Different Patterns of Strain Accumulation Across a Subduction Earthquake Segment Boundary Located at Mejillones Peninsula, N-Chile

Mejillones Peninsula of Northern Chile has been recognised as the surface expression of a major segment boundary for large subduction earthquakes. A sharp northern boundary of the 1995 rupture surface along the subduction interface is delineated underneath Mejillones Peninsula by the aftershock distribution. Additionally the b-value distribution on the fault plane defines a major asperity close to the segment boundary. The 1995 Mw=8.0 Antofagasta earthquake propagated southward after nucleating close to the segment boundary and the initial rupture was potentially accelerated by the existence of an asperity located on the seismogenic interface about 10-15 km southwest of the NEIC location. Comparing the coastal uplift signal and the strain accumulation pattern of active faults north and south of this segment boundary provides insight into the role of the subduction zone earthquake cycle in shaping the forearc crust and how changes in material properties on the seismogenic interface influence the strain accumulation along faults at the surface. In this study we present the uplift history of two Pleistocene sets of beach ridges exposed in the Mejillones Graben. The uplift history north of the segment boundary is continuous since 400 ky, whereas the uplift history south of it shows discontinuous uplift comprising phases of subsidence for the same period of time. Comparing incremental uplift rates with the mean uplift rates published for the area we find large discrepancies for the uplift south of the segment boundary, demonstrating that averaged rates do not reflect all of the processes leading to coastal uplift. Investigating slip rates of normal faults we find a slow but linearly evolving strain accumulation since 400 ky north of the segment boundary. South of it the strain accumulation pattern is episodic and comprises periods of non deformation. Additionally the fault pattern south of the segment is governed by at least three different kinematic regimes in the last 400 ky. Comparing the surface structures with the structures mapped on the seismogenic interface we propose that the existence of asperities on the interface correlating with material heterogeneities influences the strain accumulation pattern of surface faults and causes complex fault patterns and discontinuities in deformation rate. In the absence of asperities on the interface north of the segment boundary deformation rates and coastal uplift seems to be continuous on the timescale of 400 ky.