The Puerto Rico 5.8 MW Earthquake of May 16, 2010, and the Distribution of Peak Ground Motion in the Puerto Rico Island

On may 16, 2010 a 5.8 MW earthquake occurred in the northwest region of Puerto Rico Island (18.4o Lat., -67.07o Lon., and focal depth 113.1 km) at 05:16:10 UTC, reporting an intensity of VI in the city of Añasco, which is located at approximately at 15 km from its epicenter, while in some cases the intensity hardly reach an intensity of III in towns located at closer epicentral distances. This earthquake was widely felt in Puerto Rico, the eastern Dominican Republic, and The Virgin Islands. It was recorded also by 59 stations of the Puerto Rico Strong Motion Program (PRSMP) providing a reliable set of acceleration records distributed around the island. According to the USGS Centroid Moment Tensor solution, this earthquake occurred in an inclined seismic zone that dips south from the Puerto Rico Trench and that consists of subducted lithosphere of the North America plate. Earthquakes that have focal-depths between 70 and 300 km, are commonly termed "intermediate-depth" earthquakes and typically cause less damage on the ground surface above their foci than is the case with similar magnitude shallow-focus earthquakes. Large intermediate-depth earthquakes may be felt at great distance from their epicenters. In terms of the observed maximum peak ground acceleration it was: (i) 0.0651 of g at Añasco (AÑS1 station, located at an epicentral distance of 15 km in alluvial soil), and (ii) 0.2301 of g at station UTD2 located at an epicentral distance of 42 km on rock. The instrumental intensity (MMI) estimated with Wald et al., (1999) relationship was V and VII, respectively for these two previously described sites. An analysis and discussion is presented with the aim to explain the anomalous distribution of peak ground motions, which may be associated not only by local site effects due to the presence of soft soils.