Seismicity in the Upper Plate of the Northern Chilean Offshore Forearc: Evidence of Splay Fault around the Mejillones Peninsula

Abstract:

The sector of the Mejillones peninsula (MP) in northern Chile, is thought to present important onshore and offshore faults systems. Furthermore, the possibility of the existence of major splay faults is debated. To improve our understanding of the MP, we built a new catalog of aftershocks of the 1995 Antofagasta earthquake (Mw 8.1) recorded by the CINCA network. In addition, with other earthquake catalogs derived from several networks deployments around MP, we build a 3-D tomographic model to determine the physical properties of the upper crust. We found seismicity in the outer wedge of the upper crust. Part of this seismicity defines a clear lineament between 22 and 5 km depth that we identified as a splay fault. The focal mechanisms indicate a plane with NE orientation and southeast-dipping. This upper crust seismicity and the splay fault is located in areas where the Vp and Vs velocities decrease and the Vp/Vs ratio increases, could suggest a fracture zone in the outer wedge of the upper crust. Because Vs decreases more than Vp and the existence of repeating earthquakes clusters, we propose the presence of fluids in the fracture zone and splay faults. Our observations present the first clear evidence of the activation of splay faults in the outer wedge of the Northern Chilean forearc.