A Non Double-Couple Earthquake in a Subducting Oceanic Crust of the Philippine Sea Plate.

Hurukawa, Nobuo; Imoto, Masajiro

doi:10.4294/jpe1952.41.257

Cited by 15 publications

(8 citation statements)

References 23 publications

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“…The downward bending of the PHS slab in the depth range of 50-70 km shows a certain boundary of physical properties of the slab, therefore it is conceivable that this downward bending is caused by the increase in density of the oceanic crust due to the basalt-eclogite transition with the subduction of the PHS slab. Hurukawa and Imoto (1993) find a non double-couple earthquake, which is considered to be due to the sudden volume contraction, at a depth of about 60 km in the oceanic crust of the PHS slab in Kanto, and conclude that gabbroor basalt-eclogite transition in the oceanic crust caused this earthquake. The depth of the downward bending of the PHS slab estimated in our study is consistent with the depth of the non double-couple earthquake.…”

Section: Seismological Constraints On Magmatism In Southwestern Japanmentioning

confidence: 84%

Seismic tomography of the uppermost mantle beneath southwestern Japan: Seismological constraints on modelling subduction and magmatism for the Philippine Sea slab

Honda

2014

Earth Planet Sp

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Many studies have been made on the subduction of the Pacific slab and the magmatism in northeastern Japan, but not on the subduction of the Philippine Sea slab and the magmatism in southwestern Japan. Primary reasons may be that seismological networks in southwestern Japan were sparse as compared with those in northeastern Japan and that geology including volcanism of southwestern Japan is more complicated than that of northeastern Japan. However, recent instrumental development of dense seismological networks in the Japanese Islands has provided us with high quality data not only for northeastern Japan but for southwestern Japan. One of the outcomes from the development is the increase of accuracy of arrival time readings of P-and S-waves and resultant hypocenter determination. We attempt to elucidate fine image of the uppermost mantle structure beneath the Japanese Islands and to find evidence for the relation between the magmatism and subduction process. We apply travel time tomography to 216,247 P-and 98,207 S-wave arrival times observed at 1,328 seismic stations from 5,242 earthquakes in and around the Japanese Islands, and obtain three-dimensional variations of P-and S-wave velocity structure. In Chubu and Kyushu, the subducting Philippine Sea slab bends downward in the depth range of 50 to 70 km. In some nonvolcanic regions, remarkable anomalies of high Poisson's ratio (and low S-wave velocity) are seen in the depth range of 25 to 40 km near the upper boundary of the Philippine Sea slab or the Moho discontinuity, and approximately coincide with the hypocenter distribution of deep low-frequency earthquakes. The anomalies of high Poisson's ratio are also seen near the upper boundary of the Philippine Sea slab or the overlying mantle wedge down to a depth of about 60 km, but are not seen after the downward bending of the slab, in the forearc region. The anomalies are probably caused by separated fluid or hydrous minerals. These characteristics should be taken into account in numerical modelling of the subduction of young slabs (e.g., Philippine Sea slab) and associated phenomena (e.g., magmatism).

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Section: Seismological Constraints On Magmatism In Southwestern Japanmentioning

confidence: 84%

Seismic tomography of the uppermost mantle beneath southwestern Japan: Seismological constraints on modelling subduction and magmatism for the Philippine Sea slab

Honda

2014

Earth Planet Sp

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“…Inversion of P and SH waveforms yielded a moment tensor equivalent to a reverse-slip DC combined with an implosion (Figure 2). A similar but smaller (magnitude 4.6) earthquake on February 10, 1987 beneath the Kanto district, Japan, had dilatational P wave polarities over most of the focal sphere (Figure 14) [Hurukawa and Imoto, 1993]. The polarities are consistent with conical nodal planes with an apex angle of about 78 ø, implying an implosive isotropic component (the apex angle for a CLVD is 109.47ø).…”

Section: Other Shallow Earthquakesmentioning

confidence: 99%

Non‐double‐couple earthquakes 2. Observations

Miller¹,

Foulger²,

Julian³

1998

Reviews of Geophysics

197

106

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Abstract. Most studies assume that earthquakes have double-couple (DC) source mechanisms, corresponding to shear motion on planar faults. However, many wellrecorded earthquakes have radiation patterns that depart radically from this model, indicating fundamentally different source processes. Seismic waves excited by advective processes, such as landslides and volcanic eruptions, are consistent with net forces rather than DCs. Some volcanic earthquakes also have single-force mechanisms, probably because of advection of magmatic fluids. Other volcanic earthquakes have mechanisms close to compensated linear vector dipoles and may be caused by magmatic intrusions. Shallow earthquakes in volcanic or geothermal areas and mines often have mechanisms with isotropic components, indicating volume changes of either explosive or implosive polarity. Such mechanisms are consistent with failure involving both shear and tensile faulting, which may be facilitated by high-pressure, high-temperature fluids. In mines, tunnels are cavities that may close. Deep-focus earthquakes occur within zones of polymorphic phase transformations in the upper mantle at depths where stick-slip instability cannot occur. Their mechanisms tend to be deviatoric (volume conserving), but non-DC, and their source processes are poorly understood. Automatic global moment tensor services routinely report statistically significant non-DC components for large earthquakes, but detailed reexamination of individual events is required to confirm such results. INTRODUCTIONA large body of evidence connecting earthquakes with faulting comes both from field observations and from the study of seismic waves. In theory, the compressional waves radiated by a shear fault have a four-lobed pattern, with adjacent lobes alternating in polarity. This pattern, and the entire static and dynamic field of a shear fault in an isotropic medium, is identical to that produced in an unfaulted medium by a distribution over the fault surface of pairs of force couples, with each pair arranged so that its net torque vanishes. Seismologists usually specify earthquake mechanisms in terms of equivalent force systems, and shear-fault mechanisms are called "double couples" (DCs).The hypothesis that earthquake source mechanisms are DCs has become so widely accepted as to have been treated almost as a fundamental law by many seismologists. To a large extent, however, the success of the DC model has been a consequence of limitations in data quantity and quality. Recent improvements in seismological instrumentation and analysis techniques now identify earthquakes whose radiated waves are beyond•Also at U.S. Geological Survey, Menlo Park, California. doubt incompatible with DC force systems and thus with shear faulting. Well-constrained non-DC earthquakes have been observed in many environments, including particularly volcanic and geothermal areas, mines, and deep subduction zones. A companion paper [Julian et al., this issue, hereinafter referred to as paper 1] summarizes seismic source theory and...

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“…Neither of these conclusions is readily applicable to the Holsnøy rocks, however, because the pseudotachylytes clearly antedate any postgranulite facies ductile deformational features in the rocks. Similarly, field relations do not support the interpretation that the eclogitization process itself triggered the earthquakes, as has been suggested for some modern intermediate-depth earthquakes [e.g., Hurakawa and Imoto, 1993], since the best preserved occurences of pseudotachylyte lie in areas of unconverted granulite.…”

Section: Dynamic Puzzles Posed By the Pseudotachylytesmentioning

confidence: 99%

“…These changes not only influence the geophysical signature of the subducted material but also play an active role in the geodynamics of subduction and collision zones. At short timescales, subduction‐related metamorphic transitions may be responsible for intermediate‐depth earthquakes [e.g., Hori , 1990; Hurukawa and Imoto , 1993; Comte and Suarez , 1994; Kirby et al , 1996; Peacock , 2001]. Over geologic time intervals, metamorphic densification of upper crustal rocks in subduction zones and in the deep roots of mountain belts dictates the fate of tectonically buried rocks, thereby influencing the rates of crustal recycling [ Ahrens and Schubert , 1975], mountain building [ Bousquet et al , 1997], cratonization, and other geodynamic processes.…”

Section: Introduction: Metamorphic Responses To Subduction and Tectonmentioning

confidence: 99%

Processes leading to eclogitization (densification) of subducted and tectonically buried crust

Bjørnerud

Austrheim²,

Lund³

2002

J. Geophys. Res.

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[1] The transformation of subducted and tectonically buried crustal rocks to denser eclogite plays a fundamental role in the dynamics of mountain building and crustal recycling. However, a complex of partially eclogitized granulites on the island of Holsnøy in the western part of the Norwegian Caledonides reveals that such densification depends in part on the antecedent history of rock masses and that large bodies of untransformed rock can persist metastably even after long residence times in the lower crust. The rocks on Holsnøy suggest that conversion to eclogite was delayed until lower or subcrustal seismic events allowed aqueous fluids to enter the previously dry and unusually strong granulite complex. Metamorphism then occurred in a spatially heterogeneous manner, with several distinct physical processes involving feedbacks between deformation, fluid infiltration, and chemical reactions operating simultaneously in different parts of the rock mass. Field relations show that the introduction of fluids and conversion to eclogite significantly weakened the rocks. A reservoir-flux systems model is used to represent the various processes contributing to eclogitization of the granulites, which operated at vastly different rates over disparate intervals of time. The model suggests that the metamorphic process may have been brief ((1 Myr), and ultimately self-limiting, arrested by the change in rheology associated with the conversion of brittle granulite to ductile eclogite.

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A Non Double-Couple Earthquake in a Subducting Oceanic Crust of the Philippine Sea Plate.

Cited by 15 publications

References 23 publications

Seismic tomography of the uppermost mantle beneath southwestern Japan: Seismological constraints on modelling subduction and magmatism for the Philippine Sea slab

Seismic tomography of the uppermost mantle beneath southwestern Japan: Seismological constraints on modelling subduction and magmatism for the Philippine Sea slab

Non‐double‐couple earthquakes 2. Observations

Processes leading to eclogitization (densification) of subducted and tectonically buried crust

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