ALBERT

Description: The Integrated Plate Boundary Observatory Chile (IPOC) in northern Chile has been monitoring the largest seismic gap along the South American subduction zone for 10 years. When IPOC was initiated, it has been 130 years the last great earthquake in the region had occurred. And since then the Iquique gap had been accumulating a slip deficit along a 〉500 km segment of the plate boundary. Since IPOC’s inception two large events, the 2007 M 7.7 Tocopilla and the M 8.1 2014 Iquique earthquakes, have broken parts of the gap. Both events were well recorded by IPOC, produce valuable data and advance our understanding of the subduction megathrust earthquake cycle. Last year, the Helmholtz Centre for Ocean Research Kiel (GEOMAR) has been extending IPOC with the GeoSEA ocean bottom observatory. In this ambitious project deformation will be measured where it cannot be picked up by land-based instruments, i. e. far offshore near the subduction trench. This will open the crucial updip section of the subduction plate boundary to research. IPOC has thus demonstrated the necessity of long-term monitoring to observe slow or rare events, but also that tenacity and patience pay off.

Description: The Atacama Fault System (AFS) is an active trench-parallel fault system, located in the forearc of N-Chile directly above the subduction zone interface. Due to its well-exposed position in the hyper arid forearc of N-Chile it is the perfect target to investigate the interaction between the deformation cycle in the overriding forearc and the subduction zone seismic cycle of the underlying megathrust. Although the AFS and large parts of the upper crust are devoid of any noteworthy seismicity or historically documented earthquakes, at least three M=7 earthquakes in the past 10 ky have been documented in the paleoseismological record, demonstrating the potential of large events in the future. We apply a two-fold approach to explore fault activation and reactivation patterns through time and to investigate the triggering potential of upper crustal faults. 1) A new methodology using high-resolution topographic data allows us to investigate the number of past earthquakes for any given segment of the fault system as well as the amount of vertical displacement of the last increment. This provides us with a detailed dataset of past earthquake rupture of upper plate faults which is potentially linked to large subduction zone earthquakes. 2) The IPOC Creepmeter array provides us with high-resolution time series of fault displacement accumulation for eleven stations along the four most active branches of the AFS.

Description: To this date, the question of why and how a plateau-type orogen formed with massive crustal thickening at the leading edge of western South America remains one of the hotly debated issues in geodynamics. During the Cenozoic, the Altiplano and Puna plateau of the Central Andes developed during continuous subduction of the oceanic Nazca plate in a convergent continental margin setting – a situation that is unique along the 60 000 km of convergent margins around the globe. The key challenge is to understand why a first-order mechanical instability of the later plateau extent developed along the central portion of the leading edge of South America only, as well as why and how this feature developed only during the Cenozoic, although the cycle of Andean subduction had been ongoing since at least the Jurassic. Although the widespread presence of partial melts or metamorphic fluids at mid-crustal level has been suggested to indicate upper plate weakening from heating and partial melting, it is recently found that upper plate strain weakening at lithospheric scale plays a significantly larger role. This first order control is tuned by factors affecting the strength balance between the upper plate lithosphere and the plate interface of the Nazca and South American plates such as variations in trenchward sediment flux affecting plate interface coupling and slab rollback or the role of inherited structures. Late initiation of orogeny in the Eocene, however, and its sustained action over tens of million years is now found to be related to the penetration of the slab into the lower mantle around 50 Ma ago, producing a slowdown of the lateral slab migration (‚slab anchoring’), and dragging the upper plate against the subduction zone by large-scale return flow. The combination of these parameters was highly uncommon during the Phanerozoic leading to very few plateau style orogens at convergent margins like the Cenozoic Central Andes in South America or the Laramide North American Cordillera.

Description: Subduction earthquakes are the most powerful naturally occurring terrestrial processes often resulting in catastrophic fatality counts and decimation of human infrastructure. Over the past decades, great efforts have been undertaken to improve the understanding of the subduction earthquake physics. The Integrated Plate Boundary Observatory in Chile (IPOC) is a multi-instrument network installed in 2007 in the Northern Chile Seismic Gap, where a large magnitude earthquake was expected soon. On April 1st 2014, a portion of the IPOC-monitored region broke, producing the Mw 8.1 Iquique earthquake. In the year leading up to this event, IPOC’s instruments captured some unusual transient seismic and geodetic signals, resulting in a unique dataset recording the preparatory phase of a large earthquake. We combined IPOC data with satellite radar interferometry (InSAR) data to analyze not only the earthquake itself but also the interseismic phase and a detailed foreshock series before the main event. We found that the earthquake ruptured a zone on the plate interface that was highly locked before the earthquake. Additionally, we were able to characterize the aseismic (silent) slip that occurred in the two weeks leading up to the event by combining seismic and geodetic data. Application of these analyses in real-time might enable geoscientists to identify runaway processes that can precede large subduction earthquakes.