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Subducting seamounts are recognized as one of the key features influencing megathrust earthquakes. However, whether they trigger or arrest ruptures remains debated. Here, we use analog models to study the influence of a single seamount on megathrust earthquakes, separating the effect of topography from that of friction. Four different model configurations have been developed (i.e., flat interface, high and low friction seamount, low friction patch). In our models, the seamount reduces recurrence time, interseismic coupling, and fault strength, suggesting that it acts as a barrier: 80% of the ruptures concentrate in flat regions that surround the seamount and only smaller magnitude earthquakes nucleate above it. The low‐friction zone, which mimics the fluid accumulation or the establishment of fracture systems in natural cases, seems to be the most efficient in arresting rupture propagation in our experimental setting. Plain Language Summary: Seamounts ‐ extinct volcanoes ‐ are ubiquitous features of the seafloor of subducting plates. During their descent toward the mantle, seamounts are squeezed between the overriding‐ and subducting plates, creating a geometrical and mechanical discontinuity that is thought to control the largest earthquakes that occur on Earth. It is not known, however, whether they trigger or arrest earthquakes, as a variety of observations have been used to support one or the other hypothesis. Here, we tackle this subject using scaled laboratory models that allow reproduction of large earthquakes. Our models support a scenario where subducting seamounts primarily arrest rupture propagation, limiting therefore expected maximum magnitudes. Our models also suggest that subducting seamounts might influence the recurrence time of large earthquakes by decreasing their frequency. Key Points: We investigated the effect of seamount subduction on megathrust earthquakes by isolating the role of geometry from that of frictionSeamounts act primarily as barriers to earthquake propagation, promoting small ruptures and decreasing seismic couplingReduced friction displays maximum barrier efficiency, a configuration likely associated with fluids or fracture systems in nature [ABSTRACT FROM AUTHOR]

The seismic moments observed for low‐frequency earthquakes (LFEs) vary over multiple orders of magnitude, even where the LFEs occur within families of similar events. Although this variability is typically interpreted to record a scale‐limited process at the LFE source, neither the slip per LFE nor the rupture area can be determined from seismological constraints. Here, we examine incrementally developed slickenfibers that have been proposed to record LFEs in exhumed subduction zones. These structures form through repeated, micron‐scale slip events across dilational irregularities in the fault plane, which are punctuated by cementation and sealing in the interstitial space. By statistically analyzing the geometry of inclusion trails delineating slip‐parallel mineral‐growth increments, we constrain the variability in slip per inferred LFE and test end‐member hypotheses regarding the controls on LFE moments. We find that that the slickenfibers exhibit characteristic slip increments, favoring a "slip‐limited" model that requires large variability in LFE rupture areas. Plain Language Summary: Low‐frequency earthquakes (LFEs) are a category of seismic phenomena only perceptible by geophysical instruments. Understanding what causes these events, however, may help us understand larger magnitude earthquakes occurring along plate boundaries. Unfortunately, the physical processes that generate LFEs at their source depth are poorly understood. Seismological studies have shown that energy release varies substantially from one LFE to another, for example, but the causes of this variability are unclear. Given that energy release is proportional to the area of LFE rupture and the slip during the LFE, a common conceptual model argues that the variability in LFE energy release results from a wide variation in slip amount occurring over a relatively constant area between events. We utilize a unique record of slip events extracted from geological structures (inferred to record ancient LFEs) to test competing models governing energy release at the LFE source. Using a statistical test that quantifies the variability of slip increment recorded in rock samples, we find that the amount of slip during individual inferred LFEs has a strong preferred value with little variation between events. This result indicates variations in LFE energy release are determined primarily by variations in the area over which the slip occurs. Key Points: We examine slip increments associated with LFE structures in geological exposuresStatistical analysis shows that the slicken fibers record characteristic slip incrementsResults indicate slip‐limited seismic moments during low frequency earthquakes [ABSTRACT FROM AUTHOR]

Subduction zone architecture and properties are thought to control megathrust slip behavior, but few constraints on crustal structure and megathrust properties are available at sufficient resolution and depth, hindering understanding of linkages between structure and behavior. Here we present a P‐wave seismic velocity model based on wide‐angle seismic data integrated with collocated reflection imaging in the weakly coupled Shumagin Gap in the Alaska subduction zone, where a M7.8 occurred in July 2020. We show that this earthquake occurred near and below the Moho of the overriding plate where the megathrust is characterized by a 3‐ to 5‐km‐thick reflection band interpreted to represent tectonic mixing. The rheological heterogeneity of the plate boundary near and below the Moho could account for abundant interplate seismicity, repeated M7.x events, and patchiness of the 2020 rupture. Velocity variations in the overriding continental crust imply changes in rigidity that could further influence megathrust slip. Plain Language Summary: Slip on subduction zone plate boundary faults (megathrusts) produces large and destructive earthquakes, but many questions remain about what controls the size and character of these events. To address this question, we used seismic imaging data to determine subduction zone configuration and properties offshore Alaska in the area of a magnitude 7.8 earthquake that occurred in July 2020. We constrain the thickness of the crust overlying the megathrust, variations in strength within the crust, and changes in the total width and complexity of the fault zone. The M7.8 earthquake occurred on a part of the fault overlain by both continental crust and upper mantle, where imaging shows the fault zone may be particularly heterogeneous, which could explain the patchiness of slip during this event. Variations in strength of the overriding plate may also influence both shallow and deep fault slip in this subduction zone. Key Points: The megathrust intersects the continental Moho in the Shumagin Gap at ∼33 kmMegathrust heterogeneity near and below continental Moho could explain patchy slip in 2020 M7.8 event and abundant megathrust seismicityChanges in seismic velocity in the overriding crust imply changes in rigidity that could influence megathrust slip behavior [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]

Tsunamis from earthquakes of various magnitudes have affected Cascadia in the past. Simulations of Mw 7.5–9.2 earthquake constrained by earthquake rupture physics and geodetic locking models show that Mw ≥ 8.5 events initiating in the middle segments of the subduction zone can create coastal tsunami amplitudes comparable to those from the largest expected event. Our rupture and tsunami simulations reveal that the concave coastline geometry of the Pacific Northwest coastline focuses tsunami energy between latitudes 44° and 45° in Oregon. The possible coastal tsunami amplitudes are largely insensitive to the choice of slip model for a given magnitude. These results are useful for identifying the most hazardous segments of the subduction zone and demonstrate that a worst‐case rupture scenario does not uniquely yield the worst‐case tsunami scenario at a given location. Plain Language Summary: Offshore earthquakes along the Pacific Northwest coast of the U.S. and Canada (Cascadia region) can have magnitudes as high as 9.2, as was probably the case for an earthquake in the year 1700 CE that resulted in a large tsunami in Cascadia and across the Pacific Ocean. To learn more about the future tsunami hazard in the region, we design computer models of tsunamis from a wide range of earthquake scenarios. We find that almost regardless of the earthquake source details, events larger than magnitude 8.5 near the coast of Oregon can create large and widespread tsunamis along the US west coast. These are consequences of the geometry of offshore earthquake faulting and the concave shape of coastline in the region. Key Points: A Mw = 8.5 event in central Cascadia (Oregon) can create coastal tsunami amplitudes comparable to those from the largest possible eventThe concave coastline contributes to larger coastal tsunami amplitudes in central CascadiaThe choice of slip model does not significantly affect the distribution of coastal tsunami amplitudes in Cascadia [ABSTRACT FROM AUTHOR]

Deep focus earthquakes within the Tonga‐Fiji subduction system account for about two‐thirds of the global total and provide significant constraints on slab deformation. The factors controlling the intense deformation remain unclear. Here, we use two‐dimensional, time‐dependent geodynamic models to study the morphology, stress state, and thermal structure of the Tonga‐Fiji subduction zone. The results, consistent with tomographic images and focal mechanisms, demonstrate that collision between a relic slab from the Vanuatu Trench and the Tonga slab may control the steeper dip of the Tonga slab and earthquakes in the mantle transition zone. We suggest that the magnitude 8.2 and 7.9 earthquakes in 2018 mostly ruptured within the warm rim of the Tonga slab and occurred beneath the folding relic slab with high temperatures of at least ∼900°C and ∼1100°C, respectively. The findings support the hypothesis that local slab temperature likely controls rupture of deep earthquakes. Plain Language Summary: Deep earthquakes at the depths between 300 and 700 km are controversial as the increase of temperature and pressure with depth will promote ductile flow and inhibit brittle failure. Approximately two‐thirds of the deep earthquakes globally are located below the North Fiji Basin which provides significant constraints on the deformation of the Tonga slab. However, the factors controlling the intense deformation remain unclear. Here, a time‐evolving mechanical model is used to study the morphology, state‐of‐stress, and thermal structure of the Tonga‐Fiji subduction zone. The models reproduce the high‐resolution Tonga slab morphology and can better fit the focal mechanisms of the deep earthquakes, which demonstrate that collision between a relic slab detached from the Vanuatu Trench and the Tonga slab leads to the nearly vertical Tonga slab and controls the occurrences of the deep earthquakes in the mantle transition zone. We suggest that two large deep earthquakes with moment magnitude (Mw) 8.2 and 7.9 in 2018 occurred within the warm rim of the Tonga slab and beneath the high‐temperature folded relic slab, respectively, supporting the hypothesis that local slab temperature probably controls the rupture details of deep earthquakes. Key Points: Geodynamic models reproduce the high‐resolution tomographic images of the Tonga slabCollision between a relic and Tonga slabs controls the intense deformation within the two slabsSimulated thermal structures predict deep earthquake doublet (Mw 8.2 and 7.9) occurs in the warm slab areas [ABSTRACT FROM AUTHOR]

Asperities are patches where the fault surfaces stick until they break in earthquakes. Locating asperities and understanding their causes in subduction zones is challenging because they are generally located offshore. We use seismicity, interseismic and coseismic slip, and the residual gravity field to map the asperity responsible for the 2014 M8.1 Iquique, Chile, earthquake. For several years prior to the mainshock, seismicity occurred exclusively downdip of the asperity. Two weeks before the mainshock, a series of foreshocks first broke the upper plate then the updip rim of the asperity. This seismicity formed a ring around the slip patch (asperity) that later ruptured in the mainshock. The asperity correlated both with high interseismic locking and a circular gravity low, suggesting that it is controlled by geologic structure. Most features of the spatiotemporal seismicity pattern can be explained by a mechanical model in which a single asperity is stressed by relative plate motion. Plain Language Summary: At the northern Chile subduction zone, where the oceanic Nazca plate glides several centimeters per year under the South American continent, the two plates got stuck at an irregularity. This irregularity got stressed for more than 100 years until it broke in 2014 in a magnitude 8.1 earthquake. We mapped small earthquakes in the years preceding the large earthquake. We found that small earthquakes formed a ring around the irregularity. First, a half circle formed along the lower side of the irregularity, probably, because the oceanic Nazca plate is pulled down by its own weight and is thus mainly stressing along the lower side. Then, 2 weeks before the largest earthquake, a series of smaller earthquakes closed the circle around the upper side of the irregularity. Finding such irregularities between tectonic plates is important to assess earthquake risk. We also found that the irregularity can be seen in data derived from Earth's gravity field, hinting that it is due to geologic structure. Key Points: Seismicity in the years prior to and immediately before the mainshock form two halves of a "Mogi Doughnut" surrounding the main slip patchMechanical asperity model predicts stress increase where earthquakes are observedSlip region correlates with high interseismic locking and a circular gravity low, suggesting that the asperity is controlled by geologic structure [ABSTRACT FROM AUTHOR]

In the Nazca‐South American subduction zone, the subducted slab is flattened beneath northern Andes, called the Peruvian flat slab. The 2019 Mw 8.0 northern Peru intermediate‐depth normal‐faulting earthquake occurred at the leading edge of the Peruvian flat slab, where the slab rebends and sinks into greater depths. Here we investigate this earthquake by back projection analysis and finite fault inversion using seismic waveforms at teleseismic distances. The rupture process indicates that this earthquake ruptured mainly along strike (353°) ~150 km north‐northwestward within ~55 s (average rupture velocity ~2.7 km/s), resulting in two major slip areas with three high slip rate areas, which are consistent with three high‐frequency energy radiation subevents. Our study suggests that such a heterogeneous rupture may be caused by slab bending forces and dehydration embrittlement associated with morphology of the slab. Plain Language Summary: In the subduction zone, the oceanic plate and the continental plate converge, and then the oceanic plate subducts beneath the continental plate and sinks into great depths. In South America, the subducted oceanic plate is flattened beneath northern Peru instead of steeply sinking, called the Peruvian flat slab. After flattened, the oceanic plate starts to sink again. In the region where the oceanic plate starts to sink again, an earthquake with magnitude of 8.0 occurred at a depth of ~120 km on 26 May 2019. How and why this earthquake occurred at such a depth? Does the occurrence of this earthquake is related to the subducted oceanic plate? We try to answer these questions in this paper. By using two techniques, we obtained a scenario about this earthquake: In the region where the oceanic plate starts to sink again, the water in the oceanic plate releases, along with the effect of sinking to cause this earthquake. After this earthquake initiated, it ruptured mainly unilaterally north‐northwestward ~150 km with a speed of ~2.7 km/s, resulting in two large slip areas with three high slip rate areas. Such an uneven rupture may be produced by the heterogeneous structure of the slab. Key Points: Rupture process of the 26 May 2019 northern Peru earthquake is obtained from back projection analysis and finite fault inversionThe effects of slab bending forces and dehydration embrittlement together may cause this earthquake at an intermediate depthThis intermediate‐depth earthquake is probably associated with morphology of the Peruvian flat slab [ABSTRACT FROM AUTHOR]

Constraining the mechanism of earthquakes in subduction zones requires adequate estimates of source location and near‐source elastic properties. In this study, we propose a P wave autocorrelation‐based method to extract depth phase energy from teleseismic earthquakes with moment magnitude down to 4.0. We apply the method to improve location estimates of intermediate‐depth earthquakes in the Japan and northern Chile subduction zones, which represent so‐called cold and warm slabs, respectively, and which are both marked by double seismic zones. A positive correlation of slab age and double‐seismic‐zone width validates a thermally controlled model of slab morphology. The negative to normal differential times (and, thus, low or normal Vp/Vs) of the deep parts of the double seismic zones suggest that the intermediate‐depth earthquakes considered here are not due to dehydration. Plain Language Summary: Earthquake depth, which can be obtained by using depth phases, is critical for hazard mitigation and a better understanding of their mechanisms. Most techniques that use depth phases with teleseismic data require large events, even when array‐based techniques are used, which limits comprehensive studies of subduction zone seismicity. Here we propose a new autocorrelation‐based method to extract depth phase energy from teleseismic data generated by earthquakes with magnitudes as small as Mw4.0. The application to Japan and northern Chile validates the robustness of the method, and the more precise delineation of the double seismic zones reveals a positive correlation between their widths and the age of the slab when it approaches the trench. Moreover, we find the differential times of sP and pP energy, which are sensitive to the average Vp/Vs between the events and the surface, are different between the upper and the lower layers in the double seismic zone, with the lower layer mostly showing negative to normal values and thus low or normal Vp/Vs. This indicates a lack of water in the lower layer and suggests that other mechanisms are needed to interpret intermediate earthquakes rather than dehydration. Key Points: Autocorrelating teleseismic P wave improves focal depth estimates for intermediate‐depth earthquakes, with moment magnitude down to 4.0Slab age correlates with the width of the double seismic zone observed in relocated catalogs for the Japan and Chile subduction zonesThe dry lower layers in the double seismic zones call for other mechanisms to interpret the intermediate‐depth earthquakes [ABSTRACT FROM AUTHOR]