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The updip limit of seismic rupture during a megathrust earthquake exerts a major control on the size of the resulting tsunami. Offshore Northern Chile, the 2014 Mw 8.1 Iquique earthquake ruptured the plate boundary between 19.5° and 21°S. Rupture terminated under the mid-continental slope and did not propagate updip to the trench. Here, we use state-of-the-art seismic reflection data to investigate the tectonic setting associated with the apparent updip arrest of rupture propagation at 15 km depth during the Iquique earthquake. We document a spatial correspondence between the rupture area and the seismic reflectivity of the plate boundary. North and updip of the rupture area, a coherent, highly reflective plate boundary indicates excess fluid pressure, which may prevent the accumulation of elastic strain. In contrast, the rupture area is characterized by the absence of plate boundary reflectivity, which suggests low fluid pressure that results in stress accumulation and thus controls the extent of earthquake rupture. Generalizing these results, seismic reflection data can provide insights into the physical state of the shallow plate boundary and help to assess the potential for future shallow rupture in the absence of direct measurements of interplate deformation from most outermost forearc slopes. The rupture area of the 2014 Iquique earthquake offshore northern Chile was spatially limited to a region where the plate boundary is non-reflective in seismic images, indicative of low fluid pressure. In contrast, north and updip of the rupture area, a coherent highly reflective plate boundary indicates excess fluid pressure, which may inhibit strain accumulation, while strain release in the non-reflective rupture area occurs during large earthquakes. [ABSTRACT FROM AUTHOR]

The aftershock distribution of the 2014 Mw 8.1 Iquique earthquake offshore northern Chile, identified from a long‐term deployment of ocean bottom seismometers installed eight months after the mainshock, in conjunction with seismic reflection imaging, provides insights into the processes regulating the updip limit of coseismic rupture propagation. Aftershocks updip of the mainshock hypocenter frequently occur in the upper plate and are associated with normal faults identified from seismic reflection data. We propose that aftershock seismicity near the plate boundary documents subduction erosion that removes mass from the base of the wedge and results in normal faulting in the upper plate. The combination of very little or no sediment accretion and subduction erosion over millions of years has resulted in a very weak and aseismic frontal wedge. Our observations thus link the shallow subduction zone seismicity to subduction erosion processes that control the evolution of the overriding plate. Plain Language Summary: To better understand the controls on shallow seismicity and subduction erosion following large subduction earthquakes, we use marine recordings of the Mw 8.1 2014 Iquique earthquake aftershocks and long‐offset multi‐channel seismic data. By comparing the aftershock locations and seismic imaging, we observe that most aftershocks occurred in the upper continental plate and abruptly stopped in the frontal forearc. The amplitude characteristics of upper‐crust reflections indicate a fractured and fluid‐filled outer forearc, which is associated with the absence of aftershocks. Large‐scale faulting, as evidenced by disrupted reflections in the seismic image, can be correlated to upper plate seismicity. We propose that the aftershocks updip of the main earthquake area reflect active subduction erosion processes. Key Points: We investigate structure and seismicity at the updip end of the 2014 Iquique earthquake rupture using amphibious seismic dataSeismicity updip of the 2014 Iquique earthquake occurs over a broad range likely interpreted to be related to the basal erosion processesCoseismic stress changes and aftershocks activate extensional faulting of the upper plate and subduction erosion [ABSTRACT FROM AUTHOR]

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]

The subduction of seamounts and basement ridges affects the structure, morphology, and physical state of a convergent margin. To evaluate their impact on the seismo-tectonic setting of the subduction zone and the tectonic development of the lower subducting and upper overriding plate, it is essential to know the precise location of subducted topographic features under the marine forearc. Offshore Northern Chile, the Iquique Ridge represents a broad zone of complex and heterogeneous structure of variable width on the oceanic Nazca Plate, which complicates attempts to project it beneath the forearc of the Chilean subduction zone. Here we use a state-of-the-art seismic reflection data processing approach to map structures related to ridge subduction under the marine forearc with unprecedented accuracy and resolution and evaluate their impact on the deformation of both the plate boundary and the upper plate. We show that significant ridge-related topography is currently subducting south of 20.5 °S and that the combined effect of horst and graben subduction with subduction of Iquique ridge-related thickened and elevated crust causes an upward bulging of the entire upper plate from the plate interface up to the seafloor as well as the presence of kilometer-scale anticlinal structures observed in multibeam bathymetric data that are approximately aligned with horsts seaward of the trench. In the area affected by the subducting ridge, a frontal prism is absent, which may relate to frontal subduction erosion caused by the excess lower plate topography. In contrast farther towards the north, where only isolated seamounts subduct, a small frontal prism and a slope/apron sediment cover down to 3000 m water depth are found. [ABSTRACT FROM AUTHOR]