Fore-Arc Sliver Transport Mechanism in Nicaragua Revealed by Upper-Plate Earthquakes

Abstract:

Oblique convergence and strong mechanical coupling along subduction zones result in strain partitioning and the development of translating forearc terranes. Translation of the fore-arc relative to the over-riding plate is typically accommodated by strike-slip fault systems; for example, the Great Sumatran Fault, Indonesia. The Central American Fore-Arc (CAFA) is a northwestward translating (8 mm/yr to 13 mm/yr) fore-arc sliver, the result of oblique Cocos - Caribbean convergence, and Cocos Ridge collision. However, the CAFA in Nicaragua does not have the expected trench-parallel, strike-slip fault system to accommodate its relative motion with the Caribbean Plate. It has been proposed that CAFA-Caribbean dextral shear is accommodated by clockwise rotating tectonic blocks (Bookshelf Faulting), where faulting is characterized by NE-SW striking left-lateral and NW-SE striking right-lateral strike-slip faulting. Using Global Positioning System data, and a Bayesian inversion approach, we determined the kinematics and geometries of three moderate-magnitude upper-plate earthquakes in Nicaragua. We investigated the April 10th, 2014 Mw 6.1, September 15th, 2016 Mw 5.7, and September 28th, 2016 Mw 5.5 earthquakes. We find that April 10th, 2014 earthquake occurred on a NW-SE (~315°) striking fault with right-lateral coseismic slip. This is the first well-documented historical earthquake with this geometry and kinematics. The September 15th and September 28th, 2016 earthquakes were located on NE-SW (~55° & ~70°, respectively), striking faults with left-lateral and dip-slip coseismic slip. Coulomb failure stress analysis, suggests that the 2016 earthquakes were promoted by the 2014 earthquake and that the September 28th, 2016 earthquake was promoted by the September 15th, 2016 earthquake. The determination of the geometry and kinematics of these upper-plate earthquakes provides support for CAFA-Caribbean dextral shear via Bookshelf Faulting and important implications for near- to mid-real-time seismic hazard estimates.