| 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 had an average of 1.5 m of left-lateral slip along 100 km of rupture on the Swan Fault. The hypocenter was at a depth of 10 km. 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. Seven deaths were reported. The USGS released a “Did you feel it?” map showing the results of an online survey polling local’s experiences during the earthquake. Due to a lack of seismometers in the area, the best documentation for the pattern of intensity and damage are the "Did you feel it?" responses and the data collected during our field investigation. The investigation, conducted between May 30 and June 6, 2009, 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 30 km from the epicenter. The results from the field study show the severity of damage did not decrease with distance from the epicenter as predicted by distance-attenuation relations, but instead a locus of damage was observed in El Progresso, approximately 75 km directly south from the SW end of the rupture and 160 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. These intensity anomalies can be explained by two factors: (1) a localized 3.0m slip pulse that occured near the SW end of the fault that directed energy toward the mainland and; (2) local site effect, particularly the presence of hard Precambrian schists and gneisses on the island of Roatan, in contrast to soft river deposits of sand, organics, and clay beneath the city of El Progresso. This study demonstrates the impact of seismic directivity and local geologic site conditions on the observed damage patterns from the May 28, 2009 M 7.3 earthquake. |