Structural asperity focusing locking and earthquake slip along the Nicoya megathrust, Costa Rica

Abstract

On 5 September 2012, a moment magnitude 7.6 earthquake occurred along the locked megathrust interface directly beneath the Nicoya Peninsula in Costa Rica. The event was anticipated given the history of frequent large earthquakes and recent GPS results. Strain accumulation observed before and coseismic slip observed during the earthquake create a novel and important data set, offering a unique look at the strain and locking behavior around a large megathrust environment. Given this geodetic data, and a newly developed regional 3‐D subduction interface, we develop a regionally appropriate finite element model to evaluate the role of interface topography in generating interseismic locking and ultimately coseismic slip following the 2012 earthquake. We find that the interface is most strongly coupled in a patch immediately beneath the central portion of Nicoya in an area where a topographic high in the downgoing Cocos Plate is resisting further subduction. This zone is the dominant slip environment in the 2012 earthquake. Reevaluation of the interseismic locking over 62 years before the most recent event finds that the total accumulated moment potential (M0 = 3.48 × 1020 N m) well matches the total moment release in the 2012 rupture found here (M0 = 3.73 × 1020 N m, Mw 7.68), with rupture extending to the SE outside of the strongly locked region, possibly as early afterslip. In contrast to a prior study, little locking is required immediately offshore Nicoya—a conclusion important for understanding the rupture area of the 2012 event, regional tsunami potential, and relation with regionally observed shallow slow‐slip events.

Asthenosphere Flow Modulated by Megathrust Earthquake Cycles

“…This approach has brought much insight into the rheology of plate boundaries at subduction zones (e.g., Hu et al, 2014Hu et al, , 2016Klein et al, 2016;Kyriakopoulos & Newman, 2016;Masterlark, 2003;Wang, 2007), transform boundaries (e.g., Allison & Dunham, 2018;Freed & Bürgmann, 2004;Masterlark & Wang, 2002;Takeuchi & Fialko, 2013), and collision zones (Castaldo et al, 2017;Cattin & Avouac, 2000;C. This approach has brought much insight into the rheology of plate boundaries at subduction zones (e.g., Hu et al, 2014Hu et al, , 2016Klein et al, 2016;Kyriakopoulos & Newman, 2016;Masterlark, 2003;Wang, 2007), transform boundaries (e.g., Allison & Dunham, 2018;Freed & Bürgmann, 2004;Masterlark & Wang, 2002;Takeuchi & Fialko, 2013), and collision zones (Castaldo et al, 2017;Cattin & Avouac, 2000;C.…”

Rough Subducting Seafloor Reduces Interseismic Coupling and Mega‐Earthquake Occurrence: Insights From Analogue Models

“…Global studies have shown that large earthquakes (M W ≥ 7.5) preferably occur along a smooth subduction interface and that rough subducting seafloor is associated with lower seismic coupling and a creep-like behavior (Bassett & Watts, 2015;Lallemand et al, 2018;van Rijsingen et al, 2018;Wang & Bilek, 2014). In some places, geodetic measurements above subducting features show a local increase in interplate coupling (Collot et al, 2017;Kyriakopoulos & Newman, 2016), while in other places subducting topography is thought to cause a local decrease in coupling (Geersen et al, 2015;Marcaillou et al, 2016;Mochizuki et al, 2008;Singh et al, 2011). Following the classification of Lallemand et al (2018), which is based on the seafloor characteristics prior-to-subduction, Izu-Bonin-Mariana is considered almost entirely rough, while Japan-Kuril-Kamchatka is dominantly smooth.…”

Detailed spatiotemporal evolution of microseismicity and repeating earthquakes following the 2012 Mw 7.6 Nicoya earthquake

“…As anticipated, the Nicoya earthquake ruptured a seismic gap beneath the Nicoya Peninsula that locked up following a M 7.7 event in 1950 and exhibiting an apparent 50–60 year recurrence of M w 7.0+ earthquakes along the megathrust [ Protti et al ., ]. After nucleating immediately offshore [ Yue et al ., ], the Nicoya earthquake ruptured primarily under the peninsula with up to 4 m of trenchward thrust in an area that was previously mapped as locked [ Feng et al ., ; Protti et al ., ; Kyriakopoulos and Newman , ]. Local foreshocks were observed 35 min prior to the main shock within 15 km of its hypocenter, which occurred within the same 30 km diameter zone as those remotely triggered by the 27 August 2012 M w 7.3 El Salvador event about 450 km to the northwest [ Walter et al ., ].…”

“…[] derived a new structural model for the subduction zone interface along the Middle America Trench. This plate geometry model was further used to reevaluate the interseismic locking and coseismic slip [ Kyriakopoulos and Newman , ] and image afterslip both updip of the dominant coseismic slip area and downdip to deep slow‐slip events previously identified [ Hobbs et al ., ].…”