摘要:

We interpret the long‐tail‐in‐time (up to 3 minutes) decay of short‐periodPdiffas being due to multiple scattering within D″, which, for this purpose, is assumed to be an heterogeneous region with a low velocity zone just next to the core‐mantle boundary. A simple multiple scattering theory, generalized for a two‐dimensional spherical geometry, provides good agreement with observations for values of the scattering and attenuation coefficients (η) of

ISSN:0094-8276    

DOI:10.1029/96GL02225

年代:1996

数据来源: WILEY

摘要:

We consider the possibility that the velocity structure of D″ is anisotropic. The data we examined consist of seismograms from 9 deep Japanese earthquakes recorded at WWSSN receiver stations in North America. The source‐receiver combinations span distances of 70°–106° with associated S waves passing through D″ beneath Alaska. Differential travel times of the S, Scd, ScS and SKS phases are used to constrain the velocity structure in D″. Shear waves refracted by D″ are observed beyond 72.2° and provide a sensitive measurement of the velocity structure in D″. Beyond 93°, the vertically polarized (SV) and horizontally polarized (SH) shear waves often appear distinctly split, although, at distances less than 89° the components are more nearly synchronous. Near 94°, SH occurs as a double arrival. SV in this range, however, remains a single arrival roughly synchronous with the second SH arrival. We have been unable to reproduce these effects in isotropic model synthetics. Synthetics for transversely isotropic models have been computed that do match these waveforms. The anisotropy was constrained to be only within D″, with a vertical symmetry axis. We conclude that these observations may be explained by an anisotropic D″ layer. The D″ discontinuity may be due to a transition to anisotropic mantle a few hundred kilometers above

ISSN:0094-8276    

DOI:10.1029/96GL02186

年代:1996

数据来源: WILEY

摘要:

Thermal infrared multispectral scanner (TIMS) images, acquired with side lapping flight lines, provide dual angle observations of the same area on the ground and can thus be used to estimate variations in the atmospheric transmission with scan angle. The method was tested using TIMS aircraft data for six flight lines with about 30% sidelap for an area within Joshua Tree National Park, California. Generally the results correspond to predictions for the transmission scan‐angle coefficient based on a standard atmospheric model although some differences were observed at the longer wavelength channels. A change was detected for the last pair of lines that may indicate either spatial or temporal atmospheric variation. The results demonstrate that the method provides information for correcting regional survey data (requiring multiple adjacent flight lines) that can be important in detecting subtle changes in litholog

ISSN:0094-8276    

DOI:10.1029/96GL02226

年代:1996

数据来源: WILEY

摘要:

Three borehole tensor strainmeters (BTSM) installed near Parkfield and separated by more than 5km have established a good baseline of data spanning the past eight years. A significant change in the accumulation rate of shear strain (0.5 and 1.0 microstrain per year at two of the sites) commencing approximately in mid‐1993 and continuing to 1995 is evident in these data. Some supporting evidence for a change in the strain rate at that time is seen in dilatometer data, geodetic measurements, fault zone properties and microearthquake locations. There was also an increase in medium level seismic activity (M=4.7 on 10/20/92; M=3.9 on 10/26/92; m=3.5 on 3/13/93; M=4.4 on 4/3/93; M=4.8 on 11/14/93) in the Middle Mountain area. While a rainfall induced strain may have caused the observed strain anomaly, we propose a more likely explanation is that aseismic slip at depth on a section of the fault south of Middle Mountain produced the observed strain anomaly. This anomaly is unique in the current data se

ISSN:0094-8276    

DOI:10.1029/96GL02256

年代:1996

数据来源: WILEY

摘要:

Since February 1991, the Physical Oceanography Group, Tohoku University has conducted a program to monitor the oceanic current field across the Kuroshio over the Izu Ridge, south of Japan, using the Acoustic Doppler Current Profiler (ADCP) system installed on the ferry Ogasawara Maru. This ferry shuttles 58 times in a usual year between Tokyo and Chichijima in the Ogasawara (Bonin) Islands. This ADCP monitoring is regarded as one component of the Tokyo‐Ogasawara Line Experiment (TOLEX). In this paper, we will briefly describe our ADCP system and data processing procedures, and will present several aspects of the current fields using the data obtained through January 1996. During the period, the Kuroshio took the bimodal paths of 34°N and 32°30′N as mean positions on the ship course. In the sea south of the Kuroshio, many cyclonic and anticyclonic eddies with a scale of 200–400 km in diameter were observed with passing time of two–fo

ISSN:0094-8276    

DOI:10.1029/96GL02306

年代:1996

数据来源: WILEY

摘要:

The Hyogo‐ken Nanbu earthquake (JMA Magnitude = 7.2) occurred on January 17, 1995, near Kobe in Hyogo‐ken prefecture, Japan. Unusual pulsed radio emissions at 22.2 MHz (wavelength = 13.5 m) were detected with a radio interferometer at Nishi‐harima Astronomical Observatory, located about 77 km from the epicenter, both before and after this earthquake. These data indicate that one of the most probable source regions was the central part of the Nojima fault, about 25 km from downtown Kobe, and the average pulsed power appeared to be about

ISSN:0094-8276    

DOI:10.1029/96GL02307

年代:1996

数据来源: WILEY

摘要:

Close examination of the P waves from earthquakes ranging in size across several orders of magnitude shows that the shape of the initiation of the velocity waveforms is independent of the magnitude of the earthquake. A model in which earthquakes of all sizes have similar rupture initiation can explain the data. This suggests that it is difficult to estimate the eventual size of an earthquake from the initial portion of the waveform. Previously reported curvature seen in the beginning of some velocity waveforms can be largely explained as the effect of anelastic attenuation; thus there is little evidence for a departure from models of simple rupture initiation that grow dynamically from a small region. The results of this study indicate that any “precursory” radiation at seismic frequencies must emanate from a source region no larger than the equivalent of a M0.5 event (i.e. a characteristic length of ∼10 m). The size of the nucleation region for magnitude 0 to 5 earthquakes thus is not resolvable with the standard seismic instrumentation deployed in Calif

ISSN:0094-8276    

DOI:10.1029/96GL02491

年代:1996

数据来源: WILEY

摘要:

X‐ray synchrotron study of iron heated between 1400 to 2200 K over a pressure range of 40 to 60 gigapascals confirms previous reports of the transformation of the hexagonal closest packed (HCP) phase of iron (ε) to the double HCP Iron (β). The study extends the stability range of the new phase β to the highest pressure range yet achieved for such transforma

ISSN:0094-8276    

DOI:10.1029/96GL02258

年代:1996

数据来源: WILEY

摘要:

We present a local treatment of the heat transport efficiency of the base‐heated cartesian convection with a temperature‐dependent viscosity, which allows us to give a clearer interpretation of Nusselt (Nu)‐Rayleigh (Ra) numbers relation. The “local” Nusselt (Nul) and Rayleigh (Ral) numbers are defined by the local values at each boundary layer except the length scale which is the total depth. We find thatNul‐Ralrelation obtained for the convection with constant viscosity and free surfaces is nearly coincident with that of the bottom boundary layer. For top boundary layer, we can treat it as a constant viscosity convection with rigid surface for large enough viscosity contrast. This interpretation can be made by choosing an appropriate temperature drop and viscosity for the definitions

ISSN:0094-8276    

DOI:10.1029/96GL02259

年代:1996

数据来源: WILEY

摘要:

Distribution with depth of mantle anisotropy is uncertain. To better constrain this distribution, we analyse records of the mantlePsphases at the GRF array in Germany. Two components of these phases (SVandT) are detected by stacking teleseismic records of events well distributed in azimuth and distance. The detectedTcomponent signals are generated not so much by splitting ofSVas by conversion fromPtoSHat the mantle discontinuities. To reconcile these observations with previously known observations of splitting inSKS, at least three anisotropic layers are required. The data favour a model with NNE oriented fast velocity at the top of the mantle. This is consistent with the results of other seismic studies. Beneath this layer the fast direction of anisotropy changes by around 90°. Our analysis suggests as well that a layer at the base of the mantle transition zone (MTZ) is anisotropic. This anisotropy might arise in the convective boundary layer between the upper and the lower mantle

ISSN:0094-8276    

DOI:10.1029/96GL02263

年代:1996

数据来源: WILEY