T90 Megathrusts: 50 Years after the 1964 Great Alaska Earthquake—In Honor of George Plafker

Monday, 20 October 2014
8:00 AM - 12:00 PM
116/117

COMPLETING THE CYCLE?  NEAR-FIELD OBSERVATIONS OF INTERSEISMIC COUPLING, MEGATHRUST RUPTURE, POSTSEISMIC RESPONSE, AND RECOVERY OF INTERFACE COUPLING ALONG THE NICOYA PENINSULA OF COSTA RICA

Paper number: T90-13
Program: Technical Sessions
Time: 10:45 AM – 11:00 AM

Authors

Abstract

After nearly 2 decades of geophysical studies along the Nicoya Peninsula of Costa Rica, we have successfully imaged the full spatial extent of the seismogenic zone responsible for a major moment magnitude 7.6 earthquake rupture there in 2012. Unlike most subduction megathrust environs that exist mostly or entirely offshore, the Nicoya Peninsula’s location allows for unique land-based observations of the full seismogenic coupling and release zones. At the time of the earthquake the Nicoya Seismic Cycle Observatory (NSCO) was instrumented with approximately 20 seismometers and 17 continuous GPS maintained by UCSC, GT, USF, and OVSICORI. Another 24 campaign GPS sites were routinely occupied in the region with the last preseismic study occurring in 2010, before reoccupation beginning 2 days after the 2012 earthquake. Additionally, following the earthquake we measured geomorphic coseismic coastal changes associated with the event.

In summer 2012 Feng et al. [2012] published the most comprehensive map of the locked region, showing that a broad zone immediately downdip of the linear microseismic updip limit [Newman et al., 2002] extended across the central Nicoya Peninsula and downward toward the Moho intersection. Immediately updip of the microseismic lineation was a smaller locked patch beneath a segment of northern Nicoya, where a transition in subducting crustal type exists. The deeper patch ruptured entirely in the 2012 earthquake [Protti et al., 2012], and the shallow patch appears to have slipped through postseismic afterslip and large aftershocks. These previously locked patches differ in downgoing plate type, regional heatflow, interface structure, and downdip position relative to microseismicity. Do any or all of these matter in differentiating coseismic vs. postseismic rupture?

In the coming decade the Nicoya megathrust should return to typical interseismic behavior. When it does, questions remain. Will it match the model for the late interseismic locking observed before the 2012 event? If so, why? If it does, do strain residuals that remain after the removal of coseismic and postseismic motions constrain permanent deformation? And, how do these residuals correlate with sliver transport currently observed at 11 mm/yr, or uplifted coastal terraces, geologically observed to uplift between 0.1 and 2 m/ky?