How Does a Seismic Gap Control the Propagation of the Seismicity in Central Chile (31° S-31.6° S)?

In South America, the most energetic earthquakes are mainly generated along the seismogenic interface between the Nazca and the South American plates. However, there is a place in the Central Chile seismogenic zone, between latitude 31° S and 31.6° S, where almost no earthquake occurs, even using locally recorded data or from International Centre catalogues. Hence, the estimated rupture zone of the last great magnitude event of 1943 reported in the area (Illapel, Ms=7.9) should be reconsidered. In addition, this seismic gap region acts like a barrier for the propagation of seismicity. On the northern edge of the gap, a seismic swarm composed of mainly earthquakes of magnitude 5, propagated to the South between July and August 1997 but was stopped by the gap. This seismic activity was followed by the Punitaqui mainshock (October 15, 1997), an unusual normal event downdip to the gap, but in spite of its numerous aftershocks, none was located into the gap. To the South, the gap is limited by the Juan Fernandez Ridge which subducts underneath the South America plate. Tomographic results for Vp/Vs ratio, VpVs product and Qp quality factor, stress tensor analysis and bathymetric features show an EW anomaly between the coast and the trench, on the northern edge of the gap. This anomaly is likely related to a highly fractured zone with the presence of fluids where stress is not able to facilitate the propagation of a rupture.