摘要:

Paleomagnetic and rock magnetic experiments on the Triassic Yinkeng Formation and the Permian Changxing Formation sampled from the Permian‐Triassic type section, Changxing County, reveal that a high temperature primary direction is isolated in dolomitic samples from the Early Triassic but the Late Permian limestones samples appear to be remagnetized during the Jurassic. A Permian‐Triassic pole previously reported for the Changxing Formation from this section (Lin et al., 1985) is not in agreement with results from this study or other poles for that time interval reported for the Yangtze Block and may be remagneti

ISSN:0094-8276    

DOI:10.1029/93GL01242

年代:1993

数据来源: WILEY

摘要:

At the Verwey transition (Tv≈110–120 K), magnetite transforms from monoclinic to cubic spinel structure. It has long been believed that magnetic remanence and susceptibility would change markedly at Tvin the case of coarse grains but only slightly or inappreciably in the case of fine (<1 µm) grains. We find on the contrary that remanence changes at Tvby 50–80% in both large and small crystals, if they are stoichiometric. However, minor surface oxidation suppresses the transition, and the fact that fine grains oxidize more readily leads to an apparent size dependence. Our experiments used submicron magnetite cubes with mean sizes of 0.037, 0.076, 0.10 and 0.22 µm which were initially non‐stoichiometric (oxidation parameter z from 0.2–0.7). A saturation isothermal remanent magnetization (SIRM) given in a 2.5 T field at 5 K decreased steadily during zero‐field warming to 300 K with little or no indication of the Verwey transition. After the oxidized surface of each crystal was reduced to stoichiometric magnetite, the SIRM decreased sharply during warming by 50–80% around 110 K. The change in SIRM for the 0.22 µm grains was almost identical to that measured for a 1.5 mm natural magnetite crystal. Thus a 1012change in particle volume does not materially affect the remanence transition at Tvbut oxidation to z=0.3 essentially suppresses the transition. The effect of the degree of oxidation on Tvprovides a sensitive test for maghemitization in soils, sed

ISSN:0094-8276    

DOI:10.1029/93GL01483

年代:1993

数据来源: WILEY

摘要:

As determined by quantitative differential thermal analysis, the enthalpies of fusion of forsterite and spinel, are ΔHf= 142 ± 14 and 107 ± 11 kJ/mol at 2174 and 2408 K, respectively. Significant premelting has been observed for forsterite, beginning about 100 K below the congruent melting point. When heated in graphite crucibles, forsterite reacts with carbon and the products of reaction vaporize with an overall enthalpy of reaction of the order of 1600 kJ/mol. The configurational entropy of Mg2SiO4liquid decreases so rapidly with decreasing temperature that it would be almost zero at its nominal glass‐transition temperature as estimated by extrapolating viscosity data for magnesium aluminosilicate melts. Extreme cooling rates are thus needed in order to quench actual Mg2SiO4glasses having significant configurational entro

ISSN:0094-8276    

DOI:10.1029/93GL01836

年代:1993

数据来源: WILEY

摘要:

Measurements in three samples of very clean quartz sandstone in the porosity range 4–16 %, under dry and 100 % water‐saturated conditions, show that P‐ and S‐wave velocities are linearly correlated with thermal conductivity. The experimental results agree with the theoretical relation between seismic velocities (predicted by the Kuster and Toksöz model (1974)) and thermal conductivity (predicted by weighted geometr

ISSN:0094-8276    

DOI:10.1029/92GL02460

年代:1993

数据来源: WILEY

摘要:

The role of high fluid pressure as a seismogenic agent has been the subject of intense study (Hubert and Rubey, 1959; Hanshaw and Bredehoeft, 1968; Healy and Rubey, 1968; Simpson, 1976; Walder and Nur, 1984; Sibson, 1990). Of particular interest is the so‐called fault‐valve mechanism (Sibson, 1976; Sibson, 1990) a hypothesis whereby fluid pressure rises (as a result of tectonic compression and pore volume reduction) until crustal failure occurs, triggering seismic activity and upward fluid discharge. Sealing and healing of the rock matrix (Richter and Simmons, 1977; Sprunt and Nur, 1979; Angevine et al, 1982) following coseismic stress drop facilitates reaccumulation of fluid pressure, initiating another loading cycle. The fault‐valve mechanism is entertained as a plausible explanation for present‐day seismic activity in the western Transverse Ranges of California. We provide a quantitative test of the fault‐valve hypothesis that uses geologic data and rates of active tectonics for a cross‐section through an active fold‐and‐thrust belt on the flank of a developing mountain range. Rates of fluid pressure buildup and average recurrence times of large earthquakes in the fold‐and‐thrust belt are estimated to be on the order of 104Pa/yrand hundreds of

ISSN:0094-8276    

DOI:10.1029/93GL01661

年代:1993

数据来源: WILEY

摘要:

It has recently been proposed that all deep earthquakes (>400 km depth) are caused by a shear instability associated with the transformation of metastable olivine to its high‐pressure polymorphs within the cold core of subducting slabs (Green and Burnley, 1989; Kirby et al., 1991). If so, then the seismogenic zone would narrow with depth below 400 km following slab isotherms. Because the ratio of fault slip to length does not increase markedly with depth, and stress orientations indicate that deep fault planes are not parallel to the subducting slab, a fault plane large enough to generate a very large, deep earthquake would not fit within a narrow seismogenic zone and multiple fault planes would be needed. If the fault planes are not parallel due to spatial variations in stress, a deviatoric non‐double‐couple component (volume‐preserving, non‐shearing part) of the seismic moment tensor is likely to result. Thus, if the seismogenic zone narrows with depth, one would expect an increasing non‐double‐couple component (1) for large earthquakes as depth increases from 400 to 700 km, and (2) for very deep earthquakes (below 550 km depth) as seismic moment increases. Data from the Harvard CMT catalog, augmented by moment tensor inversions for the two largest deep earthquakes during the past thirty years, are examined for these features. Although significant scatter is present, the data roughly follow the trends described above, suggesting a relationship between the non‐double‐couple component of deep earthquakes and the geometry of a seismogenic zone nar

ISSN:0094-8276    

DOI:10.1029/93GL01301

年代:1993

数据来源: WILEY

摘要:

The recent Nicaragua tsunami earthquake (September 2, 1992) produced a distinct ramp‐like long‐period (up to 1000 sec) phase which begins betweenPandSwaves on displacement seismograms. In terms of ray theory, this phase consists of long‐periodP, PP, S, SS, SP, PS,etc and its propagation mechanism is similar to that of a whispering gallery. In terms of normal‐mode theory, it represents a group of higher‐mode Rayleigh waves with a group velocity close to, but slower than, that ofPwave. This phase has not been recognized as a distinct phase in the seismological practice because of clipping of seismograms for very large earthquakes. With the advent of modern wide‐dynamic range seismographs, this phase can be easily identified for all large earthquakes. In view of its use for identifying slow earthquakes, determining whether slow deformation is precursory or coseismic to the regular short‐period energy release, and determining velocity structures between the source and the station, we propose that this phase be called

ISSN:0094-8276    

DOI:10.1029/93GL01883

年代:1993

数据来源: WILEY

摘要:

Experimental observations of the seismic efficiency of a decoupled nuclear explosion in a salt dome are presented. The energy decoupling factor was calculated from local, regional and teleseismic data using the ratio of the amplitudes of the tamped (well coupled) explosion that had created the cavity and that of the decoupled one. To calculate the amplitudes of the seismic waves of a well coupled explosion of the same yield as the decoupled one, a scaling law for explosions fired within the same region was used. An energy decoupling factor near 30 is consistent with local, regional and teleseismic data. It is concluded that decoupling was only partial for the event described. Numerical calculations of partial decoupling showed that the seismic wave spectrum at an epicentral distance of 2.3 km is consistent with an amplitude decoupling factor of 20, which is consistent with the energy decoupling factor 30.

ISSN:0094-8276    

DOI:10.1029/93GL00172

年代:1993

数据来源: WILEY

摘要:

The Beppu‐Shimabara graben, in Kyushu, Japan, has active seismic regions and active volcanoes. Studies using triangulation, gravity surveys and mechanism solutions of large earthquakes show that the graben is in a state of extensive stress associated with rifting. A temporary seismic study was carried out from February 20 to March 8 in 1992 in the region, during which time many seismic signals were observed. The microearthquake seismograms recorded inside the graben have a later phase dominant on the horizontal component following the direct S‐wave. Two‐dimensional ray‐tracing was applied to investigate the cause of the later phase. The phase was identified as an S‐reflected S phase (SxS‐phase) from a discontinuity with a depth of 20 km. The reflection points are located inside the Beppu‐Shimabara graben that is in a state of extensive stress associated

ISSN:0094-8276    

DOI:10.1029/92GL02800

年代:1993

数据来源: WILEY

摘要:

The Coulomb criterion, has been widely used to infer paleostresses from fault slip data, assuming that faults are optimally oriented relative to the tectonic stress direction. Consequently if the stress direction is fixed during deformation so must be the faults. Freund [1974] has shown however that faults, when arranged in sets, must generally rotate as they slip. Nur et al. [1986]showed that large rotations require the development of new faults more favorably oriented to the principal direction of stress. This leads to older faults sets offset by younger ones, both having the same sense of slip. Because stress rotates as well, paleostress analysis must include the possible effect of stress field rotation.The combined effects of stress field rotation and material rotation were found in the Lake Mead, Nevada Fault System (LMFS). Fault inversion results imply an apparent 60° clockwise (CW) rotation of the stress field since mid‐Miocene time. In contrast, structural data from the Great Basin suggest only a 30° CW stress field rotation. By incorporating paleomagnetic declination anomalies it is inferred that slip on faults caused a local 30° counterclockwise (CCW) rotation of blocks and faults in the Lake Mead area, so that the inferred 60° CW rotation of the stress field in the LMFS is actually onl

ISSN:0094-8276    

DOI:10.1029/93GL01656

年代:1993

数据来源: WILEY