Edmundo Norabuena

SUMMARY: The South Peru subduction zone is a complex, highly active region, which has hosted four Mw 8+ earthquakes over the last 100 yr. It marks the transition between the flat slab associated with the Nazca Ridge subduction in the North and more steeply dipping subduction in the South, causing the slab to contort and affecting seismicity patterns in the region. In this study, we present the first high-density, high-quality seismic catalogue of the region between the arc and the trench, totalling 166 825 events between 2022 January 1 and 2024 December 31, including 125 467 well-located ones. We first picked and associated phases using PhaseNet and PyOcto, then located the resulting events with NonLinLoc-SSST and GrowClust3D. Finally, we derived a new slab model from the seismicity, allowing us to classify the earthquakes as upper plate (16 per cent), interface (12 per cent), lower plate (68 per cent), outer rise (0.20 per cent) and human-related (3.1 per cent). The region is broadly divided into four subregions with different seismicity patterns and slab geometries: the flat slab, with intense interface and intraslab activity, the slab transition zone, where the plate contorts to accommodate its change in geometry, the Arequipa region, with intense upper plate seismicity but very low intraslab and interface seismicity and the North Chile region, with a large band of dense intraslab seismicity. We find that in the flat slab region, the Nazca Ridge is linked to the presence of dense seismicity close to the trench, and seismic swarms hinting at the presence of slow slip. Meanwhile, the intraslab seismicity in that region is organized in trench-parallel bands which are likely related to slab bending. In the slab transition region, we image multiple orthogonal faults just south of the slab contortion, suggesting a damaged slab. Further south, in the Arequipa region, upper plate seismicity forms a large, trenchward-dipping structure seemingly connected to the Incapuquio fault at the surface. Finally, in North Chile, the deep band of intraslab seismicity appears to locate further downdip as we move to the north, perhaps reflecting changes in slab properties.

Detections of slow slip events (SSEs) are now common along most plate boundary fault systems at the global scale. However, no such event has been described in the south Peru - north Chile subduction zone so far, except for the early preparatory phase of the 2014 Iquique earthquake. We use geodetic template matching on GNSS-derived time series of surface motion in Northern Chile to extract SSEs hidden within the geodetic noise. We detect 33 events with durations ranging from 9 to 40 days and magnitudes from $M_w$~5.6 to 6.2. The moment released by these aseismic events seems to scale with the cube of their duration, suggesting a dynamic comparable to that of earthquakes. We compare the distribution of SSEs with the distribution of coupling along the megathrust derived using Bayesian inference on GNSS- and InSAR-derived interseismic velocities. From this comparison, we obtain that most SSEs occur in regions of intermediate coupling where the megathrust transitions from locked to creeping or where geometrical complexities of the interplate region have been proposed. We finally discuss the potential role of fluids as a triggering mechanism for SSEs in the area.

Wildfire occurrence has increased sharply in the last two decades in the Peruvian Andes. There is, however, little research on wildfires and their impacts. This study explores the conditions conducive to wildfire during 2020. MODIS images were collected to estimate the development of vegetation. In addition, ground-based monthly and satellite-based daily precipitation data were collected. Daily precipitation regularity was evaluated using a concentration index (CI), while monthly precipitation was used to estimate the Standard Precipitation Index (SPI). We used also the Global Vegetation Moisture Index (GVMI), which is a useful indicator of vegetation dynamics based on vegetation moisture. Our results do not indicate a direct link between rainfall regularity (lowest CI values) and development of vegetation. Although the SPI drought analysis using seasonal rainfall indicated nearly normal conditions during 2019–2020, analysis of dry-day frequency (DDF) suggests that the dry period played an important role between September and November 2020, producing conditions similar to the droughts of 2005, 2010 and 2016. GVMI also showed below-average values from April to November. We corroborate the usefulness of DDF for monitoring the potential increase in wildfire conditions. A controlled burn policy could offer a more useful way to reduce the impacts of wildfire.

The mechanisms leading to large earthquakes are poorly understood and documented. Here we characterize the long‐term precursory phase of the 1 April 2014 M_w8.1 North Chile megathrust. We show that a group of coastal GPS stations accelerated westward 8 months before the main shock, corresponding to a M_w6.5 slow slip event on the subduction interface, 80% of which was aseismic. Concurrent interface foreshocks underwent a diminution of their radiation at high frequency, as shown by the temporal evolution of Fourier spectra and residuals with respect to ground motions predicted by recent subduction models. Such ground motions change suggests that in response to the slow sliding of the subduction interface, seismic ruptures are progressively becoming smoother and/or slower. The gradual propagation of seismic ruptures beyond seismic asperities into surrounding metastable areas could explain these observations and might be the precursory mechanism eventually leading to the main shock.

The large slow‐moving landslide of Maca is located in the upper Colca valley (southern Peru), a region characterized by a well pronounced rainy period, and intense and recurrent sustained seismicity. The landslide, developed in deep lacustrine deposits, has recently accelerated, threatening the Maca village. This work aims at understanding the rupture mechanism and the causes of the recent landslide reactivation/acceleration. We present a multidisciplinary characterization of the Maca landslide that includes: (i) geological and morphological mapping in the field; (ii) remote sensing analysis using an historical aerial photograph of 1955 and the Pléiades satellite images (2013); (iii) global positioning system (GPS) including time‐series of surveys over 13 years, and continuous measurements over 14 months; (iv) a geophysical campaign with deep electrical resistivity tomography profiles acquired across the landslide mass. Our study shows that this 60 Mm³ landslide, which can be classified as a clay/silt compound landslide, moved by 15 m between 2001 and 2014 with a large inter‐annual velocity variation (up to a factor of 500) depending on the rainfall intensity. We suggest that these dramatic changes in velocity are the result of the combination of a threshold mechanism and the short intense rainy season in Peru. This study reveals three main driving factors acting at different timescales: (i) over several decades, the river course has significantly changed, causing the Maca landslide reactivation in the 1980s due to the erosion of its toe; (ii) at the year scale, a minimum amount of rainfall is required to trigger the motion and this amount controls the landslide velocity; (iii) transient changes in slide velocity may occur anytime due to earthquakes. This study particularly highlights the non‐linear behaviour of the motion with rainfall. Copyright © 2016 John Wiley & Sons, Ltd.