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The Tonga subduction zone in the south-west Pacific is the fastest convergent plate boundary in the world with the most active mantle seismicity. This zone shows unique tectonic features including Samoan volcanic lineament of plume-driven origin near the northern rim of the Tonga subducting slab. The proximity of the Samoa hotspot to the slab is enigmatic and invokes debates on interactions between the Samoa plume and the Tonga subduction. Based on long-term observations of intermediate and deep-focus Tonga earthquakes reported in the Global Centroid Moment Tensor (CMT) catalog, we provide novel detailed imaging of this region. Accurate traveltime residua of the P- and S-waves recorded at two nearby seismic stations of the Global Seismographic Network are inverted for the P- and S-wave velocities and their ratio and reveal their pronounced lateral variations. In particular, they differ for the southern and northern parts of the Tonga subduction region. While no distinct anomalies are detected in the southern Tonga segment, striking low-velocity anomalies associated with a high Vp/Vs ratio are observed in the northern Tonga segment close to the Samoa plume. These anomalies spread through the whole upper mantle down to depths of ~ 600 km. Together with the fast extension of the northern back-arc Lau Basin, slab deformation and geochemical enrichment in the northern Tonga region, they trace deep-seated magmatic processes and evidence an interaction of the Tonga subduction with the Samoa plume.

The relative contributions of scattering and viscoelastic attenuation to the apparent attenuation of seismic body waves are estimated from synthetic and observed S waves multiply reflected from Earth’s surface and the core-mantle boundary. The synthetic seismograms include the effects of viscoelasticity and scattering from small-scale heterogeneity predicted from both global tomography and from thermodynamic models of mantle heterogeneity that have been verified from amplitude coherence measurements of body waves observed at dense arrays. Assuming thermodynamic models provide an estimate of the maximum plausible power of heterogeneity measured by elastic velocity and density fluctuations, we predict a maximum scattering contribution of 43 % to the total measured attenuation of mantle S waves having a dominant frequency of 0.05 Hz. The contributions of scattering in the upper and lower mantle to the total apparent attenuation are estimated to be roughly equal. The relative strength of the coda surrounding observed ScSn waves from deep focus earthquakes is not consistent with a mantle having zero intrinsic attenuation.

In this paper, the relationship between the geodynamics and cosmic expansion is analyzed and demonstrated from the basic physical principles and various natural phenomena, and the conclusion is drawn that cosmic expansion is the dynamic force source of all planetary tectonic movements in the universe. The energy for the formation and change of landscape and the energy of earthquakes and volcanic eruptions all come from the cosmic expansion. With the cosmic expansion, the energy density of space is decreasing, the atoms and molecules of all matter in the universe are growing, and the magma is expanding and producing gases. As the earth’s internal pressure rises, the mechanical energy that accumulates within the magma forms the driving force of the earth’s various tectonic movements, and the release of these energy and matter (expansive magma and high-pressure gases) leads to the formation and the changes of the landform (such as orogenesis, epeirogenesis, the formation of the earth’s plates, the Earth expansion, the seafloor spreading, and the continental drift), as well as to earthquakes and volcanic eruptions. In this paper, the causes of all kinds of earthquakes, especially deep focus earthquakes, are given and almost all known seismic phenomena are explained, the basic principle and method of earthquake prediction are given, and the direction is pointed out for the elimination of earthquakes and the utilization of earthquake energy. Based on the same principle of physics, this paper also shows that the Ice-Age is caused by the acceleration of the speed of the motion of the solar system relative to the Milky Way in certain regions of the Milky Way. The greater the speed of the solar system relative to the Milky Way, the greater the drop in Earth surface temperature.

A weakened zone in the lithosphere plunging into the mantle can lead to an earthquake after the application of a shear stress only in the case if the effective viscosity of this zone is very low. At low viscosity, in the short time that elapses after the application of stress, significant displacements of the walls of the zone emerge causing high-amplitude seismic waves. The Andrade law describing the transient creep under constant stress applied at the initial time instant leads to very low effective viscosity a few first seconds after the initial time instant. The effective viscosity also decreases due to the temperature rise in the weakened zone caused by the dissipative release of heat. However, this process is not rapid enough to noticeably change the temperature and effective viscosity in a short time.

The rupture mechanisms of deep-focus (>300 km) earthquakes in subducting slabs of oceanic lithosphere are not well understood and different from brittle failure associated with shallow (P-wave trains recorded at dozens of globally distributed stations. If our inferences are correct, the entirety of the subducting Pacific lithosphere cannot be completely dry at deep, transition-zone depths, and other deep-focus earthquakes may also be associated with deep dehydration reactions.

Deep tectonic tremors occur at the downdip extent of the seismogenic zone due to fluid processes. Beneath the northeastern Kii Peninsula, southwestern Japan, there is an along-dip bimodal distribution of tremor. However, no constraint exists on the structures controlling that distribution. We extract detailed seismological structures from multi-band receiver functions and evaluate conditional differences in the distribution. To achieve high resolution images along the plate interface, we utilize records of regional deep-focus earthquakes from the Pacific slab. Cross-section images show the subducting oceanic plate with depth-dependent phases along the bimodal distribution, revealing a conspicuous plate interface at the updip portion and an inconspicuous interface below the mantle wedge at the downdip portion. This indicates that episodic tremors occur in the high pore-fluid plate interface below the impermeable forearc crust, and that continual tremors occur at the permeable mantle wedge corner, owing to continuous fluid supply from the oceanic crust. [Plain Language Summary] Deep slow earthquakes have mainly been detected at the deeper extent of estimated large-slip regions of large-scale regular earthquakes in the Nankai subduction zone, southwestern Japan. Epicenters of tectonic tremors are also downdip-aligned. However, some clusters of continual tremor with frequent small bursts were found at further downdip portions beneath the northeastern Kii Peninsula. The complexity of the bimodal tremor distribution poses a structural question regarding whether the tectonic tremor occurs below a mantle wedge or below the continental crust. We utilize a receiver function method that surveys subsurface velocity boundaries and evaluate detailed seismological structures around the plate interface using a multi-band analysis. Furthermore, regional deep-focus earthquake records are effectively utilized for receiver function mapping. The high-frequency cross section exhibits depth dependence of plate-interface phases, which demarcates active regions of updip events and downdip continual tremor, thus revealing that episodic tremor occurs below the continental crust and continual tremor occurs at the mantle wedge corner. The high-contrast updip interface reveals that a large amount of fluid is confined at the plate interface below the impermeable forearc crust, which may lead to active episodic slow earthquakes at updip portions.

A negative bay-shaped anomaly in radon volume activity (VA Rn) with an advance time of 10 days was recorded at a network of subsoil gas observing stations in the Petropavlovsk-Kamchatskii Test Site area before the Zhupanov earthquake (ZhE) of January 30, 2016 whose magnitude was 7.2 and the depth of focus was equal to 177 km. The anomaly was simultaneous with a positive anomaly in the concentration of molecular hydrogen. After a lapse of 98 hours after the ZhE, a sharp increase in VA Rn was recorded by gas-discharge counters at the INS site installed in a well. The increase lasted approximately 2 days and is treated as a postseismic effect in the subsoil radon field. This disturbance was followed by two periods (February 5 through 19 and February 23 to March 15) of “high-frequency” oscillations at frequencies of 0.3–6 day–1. These are thought to have been due to self-excited processes in the annular space. The oscillations are separated by a disturbance that is interpreted as a precursory anomaly before the magnitude 6.4 March 20, 2016 earthquake that occurred at a distance of ~300 km from Petropavlovsk-Kamchatskii. The precursory anomaly before the March 20 earthquake is believed to have been due to a high strain sensitivity of the hydrogeological system due to a change in the state of stress and strain in the earth after the ZhE whose epicenter was 100 km from the recording site.

Mantle seismic anisotropy beneath the Amur Plate is studied using the data of ScS waves reflected from the outer core of the Earth from local deep earthquakes in the area of five stations located in Primorye and Priamurye regions. The results of measuring the parameters of the split ScS waves near the stations show the dominance of the polarization azimuths of the fast wave along the eastern directions and are consistent with azimuthal anisotropy and the direction of motion of the Pacific Plate (300°) and Amur Plate (~120°) depending on the epicenter–station direction. In Primorye, in the vicinity of the TEY station, the polarization azimuths of the fast ScS wave dominate in the interval of NE–E directions orthogonally to the lines of the mantle flow along the complex 3D surface of the subsiding Pacific Plate. It is revealed that the delay time of ScS waves increases to 2 s and 3.4 s in the upper mantle and in the mantle transition zone, respectively, as the depth of events increases. The highest degree of anisotropy manifests itself in the upper mantle. In the case of the vertical propagation of waves under conditions of horizontal mantle flow, the difference in the arrival times of the ScS waves is the lowest. The anisotropy in the upper zone of the transition mantle can be related to the wadsleyite texture with the polarization of the fast ScS wave parallel or orthogonally to the motion of the stagnant plate. The anisotropy in the upper part of the lower mantle is associated with the texture of perovskite and periclase with the orientation of the symmetry axis and the polarization of the ScS waves in parallel to the plate subsidence and the subduction direction.

A description of the initial excitation of spheroidal mode 0S2 initiated by the 2013 Sea of Okhotsk earthquake (Mw = 8.3) is presented. It is based on the newly revealed fact of the origination of a standing P-wave in the epicenter of the Sea of Okhotsk earthquake and excitation of the traveling surface P-waves on the Earth’s surface by this wave. A new interpretation of the nature of the Earth’s free spheroidal oscillations in the case of a strong earthquake (M ≥ 7.5) is proposed. The spheroidal mode 0S2 is interpreted as a combination of standing and traveling P-waves, transferring the energy of Earth’s deformations.

We used recent seismic data and advanced techniques to investigate 3D fault geometry over the transition from the partially coupled to the fully coupled plate interface inboard of the Nankai Trough off the Kii Peninsula, Japan. We found that a gently dipping plate boundary decollement with a thick underthrust layer extends beneath the entire Kumano forearc basin. The 1 April 2016 Off-Mie earthquake (Mw6.0) and its aftershocks occurred, where the plate boundary decollement steps down close to the oceanic crust surface. This location also lies beneath the trenchward edge of an older accretionary prism (∼14 Ma) developed along the coast of the Kii peninsula. The strike of the 2016 rupture plane was similar to that of a formerly active splay fault system in the accretionary prism. Thus, the fault planes of the 2016 earthquake and its aftershocks were influenced by the geometry of the plate interface as well as splay faulting. The 2016 earthquake occurred within the rupture area of large interplate earthquakes such as the 1944 Tonankai earthquake (Mw8.1), although the 2016 rupture area was much smaller than that of the 1944 event. Whereas the hypocenter of the 2016 earthquake was around the underplating sequence beneath the younger accretionary prism (∼6 Ma), the 1944 great earthquake hypocenter was close to oceanic crust surface beneath the older accretionary prism. The variation of fault geometry and lithology may influence the degree of coupling along the plate interface, and such coupling variation could hinder slip propagation toward the deeper plate interface in the 2016 event.

International audience; The 30 May 2015 M w 7.9 Bonin Islands earthquake, Japan, is one of the largest deep-focus earthquakes ever recorded. Its occurrence, close to 700-km depth, in an area without any known historical seismicity, along with its magnitude, was a surprise to scientists. Deep earthquakes are generally believed to have few aftershocks and no foreshocks. Here, we explore the earthquake productivity in the hypocentral surroundings and detect 49 not previously identified earthquakes, 28 of which occurred during an accelerating preseismic phase that started 3 months prior to the main shock. This is the first time that such foreshock activity has been observed for a deep earthquake. The preseismic and postseismic activity suggests transformational faulting within a metastable olivine wedge (MOW) inside the slab at depth as the triggering principal mechanism for this deep earthquake sequence, the seismicity starting where the backward bending of the subducting Pacific plate is maximum. Plain Language Summary Deep earthquakes have been puzzling seismologists for almost a century, since their discovery in the 1920s. Earthquakes deeper than 50 km represent about 25% of the global seismicity, and deep-focus earthquakes are defined as those located at 300-km depth or more. They can be exceedingly large and occur at temperatures and pressures where sliding and fracture are inhibited, thus the brittle fracture/friction mechanism, valid in Earth's crust, cannot hold. Despite their abundance, the physical mechanism behind deep-focus earthquakes is still a subject of ongoing debate. The Bonin Islands earthquake (30 May 2015, M w 7.9) occurred at 680-km depth in a previously quiet area. In order to better understand why and how it occurred there, we searched for previously unknown earthquakes in the surroundings. For the first time, we identify a preseismic phase preceding such a very deep-focus earthquake. The location of the newly detected earthquakes reveals the geometry of the slab at depth, and their spatiotemporal distribution supports transformation of metastable olivine as the rupture initiation process.

На примере сейсмического события, обладающего магнитудой и механизмом глубокофокусного землетрясения у берегов Фиджи 19.08.2018, теоретически исследована связь параметров источника цунами с глубиной землетрясения. Показано, что сильные глубокофокусные землетрясения способны создавать обширные области косейсмических деформаций дна, размер которых сопоставим с баротропным радиусом деформации Россби. Волны цунами, формируемые такими источниками, обладают необычайно большой длиной и поэтому подвержены влиянию силы Кориолиса. Это влияние может варьироваться от существенного в высоких широтах до малозначительного в приэкваториальных районах. Методом численного моделирования исследовано влияние силы Кориолиса на слабые волны цунами, вызванные Фиджийским землетрясением 2018 г., Using the example of a seismic event with the same magnitude and mechanism as for the 2018 Fiji deep-focus earthquake the relationship between the parameters of a tsunami source and the depth of an earthquake was theoretically investigated. It is shown that strong deep-focus earthquakes are capable of creating extensive areas of co-seismic deformations for the ocean bottom, the size of which is comparable to the barotropic Rossby deformation radius. Tsunami waves generated by such sources have an unusually large length, and therefore they are subject to the influence of the Coriolis force. The influence of the Coriolis force on the weak tsunami waves caused by the 2018 Fiji earthquake has been studied by numerical simulation. It was found that for a virtual sea-level station located to the south of the source at latitude of 40S, the amplitude of the leading tsunami wave under the influence of the Coriolis force decreased by ≈ 30%. For virtual stations located at the equator or at latitude of 20S, taking into account the Coriolis force changes the amplitude of the waves by no more than 10%. For stations located in the equatorial zone, the effect of delaying of the manifestation of differences in sea-level fluctuations, calculated with and without Coriolis force, as compared with the entry of the leading tsunami wave was discovered. A physical interpretation of the observed delay effect is proposed., №1(24) (2019)

The M w 7.8 Ecuador earthquake on 16 April 2016 is the sixth earthquake larger than M w 7 to rupture the subduction megathrust between the Nazca and South American plates since the M w 8.8 Colombia–Ecuador earthquake in 1906. We use Interferometric Synthetic Aperture Radar (InSAR) images from Sentinel‐1A to determine coseismic surface displacements associated with this earthquake. The interferograms exhibit a relatively simple pattern of deformation, with maximum displacements of 0.7 and 0.3 m on the descending and ascending images, respectively. We invert the interferograms for both rupture geometry and slip distribution in the earthquake. We find that the data are best described by slip on a fault dipping 17° to the east and that these InSAR data cannot uniquely constrain the strike. The maximum inferred slip is just over 2.5 m at about 20 km depth, with the main slip in the depth range of about 10–25 km. The geodetic moment of our preferred slip model is 7.15×10 20 N·m, equivalent to M w 7.87. Our results suggest that there is little, if any, partitioning of the oblique plate convergence. The 2016 Ecuador earthquake is coincident with the location of the 1942 M w 7.8 earthquake, with both earthquakes most likely rupturing an asperity that also failed in the 1906 M w 8.8 earthquake.

This paper presents the results of reconstruction of the modern tectonic stress field of the northern part of the Kuril–Okhotsk region prior to the May 24, 2013 deep-focus earthquake. This earthquake is the strongest deep-focus earthquake not only in the Okhotsk region but also in the world over the entire period of seismic observations. The tectonic stress field is reconstructed using the method of cataclastic analysis (MCA) of the earthquake source mechanism data. New data on the depth variations of regional tectonic stress field are obtained.

The deep-focus Sea of Okhotsk earthquake that occurred on May 24, 2013 (h = 630 km, Mw = 8.3) was accompanied by anomalous effects that were unknown previously. A combined analysis of published data concerning the source rupture evolution and some features of the deep structure provided an explanation of some anomalous effects, such as the large number of aftershocks and the low level of ground shaking in the epicentral area. However, GPS observations revealed high coseismic vertical displacements in the area. The seafloor uplift in the Sea of Okhotsk and the adjacent coasts was 3–12 mm, peaking at the approximate center of the sea, while Kamchatka and the North Kuril Islands subsided by 3–18 mm, peaking at the Apacha station 190 km east of the earthquake epicenter. These maximum estimates are 1.2–1.8 times the analogous values (10 mm) for the Chile mega-earthquake of May 20, 1960 (Mw ~ 9.5). It is known that the large distances at which ground shaking is felt during deep-focus earthquakes are due to the fact that the body waves travel through the high-Q lower mantle. However, this does not explain the paradox of the present earthquake in the Sea of Okhotsk, viz., a constant intensity of shaking (two grades) in the range of epicentral distances between 1300 and 9500 km. The explanation requires consideration of the earth’s free oscillations excited by the earthquake.

We described the February 1, 2011 (Mw = 4.7) Zagan earthquake occurred in the area of the Zagan range. This event is one of the most significant in western Transbaikalia. Macroseismic effects of this seismic event were felt over a wide territory: the intensity of II was observed at the distance of more than 300 km. Earthquake focal parameters (hypocentral depth, seismic moment, moment magnitude and focal mechanism) calculated from the data on surface wave amplitude spectra correspond to the structural position of the origin connected with a zone of plastic flow (slip) bordering the northwest slope of the Zagan range (similar complex of the metamorphic core). The results obtained prove the existence of the continental extension processes in this area.

Earthquakes in the Indo-Burmese wedge occur due to India-Sunda plate motion. These earthquakes generally occur at depth between 25 and 150 km and define an eastward gently dipping seismicity trend surface that coincides with the Indian slab. Although this feature mimics the subduction zone, the relative motion of Indian plate predominantly towards north, earthquake focal mechanisms suggest that these earthquakes are of intra-slab type which occur on steep plane within the Indian plate. The relative motion between the India and Sunda plates is accommodated at the Churachandpur-Mao fault (CMF) and Sagaing Fault. The 4 January 2016 Manipur earthquake (M 6.7) is one such earthquake which occurred 20 km west of the CMF at ∼60 km depth. Fortunately, this earthquake occurred in a very sparse population region with very traditional wooden frame houses and hence, the damage caused by the earthquake in the source region was very minimal. However, in the neighbouring Imphal valley, it caused some damage to the buildings and loss of eight lives. The damage in Imphal valley due to this and historical earthquakes in the region emphasizes the role of local site effect in the Imphal valley.

We analyzed macroseismic data and considered the effect of extremely long range propagation of sensible shocks during the deep-focus earthquake in the Sea of Okhotsk on May 24, 2013 (Mw = 8.3). In order to explain this effect, we formulated and qualitatively solved the problem of superposition of P-waves over the radial mode 0S0 of the natural oscillations of the Earth during this earthquake. Our results confirmed the possibility of such an interpretation of the observed macroseismic effect and also allowed us to explain the fact of anomalously low decay of seismic disturbances with distance.

The 30 May 2015 Chichi-jima M8 earthquake is one of the largest deep focus earthquakes ever recorded and its depth of 682 km puts it near the base of the mantle transition zone. Before source mechanisms and slip models of this earthquake can be reliably assessed, a better understanding of the tectonic setting and structures of the region near the origin is required. Here we present evidence from receiver functions, a method of isolating subsurface material contrast with converted seismic waves, that the earthquake initiated within the upper mantle transition zone, above a significantly depressed 660 km phase boundary. Additionally, we observe multiple conversions within and below the transition zone, which we associate with seismic waves passing into and out of segments of the subducting Pacific plate. From this, we infer slab material is piling up at the base of the transition zone and segments are penetrating into the lower mantle.

In this study, the source location of the 30 May 2015 (Mw 7.9) deep-focus Bonin earthquake was constrained using P wave seismograms recorded across Japan. We focus on propagation characteristics of high-frequency P wave. Deep-focus intraslab earthquakes typically show spindle-shaped seismogram envelopes with peak delays of several seconds and subsequent long-duration coda waves; however, both the main shock and aftershock of the 2015 Bonin event exhibited pulse-like P wave propagations with high apparent velocities (~12.2 km/s). Such P wave propagation features were reproduced by finite-difference method simulations of seismic wave propagation in the case of slab-bottom source. The pulse-like P wave seismogram envelopes observed from the 2015 Bonin earthquake show that its source was located at the bottom of the Pacific slab at a depth of ~680 km, rather than within its middle or upper regions.

Robust relations correlating 10 different magnitudes of intermediate‐depth and deep‐focus earthquakes to moment magnitude are proposed, in order to be efficiently incorporated into the compilation process of homogeneous (with respect to magnitude) earthquake catalogs. By using global data available from International Seismological Centre (ISC), National Earthquake Information Center (NEIC), Comprehensive Nuclear‐Test‐Ban Treaty Organization’s International Data Centre (IDC), Institute of Physics of the Earth in Moscow, Russia, and China Earthquake Networks Center in Beijing, the performance of several widely used magnitude scales, such as body wave ( m b , m B ) and surface wave ( M s ), is examined with respect to the moment magnitude scale ( M w ). Similarly, appropriate M w ‐calibrated relations are also provided for regional magnitude scales such as the M JMA magnitude calculated by the Japan Meteorological Agency. The analysis also involves the integration of focal depth as an additional variable to some of the above magnitude‐conversion relations. This depth effect proved to be important for ISC/NEIC’s body‐wave ( m b IN) and surface‐wave ( M s IN) magnitudes, leading to significant corrections for the estimated magnitudes. More specifically, a major change in the magnitude residual variation around the depth of 230 km was identified for the case of m b IN. Furthermore, the obtained results provide important observations on the behavior of certain magnitude scales. A typical case is the m b scale reported by IDC, which shows a systematic and large bias, with respect to the published M w data values.

The earthquake that occurred on May 24, 2013, in the basin of the Sea of Okhotsk with a magnitude of 8.3 was the strongest in this region. We have modeled a possible tsunami caused by such an earthquake. The simulations confirm that the wave heights were sufficiently small because the earthquake epicenter depth was 640 km. We analyze the oscillations of the DART buoys in the vicinity of the earthquake source and show that they were not associated with the tsunami waves. Analysis of the available pressure gauge records at different points of the Sea of Okhotsk show that only in one case (Iturup Island) can the observed oscillations of the sea level with a height of approximately 4 cm be classified as tsunami waves.

The idea of the formation of deep-focus earthquakes does not fit into the framework of Mr. Reid's theory of Elastic recoil explaining the origin of earthquakes as the emission of the energy of elastic deformations by the rocks of the earth's interior due to the simple physical fact of the absence of elastic deformations at a depth of several tens of kilometers. More can be said, if humanity had not experienced the devastating catastrophes caused by deep-focus earthquakes, scientists, in the person of Cambridge University professor Mr. Jeffries, would still have argued that such events cannot take place in the bowels of the Earth. Indeed, it is a very, very dubious occupation to expect elastic deformations in hot, semi-molten rocks at depths of hundreds of kilometers. On the other hand, seismologists represented by Mr. C. Frohlich of the University of Austin, Texas continue to cling stubbornly to the idea of mythical elastic stresses of the rocks, allegedly occurring in the Wadati-Benioff subduction zones. As a result of the dispute between the two schools, deep-focus earthquakes turned out to be an established scientific fact, therefore, Mr. Jeffries and Co. were wrong. But Mr. Frohlich and Co turned out to be wrong, for the deep-focus earthquakes that occurred outside the subduction zones completely and without any hope destroyed the idea of elastic deformations at great depths. Is the intrigue preserved? Not! In fact, the process of forming a deep focus earthquake is simple and is a classic hydraulic shock. From hydrodynamics, it is known that where fluid flows through pipelines, in our case, magma flows through cracks in the Earth’s mantle, there have always been, are and will happen, hydraulic shocks.

At transition zone depths in subduction zones, deep-focus earthquakes (300–690 km depth) are thought to be associated with faulting that arises from phase transformations. In order to test the viability of this mechanism experimentally, an investigation was conducted on fayalite at high pressure, P, and high temperature, T, under deviatoric stress in order to initiate transformational faulting. Experiments were performed in a 3000-ton multi-anvil press using an 18/11 octahedral cell with 6 piezoelectric transducers mounted on the rear side of the anvils to monitor acoustic activity in situ. Acoustic emission (AE) signals were collected at a sampling rate of 40 MHz using a triggered system and a data buffer for capturing full waveforms of AE events. The use of multiple transducers distributed in a micro-seismic array allowed for events to be located within the sample based on the arrival time of signals and non-linear least squares inversion techniques. Uncertainty in location estimates were on the order of ~1 mm. The system was tested by comparing the contrasting mechanical properties of quartz beads and AgCl samples. The multi-anvil apparatus constitutes an inherently noisy environment both acoustically and electrically, therefore methods of noise reduction were developed. Results of AE experiments on Fe2SiO4 under high pressure (3.7–9.2 GPa) and high temperature (673–1273 K) conditions in the spinel stability field showed acoustic events that locate within or within 1σ of the sample in five experiments defined by the P,T envelope P = 3.9–8.4 GPa and T = 748–923 K. Optical and scanning electron microscopy of the recovered specimens displayed conjugated faulting associated with transition from fayalite (olivine structure) to ahrensite (spinel structure) and microstructural analysis revealed microstructural morphology identical to that found in similar faulting experiments on Mg2GeO4. This is the first time an olivine→spinel structured transition in a silicate mineral has demonstrated macroscopic faulting and associated microstructures, as well as acoustic activity, under conditions that nominally promote plastic deformation.

An earthquake with a reported magnitude of 4.4 (ML) was detected on 13 June 2015 in western central Alberta, Canada. This event was the third felt earthquake this year near Fox Creek, a shale gas exploration region. Our results from full moment tensor inversions of regional broadband data show a strong strike-slip mechanism with near-vertical fault plane solutions. The decomposition of the moment tensor solution is overwhelmingly double couple, while only a modest (∼20%) contribution is attributed to compensated-linear-vector-dipole. The depth of this earthquake is 3–4 km, near the base of the sedimentary layer, and the moment magnitude (M = 3.9) of this event is considerably smaller than the initial reported ML value. The hypocenter location, depth, and mechanism are favorable to a possible association between this earthquake and hydraulic fracturing operations within the Duvernay shale.

The 2014 M w 7.9 Rat Islands intermediate-depth (∼107 km) earthquake is studied using seismic observations at teleseismic distances. The nodal plane with a steep-dip angle (87°) and a strike of 308° inferred from the Global CMT solution is selected as the causative fault plane because of the consistency with the distribution of the aftershocks relocated using the Joint Hypocenter Determination method. Its slip history is constrained using broadband P and SH waveforms and long-period (3–6 mHz) seismic waves in vertical and transverse components. The rupture process associated with this plane shows a rupture propagating at ∼2 km/s. The rupture essentially breaks one large slip patch (peak slip amplitude of 3.5 m) located 30 km west of the hypocenter. The size of this slip patch extends from 80 to 140 km depth, which would be the largest depth extent of earthquake slip ever documented. We show that this earthquake breaks the entire cold core of the subducting slab defined by the depth of the 600–650 °C isotherm. In addition, the lateral extent of the rupture appears to be bounded by the heterogeneities of subducting slab. Our results further indicate that, along this segment of Aleutian Arc, the intermediate-depth earthquakes might contribute to the westward transporting motion of Rat block. Lastly, the slip distribution of this earthquake as well as that of two other well studied M w > 7.5 intermediate-depth earthquakes suggests that the coldest portion of the subducting slab, outlined by the double Wadati–Benioff zone, is seismic during large intermediate-depth earthquakes. Such a result is important for estimating the seismic hazard of future large intermediate-depth earthquakes.

Large deep-focus earthquakes (magnitude > 7.0, depth > 500 km) have exhibited strong seasonality in their occurrence times since the beginning of global earthquake catalogs. Of 60 such events from 1900 to the present, 42 have occurred in the middle half of each year. The seasonality appears strongest in the northwest Pacific subduction zones and weakest in the Tonga region. Taken at face value, the surplus of northern hemisphere summer events is statistically significant, but due to the ex post facto hypothesis testing, the absence of seasonality in smaller deep earthquakes, and the lack of a known physical triggering mechanism, we cannot rule out that the observed seasonality is just random chance. However, we can make a testable prediction of seasonality in future large deep-focus earthquakes, which, given likely earthquake occurrence rates, should be verified or falsified within a few decades. If confirmed, deep earthquake seasonality would challenge our current understanding of deep earthquakes.

The existence of earthquakes within continental lithospheric mantle remains a highly controversial topic. Here, we present a detailed set of seismological analyses confirming the occurrence of a mantle earthquake beneath the Wind River Range of central Wyoming. Combining regional waveform inversion with the analysis of the delay and relative amplitudes of teleseismically-observed depth phases, we demonstrate that the 2013 Wind River earthquake – a M W 4.7 highly-oblique thrust-faulting event – occurred at 75 ± 8 km , well beneath the base of the crust. The magnitude, mechanism, and location of this earthquake suggest that it represents simple brittle failure at relatively high temperatures within the mantle lithosphere, as a result of tectonic, rather than magmatic, processes.

The 23 June 2014 Mw=7.9 earthquake that struck the Rat Islands region of Alaska was a rare intermediate-depth event followed by a vigorous aftershock sequence. The earthquake's location within a regional network has allowed us to study it in rich detail. Double-difference relocations of aftershocks and static stress modeling reveal some intriguing features; most aftershocks are not located on the main shock rupture plane, but are concentrated in the adjacent Wadati-Benioff zone in areas of increased stress. Further, a secondary plane of seismicity aligns well with a moderately dipping nodal plane reported by the Global Centroid Moment Tensor Project, and the separation of these locations and the main shock hypocenter from the slab indicates that the main shock ruptured into the oceanic mantle.

Coseismic gravity changes stem from (1) vertical deformation of layer boundaries with density contrast (i.e., surface and Moho) and (2) density changes of rocks at depth. They have been observed in earthquakes with Mw exceeding ~8.5 by Gravity Recovery and Climate Experiment (GRACE) satellites, but those of M8 class earthquakes have never been detected clearly. Here we report coseismic gravity change of the 24 May 2013 Okhotsk deep earthquake (Mw8.3), smaller than the detection threshold. In shallow thrust faulting, factor (2) is dominant, while factor (1) remains secondary due to poor spatial resolution of GRACE. In the 2013 Okhotsk earthquake, however, factor (2) is insignificant because they occur at depth exceeding 600 km. On the other hand, factor (1) becomes dominant because the centers of uplift and subsidence are well separated and GRACE can resolve them. This enables GRACE to map vertical ground movements of deep earthquakes over both land and ocean.

Based on the analysis of different seismological data on the Sea of Okhotsk, two of the fullest catalogs of shallow-focus earthquakes are made for the historical (1735–2010; M ≥ 5.0) and instrumental (1962–2010; M ≥ 4.0) periods. The peculiarities of the lateral and depth distributions of seismicity are considered, and earthquake recurrence on three shelf zones of the Sea of Okhotsk is estimated. Maps of earthquake hypocenters are compared with the Moho depth scheme. With the data of seismic continuous profiling (SCP) and common depth point studies (CDP) taken into account, the probable relationship between the tectonic nature of seismicity in the Sea of Okhotsk and the mobility and allochthonous occurrence of the crust (divergent glide on the Okhotsk Swell, regional thrusts along the island arc margins, subsidence of the Kuril Basin’s bottom) is examined.

SUMMARY Deep focus earthquakes within the underthrust Indian lower crust beneath the Himalaya occur in very specific regions and have distinct source characteristics. The study of the source mechanisms of these earthquakes provides valuable constraints on the kinematics of deformation of the underthrust Indian Plate, and its influence on the active deformation of the overlying Himalayan wedge. One of the most significant regions of these deep focus earthquakes is beneath the Sikkim and Bhutan Himalaya. We study the source characteristics of the 2011 September 18 (Mw 6.9) deep focus Sikkim main shock and its major aftershocks using global, regional and local waveform data. We determined the focal mechanism of the main shock using moment tensor inversion of global P and SH waveforms, and ascertained the earthquake fault plane using rupture directivity from regional P-wave spectra. The main shock originated at 53 ± 4 km depth and ruptured at least 20 km thickness of the underthrust Indian lower crust. Faulting occurred on a near vertical dextral strike-slip fault oriented NW-SE (strike 127 ◦ ,d ip 81 ◦ and rake 167 ◦ ), oblique to the local strike of the Himalayan arc. The rupture initiated from the SE end of the fault and propagated to the northwest. The main shock was followed by 20 small-to-moderate aftershocks (mb > 3.0), which we relocated using phase arrival times. We computed the focal mechanisms of the larger ones (mb ≥ 3.5) using local waveform inversion. We find that all aftershocks originated SE of the main shock, between depths of 12 and 50 km, and have dominantly strike-slip mechanisms. Our results, combined with the source mechanisms of earthquakes from previous studies, reveals that the entire underthrust Indian crust is seismogenic and deforms by dextral strikeslip motion on oblique structures beneath the Sikkim and Bhutan Himalaya. These active oblique structures with transverse motion possibly mark the western boundary of the clockwise rotating ‘microplates’ in northeast India observed from GPS geodesy.