Strain Partitioning in Northwest Venezuela: Potential for a Great Quake

委内瑞拉安第斯山脉海拔5公里是委内瑞拉西北部和向东移动的加勒比板块之间斜向会聚的结果。 地球上的斜向会聚通常表现为走滑断层上的剪切断层作用和附近倾滑断层上的逆冲断层作用。 这种剪切和逆冲运动的分离在委内瑞拉西北部很明显。 S.美洲和加勒比板块被安第斯山脉的逆冲断层和走滑断层吸收，安第斯山脉两侧的逆冲断层将以11毫米/年的速度吸收会聚，安第斯山脉Bocon断层系统将以15毫米/年的速度右旋滑动。 然而，对整个安第斯山脉的GPS测量显示，这里的右旋剪切不超过12毫米/年，收敛最多为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/年的辐合仍未得到解释。 我们认为，大部分缺失的收敛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。 地震反射剖面和微地震活动表明，加勒比板块倾向于10-26东南向地幔。 现有的稀疏GPS数据表明，断层蠕动不是这个俯冲带的重要机制。因此，一个大的补丁（200公里x100公里）被锁定，并可能在未来的大地震破裂。 锁定的区域是不确定的，目前建议的主要目标是通过使用全球定位系统方法量化安第斯山脉和北方海岸之间的表面应变场来估计其下倾程度。 研究人员认为，未来地震的震级可能超过Mw=8.5。 其滑动的合成模型表明，由这一潜在地震产生的海啸将导致哥伦比亚和委内瑞拉西部的北方海岸以及中美洲、牙买加和海地的南海岸的海岸上升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述讨论的右旋和会聚速率，在安第斯山脉的右旋和会聚速率高于全新世断层的指示，并量化委内瑞拉俯冲带大地震对加勒比共同体造成的地震危险。研究人员认为，委内瑞拉西北部偶尔可能发生大地震。 在过去的500年里没有发生过任何地震，这意味着这些推断的大地震在很长一段时间内重复发生，或者目前导致我们得出这一结论的稀疏测量可能有另一种解释。 令人关切的是，委内瑞拉西北部的大地震将伴随着大海啸，这将影响委内瑞拉和哥伦比亚的东北海岸以及加勒比其他几个岛屿国家。 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，这两个岛屿都在这场破坏性海啸的直接路径上。 类似的游客密度可以在其他十几个岛屿的海滩上找到，这些岛屿将经历海啸浪潮。 大地震和海啸后，来自马拉开波湾的石油供应中断，将对全球经济产生不利影响。 两名研究生将参与这项研究：一名在委内瑞拉，另一名在科罗拉多的博尔德。委内瑞拉、哥伦比亚、CU Boulder和UNAVCO的科学家之间的密切联系使我们提议的测量成为可能，这些科学家正在整个加勒比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.