Interseismic megathrust coupling beneath the Nicoya Peninsula, Costa Rica, from the joint inversion of InSAR and GPS data

The Nicoya Peninsula, Costa Rica, was struck by a long-anticipated and gap-filling Mw 7.6 earthquake in 2012. To study interseismic strain accumulation on the megathrust beneath the Nicoya Peninsula, we present an improved interseismic coupling model by integrating interferometric synthetic aperture radar (InSAR) and GPS data. Our model reveals three strongly coupled patches. The first strongly coupled patch locates beneath the Nicoya Peninsula and ruptured during the 2012 earthquake. The second strongly coupled patch locates offshore the central Nicoya Peninsula and remained largely unbroken. However, this region is close to and possibly intermingled with shallow slow slip and tremor, suggesting that accumulated strain in this region may be released both seismically and aseismically. The third strongly coupled patch offshore of the southeastern end of Nicoya overlaps part of the coseismic rupture of the 1990 Mw 7.0 Nicoya Gulf earthquake, indicating that significant strain has re-accumulated since this event. Incorporating InSAR data provides a more refined interseismic coupling model than using GPS alone and allows for a more reliable comparison with local seismic and aseismic activities. This comparison indicates that strongly locked regions during the interseismic stage are the loci of coseismic slip, and deep slow slip and low-frequency earthquakes occur in regions of low coupling or transition zones from low to high coupling, while shallow slow slip and tremor commingle with strongly coupled regions. Our study demonstrates that InSAR data can be used to recover small long-wavelength deformation signals with refined resolution in challenging subduction zone environments when integrated with GPS observation