SITE SURVEY OF MAY 28, 2009 HONDURAN EARTHQUAKE

On May 28, 2009, at 2:24 AM local time, a M 7.3 earthquake struck off the coast of Honduras along the Motagua-Swan Fault System (MSFS), part of the boundary between the North America and Caribbean plates. The earthquake ruptured an average 1.5 m along 100 km of the Swan Fault at an approximate depth of 10 km as the result of left-lateral slip. This plate boundary has a historical average slip rate of 20 mm/year. The main shock and several aftershocks, all over M 4.4, caused 130 structures, including houses and bridges, to collapse or suffer significant damage in northern Honduras, which in turn caused at least seven deaths. The USGS provided a shake map predicting shaking intensity, but due to a lack of seismometers in the area, the best documentation for the pattern of damage was collected during a field investigation of structural damage conducted between May 30 and June 6, 2009. The investigative itinerary focused on areas with local reports of damage, including the cities of La Ceiba, El Progresso, San Pedro Sula, Puerto Cortes in northern Honduras and the island of Roatan in the Caribbean Sea, approximately 70 km north of Honduras. The results from the field study show the severity of damage did not decrease with distance from the epicenter as predicted by the shake map, but instead a focus of damage was observed in El Progresso, approximately 75 km directly south from the fault plane and 161 km from the epicenter. In fact, Roatan, just 30 km from the epicenter, registered as VI on the Modern Mercalli Intensity Scale while, El Progresso, registered as VIII. This anomaly can be explained by two factors: (1) a localized 3.0m slip pulse that occurred on the fault approximately 60 km west of the epicenter causing seismic waves to be directed toward the mainland and (2) unlike the hard rock Precambrian schists and gneisses of Roatan; El Progresso is underlain by soft river deposits of sand, organics, and clay. This study demonstrates the impact of seismic directivity and local geologic materials on the observed damage patterns from the May 28, 2009 M 7.3 earthquake.