摘要:

We have studied the evolution of material surfaces in strongly time‐dependent convection for both Newtonian and non‐Newtonian temperature‐ and depth‐dependent rheologies. A spline‐characteristic method has been employed. The method of characteristics is second‐order in time and fourth‐order in space. Our strategy is to employ a very dense grid for the material surface field with about 50 to 100 times more points than for the temperature field grid. We have detected sharp entrainment of the surrounding material into both the ascending and descending flow structures. We have observed the breakoff process involving the descending plume. Mixing takes place very differently for the two rheologies. With time, small vortical features are developed inside the Newtonian plumes, while unmixed islands still persist in the non

ISSN:0094-8276    

DOI:10.1029/96GL02182

年代:1996

数据来源: WILEY

摘要:

The partition coefficient of Fe and Mg between perovskite and magnesiowüstiteKMg‐FePv‐Mwhas been measured by adopting a B2O3catalyst technique. The present experimental results demonstrate thatKMg‐FePv‐Mwis a factor of two closer to unity than that obtained in previous studies.KMg‐FePv‐Mwincreases from 0.16 to 0.35 with increasing Mg# of magnesiowüstite from 50 to 86. On the basis of these results, the compositions of enstatite and magnesiowüstite included in diamond imply that the temperature at the top of the lower mantle is 1870–2040 K. The Mg# (100Mg/(Mg+Fe)) of perovskite and magnesiowüstite at the top of the lower mantle are 93 and 82–83, respectively, for a pyrolitic mantle model. The upper bound of the lower mantle temperature estimated in this study do not allow a larger fraction of perovskite than t

ISSN:0094-8276    

DOI:10.1029/96GL02086

年代:1996

数据来源: WILEY

摘要:

The fault parameters of the 1994 Shikotan earthquake were determined by inverting all available geodetic data using an iterative linearized least‐squares method. A one‐fault inversion shows that a shallow‐dip fault‐plane model does not reproduce the minor deformation observed on Shikotan island, while a steep‐dip model can fit all the data well. Multifault inversion shows that a fault‐plane divided into two subfaults lengthwise is the best fitting model based onAkaike's Information Criteria[AIC] for shallow‐dip faulting mechanism. This model yields a very high stress drop, 40 MPa, on the deeper subfault. For a steep‐dip model, a fault plane divided into two subfaults sideways gives the minimumAIC. All the computations for models consisting of two to eight subfaults show that the chi‐square values of the shallow‐dip models are about 10% higher than those of the steep‐dip models. Considering the high stress drop, the larger chi‐square value, and the requirements of a complex subfault system for the shallow‐dip model, we reach the conclusion that the steep‐dip model is more suitable to explai

ISSN:0094-8276    

DOI:10.1029/96GL02049

年代:1996

数据来源: WILEY

摘要:

Magmatic flow in the dense suspension of crystallizing gabbros below the free surface of basaltic magma chambers is considered from the point of view of flow mechanisms and rheology. Hyperdense suspensions (∼20% melt fraction) may arise if flat plagioclase crystals develop a strong preferred orientation induced by magmatic flow. With the help of Nomarski differential interference contrast and back scattered electron figures, we show that suspension flow is possible even for smaller melt fractions if impingements between moving crystals are reduced by chemical dissolution at their contact points. This dissolution process is rate controlling. With strain rates near 10−9s−1and viscosities near 1014–16Pa.s, such crystalline mushes should be closer to plastically deforming solids than to the overlying basaltic suspension. If we characterize magma chambers by suspension flow, no matter how small the melt fraction, magma chambers below oceanic fast spreading centers should not be restricted to a perched melt lens, but should extend to the Moho and comprise the entire volume of observed strong seismic atte

ISSN:0094-8276    

DOI:10.1029/96GL02073

年代:1996

数据来源: WILEY

摘要:

We analyze three‐dimensional (3D) particle motions of crack waves which propagate along a fracture to determine fracture orientation. Crack waves were measured at an artificial subsurface fracture at a depth of approximately 370 m in the Higashihachimantai Hot Dry Rock model field in Japan. Dispersion was observed in crack waves. Velocities of crack waves below 100 Hzare 100 m/s–150 m/s along this artificial fracture. Therefore, crack waves arrived after compressional wave and shear wave. A coherence matrix is used to analyze the 3D particle motions in the frequency domain. Noise, which is outside of the band of the detector, is excluded in the coherence matrix analysis. We examine the polarization of 3D particle motions of crack waves using Principal Component Analysis. The longest axis of an ellipsoid which approximates the 3D particle motions shows a direction consistent with estimates obtained by core‐sample measurements and tectonic stress measurements. The differences in fracture orientation obtained from the crack‐wave analysis and the core sample analysis are less than 16° in azimuth and less than 20

ISSN:0094-8276    

DOI:10.1029/96GL02051

年代:1996

数据来源: WILEY

摘要:

Forearc slivers are commonly present at obliquely convergent margins. Such slivers are often buttressed at their leading edges, but no quantitative model of the buttress effect exists. I consider the horizontal stresses in a buttressed sliver using a simple plan‐view two‐dimensional elastic model. The driving force created by the subducting plate is approximated by a body force in the direction of plate convergence. An Airy stress function solution relates the stress field to the boundary and body forces that allows the relative importance of different forces driving and resisting the sliver to be evaluated. Oblique subduction can easily cause arc‐parallel compression in the buttressed sliver, especially near the leading edge, because the arc‐parallel component of the driving force by the subducting plate is integrated over a much larger distance than its arc‐normal component. For the Southwest Kuril arc, the stress field predicted by the simple model agrees with that inferred from geological structures. It is possible that the model can also explain the stress field of the Cascadia forearc as constrained by earthquake focal mechanisms and in situ stress mea

ISSN:0094-8276    

DOI:10.1029/96GL02067

年代:1996

数据来源: WILEY

摘要:

An electric station was installed in July 1993, 4.5 km away from the VAN station of Ioannina, and recorded in the following two years a number of anomalous signals, including those of April 18 and 19, 1995, interpreted by the VAN group as Seismic Electric Signals precursors to the May 13 Kozani earthquake (West‐Macedonia). A magnetic station was also installed and clearly recorded the magnetic components of the same events. The amplitude, shape, characteristic pattern and duration, magnetic characteristics and polarisation of the anomalous signals suggest that they are generated by artificial (industrial) source

ISSN:0094-8276    

DOI:10.1029/96GL02170

年代:1996

数据来源: WILEY

摘要:

An important constraint on the size and destructive potential of earthquakes is the depth extent of rupture. Laboratory studies of the transition from unstable to stable sliding, along with observed relationships between surface heat flow and the thickness of the seismogenic crust, provide strong evidence for the significance of temperature in determining the maximum nucleation depth of large earthquakes. The June 28, 1992, Mw7.3 Landers earthquake ruptured fault segments within 20 km of 11 pre‐existing heat flow measurements, and shallowing of the base of aftershock seismicity along strike correlates with an increase in heat flow. Crustal geotherms estimated from these measurements place the base of seismicity along the 250°C isotherm. This temperature is consistent with predictions from laboratory studies of the frictional stability of Westerly granite, but estimated temperatures for the seismic‐aseismic transition along other faults within the San Andreas fault system are in the range of 350 to 400°C. Variations in country rock and fault gouge composition, together with higher slip rates, may account for this difference, although part of the Landers seismogenic crust might remain unrup

ISSN:0094-8276    

DOI:10.1029/96GL02066

年代:1996

数据来源: WILEY

摘要:

The issue concerning the dominant creep mechanism in mantle convection has been studied numerically with a rheology incorporating both linear and non‐linear elements. We have employed a rheology suggested by the recently obtained melting temperature of lower‐mantle constituents. For an effective Rayleigh number between 105and 106this type of strongly temperature and pressure‐dependent rheology induces a circulation in the Earth's mantle, which is characterized by a sluggish type of flow in the lower mantle with a, prevailing Newtonian character, while in the upper mantle much more vigorous flows are developed in a predominantly non‐Newtonian r

ISSN:0094-8276    

DOI:10.1029/96GL02065

年代:1996

数据来源: WILEY

摘要:

On December 28, 1994 a segment along the northern Japan trench failed in a Mw=7.7 underthrusting earthquake, the Sanriku‐oki earthquake. Less than 30 years earlier, the great Tokachi‐oki underthrusting earthquake of May 16, 1968 (Mw=8.2) occurred in the same region. We examine the rupture process of the recent event, using teleseismic body and surface wave data, and compare our results to the rupture characteristics of the Tokachi‐oki earthquake, as estimated by various authors in previous studies. We use an empirical Green’ function technique to obtain relative source time functions spanning a large range of the directivity parameter. The strong azimuthal dependence of the durations of these source time functions indicates a significant unilateral component of rupture in a down‐dip direction (west‐northwest). The main rupture initiated about 35 km southwest of the epicenter approximately 20 seconds after the onset of a low energy precursor. The main Tokachi‐oki rupture was also preceded by low moment release. However, it is unclear whether the low moment release preceding both earthquakes represents part of the rupture process or a distinct foreshock. The main rupture areas (asperities) of the Sanriku‐oki and the Tokachi‐oki earthquakes do not sign

ISSN:0094-8276    

DOI:10.1029/96GL02063

年代:1996

数据来源: WILEY