Search

Seismicity in the east-off Sanriku region decreased conspicuously after 26 May 1983, when the central Japan Sea earthquake with a magnitude of 7.7 occurred on the west coast of Tohoku, northeastern Japan. The decrease was restricted to shallow seismicity in the depth range of 0-40 km. For deeper earthquakes in the depth range of 41-70 km rate of earthquake occurrence rather increased. Seismicity in the land area of northern Tohoku also decreased at the same time. However, seismic activity in the Oga-Oshika tectonic zone increased remarkably after the central Japan Sea earthquake. Increase of seismicity was also observed in the sea region corresponding to the extension of the Oga-Oshika tectonic zone. We consider that the coincidence of the notable decrease of shallow seismicity in the coupling region between the Pacific plate and the land plate with the central Japan Sea earthquake occurring on the west coast of Tohoku indicates the following tectonic regime. The northern Tohoku district constitutes one rigid-like block and stress fields in the both sides of the block, e. g., stress at the east-off Sanriku subduction zone and that at the active tectonic zone along the coast of the Japan Sea, influence each other. On the other hand the increase of seismic activity in the Oga-Oshika tectonic zone and in its extension after the central Japan Sea earthquake is considered to show that the zone corresponds to the southern boundary of the northern Tohoku block and change of stress is likely to be transmitted along the zone. Seismicity in the Oga-Oshika tectonic zone has a tendency to increase always when large earthquake occurs in the zone or in its extension. It should be noted that decrease of stress in the coupling region between the Pacific and the land plates due to the occurrence of the central Japan Sea earthquake was confined to the north of the Oga-Oshika tectonic zone. Seismicity in the subduction region to the south of the Oga-Oshika tectonic zone did not show any change when the central Japan Sea earthquake occurred. It is interesting to note as well that seismicity in the Pacific coast and sea regions did not change at all by the 1964 Niigata earthquake with a magnitude of 7.5 whose focal region was located on the east coast of the Japan Sea, but to the south of the Oga-Oshika tectonic zone. These facts suggest that the eastern Japan, eastern side to the Itoigawa-Shizuoka tectonic line, does not constitute a single microplate, but is separated mechanically at the Oga-Oshika tectonic zone.

Recent improvements in the seismological networks on the Ibero-Maghrebian region have permitted estimation of hypocentral location and focal mechanisms for earthquakes which occurred at South Spain, Alboran Sea and northern Morocco of deep and intermediate depth, with magnitudes between 3.5 and 4.5. Intermediate depth shocks, range from 60 to 100 km, with greater concentration located between Granada and Malaga. Fault-plane solutions of 5 intermediate shocks have been determined; they present a vertical plane in NE-SW or E-W direction. Seismic moments of about 1015 Nm and dimensions of about 1 km have been determined from digital records of Spanish stations.P-wave forms are complex. This may be explained by the crustal structure near the station, discontinuities in the upper mantle and inhomogeneities near the source. Deep activity at about 650 km has only 3 shocks since 1954 (1954, 1973, 1990). Shocks are located at a very small region. Fault-plane solutions show a consistent direction of the pressure axis dipping 45° in E direction. For the 1990 shock seismic moment is 1016 Nm and dimensions 2.6 km. TheP-waves are of simpler form with a single pulse. The intermediate and deep activities are not connected and no activity has been detected between 100 and 650 km. The intermediate shocks may be explained in terms of a recent subduction from Africa under Iberia in SE direction. The very deep activity must be related to a sunk detached block of lithospheric material still sufficiently cold and rigid to generate earthquakes.

[1] Rupture velocity is an important source parameter, which is often difficult to determine, especially for deep-focus earthquakes where there is usually limited near-source information. To help overcome this problem, we developed a new method to estimate rupture velocities of deep-focus earthquakes with better resolution. We first carry out teleseismic P waveform inversions to determine slip distributions for a range of rupture velocities on the two nodal planes. Then forward modeling of regional data is performed using the slip distributions determined in the teleseismic inversions to estimate the rupture velocity. Using this method, we attempted to determine the rupture velocities of large deep-focus earthquakes surrounding Japan, which are well recorded on teleseismic and regional networks. Empirical Green functions are used for both the teleseismic and regional analyses. Although it is difficult to determine the rupture velocity from only the teleseismic data, the analyses including regional data show clear difference which can resolve the rupture velocity and fault geometry. For three deep earthquakes, we obtained rupture velocities of about 1∼2 km/s, which correspond to 20∼40% of the shear wave velocity and are much slower than typical values for shallow earthquakes.

[1] A new three-dimensional seismic model and relocated regional seismicity are used to illuminate the great Sumatra-Andaman Islands earthquake of December 26, 2004. The earthquake initiated where the incoming Indian Plate lithosphere is warmest and the dip of the Wadati-Benioff zone is least steep along the subduction zone extending from the Andaman Trench to the Java Trench. Anomalously high temperatures are observed in the supra-slab mantle wedge in the Andaman back-arc. The subducting slab is observed along the entire plate boundary to a depth of at least 200 km. These factors contribute to the location of the initiation of rupture, the strength of seismic coupling, the differential rupture properties between the northern and southern segments of the earthquake, and the cause of convergence in the Andaman segment.

We extend to the case of intermediate and deep earthquakes the mantle magnitude developed for shallow shocks byokal andTalandier (1989). Specifically, from the measurement of the spectral amplitude of Rayleigh waves at a single station, we obtain a mantle magnitude,Mm, theoretically related to the seismic moment of the event through $$M_m = \log _{10} M_0 - 20.$$ The computation ofMminvolves two corrections. The distance correction is the same as for shallow shocks. For the purpose of computing the frequency-dependent source correction, we define three depth windows: Intermediate (A) (75 to 200 km); Intermediate (B) (200–400 km) and Deep (over 400 km). In each window, the source correctionCS is modeled by a cubic spline of log10T.

We present a study of the lateral structure and mode of deformation in the transition between the Kuril and Honshu subduction zones. We begin by examining the source characteristics of the January 19, 1969, intermediate depth earthquake north of Hokkaido in the framework of slab-tearing, which for the December 6, 1978 event has been well documented by previous studies. We use a least-squares body wave inversion technique, and find that its focal mechanism is comparable to the 1978 event. To understand the cause of these earthquakes, which in the case of the 1978 event occurred on a vertical tear fault but does not represent “hinge” faulting, we examine the available International Seismological Centre [ISC] hypocenters and Harvard centroid-moment tensor [CMT] solutions to determine the state of stress, and lateral structure and segmentation in the Kuril and northern Honshu slabs. These data are evaluated in the framework of two models. Model (A) requires the subducting slab at the Hokkaido corner to maintain surface area. Model (B) requires slab subduction to be dominated by gravity, with material subducting in the down-dip direction. The distribution of ICS hypocenters shows a gap in deep seismicity down-dip of the Hokkaido corner, supporting model (B). From the CMT data set we find that three types of earthquake focal mechanisms occur. The first (type A) represents dip-slip mechanisms consistent with down-dip tension or compression in the slab in a direction normal to the strike of the trench. These events occur throughout the Honshu and Kuril slabs with focal mechanisms beneath Hokkaido showing NNW plungingP andT axes consistent with the local slab geometry. The second (type B) occurs primarily at depths over 300 km in the southern part of the Kuril slab with a few events in the northern end of the Honshu deep seismicity. These earthquakes have focal mechanisms with P axes oriented roughly E-W, highly oblique to the direction of compression found in the type A events, with which they are spatially interspersed. The third (type C) group of earthquakes are those events which do not fit in either of the first two groups and consist of either strike-slip focal mechanisms, such as the tearing events, or oddly oriented focal mechanisms. Examination of the stress axes orientations for these three types reveals that the compressional axes of the type C events are consistent with those of type B. The slab tearing events are just differential motion reflecting the E-W compressive states of stress which is responsible for the type B family of events. There is no need to invoke down-dip extension which does not fit the slab geometry. We conclude that these two states of stress can be explained as follows: 1) The type A events and the seismicity distribution support model (B). 2) The type B and C events upport model (A). The solution is that the slab subducts according to model (B), but the flow in the mantle maintains a different trajectory, possibly induced by the plate motions, which produces the second state of E-W compressive stress.

During 1989 the Seismological Observatory recorded and analysed 9892 earthquakes in the New Zealand region. Preliminary locations and magnitudes are now available for all these events. This is about five times the number usually analysed in previous years, thanks to the new digital recording equipment which is being installed throughout the country. No earthquakes reached magnitude 6 during the year, although one of magnitude 5.9 in Fiordland was close to that figure. This caused intensity MM VI throughout Fiordland, and lower intensities elsewhere in the southern half of the South Island. Earthquakes of magnitude 5 and greater are listed: they indicate an ongoing level of activity commensurate with New Zealand's seismic history and geographic location.

Po/So phases generated by a deep-focus earthquake (focal depth: 360 km) were observed at epicentral distances between 11° and 14° in the northwestern Pacific by an ocean-bottom seismometer (OBS) array. The apparent velocity of Po was about 8.3 km/s, which is higher than is observed for shallow-focus earthquakes. The high-frequency, long-duration features of Po/So wave trains appear to be related to observed high Q's in the deep dipping seismic zone. Lateral propagation of Po/So associated with a deep-focus earthquake seems to begin when the P wave travels up the subducting slab and reaches the upper lithosphere beneath the trench axis. Also, a waveguide phenomenon occurs in the dipping subduction slab itself. Wave trains generated near the source should be taken into consideration in the theory of Po/So generation.

SUMMARY Using more than 300 recordings of deep Tonga earthquakes from the Warramunga Seismic Array (central Australia), we have searched for short-period S-P conversions from the 670 km discontinuity arriving in the P-wave codas. The array seismograms were stacked and plotted in sections to optimize observation of S670P yielding the following results: For the northern part of Tonga (17.3°–18.4°S), S670P-phases are observed with apparent conversion depths between about 660 and 680 km. For the central part (20.2°–22.3°S), S670P is not clearly observed. For the southern part (23.1°–25.7°S), S670P is observed for conversion depths between about 660 and 700 km. Intermittent occurrence of S-P conversion may result from different characteristic source radiation patterns among the three sections. Overall uncertainties in conversion depths (resulting from unmodelled lateral velocity variations and hypocentral location errors) are perhaps a few tens of kilometres. The S-P rays must travel either within or very close to the subducted slab because of the source/receiver/slab-dip geometry, so we conclude that large (>50 km) deformations of the 670 km discontinuity do not occur beneath Tonga. Our observations appear to be inconsistent with a chemical discontinuity at 670 km, but are easily explained if the 670 km discontinuity is a strongly pressure-dependent and, perhaps, moderately endothermic phase change in both the slab and surrounding mantle.

We propose that martensitic transformations are a possible candidate for the mechanism of deep earthquakes. It is shown that such transformations are consistent with the principal features of deep earthquakes, and the fact that diffusion is quenched by large pressures makes this transformation feasible. Calculations of nucleation kinetics are carried out to show that metastable phases may be expected to exist over long times within subducting plates.

[1] We make use of rupture directivity to analyze 82 deep earthquakes (≥100 km depth) in the Tonga-Kermadec subduction zone. Identifying the fault planes for 25 of them, we are able to place new constraints on both the physical mechanism of intermediate- and deep-focus earthquakes and deformation within the subducting slab. We find that half of deep earthquakes with MW ≥ 6 have detectable directivity. We compare the obtained fault orientations with those expected for the reactivation of outer-rise normal faults and with those expected for the creation of new faults in response to the ambient stress field. Earthquakes >300 km depth match the patterns expected for the creation of a new system of faults: we observe both subhorizontal and subvertical fault planes consistent with a downdip-compressional stress field. Slip along these faults causes the slab to thicken. Rupture propagation shows no systematic directional pattern. In contrast, at intermediate depths (100–300 km), all ruptures propagate subhorizontally and all identified fault planes, whether in the upper or lower region of the double seismic zone, are subhorizontal. Rupture propagation tends to be directed away from the top surface of the slab. After accounting for the angle of subduction, the subhorizontal fault plane orientation is inconsistent with the orientation of outer-rise normal faults, allowing us to rule out mechanisms that require the reactivation of these large surface faults. Subhorizontal faults are consistent with only one of the two failure planes expected from the slab stress field, suggesting that isobaric rupture processes or preexisting slab structures may also influence the fault plane orientation. If all deformation takes place on these subhorizontal faults, it would cause the slab to thin. Assuming the slab is incompressible, this implies that the slab is also lengthening and suggests that slab pull rather than unbending is the primary force controlling slab seismicity at intermediate depths.

SUMMARY Water enters the Earth's mantle via subduction of oceanic lithosphere and sediments. A lot of this water immediately returns to the atmosphere through arc volcanism, but part, retained in Dense Hydrous Magnesium Silicates (DHMSs) and Nominally Anhydrous Minerals (NAMs) like olivine, is expected to be subducted as deep as the bottom of the upper mantle (660 km depth). Then, due to its low solubility in lower mantle minerals, water is likely to be released as a hydrated fluid during the spinel–post-spinel phase change. The dynamics of this fluid phase is investigated through a 1-D model of compaction, in which a source term has been introduced to take the fluid precipitation into account. The competition between the advective transport by the descending slab and the buoyant rise of the fluid results in three distinct situations, depending on the properties of the solid and the fluid phases. Low matrix permeability and high fluid viscosity inhibit compaction and favour the entrainment of fluid towards the deep mantle. In this case, the entire slab water content would enter the lower mantle and would be mixed at large scale. However, realistic values of the fluid viscosity and matrix permeability make this possibility unlikely. When effective, compaction results in an accumulation of fluid at and below the phase boundary. Then, depending on the value of the matrix viscosity, the situation evolves differently. Above 1020 Pa s, accumulation of fluid extends below the phase boundary and the pressure difference between the fluid and the matrix increases continuously, exceeding the yield strength of rocks. As a result, cracks would form and evolve towards the formation of dykes. In case of very low mantle viscosity, possibly due to strong grain size reduction during phase change, compaction becomes very efficient and the fluid remains confined within the phase change horizon, without increasing pressure. In the long term, this last situation appears unstable and would also evolve towards the formation of dykes. Thus, it is expected that water returns to the upper mantle by dykes propagating in the direction of the maximum compressive stress. Since maximum compressive stress appears to follow the dip of the slab below 410 km depth, we predict the formation of dykes extending from the 660 km phase change to 410 km depth. The possible existence of such dykes in slabs offers the necessary conditions for strong double couple component earthquake in the deepest part of the upper mantle.

To understand why the Bolivian earthquake of 9 June 1994 has so rapidly become one of the most thoroughly studied seismic events in history, one must know a little about both the event and the significance of when and where it occurred. (See PHYSICS TODAY, October 1994, page 17.) To begin with, the Bolivian quake occurred in the mantle, nearly 640 km beneath the Amazon basin east of the Andes. Such a large “deep‐focus” earthquake—and this one, with magnitude 8.3, was the largest such event ever recorded—can make the planet ring like a bell, exciting overtones of Earth's normal modes of oscillation that are sensitive to inhomogeneities in the core and mantle. Add in the fact that the Bolivian quake occurred after the deployment of several global and local networks of high‐quality digital seismological stations, and it becomes clear that the quake offered researchers an unprecedented opportunity to illuminate the three‐dimensional structure of Earth's mantle and core. (See the box at right.)

[1] The physical mechanism of deep earthquakes can be constrained by identifying their fault planes. Resolving the fault plane ambiguity is a classical problem in seismology, and we present a method to distinguish the fault plane using observations of source finiteness. Source finiteness is observable on seismograms at different azimuths and distances, for single-event ruptures, as variations in the apparent rupture duration and, for complex ruptures, as differences in the traveltime delay between subevents. For each earthquake, the rupture duration (or traveltime delay) will be shortest in the direction of rupture propagation and longest in the opposite direction. Rather than measuring the actual rupture duration at each station, we use a cross-correlation technique that includes a stretching factor to measure the differential rupture duration between each pair of stations. These differential measurements then allow us to identify the rupture direction, rupture velocity, and fault plane for each earthquake. First, we test the method on two synthetic earthquakes, which represent earthquakes composed of one and two events. The method works well for both examples, although attenuation can bias the determined rupture direction for the single-event case. Next, we apply this method to P waves from broadband seismograms from four intermediate- and deep-focus earthquakes composed of two subevents: the 23 January 1997 Bolivian earthquake (MW 7.1, 276 km depth), the 27 October 1994 earthquake south of the Fiji Islands (MW 6.7, 549 km depth), the 21 July 1994 Japan Sea earthquake (MW 7.3, 471 km depth), and the 11 November 1998 Fiji Islands earthquake (MW 6.3, 149 km depth). Each focal mechanism contains a subvertical and a subhorizontal nodal plane. For three of the events, our analysis shows that rupture propagated subhorizontally, and we identify the subhorizontal nodal plane as the fault plane. For the smallest event, the rupture azimuth, but not the rupture dip, is well constrained, and we cannot conclusively identify the fault plane. Rupture velocities vary from 0.18 to 0.63 of the local P wave velocity.

Deep earthquakes (earthquakes with origins deeper than 60 km) are of scientific importance and account for approximately one-quarter of all earthquakes. They are occasionally very large and damaging yet provide much of the data that constrain our knowledge of Earth structure and dynamics. This book opens with an explanation of what deep earthquakes are, their significance to science and how they were first discovered. Later chapters provide a description of deep earthquake distribution and clustering in both time and space; a review of observations about source properties; and a discussion of theories for the origin of deep earthquakes. The book concludes with a comprehensive literature review of terrestrial and lunar deep seismicity. Deep Earthquakes presents a comprehensive, topical, historical, and geographical summary of deep earthquakes and related phenomena. It will be of considerable interest to researchers and graduate students in the fields of earthquake seismology and deep Earth structure.

Determining the seismic fracture energy during an earthquake and understanding the associated creation and development of a fault zone requires a combination of both seismological and geological field data. The actual thickness of the zone that slips during the rupture of a large earthquake is not known and is a key seismological parameter in understanding energy dissipation, rupture processes and seismic efficiency. The 1999 magnitude-7.7 earthquake in Chi-Chi, Taiwan, produced large slip (8 to 10 metres) at or near the surface, which is accessible to borehole drilling and provides a rare opportunity to sample a fault that had large slip in a recent earthquake. Here we present the retrieved cores from the Taiwan Chelungpu-fault Drilling Project and identify the main slip zone associated with the Chi-Chi earthquake. The surface fracture energy estimated from grain sizes in the gouge zone of the fault sample was directly compared to the seismic fracture energy determined from near-field seismic data. From the comparison, the contribution of gouge surface energy to the earthquake breakdown work is quantified to be 6 per cent.

[1] A large (M7.2) interplate earthquake occurred on 16 August 2005 in the Miyagi-Oki region, where large interplate earthquakes with magnitude of ∼7.5 have occurred repeatedly at a recurrence interval of ∼37 yrs, NE Japan. We compared aftershock distribution of the 2005 Miyagi-Oki earthquake with that of the previous 1978 Miyagi-Oki earthquake (M7.4) using double-difference hypocenter locations. The aftershock area of the 2005 Miyagi-Oki earthquake is partly overlapped with the southern/southeastern part of that of the 1978 event. Locations of aftershock clusters of the 2005 event correspond with those of the 1978 event and those of the background seismicity. Coseismic slip area of the 2005 event estimated by seismic waveform inversion is also partly overlapped with that of the 1978 event. These observations suggest that spatial seismicity pattern on the plate boundary in subduction zone persists and the 2005 event possibly ruptured a part of the source area of the 1978 event.

[1] Transformation-induced faulting has been studied extensively in natural olivine, ice, and a variety of other olivine analog systems over the last 15 years and remains a likely candidate mechanism for initiating deep focus earthquakes. The mechanism involves the formation of microscopic packets of fine-grained denser materials from a metastable host, which then behave as mode I features that coalesce to allow bulk mode II shear failure. We present evidence for a new class of transformation microstructures that also allows shear failure at high pressures but which does not require the formation of widely distributed mode I features prior to shear failure. We have observed thin planar zones of transformation in coarse-grained (∼150 μm) Mg2GeO4 olivine that form along crystallographic planes in deformed grains at temperatures cooler than those that allow significant development of anticracks. The development of these zones is rapid and only occurs after ∼25% bulk strain. Increasing strain leads to progressive development of these zones, in turn leading to bulk shear failure once a continuous pathway of fine-grained spinel is formed that traverses the specimen. Transmission electron microscopy analysis shows that slip bands in olivine along (011) and (010) formed from dislocation pileups on these slip planes transform preferentially to a nanocrystalline aggregate of the denser spinel phase, aided by the high strain energy of these bands. This new, partially strain-driven mechanism compliments shear failure by anticracks as described in previous studies and adds new possibilities for the triggering of bulk shear failure by rapid mineralogical transformations.

We have discovered nonvolcanic tremor activity (i.e., long-duration seismic signals with no clear P or S waves) within a transform plate boundary zone along the San Andreas Fault near Cholame, California, the inferred epicentral region of the 1857 Fort Tejon earthquake (moment magnitude ~7.8). The tremors occur between 20 to 40 kilometers' depth, below the seismogenic zone (the upper ~15 kilometers of Earth's crust where earthquakes occur), and their activity rates may correlate with variations in local earthquake activity.

[1] Using the example of the 1992 M 7.3 Landers earthquake, we show that in the aftermath of a large earthquake the depth extent of aftershocks shows an immediate deepening from pre-earthquake levels, followed by a time-dependent postseismic shallowing. We use these seismic data to constrain the change in the depth of the seismic-aseismic transition with time throughout the earthquake cycle. Most studies of the seismic-aseismic transition have focussed on the effects of temperature and/or lithology on the transition either from brittle faulting to viscous flow or from unstable to stable sliding. A strain-rate dependent transient deepening of the brittle-ductile transition following a major earthquake is predicted by geological and laboratory observations. By analyzing the time-dependent depth distributions of aftershocks, we identify and quantify the temporal evolution of this transition. In the example of the Landers earthquake, its depth changes by as much as 3 km over the course of 4 years.

[1] Anomalous boundary layer structure at the core-mantle boundary (CMB) is investigated using a global set of broadband SKS and SPdKS waves from permanent and portable broadband seismometer arrays. SPdKS is an SKS wave that intersects the CMB at the critical angle for ScP, thus initiating a diffracted P wave (Pdiff) along the CMB at the core entry and exit locations. The waveshape and timing of SPdKS data are analyzed relative to SKS, with some SPdKS data showing significant delays and broadening. Broadband data from several hundred deep focus earthquakes were analyzed; retaining data with simple sources and high signal-to-noise ratios resulted in 53 high-quality earthquakes. For each earthquake an empirical source was constructed by stacking pre-SPdKS distance range SKS pulses (∼90°–100°). These were utilized in our synthetic modeling process, whereby reflectivity synthetic seismograms are produced for three classes of models: (1) mantle-side ultralow-velocity zones (UVLZ), (2) underside CMB core rigidity zones, and (3) core-mantle transition zones. For ULVZ structures, ratios of P-to-S velocity reductions of 1:1 and 1:3 are explored, where 1:3 is appropriate for a partial melt origin of ULVZ. Over 330 unique CMB boundary layer models have been constructed and tested, corroborating previous work suggesting strong trade-offs between the three model spaces. We produce maps of inferred boundary layer structure from the global data and find evidence for extremely fine-scale heterogeneity where our wave path sampling is the densest. While uncertainties are present relating to the source versus receiver sides of the SPdKS wave path geometry, our data are consistent with the hypothesis that ULVZ presence (or absence) correlates with reduced (or average) heterogeneity in the overlying mantle.

[1] An inversion method for retrieving seismic anisotropy from non-double-couple components of seismic moment tensors is presented. The method requires a set of highly accurate moment tensors of earthquakes that occurred in a homogeneous anisotropic focal area on differently oriented faults. In contrast to standard methods retrieving anisotropy from travel times or from shear wave splitting, which yield an overall anisotropy averaged along a whole ray path, the presented method yields a local value of anisotropy just in the focal area. The method is robust, being able to retrieve the orientation as well as strength of anisotropy even for low anisotropy symmetries as for orthorhombic symmetry. The method can utilize the moment tensors constrained to have zero trace, but using unconstrained moment tensors is advantageous. The method is applied to retrieving anisotropy in the Tonga subduction zone using moment tensors of deep-focus earthquakes reported in the Harvard centroid moment tensor catalog. The inversion is complemented by tests on synthetic data to assess its stability and the accuracy of the results. The inversion indicates that the subduction zone is anisotropic with orthorhombic symmetry. The orientation of the intra-slab anisotropy is defined by axes (azimuth/dip) a1 = 320°/54°, a2 = 121°/38°, and a3 = 223°/81°. The errors in the azimuth and dip are about 5°. The first and second axes lie along the downdip motion of the slab and along the normal to the slab, respectively. The strength of the P, S1, and S2 anisotropy is of 7.3 ± 1.5%, 13.4 ± 2.5%, and 12.6 ± 3.5%, respectively. The errors of anisotropy strength are only rough estimates, which reflect random but not systematic errors in the moment tensors used in the inversion. The values for anisotropy strength in the slab are remarkably higher than those observed in the surrounding mantle. The symmetry axes of anisotropy coincide with the principal stress directions in the slab. This manifests a primary impact of stress on anisotropy formation. The retrieved anisotropy in the slab can serve as an additional constraint on its structure and mineralogical composition.

[1] We measure P wave spectral amplitude ratios from deep-focus earthquakes recorded at broadband seismic stations of the Tanzania network to estimate regional variation of sublithospheric mantle attenuation beneath the Tanzania craton and the eastern branch of the East African Rift. One-dimensional profiles of QP adequately explain the systematic variation of P wave attenuation in the sublithospheric upper mantle: QP ∼ 175 beneath the cratonic lithosphere, while it is ∼ 80 beneath the rifted lithosphere. By combining the QP values and a model of P wave velocity perturbations, we estimate that the temperature beneath the rifted lithosphere (100–400 km depth) is 140–280 K higher than ambient mantle temperatures, consistent with the observation that the 410 km discontinuity in this region is depressed by 30–40 km.

[1] Recent observations of up to 6 secs of shear-wave splitting in deep focus earthquakes from the Tonga-Kermadec subduction zone provide insights into mantle dynamics near the 660 km discontinuity. Modeling of subduction body force stresses predicts large deviatoric stress (∼40 MPa) in the topmost lower mantle below a viscosity increase at the 660 km discontinuity. Finite strain calculations produce significant amounts of natural strains (∼7) in the lower mantle, and coherently aligned strain ellipses. For a viscosity model with a viscosity increase at 660 km, 5–10 secs of shear-wave splitting is predicted, compared to only 0–5 secs for uniform mantle viscosity. For a viscosity increase model, we predict that shear-wave splitting reduces from 7–10 secs to 4–6 secs when deformation above 410 km is ignored, and further reduces to 2–3 secs when deformation above 660 km is zeroed. Various conversions from finite strain to anisotropy have been explored. Predicted shear-wave splitting magnitudes are comparable to those observed from the Tonga-Kermadec subduction zone.

[1] We present a numerical model of the subducting lithosphere that provides an alternative explanation for stresses causing deep earthquakes. Our model lithosphere is composed of α olivine, β spinel, γ spinel, and perovskite + magnesiowustite. The heat conduction equation is solved to determine temperature conditions in the slab and to locate the equilibrium phase transformations in pressure-temperature space. Volumetric strains in the subducting lithosphere are calculated from the density of individual phases and from the heat released or consumed in the phase changes. These strains are used as sources of stress in the subducting lithosphere. Dislocation creep and Peierls stress creep laws are included in the viscoelastic rheology. Volumetric reductions due to equilibrium phase transformations cause high shear stress in the transition zone because of the variable viscosity inside the subducting slab. Aspects of the model shear stresses are in agreement with observations of high seismic activity in the Tonga-Wadati Benioff zone. Compression is oriented along the dip of the slab, and extension is oriented in the plane perpendicular to the compression axis. Since our model stresses agree with the seismic observations, and because the model stresses are larger than those caused by buoyancy forces, our model provides a possible explanation of stresses causing deep earthquakes. Also, our model does not need metastability of olivine to explain the occurrence of high shear stress in the transition zone.

Relocations and focal mechanism analyses of deep earthquakes (≥13 kilometers) at Kilauea volcano demonstrate that seismicity is focused on an active fault zone at 30-kilometer depth, with seaward slip on a low-angle plane, and other smaller, distinct fault zones. The earthquakes we have analyzed predominantly reflect tectonic faultingin the brittle lithosphere rather than magma movement associated with volcanic activity. The tectonic earthquakes may be induced on preexisting faults by stresses of magmatic origin, although background stresses from volcano loading and lithospheric flexure may also contribute.

[1] We systematically examined seismic data from deep earthquakes in the western and northwestern Pacific, Tonga/Fiji, and South America for signs of sharp gradients in seismic properties that could represent either a mineralogical phase change or the boundary between geochemical reservoirs in the Earth's lower mantle. This involved the stacking of short-period, vertical component teleseismic waveforms from the dense Pacific Northwest Seismic Network and searching for evidence of S-to-P conversions at sharp mantle structures between 800 and 2000 km depth. With these data, we estimate we should be able to detect seismic discontinuities with ΔVs > 2%, transition width

Deformation of solid materials affects not only their microstructures, but also their microchemistries1,2,3,4,5. Although chemical unmixing of initially homogeneous multicomponent solids is known to occur during deformation by diffusion creep4,5, there has been no report on their chemical zoning due to deformation by dislocation creep, in either natural samples or laboratory experiments. Here we report striped iron zoning of olivine ((Mg,Fe)2SiO4) in deformed peridotites, where the iron concentration increases at subgrain boundaries composed of edge dislocations. We infer that this zoning is probably formed by alignment of edge dislocations dragging a so-called Cottrell ‘atmosphere’ of solute atoms3,6,7 (iron in this case) into subgrain boundaries during deformation of the olivine by dislocation creep. We have found that the iron zoning does not develop in laboratory experiments of high strain rates where dislocations move too fast to drag the Cottrell atmosphere. This phenomenon might have important implications for the generation of deep-focus earthquakes, as transformation of olivine to high-pressure phases preferentially occurs in high-iron regions, and therefore along subgrain boundaries which would be preferentially aligned in plastically deformed mantle peridotites.

[1] Olivine within the lowest temperature regions of subducting oceanic lithosphere may persist metastably to depths well below the equilibrium phase boundary for transformation to β phase. This may produce a tapering low seismic velocity wedge within the slab in the transition zone; transformational faulting of such metastable olivine has been proposed as a possible explanation of deep earthquakes. Detecting seismic velocity structure associated with metastable olivine within a slab using seismic wave arrival time information alone has proven problematic. Motivated by previous two-dimensional numerical calculations for upgoing and downgoing seismic waves, a three-dimensional Gaussian beam method is used to assess whether the azimuthally varying teleseismic amplitude signature produced by a slab with an internal wedge of metastable olivine is distinguishable from that for a slab with equilibrium transitions. Five slab velocity structures are considered: a thermal model with horizontal phase transitions at the same depths as in ambient mantle, two models incorporating equilibrium transitions within the slab, and two models with tongues of metastable olivine. A depth- and temperature-dependent thermal conductivity term and the effect of the latent heat of transformation of olivine to β phase are included in the thermal modeling. Three-dimensional dynamic ray tracing and Gaussian beam summation are performed for events located at different depths and lateral positions for each of the slab models. For sources near 400 km depth the presence of low-velocity metastable olivine produces relatively complex azimuthal shear wave amplitude patterns compared to thermal and equilibrium models. Comparison with observed amplitudes for four events in the Kurile slab indicates that allowing for internal slab velocity complexity in the transition zone provides better agreement with observations than simple thermal and equilibrium slab models. This indicates that careful modeling of teleseismic shear wave amplitude patterns may have the resolution to detect and characterize the presence of metastable olivine in deep slabs.

[1] The shear velocity structure of the subducted Kurile slab is modeled using shear wave amplitude residual spheres. We measure long-period SH and ScSH phase amplitudes for 10 intermediate and deep focus earthquakes recorded at World-Wide Standardized Seismograph Network and Canadian Seismic Network stations from 1967 to 1986. After correction for source radiation pattern, geometric spreading, and empirical receiver/upper mantle effects, the data for intermediate depth Kurile slab events show smoothly varying amplitude residual sphere patterns with low amplitudes concentrated in the direction down the slab dip. The data are modeled using three-dimensional dynamic ray tracing and a three-dimensional Gaussian beam method. Starting models are taken from previous P and S wave arrival time residual sphere studies. Three-dimensional ray tracing indicates that ray patterns and resulting amplitudes are very sensitive to the depth extent of the slab and to the position of the event relative to the across-slab velocity gradient. This sensitivity allows us to place some constraints on the Kurile slab geometry, position of each event within the slab, and penetration depth of coherent slab structure. Our final slab model is defined by an asymmetric error function that peaks at 4% higher velocity than Preliminary Reference Earthquake Model 25 km from the top of the slab. The total width of the thermal anomaly averages 140 km. The slab is tabular and extends to 670 km in the northern half of the Kurile subduction zone. In the southern half of the subduction zone the slab flattens along the 670 km discontinuity and deflects laterally, persisting to 750 km depth.

A 2-D finite-element-method (FEM) numerical experiment of earthquake cycles at a subduction zone is performed to investigate the effect of viscoelasticity of the earth on great interplate earthquake fault slip. We construct a 2-D viscoelastic FEM model of northeast Japan, which consists of an elastic upper crust and a viscoelastic mantle wedge under gravitation overlying the subducting elastic Pacific plate. Instead of the dislocation model prescribing an amount of slip on a plate interface, we define an earthquake cycle, in which the plate interface down to a depth is locked during an interseismic period and unlocked during coseismic and postseismic periods by changing the friction on the boundary with the master-slave method. This earthquake cycle with steady plate subduction is periodically repeated to calculate the resultant earthquake fault slip.

After a subduction earthquake like the 11 March 2011 Tohoku-Oki earthquake, Earth's crust continues to deform. Scientists have been monitoring this deformation near the earthquake's rupture zone to estimate further seismic hazards.

The 1992 Nicaragua earthquake was a ‘tsunami earthquake‘; that is, it generated tsunamis disproportionately large for its surface-wave magnitude, M_s. The moment magnitude, M_w, determined from long-period (~250-s) surface waves, was 7.6, significantly larger than the 20-s M_s of 7; this M_s–M_w disparity is also characteristic of tsunami earthquakes. The Nicaragua earthquake is the first tsunami earthquake to be captured by modern broadband seismic networks, allowing us to present here seismograms of sufficiently high quality to make inferences about the rupture mechanisms. We conclude that the Nicaragua earthquake was a slow thrust earthquake which occurred on the subduction interface between the Cocos and North American plates, and because of the absence of sediments on the trench floor offshore of Nicaragua, the slip propagated up-dip all the way to the ocean bottom, exciting large tsunamis. The occurrence of slip on a plate interface filled with soft subducted sediments caused the rupture process to be slower than in ordinary subduction-zone thrust earthquakes. Our results reinforce the idea that tsunami warning systems using long-period (≥100-s) waves are necessary to reduce the hazard from this type of earthquake.

The subduction zone megathrust earthquake that struck offshore of Tohoku, Japan, in 2011 was so strong that it shifted the tilt of the Earth's axis and moved the island of Japan itself four meters. This earthquake puzzles scientists in part because the hypocenter—where the earthquake originated, deep below the epicenter—occurred on a fault previously thought to be inactive. The lack of data about the rock surrounding the hypocenter makes it hard for scientists to pinpoint specific details about how the earthquake was triggered and propagated.

We use a cluster of deep earthquakes, the fore- and aftershocks of the 23-AUG-1995 Mw=7.1 (18.8N, 145.3E, h=595 km) event in the Mariana subduction zone as source array to locate where waves scatter near the deep slab. In order to obtain the high-precision location needed in this context, we first relocate the events with a modified master event technique. Using the double beam method on the relocated events and data from the Warramunga array in Australia, we identified two non-standard S-to-P conversions arriving about 20.5 and 24 s after the P-wave. The corresponding scatterers are approximately located at a depth of 790 km (720 km) and a distance of 0.9° to the southeast (2.3° to the southwest) of the main shock. The scatterers positions correspond to the location of lithosphere subducted below the 660 km discontinuity.

We examine a new data set of reliably relocated earthquakes deeper than 400 km under Indonesia, developed by [Schoffel and Das, 1999]. The accuracy of this data allows us to demonstrate that the slab thickens, shortens and weakens before penetrating below 670 km by shearing along conjugate fault planes on the upper and lower portions of the seismic zone. We also show that the earthquakes in the lower portion of the Wadati-Benioff zone width are generally fewer and smaller than those in the central portions, with only 8% of the total moment released in the period 1976-1996 being due to earthquakes on the lower side.

Depth dependence of the source rupture duration of interplate thrust earthquakes is examined for seven subduction zones around the Pacific to explore variations in faulting properties. Multi-station deconvolutions of teleseismic P waves for moderate size earthquakes yield estimates of the source time function and centroid depth for each event. Analysis of 17 to 75 earthquakes in each region reveals a consistent trend of decreasing source duration (inferred from the source time functions, after correction for differences in total energy release) with increasing depth. Rupture duration patterns vary somewhat between subduction zones as well as along strike within a given zone, and the data have large scatter, implying significant variation in rupture processes along the interplate megathrusts, but the depth dependence appears to be robust. The rupture duration variations prompt consideration of two end-member models: 1) depth-dependent rupture velocity is caused by variations of rigidity of materials in the fault zone, while static stress drop is constant, and 2) static stress drop varies with depth while material properties and rupture velocity are constant. For the first model, the volumetrically averaged rigidity of the fault zone must increase with depth in each region by a factor of 5 between depths of 5 to 20 km. If rupture velocity is constant, the stress drop must increase by an order of magnitude over the same depth range. This systematic variation in rupture behavior with depth may reflect spatial variations in the amount, compaction and porosity of sediment in the fault zone, topography on the subducting plate, phase transitions in the fault zone materials, thermal structure of the megathrust, and varying presence of fluids in the fault zone. Such physical variations appear to control the physics of rupture propagation, leading to intrinsic dependence of rupture velocity on materials and fluids within the fault zone.

SHALLOW earthquakes are produced by brittle shear fracture of rock and/or fictional sliding on pre-existing fault surfaces1. At very high pressures, however, brittle fracture and frictional sliding are impossible because frictional resistance to movement on any potential fault surface is so great that ductile stress-relief processes accomodate strain at lower shear stresses than those necessary to activate faulting. Nevertheless, more than 20% of earthquakes with magnitudes greater than five occur at depths greater than 300km, where the pressure exceeds l0 GPa (ref. 2). Possible explanations for this paradox have been offered by several recent experimental studies of shearing instabilities associated with phase transformations at high 3–8. One of these mechanisms is associated with anticrack development during the olivine → spinel (α → γ) phase transformation in Mg2GeO4 at pressures of 1–2 GPa (refs 4–6); it is particularly attractive because it operates between mineral structures known to occur in the mantle. We show here that this mechanism also can operate in natural silicate olivine, (Mg,Fe)2SiO4, during onset of the α → β transformation at the much higher pressures at which deep earthquakes occur. These results lend strong support to the hypothesis that the anticrack mechanism is responsible for such earthquakes4.

Summary Source parameters of the 1996 Flores Sea and 1994 Fiji–Tonga deep earthquakes are derived from teleseismic body waves recorded by the global seismic network of broad-band seismograph stations. Both events consisted of several subevents. Models to approximate the spatial and temporal extent of the source process include point sources, propagating point sources and a combination of these. For the Flores Sea event, rupture lasted about 23 s and terminated some 70 km east of the nucleation point as inferred from the duration of P-wave pulses, in agreement with the findings reported by other investigators. Our preferred model suggests bilateral rupture propagation. It consists of four point sources that have variable double-couple radiation patterns and source time histories, and explains well the large compensated linear vector dipole (CLVD) component inferred from the Harvard centroid moment tensor (CMT) solution. The main moment release in the Fiji–Tonga event lasted only about 15 s. Our best model consists of two point sources with a total moment release of 2.8 × 1020 N m. Rupture propagated subhorizontally from the nucleation point to the north. The termination of rupture was located about 40 km to the north of rupture initiation. The inferred velocity of moment release in the Flores Sea and Fiji–Tonga events was 3–5 km s-1, a value which is higher than that inferred for the great Bolivian earthquake of June 1994. Other derived source parameters (static stress drop, radiated seismic energy and maximum seismic efficiency) are also significantly different from those inferred for the 1994 Bolivian event, suggesting that deep earthquake processes do not follow an easily detectable common mechanism.

A network of 60 seismographs was deployed across the Andes at approximately 23.5°S. The array was centered in the backarc; atop the Puna high plateau in NW Argentina. P and S arrival times of 426 intermediate depth earthquakes were inverted for 1-D velocity structure and hypocentral coordinates. Average velocities and v sub p/v sub s in the crust are low. Average mantle velocities are high but difficult to interpret because of the presence of a fast velocity slab at depth. Although the hypocenters sharply define a 35° dipping Benioff zone, seismicity in the slab is not continuous. The spatial clustering of earthquakes is thought to reflect inherited heterogeneties of the subducted oceanic lithosphere. Additionally, 57 crustal earthquakes were located. Seismicity concentrates in the fold and thrust belt of the foreland and Eastern Cordillera, and along and south of the El Toro-Olacapato-Calama Lineament (TOCL). Focal mechanisms of two earthquakes at this structure exhibit left lateral strike-slip mechanisms similar to the suggested kinematics of the TOCL. We believe that the Puna north of the TOCL behaves like a rigid block with little internal deformation, whereas the area south of the TOCL is weaker and currently deforming. (Résumé d'auteur)

A comprehensive set of teleseismic waveforms from two South American deep-focus earthquakes of the predigital era, the 1970 Colombia (Mw = 8.1) and 1963 Peru-Bolivia (Mw = 7.7) events, are inverted for source mechanism, seismic moment, rupture history, and centroid depth. The P and SH wave inversion of the Colombia event confirms previous work, indicating that rupture occurred on a plane that dips steeply west. Rupture direction paralleled the trend of the Wadati-Benioff zone. We decompose the source into subevents, based on a source time function which shows two major moment release pulses separated by ∼20 s. The first subevent is located near the initiation point at a depth of ∼630 km. The main moment release was located ∼70 km to the southeast and ∼20 km shallower. Rupture subsequently propagated farther southeast. The source time function has an initial subevent accounting for ∼30% of the moment release of the entire event, whereas the long-period centroid moment tensor (CMT) analysis [Russakoff et al., 1997] has the initial subevent yielding ∼50%. The high-angle nodal plane rotated ∼15° clockwise during the rupture, explaining the large compensated linear vector dipole (CLVD) component inferred from CMT solutions. Individual subevents have large CLVD and compressive isotropic components. A full moment tensor inversion of the Colombia and 1994 Bolivia events suggests that the initial subevents might contain a large non-double-couple (NDC) component. For the 1963 Peru-Bolivia event, using P waves, rupture propagated NNW for a distance of ∼70 km, parallel to the high-angle nodal plane and the trend of the Wadati-Benioff zone. The focal mechanism changed dramatically after the second subevent, causing a very large NDC component. Both events, together with the 1994 Bolivia earthquake, have a precursor separated in space and time from the main rupture and show rupture velocities varying between 3 and 4 km/s between subevents, with

We report on three intermediate depth earthquakes in the India‐Tibet collision zone, two under the Himalayan Thrust Belt (HTB) and one beneath the Indus Zangbo suture. The mb magnitudes of these three events are from 4.3 to 4.9, and are too small to be well located by conventional means. However, from modeling their broadband waveforms recorded at near‐regional distances on a temporary PASSCAL array, we can confidently confine the sources to be below the crust, between 70 and 80 km deep. The existence of these intermediate depth earthquakes in this area suggests relatively low temperatures in the mantle lithosphere. The two events under the HTB display strike‐slip mechanisms with some normal faulting component; this is quite different from the shallow thrust events typical of the same area. The principal P and T axes of all 3 fault plane solutions show roughly NS compression and EW extension, consistent with a regional stress field produced by the indenting of the India continent.

This paper describes a technique for determining the potential energy of deformed material around a future earthquake rupture, with this energy being stored during the precursory period. The basic parameters are the following: rupture length on the Earth’s surface after the earthquake has occurred , rupture depth h, and the relative block movement along the rupture strike line . We compared the results for 44 large earthquakes with those derived by determining seismic wave energy from earthquake magnitude.

Dense record sections from deep earthquakes in Fiji and Argentina recorded on hundreds of short-period stations in California at distances of 81° to 85° are used to investigate the detailed P wave velocity structure above the core-mantle boundary (CMB). In the Fiji data a secondary phase arriving 2 to 4 s after the direct P is identified as a precursor to PcP. This phase provides good evidence for a reflection off the top of a thin low-velocity layer above the CMB. Comparisons to synthetic seismograms indicate a layer thickness of 10 km and a velocity reduction of 5%–10% compared to the overlying mantle. A record section from an Argentina event does not show the PcP precursor, indicating that the low-velocity layer is not a global feature. This thin low-velocity layer is in the same place as a much larger S wave velocity anomaly in the lower mantle and is probably indicative of a boundary layer just above the CMB under the mid-Pacific.

Teleseismic body waves of the great Bolivian earthquake of June 9, 1994 are analyzed to determine the focal parameters and rupture history. Broadband seismograms reveal a complex rupture process: A small initial event (Mw 7.2) was followed about 10 s later by a large moment release pulse of about 40 s duration. Focal mechanisms determined for the mainshock indicate normal faulting with one very shallow NE dipping plane. Azimuthal variation in body-wave displacement pulse widths suggest northward rupture. From master event and body-wave inversion, the main moment release is located 25–50 km NE of the initiation point at about 650-km depth with only small depth variations between the initial and main event. This suggests that rupture was N directed on the near-horizontal plane. Because the slab along other parts of the Andean arc at about 600-km depth dips steeply, a sub-horizontal plane may imply shearing perpendicular to slab dip. Downdip compression on a sub-horizontal plane would then imply that the slab does not penetrate the 660-km seismic discontinuity, but rather, is being sheared out to the NE. This interpretation is not unique as other scenarios are also possible. Such an event immediately above the 660-km discontinuity suggests massive deformation above the discontinuity with no smooth slab penetration into the lower mantle.

The experience accumulated over a period of 20 years’ earthquake research particularly with respect to prediction suggests that many of experimental points could hardly be explained in the context of existing earthquake origination mechanical model, namely: deep focus earthquakes with hypocenters located within the plastic asthenospheric material; earthquake triggers of different physical nature from chromospheric flashes at the Sun to sharp change of Asthenosphere electricity; appearance of precursors at the distances several times exceeding the size of earthquake preparation zone in all states of matter (solid, liquid and gaseous) and any of media accessible to observation (lithosphere, lower layers of atmosphere, ionosphere, biosphere); etc.