A FAULT SYSTEM DEFINES THE NORTHEASTERN MARGIN OF THE WESTERN COSTA RICA ARC SLIVER

It has long been known that western Costa Rica near the Nicoya Peninsula is moving toward the northwest, parallel to the trench, as a forearc sliver. Little attention has been paid to the geometry, kinematics, or dynamics of the trailing edge of this sliver. We focus on the first two of these and show through well-located earthquakes, newly generated focal mechanisms, published GPS velocities and field observations, that the northeastern margin of this sliver is a right-later fault system that transects the Guanacaste arc. The location and kinematics of the fault system imply that trench-parallel motion is not limited to the forearc of the upper plate, but rather includes a substantial portion of the volcanic arc. In particular we identify the Chiripa fault south of Lake Arenal and within the active volcanic arc. This fault links to the northwest with the Cote-Arenal fault and then the Caño Negro fault and finally to the Hacienda fault very near the southern shoreline of Lake Nicaragua. The Chiripa – Cote-Arenal section is expressed as prominent lineaments in air photos and digital elevation models (DEMs), and locally as alluviated pull-apart basins hosting deflected drainage networks. Relocated earthquakes here, and new focal mechanisms, reveal a through-going strike-slip fault zone that is coincident with a profound change in trench-parallel velocity as constrained by GPS data. The Caño Negro - Hacienda section is also expressed as a prominent lineament with local pull-apart basins, however in addition, relatively fresh ignimbrite units record brittle strain gradients and fault truncations as well as fault exposures. Our results set the stage for expanded geodetic studies, focused tectonic geomorphologic work and paleoseismologic investigations. The additional work will be essential to understanding the relative contributions of Cocos Ridge collision versus coupling between the North American plate and the Caribbean Plate in driving the arc sliver. Our findings have immediate importance in the context of seismic hazards as well as mass-wasting hazards that could be triggered by shaking. This fact was brought into sharp focus with two damaging moderate-magnitude crustal earthquakes on 2 July 2016.