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The magnitude 8.7 earthquake that occurred off the coast of the Sumatra subduction zone on 11 April 2012 is shown to have had an extraordinarily complex four-fault rupture; these great ruptures represent large lithospheric deformation that may eventually lead to a localized boundary between the Indian and Australian plates. The 11 April east Indian Ocean earthquakes On 11 April 2012, two of the largest strike-slip earthquakes ever recorded — at magnitudes of 8.7 and 8.2 — occurred in the northeastern Indian Ocean, a few hundred kilometres off the coast of Sumatra. Three groups now report the analysis of seismic data from the days and months before and after these events, as well as the events themselves. Matthias Delescluse and co-authors show that these earthquakes are part of the ongoing boost of intraplate deformation between India and Australia that followed the Aceh 2004 and Nias 2005 megathrust earthquakes. They conclude that the Australian plate, driven by slab-pull forces at the Sunda trench, is gradually detaching from the Indian plate. Han Yue and colleagues show that the 11 April event involved a complex four-fault rupture lasting several minutes, followed two hours later by a magnitude-8.2 aftershock. These great ruptures on a lattice of strike-slip faults that extends through the crust and into the upper mantle represent large lithospheric deformation that may eventually create a localized boundary between the Indian and Australian plates. Fred Pollitz and colleagues show that, in the six days following 11 April, the global rate of remote earthquakes with magnitudes greater than 5.5 increased nearly fivefold, and events up to magnitude 7 seem to have been triggered. The unprecedented delayed triggering power of this earthquake may arise from its strike-slip source geometry, or because it struck at a time of an unusually low global earthquake rate and increased the number of nucleation sites that were very close to failure. The Indo-Australian plate is undergoing distributed internal deformation caused by the lateral transition along its northern boundary—from an environment of continental collision to an island arc subduction zone 1 , 2 . On 11 April 2012, one of the largest strike-slip earthquakes ever recorded (seismic moment magnitude M w 8.7) occurred about 100–200 kilometres southwest of the Sumatra subduction zone. Occurrence of great intraplate strike-slip faulting located seaward of a subduction zone is unusual. It results from northwest–southeast compression within the plate caused by the India–Eurasia continental collision to the northwest, together with northeast–southwest extension associated with slab pull stresses as the plate underthrusts Sumatra to the northeast. Here we use seismic wave analyses to reveal that the 11 April 2012 event had an extraordinarily complex four-fault rupture lasting about 160 seconds, and was followed approximately two hours later by a great ( M w 8.2) aftershock. The mainshock rupture initially expanded bilaterally with large slip (20–30 metres) on a right-lateral strike-slip fault trending west-northwest to east-southeast (WNW–ESE), and then bilateral rupture was triggered on an orthogonal left-lateral strike-slip fault trending north-northeast to south-southwest (NNE–SSW) that crosses the first fault. This was followed by westward rupture on a second WNW–ESE strike-slip fault offset about 150 kilometres towards the southwest from the first fault. Finally, rupture was triggered on another en échelon WNW–ESE fault about 330 kilometres west of the epicentre crossing the Ninetyeast ridge. The great aftershock, with an epicentre located 185 kilometres to the SSW of the mainshock epicentre, ruptured bilaterally on a NNE–SSW fault. The complex faulting limits our resolution of the slip distribution. These great ruptures on a lattice of strike-slip faults that extend through the crust and a further 30–40 kilometres into the upper mantle represent large lithospheric deformation that may eventually lead to a localized boundary between the Indian and Australian plates.

Continental mantle earthquakes are uncommon but hold important clues for understanding lithospheric rheology. Few of these earthquakes (<10) have been documented in western North America, though it is likely more exist owing to difficulties in resolving focal depth for small earthquakes. Mapping their distribution in western North America is key to better characterizing cratonic evolution in this area. Here, we evaluate 25 nominally lower crustal and upper mantle earthquakes in the Milk River region of southwestern Alberta. We absolutely relocate each earthquake and compare depths to published estimates of Moho depth and find 17 earthquakes locate ∼8–16 km below the Moho. High‐precision relative relocations and first‐motions define a steeply west‐dipping, Moho‐crossing normal fault. We hypothesize these earthquakes are related to regional mantle flow patterns interacting with lithospheric edges and are part of a larger trend of upper mantle seismicity along the western edge of North American cratonic lithosphere.

Subduction zone plate boundary megathrust faults accommodate relative plate motions with spatially varying sliding behavior. The 2004 Sumatra‐Andaman (Mw 9.2), 2010 Chile (Mw 8.8), and 2011 Tohoku (Mw9.0) great earthquakes had similar depth variations in seismic wave radiation across their wide rupture zones – coherent teleseismic short‐period radiation preferentially emanated from the deeper portion of the megathrusts whereas the largest fault displacements occurred at shallower depths but produced relatively little coherent short‐period radiation. We represent these and other depth‐varying seismic characteristics with four distinct failure domains extending along the megathrust from the trench to the downdip edge of the seismogenic zone. We designate the portion of the megathrust less than 15 km below the ocean surface as domain A, the region of tsunami earthquakes. From 15 to ∼35 km deep, large earthquake displacements occur over large‐scale regions with only modest coherent short‐period radiation, in what we designate as domain B. Rupture of smaller isolated megathrust patches dominate in domain C, which extends from ∼35 to 55 km deep. These isolated patches produce bursts of coherent short‐period energy both in great ruptures and in smaller, sometimes repeating, moderate‐size events. For the 2011 Tohoku earthquake, the sites of coherent teleseismic short‐period radiation are close to areas where local strong ground motions originated. Domain D, found at depths of 30–45 km in subduction zones where relatively young oceanic lithosphere is being underthrust with shallow plate dip, is represented by the occurrence of low‐frequency earthquakes, seismic tremor, and slow slip events in a transition zone to stable sliding or ductile flow below the seismogenic zone. Key Points Seismic radiation from megathrust earthquake rupture varies with depth A 4‐domain model of radiation segmentation is introduced for megathrusts Strong‐ground motions originate from the down‐dip region

Earthquakes in continental regions overwhelmingly occur in the crust where low pressure and temperature promote brittle failure in response to tectonic stress. In rare cases, primarily in the thickened lithosphere near the Himalayas and Tibet, continental earthquakes occur in the uppermost mantle, perhaps implying an abnormally deep brittle‐ductile transition zone created by relatively low temperatures (≲600°C) and the increased strength of olivine‐rich mantle rocks. Here we present evidence for nine mantle earthquakes—only four of which were previously recognized—along the edge of the Wyoming Craton in the western U.S. Eight of the nine earthquakes occurred >15 km beneath the Moho where temperatures are likely above 700°C. We infer a mixture of brittle and ductile (thermal runaway) source processes facilitated by elevated strain rates from regional or edge‐driven mantle convection, which is thought to be a primary force behind crustal seismicity in the Intermountain West. Plain Language Summary Continental earthquakes typically occur in the uppermost 10 km of the crust. In rare cases, they can occur deeper in the uppermost mantle. The Intermountain West of the U.S. provides convincing evidence of intraplate upper mantle seismicity on the edge of the ancient Wyoming Craton in Utah and Wyoming, a portion of the stable interior of the North American continent. We present the most up to date map of upper mantle seismicity—nine confirmed events in total—beneath the Wyoming Craton through verification of earthquake depths and comparisons to crustal thickness. We find that these earthquakes are likely occurring in ductile mantle material at temperatures exceeding 700°C and are located in areas exhibiting rapid changes in lithospheric thickness. These earthquakes are likely facilitated by regional or localized mantle convective forces interacting with complex lithospheric structure, which is suspected to be a leading cause of crustal seismicity in the Intermountain West. Key Points We document the occurrence of nine small earthquakes in the upper mantle beneath Wyoming and Utah between 1979 and 2023 The earthquakes occurred along the western edge of the Wyoming Craton at relatively high temperatures (>700°C) Brittle and ductile (thermal runaway) source processes are likely facilitated by relatively high strain rates from mantle convection

Teleseismic short‐period (0.5–5 s) P waves from the 27 February 2010 Chile earthquake (Mw 8.8) are back projected to the source region to image locations of coherent short‐period seismic wave radiation. Several receiver array configurations are analyzed using different P wave arrivals, including networks of stations in North America (P), Japan (PKIKP), and Europe (PP), as well as a global configuration of stations with a broad azimuthal distribution and longer‐period P waves (5–20 s). Coherent bursts of short‐period radiation from the source are concentrated below the Chilean coastline, along the downdip portion of the megathrust. The short‐period source region expands bilaterally, with significant irregularity in the radiation. Comparison with finite fault slip models inverted from longer‐period seismic waves indicates that the regions of large slip on the megathrust are located updip of the regions of short‐period radiation, a manifestation of frequency‐dependent seismic radiation, similar to observations for the great 2011 Tohoku earthquake (Mw 9.0). Back projection of synthetic P waves generated from the finite fault models demonstrates that if the short‐period energy had radiated with the same space‐time distribution as the long‐period energy, back‐projection analysis would image it in the correct location, updip. We conclude that back‐projection imaging of short‐period signals provides a distinct view of the seismic source that is missed by studies based only on long‐period seismic waves, geodetic data, and/or tsunami observations. Key Points Short‐period energy was radiated downdip of the major slip release Rupture velocity was faster for the short‐period energy release Back projection provides information that is complementary to slip models

The solid inner core, suspended within the liquid outer core and anchored by gravity, has been inferred to rotate relative to the surface of Earth or change over years to decades based on changes in seismograms from repeating earthquakes and explosions 1 , 2 . It has a rich inner structure 3 – 6 and influences the pattern of outer core convection and therefore Earth’s magnetic field. Here we compile 143 distinct pairs of repeating earthquakes, many within 16 multiplets, built from 121 earthquakes between 1991 and 2023 in the South Sandwich Islands. We analyse their inner-core-penetrating PKIKP waves recorded on the medium-aperture arrays in northern North America. We document that many multiplets exhibit waveforms that change and then revert at later times to match earlier events. The matching waveforms reveal times at which the inner core re-occupies the same position, relative to the mantle, as it did at some time in the past. The pattern of matches, together with previous studies, demonstrates that the inner core gradually super-rotated from 2003 to 2008, and then from 2008 to 2023 sub-rotated two to three times more slowly back through the same path. These matches enable precise and unambiguous tracking of inner core progression and regression. The resolved different rates of forward and backward motion suggest that new models will be necessary for the dynamics between the inner core, outer core and mantle. Matching seismic waveforms show that the inner core of Earth gradually super-rotated from 2003 to 2008, and then more slowly sub-rotated from 2008 to 2023 back through the same path.

Earth’s inner core acquires texture as it solidifies within the fluid outer core. The size, shape and orientation of the mostly iron grains making up the texture record the growth of the inner core and may evolve over geologic time in response to geodynamical forces and torques 1 . Seismic waves from earthquakes can be used to image the texture, or fabric, of the inner core and gain insight into the history and evolution of Earth’s core 2 – 6 . Here, we observe and model seismic energy backscattered from the fine-scale (less than 10 km) heterogeneities 7 that constitute inner core fabric at larger scales. We use a novel dataset created from a global array of small-aperture seismic arrays—designed to detect tiny signals from underground nuclear explosions—to create a three-dimensional model of inner core fine-scale heterogeneity. Our model shows that inner core scattering is ubiquitous, existing across all sampled longitudes and latitudes, and that it substantially increases in strength 500–800 km beneath the inner core boundary. The enhanced scattering in the deeper inner core is compatible with an era of rapid growth following delayed nucleation. We create a three-dimensional model of inner core fine-scale heterogeneity, showing that inner core scattering is ubiquitous and that it substantially increases in strength 500–800 km beneath the inner core boundary.

We used the Yellowknife seismic array (YKA) to measure the slowness of 1,371 P and Pdiff waves from earthquakes occurring in the circum‐Pacific region. The corresponding anomalies in P‐velocity show a sharp reduction of up to 6% across a patch of the lowermost mantle beneath the Northwest Pacific with lateral dimensions of several hundred kilometers. The location of this ultra low velocity zone (ULVZ) correlates with a long‐wavelength compositional boundary revealed by probabilistic mantle tomography. We interpret the ULVZ as partial melt created by paleo‐slab material that is being swept laterally from northwestern Pacific subduction zones towards the large, chemically distinct province beneath the south‐central Pacific.

The short‐period (0.5–2 s) seismic radiation properties of the August 15 (23:40:57 UTC) 2007 Mw8.0 Pisco, Peru earthquake are imaged by back‐projecting P waves recorded at 374 elements of USArray deployed in western North America at distances of 54°–74° from the source region. The coherent short‐period seismic energy release has two main intervals similar to moment‐rate functions determined by inversion of longer‐period teleseismic body waves; however, the spatial locations of the coherent bursts of short‐period energy release are located north and down‐dip of the region of major slip. The contrast between short‐ and long‐period seismic radiation properties of the Pisco earthquake is more subtle than for the 2011 Mw9.0 Tohoku earthquake, but provides further support for the idea of depth‐dependent changes in sliding behavior during megathrust ruptures. Key Points Down‐dip portion of the 2007 Pisco earthquake was enriched in high frequencies Observations are similar to those made for other recent great earthquakes Along dip rupture segmentation is common in megathrust events

Delineating Deep Faults. Most large, damaging earthquakes initiate in Earth's crust where friction and brittle fracture control the release of energy. Strong earthquakes can occur in the mantle too, but their rupture dynamics are difficult to determine because higher temperatures and pressures play a more important role. Ye et al. (p. 1380) analyzed seismic P waves generated by the 2013 Mw 8.3 Sea of Okhotsk earthquake-the largest deep earthquake recorded to date-and its associated aftershocks. The earthquake ruptured along a fault over 180-kilometer-long and structural heterogeneity resulted in a massive release of stress from the subducting slab. In a set of complementary laboratory deformation experiments, Schubnel et al. (p. 1377) simulated the nucleation of acoustic emission events that resemble deep earthquakes. These events are caused by an instantaneous phase transition from olivine to spinel, which would occur at the same depth and result in large stress releases observed for other deep earthquakes.