摘要:

Several recent inversion studies have clearly indicated the lack of resolving power of the normal mode data set and the possible trade‐offs among the various parameters. These studies have also shown that the final model is as dependent on the starting model as on the data set. It is therefore important to incorporate body wave data into any inversion scheme not only to gain resolution but also to reduce trade‐offs between density and velocity. An earth model based on special studies of the structure of the mantle and core is inverted to be consistent with both body wave data and a representative set of normal mode observations (437 modes). The resulting model has a 40‐km‐thick upper mantle lithospheric lid terminating at 61 km, with high density (3.5 g/cm3) and seismic velocities (8.38 and 4.71 km/s), a pronounced upper mantle low‐velocity zone (LVZ) of 180‐km thickness, and transition regions of rapid velocity increase at 375–425, 500–550, and 650–675 km. There are also anomalous gradients between 700 and 1200 km. This model, C2, is slow by about 0.6 and 2–4 s forPandSwaves, respectively, in comparison with body wave solutions which have a greater continental bias. The major features of the upper mantle can be explained by partial melting (LVZ) and the successive transformation of an olivine‐pyroxene mantle to β spinel, γ spinel, and garnet and further phase changes below 750 km. In addition to the radial inhomogeneities in the upper mantle there is evidence for inhomogeneity at the base of the mantle, the top of the core, and the regions on each side of the outer c

ISSN:0148-0227    

DOI:10.1029/JB081i008p01461

年代:1976

数据来源: WILEY

摘要:

Sixty‐five Brunhes age lava flows from Easter Island are analyzed for secular variation statistics and mean geomagnetic field parameters. The present rotation axis is enclosed within the 95% confidence limits of the mean virtual geomagnetic pole (θ = 87.4°, ϕ = 204.2°;dm= 4.13°,dp= 2.57°). The angular standard deviation of a subset of 51 poles is 12.8° with upper and lower confidence limits of 14.8° and 11.3°. The secular variation of the young Easter Island basalts lies above a pure dipole wobble component of secular variation but below the lower limit for all proposed models. Thus it appears that the Pacific secular variation low extends into the southeast Pacific but is not as pronounced as it is aro

ISSN:0148-0227    

DOI:10.1029/JB081i008p01476

年代:1976

数据来源: WILEY

摘要:

In studies of the source mechanism of small intraplate earthquakes the analysis of surface wave spectra is extremely valuable because reliablePwave solutions of these earthquakes are difficult to obtain. To determine the source mechanism using surface waves alone, phase and amplitude spectra must be analyzed. In this study of the southeastern Missouri earthquake of October 21, 1965, we examine the phase spectra of Rayleigh waves over periods of 20–50 s to follow up Mitchell's (1973) analysis of spectral amplitudes. It is shown that using known regionalized dispersion curves to remove propagation effects, we can isolate focal phase spectra and determine the mechanism of the earthquake. The results provide conclusive evidence for normal faulting with a NNW‐SSE axis of tension. Thus our analysis of phase spectra gives results which are complementary as well as supportive to those obtained by Mitchell (19

ISSN:0148-0227    

DOI:10.1029/JB081i008p01483

年代:1976

数据来源: WILEY

摘要:

For many situations of practical interest, the seismicity that results from ground deformation associated with a change in volume, ΔV, is related to the volume change by ΣM0=Kμ∣ΔV∣, where ΣM0is the sum of the seismic moments of the earthquake population, μ is the modulus of rigidity, andKis a factor close to 1. A necessary condition for this relationship is that the change in volume is accommodated only by seismic failure. The relationship between seismic moment and volume change was used to analyze earthquake sequences resulting from ground deformation that could most easily be described as volume changes. First, in the East Rand Proprietary Gold Mine, South Africa, total seismic moments of mine tremors were found to be in good agreement with total moments inferred from the volume of rock mined. Second, annual sums of seismic moments for the Denver earthquakes were compared with yearly total moments estimated from the volume of fluid injected at the Rocky Mountain arsenal well. There was good agreement between calculated and observed moments from 1962 to 1965 when annual rates of fluid injection were high. Nearly all of the Denver earthquakes in these years correspond to the accommodation of injected fluid at depth. The association between earthquakes that occurred after February 1966, when the fluid injection was stopped, and the Rocky Mountain arsenal well is not clear. Finally, the total seismic moment for earthquakes near Matsushiro, Japan, that occurred from March to September 1966 was found to be in good agreement with the moment calculated from the expansion of the seismic source region determined from measurements of horizontal and vertical ground movement. Energy changes were calculated for the volumetric changes considered here and compared with the total energy radiated in the seismic waves. The seismic energies ranged from 0.2% to 3% of the total energ

ISSN:0148-0227    

DOI:10.1029/JB081i008p01487

年代:1976

数据来源: WILEY

摘要:

The axis of the East Pacific Rise (EPR) in the area of detailed survey, 5°−7°S, is generally marked by a topographic block about 300 m high and 15–20 km wide. This axial block trends 14° through much of the survey area, but at 5.8°S it changes to a regional trend of 22° which continues to the north. A 200‐m‐high step (downdropped to the west) lies subparallel to and 20 km west of that part of the axial block which trends 14° and dies out to the north where the axial trend changes. Large fault blocks 1000 m high and 6–10 km wide occur 175 km west of the axis in the survey area, and a topographic depression of similar dimensions occurs 80 km east of the rise axis. Identification of magnetic anomalies here on the magnetic equator is complicated by the electrojet effect which produces a large and irregular diurnal variation in the earth's magnetic field. The few identifications possible indicate a whole spreading rate of 153 mm/yr for the EPR at 6° S. Mapped anomalies reveal a trend of 42° for the Gilsa anomaly, a 20° deviation from the present regional axis trend, and show the present topographic axis to be 10 km west of a central position between the flanking Brunhes/Matuyama anomalies. At about 6.5°S the rise axis is offset 55 km right laterally along a fracture zone which is poorly defined bathymetrically. A 1000‐γ magnetic anomaly trends 108° along the fracture zone and coincides with its active portion. Both the bathymétric and the magnetic expressions of the fracture zone terminate a few tens of kilometers outside the offset axes, suggesting a change in the process that formed the fracture zone a few hundred thousand years ago. These data, combined with the areal distribution of earthquake epicenters, suggest that (1) during the past 1.7 m.y. the trend of the spreading axis has changed from highly oblique to more orthogonal to the spreading direction, (2) during the past 0.69 m.y. the spreading axis has shifted 10 km to the west, and (3) at present, that portion of the rise axis trending 14° is unstable and is in the process of reorienting to the regional trend of 22°. The westward shift of the rise axis may have occasioned the change in the morphologic and magnetic signatures

ISSN:0148-0227    

DOI:10.1029/JB081i008p01495

年代:1976

数据来源: WILEY

摘要:

A 2330‐m‐thick sequence of basaltic lava flows near Neskaupstadur, eastern Iceland, yields K‐Ar ages ranging from 11.8 m.y. to about 8.4 m.y. A linear regression model of uniform rate of accumulation of the lava pile with time best fits these data. A growth of 690 m/m.y. is indicated, one lava having erupted on average every 13,000 years. These rates closely resemble those determined for the Borgarfjördur region of western Iceland for the periodt= 7–2 m.y., and thus it is shown that steady state crustal growth has occurred in Iceland on both sides of the mid‐Atlantic ridge for at least 10 m.y. The paleomagnetic polarities in the sequence provide a geomagnetic polarity history which matches closely that derived by Blakely (1974) using sea floor spreading analyses of marine magnetic anomalies. Anomaly 5 is recorded in the section and has lower and upper boundaries oft= 10.03 (±0.34) m.y. and not more thant= 8.70 (±0.36) m.y., respectively. As was proposed by Blakely (1974), at least one brief period of reversed polarity is present with

ISSN:0148-0227    

DOI:10.1029/JB081i008p01505

年代:1976

数据来源: WILEY

摘要:

Stabilized linear inverse theory is applied to the problem of determining the topography of a subsurface density anomaly from Bouguer gravity observations. The topography is assumed to take the form of a step function in two dimensions, and model predictions are compared directly with the raw data (no smoothing or interpolation is necessary). The concept of resolving power, as introduced by Backus and Gilbert (1970), is extended to problems with fixed linear constraints. The nonuniqueness characteristic of gravity data proves to be conveniently represented by the resolving kernels and poses no problem of numerical instability or poor convergence even where data are sparse. The method is applied to gravity data for Chuckwalla Valley, California, a sedimentary basin with positive gravity anomaly near the center. Although resolution is generally poor, a rather pronounced peak in the basement topography is well resolved and is the cause of the positive anomaly. A single constraint imposed by the depth to basement in one well improves the resolving power of the data significantly.

ISSN:0148-0227    

DOI:10.1029/JB081i008p01513

年代:1976

数据来源: WILEY

摘要:

Seismic refraction measurements were carried out in 1970–1971 near Byrd Station, Antarctica, with a specific emphasis on the determination of seismic wave attenuation. The primary difficulty in interpreting field results is the extreme sensitivity of the observed attenuation to small variations in the seismic velocity gradient and its first derivative. Detailed velocity analysis, based on multiple surface‐reflected arrivals, has made possible an estimate of the average internal friction for compressional waves between 100‐ and 500‐m depth (temperature, −28°C):QP−1= 0.0014 at 136 Hz, with upper and lower standard error estimates of +0.0005 and −0.0008, respectively. This value has been compared with laboratory measurements by Kuroiwa (1964) on cold slightly impure polycrystalline ice. The comparison required extrapolation of Kuroiwa's results to lower frequency, a correction for the difference between shear and compressional wave absorption, and an estimate of the amount of salt contained in ice frozen from a known solution. The agreement between field and laboratory results is good for the ionic impurity concentration measured in the ice near Byrd Station. All field measurements ofQP−1by reflection shooting in Antarctica and Greenland, although they cannot be assigned to a particular temperature, are also in satisfactory agreement, as are some previous determinations by refraction shooting in Greenland, although some from the high interior part of the Greenland ice sheet are seriously discrepant, probably because of inadequate knowledge of the seismic velocity gradient. We conclude that with the possible (but unlikely) exception of the central Greenland ice sheet, cold polar ice sheets exhibit an internal friction in the frequency range 50–200 Hz that is completely explained by the two fundamental mechanisms in polycrystalline ice: molecular relaxation (dominant at relatively cold temperatures) and grain boundary viscosity (dominant near the melting point). There is no evidence of any additional mechanism associated with internal structures such as bubbles or microcracks. Appendices 1 and 2 are available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document J76‐003; $1.00. Payment

ISSN:0148-0227    

DOI:10.1029/JB081i008p01519

年代:1976

数据来源: WILEY

摘要:

A self‐potential survey made in the Long Valley caldera produced an anomaly derived from a dipolar source superimposed on potentials negative in polarity in relation to the area outside the caldera. The dipolar anomaly, consisting of negative and positive components differing amplitude by approximately 1 V, is centered over a resurgent dome in the west central part of the caldera. The exact nature of the potential source is unknown; however, electrofiltration processes caused by movement of heated groundwater that gives rise to streaming potentials are thought to be the principal cause of the dipolar anomaly. Diffusion potentials resulting from concentration differences between rising volcanic water and descending meteoric water may be responsible for the negative potentials measured within the caldera. Potential increases as high as 1100 mV were observed across the perimeter of the calder

ISSN:0148-0227    

DOI:10.1029/JB081i008p01527

年代:1976

数据来源: WILEY

摘要:

Surface‐ship and submarine pendulum gravity measurements have been compiled in a new free air gravity anomaly map of the central Pacific Ocean in the region of Hawaii. The main features of the map are large amplitude positive anomalies (up to +700 mGal) over the Hawaiian ridge, large amplitude negative anomalies (up to −136 mGal) flanking the ridge, and a broad belt (>250 km) of positive anomalies (+25 to +50 mGal) bordering the negative anomalies. The map has been used to construct 1°×1°, 5°×5°, and 10°×10° free air anomaly averages. The main feature of the 5°×5° average map is a long‐wavelength (∼2200 km) positive anomaly (up to +15 mGal) over the southeastern end of the Hawaiian ridge. A long‐wavelength positive anomaly is also seen on the 10°×10° average map, which agrees well with the Gem 6 satellite‐derived solution to harmonic degree 16. Computations suggest that crustal structure of the Hawaiian ridge is unlikely to contribute significantly to these long‐wavelength positive anomalies. The positive anomalies correlate closely with the Hawaiian swell upon which the Hawaiian ridge is superimposed. The regression lines representing 1°×1° and 5°×5° averages of gravity against topography slope at 21 mGal/km and 22 mGal/km, respectively. These slopes are smaller than those over other regions where the lithosphere is warped for large distances, suggesting that if the swell is warped, it must be compensated. The form which the compensation takes is uncertain, but it may be related to some pattern of flow beneath the lithosphere which maintains both the swell and the associated

ISSN:0148-0227    

DOI:10.1029/JB081i008p01533

年代:1976

数据来源: WILEY