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Subduction of the Nazca plate results in the uneven distributions of earthquakes and arc volcanoes along the South America's western margin. Here, we construct a high‐resolution shear‐wave velocity model from immediately offshore to the backarc in South America, using advanced full‐wave ambient noise tomography. Our new model confirms and provides further constraints on three major features, including (a) extensive low‐velocity anomalies within the continental crust, (b) two high‐velocity flat slab segments located beneath southern Peru and central Chile, and (c) complex slab geometry at flat‐to‐normal transitional subduction. The flat slab segments roughly correlate with the volcanic gaps but not with the seismicity gaps. We suggest that variations of slab geometry along strike and down dip have significantly modified the flow patterns within the mantle wedge. Subduction of oceanic ridges has altered the slab dehydration processes, which can influence the distribution of arc volcanism and the occurrence of intermediate‐depth earthquakes. Plain Language Summary: The subduction of the oceanic Nazca plate beneath the South American continent results in abundant earthquakes and arc volcanoes, which are unevenly distributed along the margin. There is a lack of intermediate‐depth earthquakes and arc volcanoes beneath Peru and central Chile/Argentina. How the properties of the subducted slab influence the distribution patterns of earthquakes and arc volcanoes remains as a fundamental question to be answered. In this study, we use an advanced seismological imaging method to construct a high‐resolution seismic velocity model within the South American margin. Our new model reveals two flat subduction regions beneath southern Peru and central Chile, which roughly correlate with the observed gaps of arc volcanoes but not of the earthquakes. Our model suggests that the slab could be either torn or distorted when transiting from flat to normal subduction. We propose that the variations of slab geometry have significantly modified the mantle flow patterns within the mantle wedge, resulting in the uneven distributions of arc volcanoes. The subduction of the Nazca Ridge and the Juan Fernandez Ridge has played a critical role in controlling the occurrence of the intermediate‐depth earthquakes. Key Points: Strong low‐velocity anomalies distribute extensively within the continental crust along latitudes of 12°–28°S at 15–35 km depthsThe subducting slab is divided into four segments along the margin including two normal sections and two nearly flat sectionsThe two flat slab segments beneath southern Peru and central Chile correlate well with the volcanic gaps but not with the seismic gaps [ABSTRACT FROM AUTHOR]

The state‐of‐stress within subducting oceanic plates controls rupture processes of deep intraslab earthquakes. However, little is known about how the large‐scale plate geometry and the stress regime relate to the physical nature of the deep intraslab earthquakes. Here we find, by using globally and locally observed seismic records, that the moment magnitude 7.3 2021 East Cape, New Zealand earthquake was driven by a combination of shallow trench‐normal extension and unexpectedly, deep trench‐parallel compression. We find multiple rupture episodes comprising a mixture of reverse, strike‐slip, and normal faulting. Reverse faulting due to the trench‐parallel compression is unexpected given the apparent subduction direction, so we require a differential buoyancy‐driven stress rotation, which contorts the slab near the edge of the Hikurangi plateau. Our finding highlights that buoyant features in subducting plates may cause diverse rupture behavior of intraslab earthquakes due to the resulting heterogeneous stress state within slabs. Plain Language Summary: A key type of tectonic boundary is where two plates collide with one sinking into the mantle beneath. These subduction zones generate the world's largest earthquakes. Quantifying stress in the subducting plate ("slab") is important because slabs drive the global plate‐tectonic system, and large earthquakes can occur within them. These earthquakes can cause strong shaking, and when occurring near cities, can lead to damage. However, mapping stress is challenging as we cannot directly "see" inside deep slabs. Our best indications of slab stress come from earthquakes themselves. A magnitude 7.3 earthquake north of New Zealand in 2021 generated a distinct pattern of seismic waveforms at seismometers installed worldwide. We used these seismic records to probe the earthquake, providing a new view of stress in subduction zones. We found the earthquake generated both vertical and horizontal motions along faults, driven by compressional and extensional stresses deep within the slab. The compressional part is oriented 90 degrees from the subduction direction, which is opposite to the usual compression in subduction zones. This unusual direction of compression can be explained by subduction of a thickened and buoyant part of the Pacific plate, known as the Hikurangi plateau. Key Points: A moment magnitude 7.3 2021 East Cape, New Zealand intraslab earthquake comprised multiple rupture episodes with different faulting stylesThe complex rupture comprises components of shallow trench‐normal extension and unexpectedly, deep trench‐parallel compression in slabThe trench‐parallel compression likely reflects stress rotation at a buoyancy contrast that drives slab contortion [ABSTRACT FROM AUTHOR]

Deep‐focus earthquakes provide insight into how subducting slabs deform over a range of spatial and temporal scales as they descend into the mantle. This study uses a 4D source imaging approach to determine centroid locations of the 2015 Mw 7.9 Bonin Islands deep‐focus earthquake and its aftershock sequence. Imaged sources of the mainshock show a complex rupture, but one that is compatible with a sub‐horizontal rupture plane. Previously undetected early aftershocks are imaged down to depths of approximately 750 km and represent the first reported earthquakes that initiate in the lower mantle. These events and a previously reported group of shallower distal aftershocks occur at the lower and upper boundaries of an imaged slab segment that deforms as it penetrates into the lower mantle. We hypothesize that mainshock failure allowed gravitational settling of the slab segment to occur which produced the distal aftershock sequences. Plain Language Summary: The causes of earthquakes below depths of approximately 60 km inside the Earth have been debated for nearly a century. This study reports on the first detected earthquakes with source locations in the lower mantle. These observations provide new insights into the mechanisms that can produce earthquakes deep inside the Earth. Key Points: A 4D back‐projection method is used to image the 2015 Mw 7.9 Bonin Islands deep‐focus earthquake and its aftershock sequenceEarly aftershocks detected down to ∼750 km depth are the first reported events that initiate in the lower mantleDistal aftershocks are hypothesized to occur at the top and bottom boundaries of a settling slab segment in response to the mainshock [ABSTRACT FROM AUTHOR]