摘要:

Regional seismograms of intermediate‐depth earthquakes (50≤H≤80 km) that occur below the Central Mexican Plateau show a phase which, at epicentral distances of 150 to 450 km, arrives about 15 to 20 sec after the P wave and 5 to 12 sec before the S wave. This phase was previously interpreted as a seismic wave refracted from a dipping interface below the source, thus apparently providing direct evidence of the structural location of the subducted Cocos plate below México. The phase was called the plate wave. Recent intermediate‐depth events have given rise to better quality data recorded by some newly‐installed broadband seismographs. An analysis of these and previous data strongly suggests that the phase is an S‐to‐P converted phase at the free surface and, therefore, provides no information regarding the s

ISSN:0094-8276    

DOI:10.1029/94GL03383

年代:1995

数据来源: WILEY

摘要:

Short‐period seismograms of Tonga deep earthquakes recorded by Japanese and Californian seismic networks are stacked to identify the S‐P converted wave associated with the 660‐km discontinuity. The travel‐time difference between this S‐P converted wave and the direct P wave is used to constrain the depth of the 660‐km discontinuity. Analysis of a total of 29 events produced a detailed topographical map of the discontinuity beneath the Tonga subduction zone. Two events which exhibit clear S‐P conversions in both Japan and California data are selected to show directly the depth variations of the 660‐km discontinuity adjacent to the subducting slab. The S‐P conversion points on the ray paths to Japan are observed to be approximately 10 to 30 km deeper than the conversion points on those to California, which represents direct evidence for a slab‐induced depression of the 6

ISSN:0094-8276    

DOI:10.1029/94GL03332

年代:1995

数据来源: WILEY

摘要:

Earthquake recurrence data from the Pallett Creek and Wrightwood paleoseismic sites on the San Andreas fault appear to show temporal variations in repeat interval. These sites are located near Cajon Pass, southern California, where detailed mapping has revealed geomorphically and structurally expressed domains of alternating extension and contraction respectively associated with releasing and restraining bends of the San Andreas fault. We investigate the interaction between strike‐slip faults and auxiliary reverse and normal faults as a physical mechanism capable of producing such variations. Under the assumption that fault strength is a function of fault‐normal stress (e.g. Byerlee's Law), failure of an auxiliary fault modifies the strength of the strike‐slip fault, thereby modulating the recurrence interval for earthquakes. In our finite element model, auxiliary faults are driven by stress accumulation near restraining and releasing bends of a strike‐slip fault. Earthquakes occur when fault strength is exceeded and are incorporated as a stress drop which is dependent on fault‐normal stress. The model is driven by a velocity boundary condition over many earthquake cycles. Resulting synthetic strike‐slip earthquake recurrence data display temporal variations similar to observed paleoseismic data within time windows surrounding auxiliary fault failures. Although observed recurrence data for the two paleoseismic sites are too short to be definitive about the temporal variations or the physical mechanism responsible for it, our simple model supports the idea that interaction between a strike‐slip fault and auxiliary reverse or normal faults can modulate the recurrence interval of events on the strike‐slip fault, possibly producing short term variations in earthquake recu

ISSN:0094-8276    

DOI:10.1029/94GL03393

年代:1995

数据来源: WILEY

摘要:

The method of nonlinear waveform‐fitting was extended to include changes in the depth of the Moho. Studies performed on synthetic data at 10° epicentral distance showed that it is theoretically possible to determine Moho‐depth with an accuracy of less than a kilometer if the frequency range extends to 0.12 Hz. We applied this method on several broadband records from two earthquakes in Western United States to invert for crustal thickness. Waveform‐fitting of Rayleigh waves for epicentral distances between 3° and 10° for individual paths in this region show that the average Moho‐depth can be resolved to 1–2 km. It was possible to obtain good waveform fits for frequencies between 0.02–0.08 Hz. We conclude that broadband seismic data from local events allow us to determine Moho‐depth independently from crustal she

ISSN:0094-8276    

DOI:10.1029/94GL02845

年代:1995

数据来源: WILEY

摘要:

Given that seismic coda is largely the result of scattering near the receiver, a tie to geological complexity,i.e., to crustal heterogeneity, is expected, but a systematic and predictive understanding of the important aspects of geology is lacking. We have developed a technique for mapping the origins of coda energy as recorded by regional networks that provides insight into the proximal generation of seismic coda. Here, the role of topography in the production of surface‐propagating coda energy, both as a scatterer and as a proxy of shallow geologic structure, is examined for the portion of the San Andreas fault system within the Southern California Seismic Network. We find significant correlation ofPtoRgscattering potential with topographic roughness that extends to relatively subtle topography. The correlation peaks at a migration velocity of ∼2.9 km/s, confirming the identification of scatteredRgenergy and offering an alternative means of estimating regional shallow velocity struct

ISSN:0094-8276    

DOI:10.1029/95GL00162

年代:1995

数据来源: WILEY

摘要:

We propose a new method for defining segmentation of plate boundaries and faults, based on the directions of the stress tensor. Estimates for these directions are obtained by minimizing the average misfit between the theoretical and observed slip directions on fault planes of earthquake focal mechanisms. The misfit,f, for an individual earthquake is the parameter we use for defining the segmentation of plate boundaries. We hypothesize that the stress directions along plate boundaries, and faults, are uniform within segments, but different from other segments. If this is true, a cumulative plot offas a function of space along strike will show constant, but different, slopes for each segment. The significance of the difference between segment‐slopes can be estimated by the standard deviate z‐test. Applying this method to the San Andreas fault from the Carrizo plains to its southern end, we identify quantitatively the same four boundaries between segments as proposed based on non‐quantitative tectonic considerations, plus one additional segment boundary. We interpret the relatively uniform, but segmented, distribution of stress directions as due to the changing strike, and possibly changing fault surface properties. Whether great earthquake ruptures, or their major asperities, may terminate at segment‐boundaries, should be determined along faults that recently generated large earthquakes. This method of defining fault segmentation also allows identification of volumes with uniform stress directions, suitable for inversion for stress orientations, with a minimum of computing time. And finally the method affords an alternative estimate of the significance of differences in stress dir

ISSN:0094-8276    

DOI:10.1029/95GL00074

年代:1995

数据来源: WILEY

摘要:

Magnetic hysteresis data from young mid‐ocean ridge basalts include samples with saturation remanence to saturation magnetization (Mrs/Ms) ratios greater than 0.5, the theoretical limit for an assemblage of single domain grains with uniaxial anisotropy. Under the usual assumption of dominant uniaxial anisotropy, the narrow single domain grain size distribution implied by these high Mrs/Ms values is difficult to reconcile with petrographic and remanence data that suggest the presence of larger multidomain grains. Dominant cubic anisotropy provides a plausible explanation for the high Mrs/Ms ratios, and if generally valid, requires reinterpretation of granulometric and domain state inferences made from hysteresis dat

ISSN:0094-8276    

DOI:10.1029/95GL00263

年代:1995

数据来源: WILEY

摘要:

Rocks generally have a percolation porosity at which they lose rigidity and fall apart. Percolation behavior is a purely geometrical property, independent of any physical properties, and is a powerful constraint on any valid velocity‐porosity relation. We show how the conventional Differential Effective Medium (DEM) theory can be modified to incorporate percolation of elastic moduli in rocks by taking the material at the critical porosity as one of the constituents of a two‐phase composite. Any desired percolation porosity can be specified as an input. In contrast, the conventional DEM model always predicts percolation at a porosity of either 0 or 100 percent. Most sedimentary rocks however have intermediate percolation porosities and are therefore not well represented by the conventional theory. The modified DEM model incorporates percolation behavior, and at the same time is always consistent with the Hashin‐Shtrikman bounds. The predictions compare favorably with laboratory sandstone

ISSN:0094-8276    

DOI:10.1029/95GL00164

年代:1995

数据来源: WILEY

摘要:

Melting determinations on spectrographic grade RbCl by differential thermal analysis in an internally‐heated, gas‐media apparatus gave the following constants for the Simon equation,P−Po= A[(T/To)c−1] (bars, Kelvin). For RbClI=L,Po= 1,To= 993.45, A = 8122, c = 5.02, sd = 0.5 K. For RbClII= L,Po= 7750,To= 1131.15, A = 5061, c = 6.40, sd = 1.0 K. The solid‐solid transition RbClI=IIdetermined by DTA is described byP= 4490 +T/0.35. The best‐fit triple point lies on a liquidus cusp at 1134 K, 7790 b. The results for melting are 3–7 K higher than earlier results. The new determinations are useful in the calibration of experimental apparatus at moderate pressure. They confirm that temperature calibration cannot explain divergent liquidus determinations for the

ISSN:0094-8276    

DOI:10.1029/94GL03392

年代:1995

数据来源: WILEY

摘要:

Analysis of the frequency‐size relationship of explosive volcanic eruptions for the past 2000 years reveals that the time‐averaged eruptive mass flux of tephra from subaerial explosive eruptions is 3×104kg s−1. This mass flux corresponds to a thermal energy release of 4×1010W. Half of the long‐term (>10³ year) mass and thermal energy release from explosive volcanic eruptions is accounted for by eruptions of>10 km

ISSN:0094-8276    

DOI:10.1029/95GL00052

年代:1995

数据来源: WILEY