PEAK GROUND ACCELERATION RESPONSE OF THREE MODERATE MAGNITUDE EARTHQUAKES AND THEIR IMPLICATION TO LOCAL SITE EFFECTS IN THE PUERTO RICO ISLAND

Strong-motion accelerograms were processed to estimate the Peak ground accelerations (PGA’s) in order to study the PGA’s distribution due to three earthquakes of magnitudes ranging from 5.3 to 5.8, occurred in 2010 and 2011, in La Mona Passage, and in the northwest and northeast region of Puerto Rico Island. An analysis 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.

The standard procedure for processing strong-motion accelerograms which includes volume- I, II, and III (V-I, V-II, and V-III) was adopted. V-I performs elementary corrections for time and fixed base line, and to scale the raw acceleration data to seconds and the acceleration in g’s (or cm/s/s). V-II performs instrument and base line corrections and calculates ground velocity and displacement. V-III calculate true velocity spectra, Fourier amplitude and pseudo-velocity spectra.

PGA’s were estimated using the procedure above described, and instrumental intensity (MMI) were estimated also with Wald et al., (1999) relationship.

These earthquakes were widely felt in Puerto Rico, the eastern Dominican Republic, and The Virgin Islands. The 5.8 magnitude earthquake (largest of the three), according to the USGS Centroid Moment Tensor solution, occurred at focal depth of 113 Km in an inclined seismic zone that dips south from the Puerto Rico Trench and that consists of sub-ducted lithosphere of the North America plate. Earthquakes with focal-depths in the range of 70- to 300-km, are commonly termed "intermediate-depth" earthquakes. They 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.