scientists in Venezuela, in Colombia, CU Boulder and in UNAVCO, who are installing and maintaining a new GPS network (COCONET) throughout the Caribbean.