摘要:

The focal mechanisms of 120 larger earthquakes (mb≥ 5.7) of the Kurile‐Kamchatka arc for the period 1962–1973 illustrate particular features of the subduction of the Pacific plate beneath this island arc system. The faulting characteristic of normal‐depth earthquakes is consistent with convergence of the oceanic plate in the direction N60°W with respect to the continental plate. Shallower foci under the trench are tensional; those seaward of the trench and related to the Hokkaido rise are compressional. Intermediate‐depth foci separate into those which represent either axial compression with respect to the plate or axial tension. The occurrence of the two types in close spatial proximity may possibly correspond (cf. Veith, 1974b) to differences in the orientation of principal stresses due to volume change across the olivine‐spinel phase transition boundary. In the northern Kuriles, deep‐focus earthquakes are compressional with respect to the axis of the descending plate. In the vicinity of the Hokkaido corner, east‐west striking normal faults and northward dipping tension axes in intermediate‐ and deep‐focus earthquakes imply hinge faulting and contortion of the Pacific plate between the more moderately dipping Japanese segment of the plate and the steeper dipping Kurile Islands segment. Recurrent Tokachi‐Oki earthquakes seaward of the Hokkaido corner indicate that the direction of convergence of the Pacific plate is unaffected by the presence of the junction of the two island arc systems. Equal‐area projections are available with entire article on rrticrofiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document J76‐001; $1.00. Pa

ISSN:0148-0227    

DOI:10.1029/JB081i002p00297

年代:1976

数据来源: WILEY

摘要:

We present 51 heat flow measurements and two geophysical profiles across the West Philippine and Parece Vela basins. We show that both regions have a variable heat flow but that the scatter decreases markedly if we accept as reliable only measurements in areas of uniform sediment drape. Extending this argument to the deep ocean floor shows that the heat flow in these two marginal basins is not necessarily higher than that for deep ocean floor of the same age. On the other hand, the mean depth of both basins is greater and the oceanic crust thinner than the depth and crust of ocean floor of the same age. In the absence of a significant free air gravity anomaly over both basins, we suggest that the thinner crust may account for most of the increase in depth of the two basins. However, more refraction studies are needed to substantiate this difference before this explanation can be unreservedly accepted.

ISSN:0148-0227    

DOI:10.1029/JB081i002p00309

年代:1976

数据来源: WILEY

摘要:

Cavities, surface topography, and inhomogeneities in elastic constants all distort the strains and tilts produced by stresses in the earth. Hence locally observed strains and tilts deviate from those calculated on the basis of simple earth models. We investigate these deviations on the assumption that the scale of these inhomogeneities is small in comparison with that of the stress fields. The results are applicable to observations of free modes, earth tides, and regional tectonic stresses. Analytical results are given for the deformation of an ellipsoidal cavity in a homogeneous medium in which the strain is uniform at large distances from the cavity. One important result is that greatly magnified strains are expected across narrow cavities. Finite element techniques are employed to study two features of importance in actual cavities, the influence of the corner in a tunnel of square cross section and the influence of the end face in a cylindrical tunnel with plane ends. The floor of the perfectly regular square tunnel remains flat, but irregularities produce large local tilts. Longitudinal strain in the cylindrical cavity is magnified near its ends, and most of the anomalous strain is concentrated within one tunnel diameter of the end. Topographic effects are also studied by finite element techniques. These are found to be very large and to result in strain magnifications in valleys, strain diminutions in hills, and strain‐induced tilts which are different for vertical and horizontal surfaces. In one case, regional compression actually results in local tension, and even a 1 in 10 (5.7°) slope produces a 36% perturbation in strain. Geological effects are investigated for a profile near Boulder, Colorado, where there is a very abrupt transition between the sedimentary rocks of the plains and the basement rocks of the Front Range of the Rocky Mountains. The surface strains perpendicular to the boundary fall from 177% to 37% of their regional values on crossing this boundary, and strain‐induced tilts attain values of about 0.5 times the regional strain in its vicinity. Long‐base strain meters and tiltmeters can give good results in underground sites, but short‐base instruments are very susceptible to local effects arising from geometric irregularities or rock inhomogeneities. Corrections for local topography are required for most installations. This topographic effect can be exploited to measure regional strains with tiltmeters, while the geological effects provide the basis of a technique for using earth tide observations to explore geological s

ISSN:0148-0227    

DOI:10.1029/JB081i002p00319

年代:1976

数据来源: WILEY

摘要:

Tiltmeters consisting of a servoed bubble trapped under an optical flat are described, together with techniques used for calibrating, orienting, and installing such instruments. The long‐term stability of one such (quartz) instrument is such that a 2‐μ rad annual tilt of the Poorman Hill can be measured; however, we cannot tell whether an apparently steady tilt of 3 μ rad/yr is real or due to instrumental drift. A beryllium copper instrument drifted at a more rapid but relatively constant rate of 22 μ rad/yr. The two instruments agree at the 10−7‐ad level over a 2‐month interval, after a linear term has been removed from both records. Earth tide tilts observed with the quartz instrument before and after construction of a concrete block enclosure, and with the beryllium copper instrument at two different sites in close proximity to the quartz instrument, all differ in amplitude and phase. The difference between a pair of observations at a single tidal frequency approximates a periodic unidirectional tilt with maximum excursion near the time of greatest earth strain. For a given pair of observations the direction is the same at all frequencies, but this direction differs for different observation pairs. These differences are ascribed to local tilts generated by tidal strains acting on the geometrical irregularities of the observing site. Tilts due to ocean loads and coupling of strain with local geology and topography have been computed, but it is not possible to calculate the very local site effects owing to the complex and irregular nature of the site. Strain‐induced tilts are believed to be the major source of error in tilt measurement; the larger‐scale couplings are probably calculable in most cases, but the very local site effects must be avoided by the use of long‐base instruments in either tu

ISSN:0148-0227    

DOI:10.1029/JB081i002p00329

年代:1976

数据来源: WILEY

摘要:

The geothermal heat flux has been measured at 15 sites in the inlets of southwestern British Columbia by using the ocean probe technique. Corrections have been applied for variations in bottom water temperature, sedimentation, thermal refraction by the sediment prism, topography, warm‐rim effect, Pleistocene thermal history, uplift, and erosion. The accuracy of the values is about ±0.3 μcal cm−2s−1(±13mW m−2). The relative accuracy between the values is about half of this range. The data indicate a pattern of low heat flow from the coast inland about 200 km to the heads of the inlets (13 values average 0.9 μcal cm−2s−1or 37 mW m−2). Stations at the heads of two inlets (1.5 μcal cm−2s−1or 63 mW m−2) mark the transition to high heat flow further inland (about 2.0 μcal cm−2s−1or 84 mW m−2). From west to east the heat flow transition coincides with the first occurrence of recent volcanic centers and hot springs, the transition from high to low Bouguer gravity, and the transition to low deep crustal and upper mantle electrical resistivity and to recent regional uplift. The coast low heat flow is explained partly by low crustal radioactive heat production and in part by the heat sink effect of the cold oceanic Juan de Fuca plate being subducted along the continental margin. The high inland heat flow probably is produced by the upward convective transport of heat by magma and thinning of the lithosphere that occurs when the sinking plate reaches a depth where temperatures are sufficient for partial melting. Low mantle temperature and thus high density in both the continental lithosphere and the underlying subducted oceanic lithosphere under the coast zone explain the high Bouguer gravity and thick crust in this region compared with the low gravity and thin crust inland. To the north of about 51°N where at present there is transform fault motion along the continental margin, there probably was subduction prior to about 10 m.y. ago. The gravity and crustal thickness data suggest that the high density in the subducted oceanic lithosphere has disappeared but that upwelling and a hot thin continental lithosphere persist inland of

ISSN:0148-0227    

DOI:10.1029/JB081i002p00337

年代:1976

数据来源: WILEY

摘要:

Radioactive disequilibria between Ra and Th short‐ and intermediate‐lived isotopes belonging to the232Th and238U decay series have been measured in lavas and bombs erupted since 1971 at Etna and Stromboli volcanoes. They indicate that Ra was enriched in relation to Th in the erupted fraction of the magmas by processes of magma formation, differentiation, and eruption. Equations relative to one‐stage, twostage, and continuous Ra enrichment were derived. Application of these equations to Etna and Stromboli data indicates either two (or more) stages or an exponentially growing Ra over Th enrichment. All models require a late event of Ra enrichment occurring at, or shortly before, the eruption. By the two–stage model the first major event of Ra enrichment was dated at 30–40 years for both volcanoes. The exponential growth model furnishes the age of the beginning of Ra enrichment within the erupted fraction of the magmas, which is some hundred years for Etna and 500–700 years for Stromboli. Although the volcanological meaning of the obtained dates is still uncertain, it is noteworthy that the same models of magmatic histories apply to two volcanoes which are very different in terms of both chemical composition of magmas and

ISSN:0148-0227    

DOI:10.1029/JB081i002p00350

年代:1976

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/JB081i002p00359

年代:1976

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/JB081i002p00360

年代:1976

数据来源: WILEY