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On April 2014 the Iquique Mw 8.1 earthquake ruptured the interpolate contact between the oceanic Nazca and continental South American plates offshore northern Chile between 19.5°S to 21°S in April 2014. This earthquake did not fully release the strain accumulated since the last great megathrust (Mw 8.8) in 1877 and had left an unbroken segment in the south. From December 2014 to November 2016, we deployed an offshore network of 15 Ocean Bottom Seismometers (OBS) that covered the rupture area and the unbroken southern segment using the Chilean Navy ship OPV Toro and R/V Sonne. That data set is supplemented by five weeks of data from 67 OBS installed for a controlled source seismic experiment during cruise MGL1610 of the R/V Marcus Langseth in late 2016. Data acquired onshore by stations from of the IPOC (Integrated Plate Boundary Observatory Chile) and CSN (Chilean Seismological Service) networks are also included.We present first results of this ongoing project, which include double-difference hypocenter relocations based on waveform cross-correlation. Most of the seismicity occurs between 19.5 and 21°S up-dip of the patch of maximum coseismic slip during the 2014 earthquake, while the seismicity in seismogenic depths is highly concentrated forming well-defined clusters. The observed seismicity provides constraints on the structure of the marine forearc and enables us to relate the seismicity distribution to the background seismicity, seafloor morphology, and regional tectonics. [ABSTRACT FROM AUTHOR]

Little is known about the movement of offshore faults because of the absence of direct, underwater geodetic measurements. Furthermore, satellite geodesy techniques such as GPS cannot be applied underwater due to the strong attenuation of electromagnetic signals. However, capturing fault movement is crucial to determine the state of locking which reveals whether a fault is creeping or accumulating elastic energy. Here we show that horizontal crustal strain on the seafloor can be continuously measured with mm-precision over periods of years and dozens of baselines allowing to resolve tectonic deformation quasi-in-situ on the seafloor. To this effect, we installed an acoustic ranging network across the central segment of the North Anatolian Fault in the Marmara Sea offshore metropolitan Istanbul. The seafloor observation shows that the fault is locked with an upper bound of the local creep rate of 2 mm/a. The absence of any significant fault displacement on the seafloor for 2.5 years together with sparse local seismicity from OBS observation reveal that the fault is currently locked and therefore is accumulating strain. The slip-deficit since the last known rupture in 1766 of at least 4 m is sufficient to trigger an earthquake up to magnitude 7.4. The rate of strain accumulation on the central segment of the North Anatolian Fault in the Marmara Sea had previously been extrapolated from onshore observations or inferred from the absence of seismicity, but results remained inconsistent. Our study for the first time fills part of this data gap through in-situ seafloor geodetic measurements and demonstrates the urgent need to conduct similar studies in regions with a high-hazard potential from active faults offshore. [ABSTRACT FROM AUTHOR]