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The Nicoya Peninsula in Costa Rica is one of the few places on Earth where the seismically active plate interface of a subduction zone is directly overlaid by land rather than ocean. At this plate interface, large megathrust earthquakes with magnitudes greater than 7 occur approximately every 50 years. Such quakes occurred in 1853, 1900 and 1950, so another large earthquake had been anticipated 1,2 . Land-based Global Positioning System 3,4 (GPS) and seismic 5-7 measurements revealed a region where the plate interface was locked and hence accumulated seismic strain that could be released in future earthquakes. On 5 September 2012, the longanticipated Nicoya earthquake occurred in the heart of the previously identified locked patch. Here we report observations of coseismic deformation from GPS and geomorphic data along the Nicoya Peninsula and show that the magnitude 7.6 Nicoya earthquake ruptured the lateral and down-dip extent of the previously locked region of the plate interface. We also identify a previously locked part of the plate interface, located immediately offshore, that may not have slipped during the 2012 earthquake, where monitoring should continue. By pairing observations of the spatial extent of interseismic locking and subsequent coseismic rupture, we demonstrate the use of detailed near-field geodetic investigations during the late interseismic period for identifying future earthquake potential.The interface between convergent plates produces most of the world's largest earthquakes, threatening local inhabitants and global populations through destructive shaking and tsunami generation, as demonstrated by the recent 2011 M w 9.0 Tohoku-Oki and 2004 M w 9.15 Sumatra-Andaman earthquakes and tsunami. Owing to the significant societal impacts, geoscientists endeavour to understand the driving and locking mechanisms controlling subduction zone seismicity. The shallow earthquakegenerating portion of the subduction interface, hereafter referred to as the megathrust, is difficult to characterize because it is relatively inaccessible, spans great lengths of continental margins and requires detailed near-field observations primarily in the marine environment.

Significance Recent destructive megathrust earthquakes and tsunamis in Japan and Sumatra indicate the difficulty of forecasting these events. Geodetic monitoring of the offshore regions of the subduction zones where these events occur has been suggested as a useful tool, but its potential has never been conclusively demonstrated. Here we show that slow slip events, nondestructive events that release energy slowly over weeks or months, are important mechanisms for releasing seismic strain in subduction zones. Better monitoring of these events, especially those offshore, could allow estimates of the size of future earthquakes and their potential for damaging tsunamis. However, the predictive value of slow slip events remains unclear.

[1] GPS observations in south Iceland between 1994 and 2003 are compared with twodimensional elastic half-space and viscoelastic coupling models for two parallel rift zones, representing the Western and Eastern volcanic zones (WVZ, EVZ). GPS data from the Hreppar block, between the WVZ and EVZ, fit a rigid block model within uncertainties. Spreading rates across the WVZ increase from 2.6 ± 0.9 mm/yr in the northeast to 7.0 ± 0.4 mm/yr in the southwest. Conversely, spreading rates in the EVZ decrease from 19.0 ± 2.0 mm/yr in the northeast to 11.0 ± 0.8 mm/yr in the southwest, the direction of ridge propagation. Summed extension rates across the two rift zones are approximately constant and equal to the total plate rate, $18-20 mm/yr, consistent with a simple propagating ridge model whereby the WVZ is deactivating in the direction of EVZ propagation. The coupling model confirms results from the simple elastic half-space model, including relatively shallow locking depths (<5 km) beneath the rift zones, and allows for an estimate of mean viscosity ($10 19