摘要:

Vertically‐polarized S‐waves (SV) attenuate rapidly as they diffract around the Earth's core, whereas horizontally‐polarized S‐waves (SH) propagate to large distances in the core shadow zone. The amplitude decay of diffracted SV signals is so strong that few studies have been made of these phases, despite their acute sensitivity to velocity structure just above the core‐mantle boundary. We analyze SV signals in the penumbra of the core's shadow, finding systematic waveform complexities indicative of local stratification and apparent anisotropy at the base of the mantle, with implications for the dynamic processes in this internal bound

ISSN:0094-8276    

DOI:10.1029/91GL01691

年代:1991

数据来源: WILEY

摘要:

The Loma Prieta earthquake (Ms=7.1, 17 October 1989) provided unique near field borehole tensor strain observations. The medium term data at the San Juan Bautista site showed a clear anomalous change in the fault parallel shear strain rate beginning about a year before the event. The anomaly ultimately reached 30% of the coseismic offset. The signal shows similarities with changes in strain rates reported from the geodetic record, suggesting a broad regional anomaly. The limited spacial sampling available prevents determination of a causal link useful for prediction between these data and the earthquake.

ISSN:0094-8276    

DOI:10.1029/91GL01846

年代:1991

数据来源: WILEY

摘要:

An earthquake which occurred off the northern Mozambique gives a peculiar feature of P‐waves recorded by WWSSN long period instruments. All the P‐waveforms can be divided into two parts: the amplitude of the first arriving phase varies from station to station, which is consistent with the normal fault type solution of the Harvard CMT solution determined by the long period GDSN data. The amplitude of the second phase, in contrast, is fairly constant over all the stations, inconsistent with the normal fault solution. From these observations and data analyses, we propose that the source process of this event is a combination of a normal fault and a subsequent isotropic source starting with an explosion followed by a contraction or a corruption with each net moment of 2 ∼ 3 × 1025dyne.cm at the assumed depth of 7.5 km. Our image of these source mechanisms is consistent with the known geological setting around the epicenter: grabens with many dikes or intr

ISSN:0094-8276    

DOI:10.1029/91GL01441

年代:1991

数据来源: WILEY

摘要:

The installation of very broadband seismic stations makes it possible to recover the source parameters of small earthquakes (2.5

ISSN:0094-8276    

DOI:10.1029/91GL01804

年代:1991

数据来源: WILEY

摘要:

The Fourier spectral centroid formula for instantaneous frequency is extended to all times and to all analytic signals and is in a convenient computable form. Examples using two different synthetic even seismic wavelets show that the peaks and troughs of their instantaneous frequency may occur at the envelope maxima or minima, with possible singularities at the envelope minima. For a purely even or odd wavelet, either a turning point or a point of inflection occurs at the wavelet center of the instantaneous frequency and envelope.

ISSN:0094-8276    

DOI:10.1029/91GL01531

年代:1991

数据来源: WILEY

摘要:

Here we present a new approach for estimating the slowness vector of incident P‐waves from records of a single or an array of 3‐component (3C) seismometers. The slowness vector is extracted by using a Maximum Likelihood (ML) inversion scheme where the signal model incorporates the transfer function of the layered crust beneath the receivers. The unknown time function of the incident plane wave is excluded from the inversion. In the case of array observations both polarization and moveout times are utilized. The scheme was tested on records of NORESS 3‐component stations from teleseismic and local events and the obtained slowness estimates are closer to the expected values than estimates derived by conventional me

ISSN:0094-8276    

DOI:10.1029/91GL01725

年代:1991

数据来源: WILEY

摘要:

Ultrasonic measurements of compressional wave velocities for six komatiitic melts containing 13–30 wt% MgO are reported in the frequency range 3–20 MHz and in the temperature range 1280–1661°C. Melts with less than about 20 wt% MgO have velocities that are 6% lower and relaxed compressibilities,βr, that are 15% higher than melts with more than about 20 wt% MgO. The general trend of increasing velocity with increasing MgO content correlates well with increasing depolymerization (decreasing molar volume) of the melts as reflected in the NBO/T, (CaO + MgO)/Al2O3, and Al/(Al+Si) ratios. The value of dβr/dw(w=wt% MgO) for the high MgO melts is more than four times greater than dβr/dwfor the low

ISSN:0094-8276    

DOI:10.1029/91GL01803

年代:1991

数据来源: WILEY

摘要:

Rapid compaction creep has been achieved in fine‐grained quartz aggregates during hydrothermal isostatic pressing (HTIP) at 1200K, a confining pressure of 300MPa and a pore water pressure of 200 MPa. While raising the pressure and temperature over a period of three hours, the porosity decreased by over 50%. During subsequent HTIP for periods up to 4 hours, further porosity reduction occurred at compaction creep rates between 10−5and 10−6s−1. Changes in grain shapes, grainsize distribution, pore geometry and microstructures on grain surfaces, together with an absence of substantial brittle or intragranular plastic deformation, indicate that compaction creep has occurred largely by dissolution‐precipitation processes. Interpenetration of grains and grain‐shape truncation are well‐developed at grain contacts. Arrays of fine‐scale ridges, plateaus and flat‐bottomed pores on interfaces indicate that the fluid distribution on actively dissolving interfaces is a dynamic structure that is not controlled entirely by equilibri

ISSN:0094-8276    

DOI:10.1029/91GL01802

年代:1991

数据来源: WILEY

摘要:

Volcanoes are an important source of gases and aerosols in the atmosphere. Significant quantities of trace elements are emitted as vapor species [Nriagu, 1989; Symondset al.,1987] and are strongly enriched in the gas relative to the magma [Tazieff and Sabroux, 1983; Croweet al.,1987]. After eruption the trace elements condense on ash and other particles or they form sublimates and agglomerates. Here we report on the emission of gold (Au) from Mount Erebus, Antarctica. Although the flux of emitted Au is low compared to other volcanoes, crystalline particulate Au has been found in the plume near the crater, in ambient air up to 1000 km from the volcano and in near surface samples. Vapor phase transport of Au may occur as a chloride species and could be an important transport mechanism in crystallizing magmatic intrusions.

ISSN:0094-8276    

DOI:10.1029/91GL01928

年代:1991

数据来源: WILEY

摘要:

Magnetic remanence and bulk susceptibility measurements are reported from 93 samples spanning the uppermost 27m of the Red Clay underlying the loess sequence at Baoji, Shaanxi province, China. If a new, previously undetected, short (about 20ka) excursion or reversed interval near 3.3Ma be accepted, then a reasonable fit to the standard time scale emerges. A linear accumulation rate of 1.5cm/ka is implied and the correlation coefficient is 0.992. No significant hiatus occurs between the Red Clay and the overlying loess, so the 27m investigated reach back to about 4.3Ma. Fourier analysis of the bulk susceptibility data yields a strong peak near 400ka in agreement with astronomical calculations of the orbital parameters responsible for climatic forcing.

ISSN:0094-8276    

DOI:10.1029/91GL01800

年代:1991

数据来源: WILEY