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海板块斜向会聚的结果。地球上的斜向会聚通常表现为走滑断层上的剪切断层和附近倾滑断层上的逆冲断层。这种剪切和逆冲运动的分离在委内瑞拉西北部很明显。如果南美洲和加勒比海板块之间的相对运动都被安第斯山脉的逆冲和走滑断裂吸收了19.6 mm/年，安第斯山脉两侧的逆冲断层将吸收11 mm/年的会聚，安第斯博肯断裂系统将以15 mm/年的速度右滑。然而，横跨安第斯山脉的GPS测量显示，这里的右旋切变不超过12毫米/年，会聚最多为5毫米/年。因此，安第斯山脉和西北海岸之间约3毫米/年的切变和5毫米/年的会聚仍然下落不明。我们认为，许多缺失的会聚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活动表明，加勒比海板块向南东倾斜10-26度进入地幔。现有的稀疏GPS数据表明，断层蠕动不是该俯冲带上的重要机制。结果，一个大片(200千米x100千米)被锁定，并可能在未来的大地震中破裂。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表面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8.5兆瓦。其滑动的综合模型表明，这场潜在地震引发的海啸将在哥伦比亚北部海岸和委内瑞拉西部以及中美洲、牙买加和海地的南向海岸造成1-10米的海岸抬高。其影响的严重程度取决于向下锁定的破裂距离海岸的距离，以及自上次地震以来储存的潜在滑动，我们知道这至少有500年，甚至可能更长。这项工作的测量旨在确认和完善上面讨论的右旋和会聚速率，这些速率在安第斯山脉高于全新世断层所指示的速率，并量化委内瑞拉俯冲带的大地震对加勒比海社区构成的地震危险。研究人员认为，委内瑞拉西北部可能偶尔会发生大地震。过去500年中没有发生过一次地震，这意味着要么这些推测的大地震会在很长一段时间内重复发生，要么可能是目前导致我们得出这一结论的稀疏测量结果可能有另一种解释。令人担忧的是，委内瑞拉西北部的大地震将伴随着大海啸，这将影响委内瑞拉北部海岸和哥伦比亚以及加勒比海其他几个岛国。这里一场大地震的影响实际上是全球性的：每年有200万游客前往牙买加，150万游客前往阿鲁巴，这两个岛屿都位于这场破坏性海啸的直接路径上。在其他十几个将经历海啸激增的岛屿的海滩上，也可以找到类似密度的游客。在一场大地震和海啸之后，马拉开波湾石油供应中断，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博尔德。我们建议的测量是由于委内瑞拉、哥伦比亚、科罗拉多州立大学博尔德分校和联合国志愿人员组织的科学家之间的密切联系，他们正在整个加勒比地区安装和维护一个新的全球定位系统网络(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.