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On April 25, 2015 a major (M W 7.9) thrust earthquake ruptured the deeper portion of the seismogenic plate boundary beneath Nepal along which India is underthrusting Eurasia. An M W 7.2 aftershock on May 12, 2015 extended the eastern, down-dip edge of the rupture. These destructive events caused about 9000 fatalities and 23,000 injuries. The overall rupture zone is about 170 km long and 40–80 km wide. This region of the plate boundary previously experienced a large earthquake in 1833, and in 1934 a larger M S 8.0 event located to the east ruptured all the way to the surface. The Main Himalayan Thrust (MHT) on which slip occurred in 2015 has a very low dip angle of ~ 6°, and the depth of the mainshock slip distribution is very shallow, extending from ~ 7 to ~ 18 km. The shallow dip and depth present challenges for resolving faulting characteristics using teleseismic data. We analyze global teleseismic signals for the mainshock and aftershock to estimate source parameters, evaluating the stability of various procedures used for remotely characterizing kinematics of such shallow faulting. Back-projection and finite-fault slip inversion are used to assess the spatio-temporal rupture history and evidence for frequency-dependent radiation along dip. Slip zone width constraints from near-field geodetic observations are imposed on the preferred models to overcome some limitations of purely teleseismic methods. Radiated energy, stress drop and moment rate functions are determined for both events. [ABSTRACT FROM AUTHOR]

The South Scotia Ridge Transform (SSRT) plate boundary between the Scotia and Antarctic plates experienced large strike-slip earthquakes on August 4, 2003 (Mw 7.6) and November 17, 2013 (Mw 7.8). These events have overlapping aftershock zones, which is unusual. A 36°-45° southward dipping fault zone ruptured with left-lateral displacements in each event along the northern margin of the South Orkney micro-continent near 60°S. Slip distributions for the two events are determined using teleseismic body and surface wave recordings along with constraints from GPS ground motion recordings at station BORC on Laurie Island (South Orkney Islands), just south of the SSRT. The aftershock distributions, high-frequency back-projections, and unconstrained body wave finite-fault inversions permit significant overlap of the 2003 and 2013 slip zones; however, the GPS static displacements resolve differences in the large-slip regions of the two ruptures. The 2013 earthquake sequence along the SSRT initiated with Mw 6.1 (November 13) and Mw 6.8 (November 16) foreshocks located ~50 km west of the mainshock hypocenter, and had aftershocks extending ~250 km eastward. The rupture spread primarily eastward at ~2.5 km/s with a total rupture duration of about 120 s, with two distinct patches of large-slip located northwest and northeast of the South Orkney Islands. The rupture swept past BORC, with high-rate GPS (HRGPS) ground motion recordings capturing the time-varying slip history of the faulting. Traditional GPS data require that the largest-slip region of the shorter rupture in 2003 is located in the gap NNE of BORC between the two patches that ruptured in 2013. There appears to be some overlap of lower slip regions. The complementary slip distributions comprise a relatively uniform offset along this portion of the SSRT, which is one of the most seismically active regions of the entire Antarctic plate boundary. [ABSTRACT FROM AUTHOR]

The Santa Cruz Islands region has high seismicity near a 90° bend in the boundary between the Pacific and Australian plates. Southward, along the Vanuatu island arc, the Australian plate under-thrusts the Pacific plate in a well-defined subduction zone. Westward, a transpressional, predominantly transform boundary extends to the southern Solomon Islands subduction zone. The Santa Cruz Islands region has upper plate strike-slip and normal faulting, plate boundary under-thrusting, outer rise extensional faulting, and intraplate compressional faulting. On February 6, 2013 the largest under-thrusting earthquake (Mw 8.0) that has been instrumentally recorded in the region ruptured the megathrust. The epicenter (10.738°S, 165.138°E) is about 1° north of epicenters of prior large shallow under-thrusting events in Vanuatu in 1934 (M~7.8) and 1966 (MS 7.9), and there is overlap of all three events' aftershock zones, but not their large-slip regions. At least 10 lives were lost, with 6 more missing, due to tsunami ~1.5m high striking the town of Lata and several villages on the main Santa Cruz Island of Nendö (Ndeni) and a nearby small island Nibanga. Inundation of 500m flooded the Lata airport. The tsunami was well-recorded by DART buoys spanning an unusually wide three-quadrant azimuthal aperture. Iterative modeling of teleseismic broadband P waves and the deep-water tsunami recordings resolves the slip distribution. There are two large-slip patches with a southeastward rupture expansion at about 1.5km/s, with the second patch appearing to have ruptured with large slip at the trench. The event has relatively low short-period seismic wave energy release, but a typical overall moment-scaled total energy. The shallow rupture depth may be associated with the low level of short-period energy, and a near-total stress drop may account for a lack of underthrusting aftershocks among a highly productive aftershock sequence that included three events with Mw ≥7.0. [ABSTRACT FROM AUTHOR]

The Pacific/North American plate boundary is undergoing predominantly right-lateral strike–slip motion along the Queen Charlotte and Fairweather transform faults. The Queen Charlotte Fault (QCF) hosted the largest historical earthquake in Canada, the 1949 M S 8.1 strike–slip earthquake, which ruptured from offshore northern Haida Gwaii several hundred kilometers northwestward. On January 5, 2013 an M w 7.5 strike–slip faulting event occurred near the northern end of the 1949 rupture zone. Along central and southern Haida Gwaii the relative plate motion has ∼20% oblique convergence across the left-stepping plate boundary. There had been uncertainty in how the compressional component of plate motion is accommodated. The October 28, 2012 M w 7.8 Haida Gwaii earthquake involved slightly (∼20°) oblique thrust faulting on a shallow (∼18.5°) northeast-dipping fault plane with strike (∼320°) parallel to the QCF, consistent with prior inferences of Pacific Plate underthrusting beneath Haida Gwaii. The rupture extended to shallow depth offshore of Moresby Island beneath a 25–30km wide terrace of sediments that has accumulated in a wedge seaward of the QCF. The shallow thrusting caused seafloor uplift that generated substantial localized tsunami run-up and a modest far-field tsunami that spread across the northern Pacific, prompting a tsunami warning, beach closure, and coastal evacuation in Hawaii, although ultimately tide gauges showed less than 0.8m of water level increase. The mainshock rupture appears to have spread with a ∼2.3km/s rupture velocity over a length of ∼150km, with slip averaging 3.3m concentrated beneath the sedimentary wedge. The event was followed by a substantial aftershock sequence, in which almost all of the larger events involve distributed intraplate normal faulting extending ∼50km oceanward from the QCF. The highly oblique slip partitioning in southern Haida Gwaii is distinctive in that the local plate boundary-parallel motion on the QCF may be accommodated either by infrequent large strike–slip ruptures or by aseismic creep, as seems to be the case for deeper oblique relative plate motion beneath Haida Gwaii, while the sedimentary terrace accumulates plate boundary-perpendicular compressional strain that releases in almost pure thrust faulting earthquakes, seaward of the QCF. [ABSTRACT FROM AUTHOR]

Highlights: [•] The December 7, 2012 Japan Trench earthquake was an intraplate doublet. [•] The doublet involved a thrust sub-event followed by a normal fault sub-event. [•] The complex faulting is resolved by multiple sub-event waveform inversions. [•] The deep thrust event is a rare type of faulting attributed to earlier events. [•] Other recent large deep intraplate thrusting below trenches is discussed. [Copyright &y& Elsevier]

Abstract: A large, high quality P-wave data set comprising short-period and broadband signals sampling four separate regions in the lowermost mantle beneath the Cocos plate, Mexico, the central Pacific, and the north Pacific is analyzed using regional one-dimensional double-array stacking and modelling with reflectivity synthetics. A data-screening criterion retains only events with stable PcP energy in the final data stacks used for modelling and interpretation. This significantly improves the signal stacks relative to including unscreened observations, allows confident alignment on the PcP arrival and allows tight bounds to be placed on P-wave velocity structure above the core–mantle boundary (CMB). The PcP reflections under the Cocos plate are well modelled without any ultra-low velocity zone from 5 to 20°N. At latitudes from 15 to 20°N, we find evidence for two P-wave velocity discontinuities in the D″ region. The first is ∼182km above the CMB with a δln V p of +1.5%, near the same depth as a weaker discontinuity (<+0.5%) observed from 5 to 15°N in prior work. The other reflector is ∼454km above the CMB, with a δln V p of +0.4%; this appears to be a shallower continuation of the joint P- and S-wave discontinuity previously detected south of 15° N, which is presumed to be the perovskite to post-perovskite phase transition. The data stacks for paths bottoming below Mexico have PcP images that are well matched with the simple IASP91 structure, contradicting previous inferences of ULVZ presence in this region. These particular data are not very sensitive to any D″ discontinuities, and simply bound them to be <∼2%, if present. Data sampling the lowermost mantle beneath the central Pacific confirm the presence of a ∼15-km thick ultra-low velocity zone (ULVZ) just above the CMB, with δln V p and δln V s of around −3 to −4% and −4 to −8%, respectively. The ULVZ models predict previous S-wave data stacks well. The data for this region indicate laterally varying V p discontinuities in D″, with one subregion having a δln V p of 0.5% 140km above the CMB. Beneath the north Pacific, the PcP arrivals are compatible with only weak ULVZ (δln V p ∼0 to −3%), and there is a weak D″ reflector with δln V p =0.5%, near 314km above the CMB. These results indicate localized occurrence of detectable ULVZ structures rather than ubiquitous ULVZ structure and emphasize the distinctiveness between the large low shear velocity province under the central Pacific and circum-Pacific regions. [Copyright &y& Elsevier]

Seismological detections of complex structures in the lowermost mantle boundary layer (the D″ region) motivate a conceptual model of a compositionally stratified thermo-chemical boundary layer (TCBL) within which lateral temperature variations (sustained by large-scale mid-mantle flow) cause variations of partial melt fraction. Partial melt fractions of from 0 to 30% in the TCBL occur due to the eutectic of the boundary layer material lying below the high temperature at the core–mantle boundary (CMB), coupled with the presence of steep thermal gradients across the boundary layer. Regions of the TCBL with the highest temperatures have extensive partial melting, producing lateral chemical variations and strong effects on seismic velocities and boundary layer dynamics. This TCBL concept provides a relatively simple framework that can plausibly account for diverse seismological observations such as: predominance of large-scale volumetric elastic wave velocity heterogeneity in the boundary layer, laterally extensive, but intermittent abrupt shear and compressional velocity increases and decreases at the top of the D″ region, small-scale topography of D″ velocity discontinuities, thin ultra-low velocity zones at the CMB, widespread shear wave anisotropy in D″, bulk sound velocity anomalies detected in low shear velocity regions of D″, and large-scale upwellings from the most extensively melted regions of the boundary layer. Neutral or negative buoyancy of the partial melt in the TCBL is required, along with an increase in bulk modulus and some density increase of the boundary layer material relative to the overlying mantle. More complex models, in which the partially melted chemical boundary layer is laterally displaced by mid-mantle downwellings are also viable, but these appear to require additional special circumstances, such as a phase change, efficient segregation of slab crustal material, or abrupt onset of anisotropy, in order to account for rapid velocity increases at the top of D″. The precise nature of the compositional anomaly, or anomalies, in D″ and the eutectic composition of this zone are yet to be determined, but it appears likely that partial melting within the lowermost mantle plays a paramount role in the observed seismic velocity heterogeneity. Increased resolution geodynamic calculations of dense boundary layer structures with partial melting (and attendant viscosity reductions and chemical heterogeneity) are needed to quantify this TCBL concept. [Copyright &y& Elsevier]

The D″ region in the lowermost mantle beneath the Caribbean and Central America is investigated using shear waves from South American earthquakes recorded by seismic stations in North America. We present a large-scale, composite study of volumetric shear velocity heterogeneity, anisotropy, and the possible presence of a D″ discontinuity in the region. Our data set includes: 328 S(Sdiff)-SKS differential travel times, 300 ScS-S differential travel times, 125 S(Sdiff) and 120 ScS shear wave splitting measurements, and 297 seismograms inspected for Scd, the seismic phase refracted from a high-velocity D″ layer. Broadband digital data are augmented by high-quality digitized analog WWSSN data, providing extensive path coverage in our study area. In all, data from 61 events are utilized. In some cases, a given seismogram can be used for velocity heterogeneity, anisotropy, and discontinuity analyses. Significant mid-mantle structure, possibly associated with the ancient subducted Farallon slab, affects shear wave travel times and must be corrected for to prevent erroneous mapping of D″ shear velocity. All differential times are corrected for contributions from aspherical mantle structure above D″ using a high-resolution tomography model. Travel time analyses demonstrate the presence of pervasive high velocities in D″, with the highest velocities localized to a region beneath Central America, approximately 500–700 km in lateral dimension. Short wavelength variability overprints this general high-velocity background. Corrections are also made for lithospheric anisotropy beneath the receivers. Shear wave splitting analyses of the corrected waveforms reveal D″ anisotropy throughout the study area, with a general correlation with heterogeneity strength. Evidence for Scd arrivals is pervasive across the study area, consistent with earlier work, but there are a few localized regions (100–200 km) lacking clear Scd arrivals, which indicates heterogeneity in the thickness or velocity gradients of the high-velocity layer. While small-scale geographic patterns of heterogeneity, anisotropy, and discontinuity are present, the details appear complex, and require higher resolution array analyses to fully characterize the structure. Explanations for the high-shear wave speeds, anisotropy, and reflector associated with D″ beneath the Caribbean and Central America must be applicable over a lateral scale of roughly 1500 km2, the dimension over which we observe coherent wavefield behavior in the region. A slab graveyard appears viable in this regard. [Copyright &y& Elsevier]

• The 2020 M W 7.8 Alaska earthquake occurred in the eastern portion of the Shumagin Gap. • It involved a patchy rupture of the deeper portion of the subduction megathrust. • It has larger magnitude and longer duration compared to the last major event in 1917. • Modest coseismic slip is compatible with geodetic estimates of low seismic coupling. • Further efforts to estimate the seismic coupling of the shallow interface are warranted. The earthquake potential of the Shumagin seismic gap along the Alaska Peninsula (∼162°W to ∼158.5°W) has been debated for more than 40 years. On 22 July 2020, the eastern half of the gap hosted an M W 7.8 earthquake involving a patchy rupture of the megathrust in the depth range of 20 to 45 km. The space-time slip distribution is determined by joint inversion of teleseismic P and SH waves and static displacements from regional GPS stations. The event initiated near the epicenter of the 10 November 1938 (M W 8.2) event, and ruptured westward, with little/no overlap with the 1938 rupture zone. The main slip patch has peak slip of ∼3.8 m below the Shumagin Islands, and produced ∼30 cm uplift and ∼25 cm SSE horizontal displacement on Chernabura Island. The slip model predicts well the small (<1 cm) tsunami signals persisting for more than ten hours observed at deep-water DART seafloor pressure recordings along the Alaska-Aleutian arc. Aftershocks with depths from 20 to 40 km fringe the large-slip patches, and show westward concentration during the first month after the mainshock. Aftershocks up-dip of the 1948 M W 7.1 event contribute to the high level of modest-size background seismicity extending to the trench in the region of very low seismic coupling (0.0–0.1) in the western Shumagin gap east of the 1 April 1946 (M W 8.6) rupture zone. The 31 May 1917 event is the last major earthquake to rupture the eastern half of the Shumagin gap, and has a lower surface wave magnitude (M S G-R 7.4, horizontal components) compared to the 2020 event (M S G-R 7.7, vertical components). Comparison of instrument-equalized waveforms for the 1917 and 2020 events indicates similar size contrast and differences in overall rupture duration and slip complexity. The 2020 rupture has average slip of ∼1.9 m over the 3600 km2 region with co-seismic slip ≥1 m. This is much less than the ∼6.7 m of potentially accumulated slip deficit since 1917, consistent with geodetic estimates of low average seismic coupling coefficient of 0.1–0.4. The megathrust seaward of the 2020 event has low seismicity and may either be aseismic or capable of comparable size ruptures. Comparisons are made with other subduction zones that have experienced relatively deep megathrust slip in regions with moderate seismic coupling. [ABSTRACT FROM AUTHOR]

• The 2020 Paramushir rupture involved near-trench intraplate compressional faulting. • It occurred seaward of a large-slip region in the 1952 M W 9.0 Kamchatka rupture. • Temporal variations of near-trench activity occur relative to interplate thrusts. • Large compressional activity is more common before and long after major thrusts. • Compressional events along coupled zones suggest interplate strain accumulation. Large compressional-faulting earthquakes located relatively deep in oceanic lithosphere entering subduction zones are primarily caused by plate bending stress, but their timing, depth extent and size can be influenced by temporally-varying shear stress on the plate boundary. The 25 March 2020 M W 7.5 event in the Pacific plate seaward of Paramushir Island (northern Kuril Islands), is among the largest recorded events of this type. Its rupture extends along a large-slip region in the southwestern portion of the 1952 Kamchatka M W 9.0 rupture zone. This region has somewhat lower interplate coupling than the megathrust fault along Kamchatka to the northeast, but there could be 68 yrs of strain accumulation. The 2020 event is considered in the context of the 24 recorded major (M W ≥ 7.0) near-trench intraplate compressional-faulting events. An updated compilation of temporally varying near-trench intraslab faulting relative to major interplate ruptures indicates that the stress cycles on the plate boundary influence both extensional and compressional near-trench faulting caused by plate bending. Particularly noteworthy are such events seaward of areas presumed to be in an advanced stage of their seismic cycle, including relatively shallow compressional events along the 1944 M W 8.1 Tonankai, Japan rupture zone, along with activity along the 1952 Kamchatka and 1922 Chile rupture zones. [ABSTRACT FROM AUTHOR]

The 26 May 2019 Peru (M W 8.0) earthquake struck within the nearly-horizontal underthrust Nazca plate at depths from ∼110 to 150 km below the upper Amazon, near a steep bend in the plate where it plunges down to a deep earthquake zone. Little prior seismicity occurred in this region, but large intraslab events with similar normal-faulting mechanisms have occurred to the west. The event is situated in a similar slab position to the 2017 Puebla-Morelos, Mexico earthquake, but the remote location resulted in limited loss of life and damage. Back-projection imaging and finite-fault inversion based on teleseismic data suggest a brittle and energetic rupture process with unilateral expansion northward over a 170-km-long zone at a rupture speed, V r ∼3 km/s, with three normal-faulting patches of up to ∼4.5 m slip. Despite the mainshock size, it produced only three M4.0+ aftershocks within 300 km (one nearby); the aftershock productivity of the 2019 Peru earthquake is very low even among all large intermediate-depth earthquakes, but similar to that for other large Peruvian intraslab events. Comparison of aftershock productivity of the Peru event with that of global large earthquakes in various tectonic settings suggests that the low aftershock productivity can largely be attributed to regionally homogeneous faulting systems and relatively uniform stress state in the flat Peru slab. • The 2019 M W 8.0 Peru event occurred within the flat slab with little prior activity. • Rupture expanded unilaterally northward ∼170 km with three distinct slip patches. • Only three M ≥ 4.4 aftershocks were detected within a month after the great event. • Other large deep events in the Peru flat slab also have low aftershock productivity. • Homogeneity in faulting geometry and stress state accounts for the low productivity. [ABSTRACT FROM AUTHOR]

Most seismological models for the interior of Mars lack an upper mantle low velocity zone. However, there is expected to be a large thermal gradient across the stagnant conductive lid (lithosphere) of Mars. This gradient should tend to decrease elastic wave velocities with increasing depth, with this effect dominating the opposing tendency caused by increasing pressure with depth because Mars has low gravity. An upper mantle lithosphere with a low velocity zone (LVZ) beneath a thin high velocity “seismic lid” is thus predicted. The upcoming NASA InSight mission includes a three-component seismometer, which should provide the first opportunity to directly detect any lithospheric LVZ in Mars. Seismic wavefields expected for Mars mantle velocity structures with or without a strong LVZ are very distinct. The LVZ models predict shadow zones for high-frequency seismic body wave phases such as P , S , PP and SS , etc. The most diagnostic waves that can be used to evaluate presence of a lithospheric LVZ given a single seismometer are intermediate-period global surface waves, which travel along the great circle from a seismic source to the seismometer. An LVZ produces distinctive dispersion, with a Rayleigh wave Airy phase around 100 s period and very different surface wave seismograms compared to a model with no LVZ. Even a single observation of long-period surface waves from a known range can be diagnostic of the lithospheric structure. Establishing the existence of an LVZ has major implications for thermal evolution, volatile content and internal dynamics of the planet. [ABSTRACT FROM AUTHOR]

• Innovative joint inversion techniques resolve complementary co-seismic slip and afterslip distributions. • Mainshock triggering is likely dominated by a foreshock cascade mechanism. • Early afterslip shows distinct seismically or aseismically dominated behavior on orthogonal faults. • Co-seismic stress changes cause afterslip migration on the SW trending fault. Cascade-up and/or slow-slip processes are commonly believed to control interactions between foreshocks, mainshocks and aftershocks, but their relative contributions remain poorly resolved. Discrimination between these processes will shed light on the understanding of earthquake physics, which requires exceptional observations of earthquake sequences. The well-recorded July 2019 Ridgecrest, California foreshock-mainshock-aftershock earthquake sequence provides such an opportunity. We perform simultaneous inversion of the July 4th M W 6.4 foreshock and July 5th M W 7.1 mainshock kinematic rupture models using SAR, strong motion, and GPS data. We also invert for afterslip models following the M W 6.4 foreshock and the mainshock, respectively, by developing an inversion method that utilizes strainmeter, SAR and daily GPS time series. The inversion results show that the overall sequence involves no less than six fault segments, which include a main northwest-trending fault and secondary faults with sub-parallel and orthogonal geometry to the main fault. Co-seismic slip and afterslip have complementary patterns on the faults. During the early post-seismic period following the M W 6.4 foreshock and the mainshock, moment release on the southwest-trending fault is dominated by aseismic slip, in contrast to the predominantly seismic slip on the northwest-trending fault. The mainshock appears to be triggered by a cascade migration of foreshocks on a northwest-trending fault. Slip on the southwest-trending fault migrates from the fault junction at the northeast end (following the M W 6.4 foreshock) to the southwest end (following the mainshock) during the afterslip interval. The dual-mode (seismic versus aseismic) slip phenomena appear to be driven by co-seismic stress changes produced by the major events. [ABSTRACT FROM AUTHOR]