Seismicity in the upper plate of the Northern Chilean offshore forearc: Evidence of splay fault south of the Mejillones Peninsula

The region around the Mejillones Peninsula is characterized by major onshore and offshore fault systems. The existence of major splay faults in these fault systems has been debated. To improve our undestanding of seismotectonic processes affecting the MP, we built a new catalog of aftershocks of the 1995 Antofagasta earthquake (Mw 8.1) recorded by the CINCA seismic network. In addition, we build a three-dimensional tomographic model to determine the physical properties of the upper plate using arrival times from our new catalog combined with other earthquakes catalogs derived from several networks deployments across the MP. We detected seismicity in the outer wedge of the upper plate. The seismicity is partially defined by a clear lineament between 22 and 5 km depth that we defined as a splay fault. The focal mechanisms indicate a rupture plane with NE orientation and SE dip. The seismicity in the upper plate and splay fault are located in the areas in which the Vp and Vs velocities are decreased and the Vp/Vs ratio is increased, suggesting a fractured fault zone within the outer wedge of the upper plate. We propose the presence of fluids in the fracture fault zone within the outer wedge because the decrease in Vs larger than that of Vp and repeating earthquakes clusters were detected. Our observations present clear evidence of the activation of the splay fault in the outer wedge of the North Chilean offshore forearc.