摘要:

The November 20,1960 Peru earthquake excited a tsunami that was anomalously large for an earthquake of moderate magnitude (M6.75 PAS). Body wave first motions combined with observed Love to Rayleigh wave spectral ratios indicate slip along a shallowly dipping thrust fault rather than a single force mechanism suggestive of a landslide. Deconvolution ofSHwaveforms shows a time function consisting of 4 subevents with a total duration of 110 s. Using this time function, body waveform inversion yields a very shallowly dipping thrust faulting mechanism (strike 0°, dip: 6°, slip: 100°) and a seismic moment of 1.9 × 1027dyn cm. However, the time function obtained from the band limited body waveforms cannot fit the surface wave data which show a large increase in amplitude with period and a moment of 3.4 × 1027dyn cm. Both types of data can be modeled by combining the body wave time function and a 130 s cosine time function representing a longer period component. This event represents the largest earthquake to occur along this relatively aseismic portion of the Peruvian coast. The tsunami excitation is not anomalous relative to the measured seismic moment and moment magnitude (MW7.6). The disparity between tsunami height and surface wave magnitude results from underestimation of the earthquake size by conventional magnitude scales due to the long source dura

ISSN:0094-8276    

DOI:10.1029/GL017i006p00661

年代:1990

数据来源: WILEY

摘要:

We investigate anomalies of both the phase velocity and the amplitude of surface wave that depend on the direction traversed along the great circle. Pairs of observed Rayleigh wave packets are convolved to show that the great circle phase velocity may depend on the propagation direction. We calculate synthetic seismograms using the eigensolutions obtained by the variational method to see if these anomalies are reproduced by present laterally heterogeneous earth models. The results show that present models can reproduce these anomalies qualitatively.

ISSN:0094-8276    

DOI:10.1029/GL017i006p00665

年代:1990

数据来源: WILEY

摘要:

Attenuation parameter Q of five singlets for the spheroidal mode0S2(m=−2, −1, 0, 1, 2) are measured from a strain recording for the 1960 Chilean earthquake and three gravimeter recordings for the 1977 Indonesian (Sumbawa) earthquake. From the variation of modal Q as a function of m, three spherical harmonic components of radially averaged Q, angular degree and order (0,0), (2,0) and (4,0), can be determined. The solution indicates lower Q values in the polar regions than in the equatorial regions of the lower mantle, although error bars for (2,0) and (4,0) are large. Assuming a thermally activated process with the activation energy of about 100 kcal/mol, this variation of Q is consistent with a few tens of degrees of lateral temperature variations. With better0S2data, large scale temperature variation in the lower mantle can be constrained in princi

ISSN:0094-8276    

DOI:10.1029/GL017i006p00669

年代:1990

数据来源: WILEY

摘要:

A promising technique for high resolution geophysical imaging has been proposed which is based on the inversion of an exact solution to the reduced wave equation. The method is known as backpropagation because it propagates a received waveform back into the host formation mapping wavespeed variations. The implementation of backpropagation is complicated by the fact that it requires a complete characterization of the wave source and the wave propagation properties of a homogeneous host medium. The imaging procedure can be simplified by utilizing a zeroth—order phase approximation which allows implementation with first arrival times. Numerical experiments and a field study suggest that this approach is a practical means for high resolution subsurface imagin

ISSN:0094-8276    

DOI:10.1029/GL017i006p00673

年代:1990

数据来源: WILEY

摘要:

We have measured the extensional modulus (ratio of stress to strain,E) and the fractional energy loss per radian (QE‐1) as functions of frequency in cylindrical samples of two solids: a room‐dry sandstone (Berea) and an acrylic plastic (Lucite). In the seismic frequency range (0.1–100 Hz),QE‐1for the sandstone is constant, while the sandstone modulus follows a power law of frequency, allowingQE‐1to be independently estimated from the slope of logEversus logf. In the same frequency range,QE‐1for the acrylic plastic is not constant but exhibits a slight relaxation peak centered about 2 Hz. In this caseQE‐1can be estimated from the derivative,dlogE/dlogf, i.e., the above power law appears to hold independently at each point in the frequency domain even when

ISSN:0094-8276    

DOI:10.1029/GL017i006p00677

年代:1990

数据来源: WILEY

摘要:

Seismograms of noise recorded simultaneously at the surface and at two subsurface stations (335m and 945m below surface level) show that high frequency (1–100 Hz) background noise is reduced at the subsurface stations. Seismometers at all three stations are well coupled to hard crystalline bedrock. Most of the noise reduction occurs between the surface and 335m depth. Between 1 and 3 Hz, seismic noise correlates well with wind speed at all levels. Above 3 Hz, wind speed up to 8 m/sec does not influence high frequency noise levels at 335m or 945m depths. At the surface, however, high frequency noise is associated with wind speeds in excess of 2 m/s. A site resonance is characteristic of the surface spectra, but is not characteristic of the subsurface sites. Spectral signal‐to‐noise (S/N) ratios, measured for S‐waves from a regional earthquake (Δ = 530 km,mb= 4.1), are roughly equal from 1–10 Hz at all depths. Above 10 Hz, however, S/N is greater at the subsurface stations with S/N greater than 1 up to 30 Hz for the surface station, ≈ 50 Hz for the 335m station, and ≈ 70 Hz for the 945m station. Results indicate the advantage of employing high frequency sensors sufficiently below the surface to reduce the level of noise and to improve the S/N ratio in the high frequency range. Such an advantage could be critically important for nuclear test detection and

ISSN:0094-8276    

DOI:10.1029/GL017i006p00681

年代:1990

数据来源: WILEY

摘要:

Results from recent high pressure experiments have shown δTin the relationship to be constant over significant compressions, one of the original assumptions required to make this relationship valid. We have re‐examined the original derivation of this equation and present a simpler derivation eliminating the previous assumptions. Furthermore, high pressure experiments find (∂lnα/∂lnρ)Tis equal in value to l/αKT(∂KT/∂T)Pderived from experiments at ambient pressures. From values of −δT= l/αKT(∂KT/∂T)Pat high temperatures, it is asserted that −(∂lnα/∂lnρ)Tmust lie between 4 and 6 in the lower mantle, a value significantly higher than prev

ISSN:0094-8276    

DOI:10.1029/GL017i006p00685

年代:1990

数据来源: WILEY

摘要:

Recent experimental work has shown that the pressure dependence of the thermal expansion coefficient can be expressed as: where δT, the Anderson‐Gruneisen parameter, is assumed to be independent of pressure, and for the materials studied has a value that lies between 4 and 6. Calculation of δTfrom seismic data, however, appears to suggest a contradictory value of between 2 and 3 for mantle‐forming phases. Using an atomistic model based on our previously successful many‐body interatomic potential set (THBl), we have performed calculations to obtain values of δTfor four major mantle‐forming minerals. Our model results are in excellent agreement with experimental data, yielding values of between 4 and 6 for forsterite and MgO, and values in the same range for MgSiO3−perovskite and γ‐Mg2SiO4. Moreover, the calculations confirm that δTis indeed constant with pressure up to the core‐mantle boundary. The apparent conflict between the values of δTpredicted from seismic data and those obtained from experiment, and now from

ISSN:0094-8276    

DOI:10.1029/GL017i006p00689

年代:1990

数据来源: WILEY

摘要:

It has been argued that most of the Earth's potassium was segregated into the outer core and that the radioactive decay of40K provided the heat source for the geodynamo. This idea rests on the assumption that the affinity of potassium for sulfur or metallic iron is enhanced at high pressure. In this paper, the high pressure electronic structures of K in sulfide, iron sulfide and metallic iron coordination environments were determined from molecular orbital (SCF‐Xα‐SW) calculations on (KS8)15−, (KS8Fe6)3−and KFe14clusters. It is shown that, even at high pressure, potassium cannot alloy with metallic iron. Although a high‐pressure electronic transition may enhance the potassium‐sulfur chemical bond, the electronic structure of the KS8Fe6cluster shows that this electronic transition cannot happen in an iron sulfide melt. Consequently, potassium will not have an enhanced affinity for sulfur in planetary interiors. If the lower mantle were more reducing, potassium might be excluded from the silicate phases by more strongly lithophile elements and segregated into a metal sulfide phase in the outer core (cf. the phase assemblages in enstatite chondrites). Given the oxidation state of the Earth, however, it is unlikely that significant quantities of potassium have been incorporated into the outer core. The Earth, like the moon and the eucrite parent body, is depleted in potassium. An alternative heat source (e.g., the radioactive decay of U and Th) must be invoked to explain t

ISSN:0094-8276    

DOI:10.1029/GL017i006p00693

年代:1990

数据来源: WILEY

摘要:

Paleomagnetic data were obtained from two oppositely dipping carbonate sections of the Triassic Prida Formation, lower Star Peak Group, in the Humboldt Range of west‐central Nevada. Progressive demagnetization of the rocks indicated that the remanent magnetization consists of three or fewer vector components. A characteristic component, with the highest unblocking temperatures, was most effectively isolated by low temperature (200–300°C) thermal followed by progressive alternating field demagnetization. The sections yield in situ mean directions that differ significantly, but that have similar inclinations (Star Canyon: D=311, I=+34, α95=5, k=22, N=38 samples; Fossil Hill: D=281, I=+33, α95=2°, k=2814, N=3 section means, 154 samples). The directions diverge upon simple structural correction, from which we infer that at least one and probably both of the sections were remagnetized, probably in latest Middle to Late Jurassic time. These data cannot be used to indicate significant southward translation of Golconda allochthon rocks, as inferred from magnetizations in underlying volcanic rocks. The translation interpretation may require revision if magnetizations in the volcanic rocks are also se

ISSN:0094-8276    

DOI:10.1029/GL017i006p00697

年代:1990

数据来源: WILEY