[1] To understand controlling factors for seismogenesis and strain-accumulation in subduction megathrusts we examine seismicity patterns across Nicoya, Costa Rica, to determine the overall and spatial variability in the earthquake frequency-magnitude distribution along the interface. The mean reduction in earthquake activity with magnitude, b-value, is higher (1.06) than global subduction zone averages (0.6 -0.8), suggesting the interface here is weakly coupled. Strong spatial variations in b are anticorrelated (À0.53) with geodetic estimates of interface locking of E. Norabuena et al. (2004). High b prevails in two regions, including the subducted Fisher seamount chain. A broad zone of reduced b is observed at and offshore the central Nicoya coast; extending towards an imaged locked patch. These results suggest b-value studies may be useful in identifying regions of increased interface locking in subduction zones, which may indicate regions capable of large slip in future large earthquakes.

We provide a detailed, seismically defined three-dimensional model for the subducting plate interface along the Middle America Trench between northern Nicaragua and southern Costa Rica. The model uses data from a weighted catalog of about 30,000 earthquake hypocenters compiled from nine catalogs to constrain the interface through a process we term the "maximum seismicity method." The method determines the average position of the largest cluster of microseismicity beneath an a priori functional surface above the interface. This technique is applied to all seismicity above 40 km depth, the approximate intersection of the hanging wall Mohorovičić discontinuity, where seismicity likely lies along the plate interface. Below this depth, an envelope above 90% of seismicity approximates the slab surface. Because of station proximity to the interface, this model provides highest precision along the interface beneath the Nicoya Peninsula of Costa Rica, an area where marked geometric changes coincide with crustal transitions and topography observed seaward of the trench. The new interface is useful for a number of geophysical studies that aim to understand subduction zone earthquake behavior and geodynamic and tectonic development of convergent plate boundaries.

moment magnitude M W 6.4 crustal earthquake in northwestern Peloponnese, Greece, was a strong-shaking dextral strike-slip event with teleseismic broad-band and highfrequency energy magnitudes M e of 6.8 and 7.2, respectively. A high stress drop 5-10 times the global average is associated with excessive high-frequency energy. The NE-SW trending fault plane shown by the aftershock distribution and focal mechanism is not associated with previously mapped faults, and no obvious coseismic surface rupture was discovered. Contrasting the enhanced rupture energy, the event created no substantial coseismic or post-seismic surface deformation, likely due to a fault buried below a detached thick and compositionally weak flysch layer. Comparative spatial analysis including over 30 regional strike-slip events between 1965 and 2009 reveals a NE-SW striking diffuse transform fault zone subparallel to the Cephalonia Transform Fault. The dextral sense of motion along the transform zone is consistent with the ongoing Global Positioning System (GPS)-derived deformation along the West Hellenic Arc and the motion on the Cephalonia Transform Fault. Characterizing this system is important to constraining the seismic hazard near Patras, a major port city immediately NE of the 2008 event.

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