Mechanism Of Deep-focus Earthquakes Research Articles

Unsupervised machine learning reveals slab hydration variations from deep earthquake distributions beneath the northwest Pacific

Although transformational faulting in the rim of the metastable olivine wedge is hypothesized as a triggering mechanism of deep-focus earthquakes, there is no direct evidence of such rim. Variations of the b value – slope of the Gutenberg-Richter distribution – have been used to decipher triggering and rupture mechanisms of deep earthquakes. However, detection limits prevent full understanding of these mechanisms. Using the Japan Meteorological Agency catalog, we estimate b values of deep earthquakes in the northwestern Pacific Plate, clustered in four regions with unsupervised machine learning. The b-value analysis of Honshu and Izu deep seismicity reveals a kink at magnitude 3.7–3.8, where the b value abruptly changes from 1.4–1.7 to 0.6–0.7. The anomalously high b values for small earthquakes highlight enhanced transformational faulting, likely catalyzed by deep hydrous defects coinciding with the unstable rim of the metastable olivine wedge, the thickness of which we estimate at sim1 km.

A laboratory nanoseismological study on deep-focus earthquake micromechanics.

Global earthquake occurring rate displays an exponential decay down to ~300 km and then peaks around 550 to 600 km before terminating abruptly near 700 km. How fractures initiate, nucleate, and propagate at these depths remains one of the greatest puzzles in earth science, as increasing pressure inhibits fracture propagation. We report nanoseismological analysis on high-resolution acoustic emission (AE) records obtained during ruptures triggered by partial transformation from olivine to spinel in Mg2GeO4, an analog to the dominant mineral (Mg,Fe)2SiO4 olivine in the upper mantle, using state-of-the-art seismological techniques, in the laboratory. AEs' focal mechanisms, as well as their distribution in both space and time during deformation, are carefully analyzed. Microstructure analysis shows that AEs are produced by the dynamic propagation of shear bands consisting of nanograined spinel. These nanoshear bands have a near constant thickness (~100 nm) but varying lengths and self-organize during deformation. This precursory seismic process leads to ultimate macroscopic failure of the samples. Several source parameters of AE events were extracted from the recorded waveforms, allowing close tracking of event initiation, clustering, and propagation throughout the deformation/transformation process. AEs follow the Gutenberg-Richter statistics with a well-defined b value of 1.5 over three orders of moment magnitudes, suggesting that laboratory failure processes are self-affine. The seismic relation between magnitude and rupture area correctly predicts AE magnitude at millimeter scales. A rupture propagation model based on strain localization theory is proposed. Future numerical analyses may help resolve scaling issues between laboratory AE events and deep-focus earthquakes.

Evolution of the Downgoing Lithosphere and the Mechanisms of Deep Focus Earthquakes

Summary The thermal evolution of the lithospheric slab at subduction zones and its geophysical effects are numerically calculated. An alternating-direction, implicit, finite-difference scheme is used to compute the thermal models taking into account all heating sources and phase boundaries. These models, with the appropriate spreading rates and dip angles, are compared with different island arc systems. Temperatures inside the slab are strongly controlled by the conductivity and by the time elapsed since the initiation of descent. The depth to which temperature anomalies persist is generally about 700 km or less. The thermal results are used to construct the seismic velocities and ray paths, density anomalies, and the resulting stress distribution. Comparing the theoretical stress distribution and the focal mechanism studies of intermediate- and deep-focus earthquakes indicates the importance of both the mantle's rheology and the temperature dependence of the slab's elastic properties. The intermediate and deep focus earthquakes are located along the coolest region of the slab. The theoretical results explain the source mechanisms and the orientation of principal stresses under major island arcs.

Evaluation of the Nucleation Rate of Olivine

AbstractPhase transformation of (Mg0.89Fe0.11)2SiO4 olivine, the proposed major composition of upper mantle, is related to the mechanism of deep focus earthquakes and the interaction between subduction zones and their ambient mantle. Nucleation rate and growth rate are two important parameters in the computation of the phase transformation. However, the nucleation rate has not been derived from the experimental data partly for technical difficulties. We modify the dimensionless parameter describing the impingement under grain boundary nucleation through physical analysis on the image of the impingement. Based on this modification and the experimental data, we first evaluate the nucleation of olivine. By fitting the new nucleation rates, we get the parameters K0 and γø1/3. The best fitting results are K0 = 6.9 × 1024s−1·m−2·K−1, γø 1/3=0.16 J·m−2.

高温高圧鉱物物性 Ｄｒｃｋａｍｅｒ型高圧装置を用いたＸ線その場観察とその地球物理学への応用

In situ X-ray observation using Drickamer type high pressure apparatus and its application to geophysics are discussed in detail. At first, characteristics of Drickamer apparatus are introduced by comparing with diamond anvil cell and multi anvil apparatus. Then, two topics are discussed as its applications to geophysical problems: One is the mechanism of deep-focus earthquakes and the other is the phase relation of iron which thought to be dominant component of the Earth's inner core. Nature of deep-focus earthquakes has been enigmatic. Two models describing the mechanism of deep earthquakes are proposed recently based on high pressure experiments and seismological measurements of the structure of subduction zone. These two models, frictional instability caused by pore water pressure and transformational faulting, are briefly introduced, and discussed in relation to differential stress measured by the Drickamer apparatus. Recently, the existence of two new phases of iron is proposed. One is a high temperature phase with dhcp structure, which appears at pressures of above 30GPa. The other is a very high pressure and temperature phase above 200GPa and 4000K with bcc structure which is proposed theoretically. Whether the former new phases exist or not is discussed together with X-ray and electrical resistance data obtained by the Drickamer apparatus.

蛇紋石の非晶質化と深発地震

蛇紋石の非晶質化と深発地震

Rupture Characteristics of the Deep Bolivian Earthquake of 9 June 1994 and the Mechanism of Deep-Focus Earthquakes

The M(w) = 8.3 deep (636 kilometers) Bolivian earthquake of 9 June 1994 was the largest deep-focus earthquake ever recorded. Seismic data from permanent stations plus portable instruments in South America show that rupture occurred on a horizontal plane and extended at least 30 by 50 kilometers. Rupture proceeded at 1 to 3 kilometers per second along the down-dip azimuth of the slab and penetrated through more than a third of the slab thickness. This extent is more than three times that expected for a metastable wedge of olivine at the core of the slab, and thus appears to be incompatible with an origin by transformational faulting. These large events may instead represent slip on preserved zones of weakness established in oceanic lithosphere at the Earth's surface.

Introduction to the Special Section on the Transition Zone

“The transitional layer appears to hold the key to a number of major geophysical problems.” This often quoted phrase, from the abstract of Birch's [1952, p. 227] work on the transition zone, is still true today, some 42 years later. In fact, the many studies related to the transition zone since Birch's work have often raised more challenging questions than they have answered. These questions include: Does the lower boundary of the transition zone mark a change in bulk composition between the upper and lower mantle? What is the mechanism of deep focus earthquakes and why does seismic activity cease near the bottom of the transition zone? How deeply are volatiles recycled into the mantle and does the transition zone serve as a volatile rich reservoir? Our view of the transition zone places important constraints on these and many other questions.

高圧実験による深発地震発生機構の解明

高圧実験による深発地震発生機構の解明

A new self-organizing mechanism for deep-focus earthquakes

THE mechanism of deep-focus earthquakes has been a puzzle since their discovery almost 70 years ago1, 2, because brittle fracture and frictional sliding at depths in excess of 100–200 km would require unrealistic rock strengths3, 4. Rock strength does increase with pressure, but a few hundred MPa (equivalent to 10–20 km depth) suffices to inhibit most fracture, and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength. A range of mechanisms has been proposed for deep earthquakes, including plastic instabilities5–7, shear-induced melting8–11 and instabilities accompanying recrystallization12, 13 or polymorphic phase transformation14–23. Each of these proposed mechanisms has exhibited certain inherent weaknesses (see Kirby22 for review and discussion). Here we report experimental observations of high-pressure faulting of metastable Mg2GeO4 olivine as it undergoes incipient transformation to a spinel 24, 25 structure. We present a model in which this faulting arises from the generation, propagation and linking-up of spinel-filled anticracks26. When applied to the olivine → spinel transformation in the Earth's mantle, the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding.

Stress distribution due to olivine‐spinel phase transition in descending plate and deep focus earthquakes

The stress field due to olivine‐spinel phase transition in and around a descending plate is studied based on the calculations for an equilibrium phase transition with a constant Clapeyron's slope and for a nonequilibrium transition taking a kinetic effect into consideration. The material concerned is assumed to be a viscoelastic body whose physical coefficients are functions of temperature and pressures. The calculated stress distribution for the equilibrium transition is characterized by a compressional stress near the upper and/or lower surfaces of the plate and by a tensional stress at the central part in the depth range from 200 to 550 km, the magnitude of principal stress being greater than 0.5 GPa. However, for the nonequilibrium transition a tensional stress predominates near the upper and lower surfaces, and the central part is compressional in the depth range from 300 to 600 km, unlike the case of equilibrium transition. The maximum principal stress in the latter case is greater than 2 GPa, which is much more than that for the equilibrium transition. The principal axes in the high stressed region are oriented almost parallel to the descending direction in both cases. Comparison of the calculated results to the actually observed seismicity‐depth relation and to focal mechanisms of deep focus earthquakes in descending plates beneath island arcs shows that the nonequilibrium phase transition agrees better with the observations.

Shear instability in a viscoelastic material as the cause of deep focus earthquakes

The mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs. We assume subducting slabs deform such that the spatially averaged strain rate becomes time independent. Furthermore, we assume that rheology depends on temperature in the manner of an activation process. Using this model, we investigate quantitatively the conditions under which shear instability takes place, i.e., the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs. First, we study instability in a material whose rheological properties are spatially homogeneous. Shear instability takes place in a homogeneous material when exceeds a certain critical level , which depends strongly on the stress in the slabs but does not depend on the detail of the creep law (the value of the activation energy). Next, we study instability in a material that contains shear zones similar to those often observed by field geologists, i.e., a thin layer made of material with lower vicosity than the surrounding region. We find that shear zones trigger shear instability even for the case of provided that G, the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones, exceeds 1, and that f, the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones, becomes lower than 1. Applying this condition to subducting slabs with shear zones, we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude. From these results, and from an estimate of in subducting slabs, we conclude that shear instability is a mechanism likely to induce deep focus earthquakes.

Creep mechanism in Mg2GeO4: Effects of a phase transition

We deformed polycrystalline Mg2GeO4 olivine and spinel in a Griggs‐type solid medium apparatus. Flow of Mg2GeO4 olivine can be represented by = 6.5×l07 σ3.5 exp(−105(kcal/mol)/ RT) where σ (kbar) is the differential stress and (s−1) is the natural strain rate. For Mg2GeO4 spinel the flow law is = 2.6×104 σ2exp(−73(kcal/ mol)/RT). The low stress exponent and activation enthalpy coupled with fine grain size (3 μm) suggest that Mg2GeO4 spinel deformed by a superplastic mechanism. Flow parameters for the olivine phase suggest a dislocation creep mechanism. Comparison of theoretical superplastic flow laws for Mg2GeO4 olivine with the spinel phase data suggests that strain rates in Mg2GeO4 spinel are only about a factor of 3 lower than for Mg2GeO4 olivine of the same grain size. A similar estimate holds for dislocation creep of the two phases if it is controlled by diffusion. Transformation from Mg2GeO4 olivine to spinel reduced grain size to approximately 3 μm. Thus we might expect a similar reduction in grain size in the earth's transition zone which could result in superplastic deformation of the transformed phase and cause a weak ‘decoupling’ zone at the transition boundary in the mantle. Superplasticity brought about by transformation‐induced reduction in grain size may also provide a mechanism for deep focus earthquakes and an explanation for the correlation observed between their distribution with depth and the depths of phase transitions within the mantle.

The fate of the downgoing slab: A study of the moment tensors from body waves of complex deep-focus earthquakes

Two large, complex deep-focus earthquakes are analyzed, using body waves, to determine the relative locations and equivalent forces of the component events. Details of the source mechanism determination are given, with special emphasis on the means of comparing results for double-couple and more general models. The formalism includes a description of a method for calculating the confidence ellipses around the principal axes of the moment tensor. An interpretation of the results suggests that the mechanism of deep-focus earthquakes is controlled by the buoyancy of the slab and the change in viscosity at the 650 km discontinuity.

Deep focus earthquakes as triggered dislocation processes

The dependence of stress drop on magnitude and distribution of first motions from long and short period seismographs suggests a trigger mechanism for deep focus earthquakes that might be different from the main dislocation process. Fracture experiments with two-dimensional plates of plastics were carried out to study a triggered fracture process. A high energy rubidium pulse laser served as source for the trigger energy. It was possible to initiate fracture in the stress range 0.2–0.8 of the strength of the material. Inhomogeneity of the stress field was found to be essential in order to obtain a fracture of finite extent. The velocity of crack propagation and the acceleration of the pulses radiated from the trigger phase and main phase were measured as a function of the applied stress.

The pyroxene-garnet transformation in the earth's mantle

An investigation of the solid solubility of pyroxenes in garnet structures in the pressure range 0–250 kb and at about 900°C, is described. At about 100 kb, the solid solubility of enstatite in pyrope increases suddenly, resulting in formation of a garnet of composition Mg 3(AlMg 0.5Si 0.5)Si 3O 12. Analogous relationships are found to exist between calcium-rich pyroxenes and garnets. The results imply the occurrence of a type of pyroxene-garnet transformation, in which the garnets are characterized by partial 6-fold coordination of silicon. The transformation is accompanied by a density increase of about 10%. A variety of evidence strongly indicates that these new garnets are thermodynamically stable. The transformation has also been demonstrated in a typical eclogite, which, at a pressure of 127 kb, crystallizes to an assemblage of 95% garnet and 5% pyroxene, with a mean density of 3.75 g/cm 3. The pyroxene-garnet transformation may be important in the earth's mantle where it would occur at a depth of about 350 km, thus marking the beginning of the transition zone. A speculative model is also presented whereby the occurrence of the garnet-pyroxene transformation in blocks of eclogite which are sinking through the mantle provides a mechanism for deep-focus earthquakes.

On the mechanism of deep-focus earthquakes

Обясняется возникновение глубоких землетрясений на основе гипотеза автора по увеличении объема Земли. Получается ответ на тот вопрос, почему существует минимум сейсмичности между глубоких и мельких землетрясений. Получается и объяснение о механизме возникновения глубоководных грабенов, которое согласуется с наблюдениями Юинга (Ewing) и сотрудников.

Materials for the study of deep-focus earthquakes*

Summary (Abstract) Travel times of normal earthquakes are made the basis for calculated travel times for shocks at depths down to 800 km. These are presented in tabular form, together with certain auxiliary data. Theoretical discussion is given for the critical distances at which pP and PP, sP and SP, etc., coincide, and below which these phases should not occur. These distances are not focal points (caustics). Methods are given for determining epicenter, depth, and origin time, and are applied to a selected group of shocks. The results of different methods agree very well with one another and with the calculated travel-time data. A preliminary report is given on certain characteristics of the shocks studied. The mechanism of deep-focus earthquakes is discussed briefly, and it is concluded that normal and deep-focus earthquakes are probably brought about by the same forces.