Strain Partitioning in Northwest Venezuela: Potential for a Great Quake

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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.