The Seismogenic Coupling Zone in Southern Central Chile, 38° S: A Reflection seismic image of the subduction zone (Project TIPTEQ)

The multi-disciplinary project TIPTEQ (from The Incoming Plate to mega-Thrust EarthQuake processes) investigates the seismogenic coupling zone in Southern Central Chile and the associated subduction zone processes between the Pacific Ocean and the volcanic arc. The reflection seismic component of TIPTEQ includes a 110 km long profile which spans from the coast over the down-dip end of the seismogenic coupling zone, crossing the 1960 Valdivia earthquake hypocentre. 180 three-component geophones were deployed (100 m spacing) along an 18 km wide spread whereof 4.5 km were shifted in a daily roll-along. With 100 borehole shots, 1.5 km apart, this up to 8-fold covered line delivers a high-resolution image of the seismogenic coupling zone. 15 additional shots in an expanding spread profiling configuration focussed on the seismogenic coupling zone. SH wave source signals were generated to yield an improved picture of the petrophysical contrasts within the system. The SPOC-South wide-angle data velocity model is combined with a first-break tomography velocity model to get an advanced migration image. The subducting Nazca plate can be traced from a depth of 25 km below the coast down to a depth of 50 km at the eastern end of the profile. Structural evidence suggests that material is transported down in a subduction channel. From slow uplift of the Coastal Cordillera we conclude that basal accretion of parts of this material controls the seismic architecture and growth of the south Chilean crust. Between depths of 5 to 25 km several bright reflectivity spots can be seen in the upper plate, which may suggest fluid traps in the accretionary wedge. The tomographic p-wave velocity model reaches approximately 10 km depth. Its segmentation corresponds to the geological units mapped at surface. The sediment thickness in the Central Valley is approx. 3 km, and we see prominent fault systems like the Lanalhue fault zone also in the tomographic model. At present, almost no seismicity is observed along the entire, approximately 130 km wide seismogenic coupling zone, which could point to a higher coupling and stress accumulation in the region.