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SUMMARY We report results from a year‐long deployment of broad‐band seismometers in Scotland to study the mantle structure in the northern United Kingdom. The region is tectonically inactive and its underlying mantle transition zone should not show unusual structure due to thermal perturbations caused by Cenozoic subduction or plume‐related volcanism. The estimated 410 and 660 km discontinuity depths, estimated by receiver function stacking, are 414 ± 5 and 655 ± 14 km, typical of average worldwide values. There is no evidence for a discontinuity at 520 km. The 660 km discontinuity conversion is weaker than global Earth models predict by factors of 1.65–2.97, which is significant given amplitude measurement uncertainties. Either significant attenuation below 410 km or regional, non‐thermal variations in 660 km discontinuity properties that decrease its velocity and density jumps to 1.9 per cent ΔVp, 4.8 per cent ΔVs and 4.8 per cent Δρ, or that broaden it to 13–35 km width could account for the lower than expected amplitudes. Lateral variations in transition zone water content around the 660 km discontinuity may be responsible for the broadening.

In 2004 and 2005 the Puerto Rico Seismic Network (PRSN), Puerto Rico Strong Motion Program (PRSMP) and the Geology Department at the University of Puerto Rico-Mayagüez (UPRM) collaborated with the U.S. Geological Survey to study near-surface shear-wave (Vs) and compressional-wave (Vp) velocities in and around major urban areas of Puerto Rico. Using noninvasive seismic refraction-reflection profiling techniques, we acquired velocities at 27 locations. Surveyed sites were predominantly selected on the premise that they were generally representative of near-surface materials associated with the primary geologic units located within the urbanized areas of Puerto Rico. Geologic units surveyed included Cretaceous intrusive and volcaniclastic bedrock, Tertiary sedimentary and volcanic units, and Quaternary unconsolidated eolian, fluvial, beach, and lagoon deposits. From the data we developed Vs and Vp depth versus velocity columns, calculated average Vs to 30-m depth (V S 30), and derived NEHRP (National Earthquake Hazards Reduction Program) site classifications for all sites except one where results did not reach 30-m depth. The distribution of estimated NEHRP classes is as follows: three class "E" (V S 30 below180 m/s), nine class "D" (V S 30 between 180 and 360 m/s), ten class "C" (V S 30 between 360 and 760 m/s), and four class "B" (V S 30 greater than 760 m/s). Results are being used to calibrate site response at seismograph stations and in the development of regional and local shakemap models for Puerto Rico.

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The distribution of 268 small earthquakes recorded during a 1-year period in the western part of Puerto Rico indicates that deformation is occurring along well defined surface or near-surface geologic structures. The distribution of hypocenters ranges in depth from near surface to a maximum depth of about 30 km, although a diffuse southdipping seismic zone beneath Puerto Rico extends up to a maximum depth of 155 km.Focal mechanism solutions suggest that the Puerto Rico block itself is deforming in response to active northeastsouthwest directed horizontal compressional stresses with an apparent NNE-SSW rotation to the south.The island of Puerto Rico seems to behave like a small microplate caught in a wide transform margin between the North American and Caribbean plates. The margins involve westerly trending, strike-slip faulting along the Muertos Trough and Puerto Rico Trench.Magnitudes were determined using the signal duration method, and a b value of approximately .74 was determined for earthquakes within the western Puerto Rico region.