摘要:

A series of experiments has been conducted during the last decade to develop a radio interferometry system capable of measuring crustal and rotational motions of the earth, as well as source positions for a reference frame based on compact extragalactic radio sources. With the exception of one session between Big Pine, California, and Westford, Massachusetts, the observing stations have been those of NASA's Deep Space Network in California, Spain, and Australia. Approximately 2400 observations of extragalactic radio sources were made between August 1971 and February 1980 during 48 separate sessions. A single multiparameter fit was applied to the observed values of delay and delay rate to extract astrometric and geophysical parameters from this decade‐long sequence. The fit produced estimates of 684 parameters, including station locations, radio source positions, polar motion, universal time, the precession constant, and solid earth tides. The a priori model included gravitational bending, the 1980 International Astronomical Union (IAU) nutation series, the 1976 IAU expressions for Greenwich mean sidereal time and precession, Bureau International de l'Heure estimates of universal time and polar motion, and monthly mean values for zenith troposphere delay. The rms residuals were 0.52 ns for delay and 0.30 ps/s for delay rate. Intercontinental baseline lengths have been determined with formal uncertainties of 4 to 10 cm. Universal time and polar motion were measured at 49 epochs, with formal uncertainties (for the more recent data) of 0.5 ms for UT1 and 6 and 2 milliseconds of arc (mas), respectively, for theXandYcomponents of polar motion. Our 1971–1980 data produced an estimate of the lunisolar precession constant that is smaller than the 1976 IAU value by 3.8 mas/yr with an approximate accuracy (1σ) of 2 mas/yr. The earth tide results agree with the commonly accepted va

ISSN:0148-0227    

DOI:10.1029/JB089iB09p07597

年代:1984

数据来源: WILEY

摘要:

We give a theoretical development of a uniformly valid solution for describing the polar motions of a layered viscoelastic earth. Such solutions require the usage of two classes of eigenspectra to describe the processes of rotational deformation and viscous relaxation. One set consisting of real eigenvalue describes the isostatic relaxation of the mantle by viscous creep. The second one governs the readjustment of the rotational axis of the viscoelastic earth, and the eigenspectrum is complex valued. These solutions are capable of describing rotational phenomena ranging from Chandler wobble excitation to long‐term polar drift. A comparative study is conducted between the complete solution for polar wander and one in which a number of the rotational modes has been truncated. For a four‐layer model consisting of an elastic lithosphere, a two‐layer, adiabatically stratified viscoelastic mantle, and an inviscid core, such a comparison shows that at most a 30% difference exists in the viscosity solutions of the lower mantle, which are obtained by fitting the theoretical predictions to the observed polar wander data. Although the polar speeds from the various models can vary by as much as a factor of 2, the differences in the inverted viscosity solutions are sharply reduced upon fitting the calculations with the actual data for the time window, between 5 and 7×103years, which characterizes the hiatus between the termination time of the deglaciation period and today. The averaged polar drift from typical glacial cycles in the Pleistocene shows an initial magnitude of 0(1°/m.y., which after a few million years decays to a steady state value of a few tenths of a degree per millio

ISSN:0148-0227    

DOI:10.1029/JB089iB09p07609

年代:1984

数据来源: WILEY

摘要:

When two physical systems are dynamically coupled, the eigenmodes for the combined system will differ, in general, from the modes for the two original, uncoupled systems. This paper examines certain normal modes of the coupled earth and ocean system and discusses differences from corresponding results for the two individual systems. Particular attention is paid to those modes which involve wobble of the solid earth. It has been suggested that the coupled system may possess a long—period wobble mode, not part of the spectrum for the solid earth alone, which might be identified as the Markowitz wobble. The results presented below do not support that conclusio

ISSN:0148-0227    

DOI:10.1029/JB089iB09p07621

年代:1984

数据来源: WILEY

摘要:

Vertical displacements due to periodic reverse faulting events in an elastic plate overlying a viscoelastic (Maxwell) half space are obtained and compared with the observed deformation cycle (coseismic strain release, postseismic transients, interseismic strain accumulation) from Japan. The viscoelastic effects, including the influence of buoyant restoring forces, are obtained using the method developed by Rundle, and plate convergence and strain accumulation are incorporated following the procedure suggested by Savage. The resulting deformation cycle is compared with that of an analogous elastic half‐space dislocation model in which postearthquake effects are due to transient aseismic slip below the coseismic fault. Cyclic deformation is similar but not identical for the two models, and observations from southwest Japan suggest the superiority of the viscoelastic coupling model. In particular, inclusion of the effects of steady state flow in the asthenosphere overcomes a defect of the elastic half‐space model and results in agreement with the observed interseismic movement pattern. Several aspects of the postseismic deformation, its landward migration, and its transition to the interseismic phase of the cycle are explained as well, but the short duration of near‐trench transients relative to those observed farther inland is not matched. The success of a buried slip model in explaining early postseismic near‐trench movements and asthenospheric flow in accounting for cumulative postearthquake transient motions suggests the existence of a transition zone between lithosphere and asthenosphere whose behavior is brittle/elastic in the short term and ductile for longer‐term deformation, and such a modification may reconcile remaining discordant observations. However, reasonable variations in coupling model parameters cannot account for observed differences in the deformation cycle in other parts of Japan, and these regional differences remain un

ISSN:0148-0227    

DOI:10.1029/JB089iB09p07631

年代:1984

数据来源: WILEY

摘要:

Recent excavation and new radiocarbon dates of sediments at Pallett Creek are the basis for new conclusions regarding the late Holocene history of the San Andreas fault. Systematic dissection of a 50‐m‐long, 15‐m‐wide, 5‐m‐deep volume of earth, centered on the fault, enables documentation in three dimensions of fault patterns, lateral offsets, and vertical deformation associated with large earthquakes of the past. The excavations expose evidence for 12 earthquakes that occurred between about 260 and 1857 A.D., with an average recurrence interval of about 145 years. Prehistoric slip events that occurred in 1720±50, 1550±70, 1350+50, 1080±65, and 845±75 A.D. have lateral offsets that are comparable to those of the most recent great earthquake of 1857. Thus all of these events represent earthquakes of large magnitude. The lateral offsets of two other events, in 935±85 and 1015±100 A.D., are an order of magnitude smaller and may be interpreted in several ways with regard to the size of these events. The new data constrain the average recurrence interval for large earthquakes at this site to between 145 and 200 years but suggest a monotonic decrease in individual intervals to below this range during the past 900 years. On the basis of these data, the probability of a large earthquake with surficial fault rupture at this site is between 0.2 and 5% during 1984 and 7 and 60%

ISSN:0148-0227    

DOI:10.1029/JB089iB09p07641

年代:1984

数据来源: WILEY

摘要:

A 100×40 km trilateration network extending from Bishop, California, to near Hawthorne, Nevada, crosses the east end of the Long Valley caldera, site of renewed magma inflation in the 1979–1980 interval, and spans most of the White Mountain seismic gap. The network was surveyed in 1972, 1973, 1976, 1979, 1980, and 1982. The 1980 survey may be contaminated by a scale error. In addition, leveling surveys across the caldera have been run in 1932, 1957, 1975, 1980, 1982, and 1983. Interpretation of the deformation is complicated by the occurrence of the May 1980 Mammoth Lakes earthquake sequence (four earthquakesML≥6) at the south edge of the caldera as well as other moderate earthquakes within the White Mountain seismic gap. The vertical deformation is largely accounted for by 0.10‐ to 0.15‐km3expansion of a spherical magma chamber 8–10 km beneath the resurgent dome within the Long Valley caldera sometime between July 1979 and September 1980 with an additional expansion of perhaps 0.05 km3between September 1980 and July 1982. Some additional sources of deformation within the aftershock zone of the Mammoth Lakes earthquakes seem to be required to explain the horizontal deformation. We show that right‐lateral slip on vertical faults extending WNW from each of the three largest earthquakes in the Mammoth Lakes sequence provides the required additional deformation, but this solution is by no means unique. There are simply too few data to define the rather complex deformation that apparently occurred within the aftershock zone. There is little doubt, however, that inflation of a magma chamber beneath the resurgent dome within the Long Valley caldera was involved in the

ISSN:0148-0227    

DOI:10.1029/JB089iB09p07671

年代:1984

数据来源: WILEY

摘要:

The compensated linear vector dipole (CLVD) mechanism (Pwave radiation with conical nodal surfaces) found for the Mammoth Lakes earthquakes of May 1980 has been attributed to dike intrusion by Julian and his colleagues. There is a concern about this interpretation because magma intrusion through a crack should radiate compressional first motion everywhere. Chouet's calculation on a magma intrusion model, however, showed that the main dynamic motion as well as the final static displacement have patterns similar to the CLVD mechanism. We found an example of observed radiation pattern similar to the CLVD mechanism from eruption earthquakes at Mount Aso obtained by Sassa, and we interpret the observation as misidentification of the main motion as the initial motion. In searching for a similar misidentification for the Mammoth Lakes earthquakes we found that there are numerous reports of compression for short‐periodPwaves at stations where long‐periodPwaves show unequivocal dilatation. Furthermore, we found that the compressional short‐period first motion is systematically earlier than the dilatational one. These observations can be explained if the source of magma intrusion generated compressional first motions which were very weak and of high‐frequency contents in the direction where long‐periodPwaves are dilatational. The seismic moment of the Mammoth Lakes earthquakes suggests an involvement of magma volume of the order of 10−2km3. However, we have not yet observed long‐period events or volcanic (harmonic) tremors which are often observed at active volcanos. We review recent advances in theories and observations on volcanic tremor and conclude that for the volume of magma reservoir suggested for the Mammoth Lakes area a fluctuation of pressure of the order of 10−2bar would generate measurable tremor. This conclusion is obtained under the assumption that both the viscosity of magma and the radiation loss of the magma reservoir are low enough for the excitation of a harmonic tremor. We found that the concept of “reduced displacement” introduced by Aki and Koyanagi for body waves and by Fehler for surface waves is very useful for estimating the physical parameters of a vol

ISSN:0148-0227    

DOI:10.1029/JB089iB09p07689

年代:1984

数据来源: WILEY

摘要:

The precursory swarm, three mainshocks (M = 7.2,6.7, 7.2), and aftershocks of the Songpan earthquakes have been reanalyzed using both local and teleseismic data. The three mainshocks of this sequence occurred on the Huya fault over a 7‐day period. Relocations of the aftershocks using local arrival times show that three fault strands were activated during this sequence. Each mainshock occurred on a separate strand, each one south of the strand activated in the previous mainshock, and the aftershock zones of each mainshock appear to abut rather than overlap. Fault plane solutions determined by matching teleseismic P waveforms at World‐Wide Standard Seismograph Network stations with synthetic seismograms are consistent with the observed aftershock zones. The first and third mainshocks (M0= 1.3 ×1 019and 8.4 × 1018N m, respectively) showed almost identical senses of motion, a combination of reverse and left‐lateral strike‐slip motion, on parallel strands, striking N15°W, that were separated by a large rightstepping en echelon offset. The second mainshock (M0= 4.0 × 1018N m), occurred in this offset on a fault at a steep angle (∼125°) to the other two strands and showed almost pure reverse motion. Differences in the orientations of the slip vectors of the three mainshocks show that the first mainshock increased the normal and shear stresses on the fault segment that moved in the second mainshock and that the second mainshock decreased the normal stress on the fault segment activated by the third mainshock. These changes in normal stresses may have given rise to the longer time between the first and second events (5 days) as compared with the time between the second and third events (30 hours). A precursory swarm that preceded the Songpan sequence by 3 years occurred in a volume that surrounded the northernmost part of the planar aftershock zone. The time between the start of the swarm and the mainshocks and the magnitude of the largest event in the swarm are similar to those seen for precursory swarms in Sovie

ISSN:0148-0227    

DOI:10.1029/JB089iB09p07697

年代:1984

数据来源: WILEY

摘要:

The common occurrence of high‐pressure tectonic terranes along the Pacific coast and other convergent origins suggests that the evolution of subduction complexes may be an important phase in the development of continents. In this study we model the steady state thermal structure in an active subduction complex. Following Cloos, we treat the movements of sediments in deep subduction complexes as laminar flow in a viscous fluid. Driven by the descending plate and blocked by the hanging wall of the nonsubducted plate, a part of the sedimentary pile is forced to convect. Temperatures in subduction complexes are determined by the velocity of flow, radiogenic heating, and heat flow across the boundaries of these complexes. Heating becomes important when the rate of flow is small, of the order of 10 mm yr−1. Under such conditions, temperatures in subduction complexes are suitable for the formation of the characteristic metamorphic facies and may lead to the progression in metamorphic age and grade observed in many high‐pressure tectonic terranes. Fast convection cools the accretionary prisms; convective cooling can be so effective that temperatures even in the deepest interiors of subduction complexes may become too low for the formation of the metamorphic mineral assemblages that are currently used as criteria to identify sediments that experienced deep burial in the past. The results of this study suggest that thermal modeling and prediction of the distribution of metamorphic grades and ages of sediments in subduction complexes can provide powerful tests on hypotheses on the evolution of these comp

ISSN:0148-0227    

DOI:10.1029/JB089iB09p07709

年代:1984

数据来源: WILEY

摘要:

We have examined teleseismic earthquake locations reported by the International Seismological Centre (ISC) for the Middle America region and selected 220 as the most reliable. These hypocenters and other data are used to delineate the deep structure of the subducted Cocos Plate. The results indicate that the subducted plate consists of three major segments: Segment I extends from the Panama Fracture Zone to the Nicoya Peninsula. The structure of this segment is poorly defined. Segment II is the largest and best‐defined segment. This segment consists of two parts, IIA and IIB. Part IIA extends from the Nicoya Peninsula to western Guatemala and is very well defined and continuous in structure. Its strike follows the curvature of the trench and dips at about 60°. Part IIB extends from western Guatemala to Orizaba, Mexico. The dip of this part of the segment decreases slightly toward the northwest, and its strike is more northward than that of the trench. Segment III extends from Orizaba to the Rivera Fracture Zone, and is not well defined due to a lack of earthquake activity beneath about 100 km. Its orientation differs markedly from segment II and strikes somewhat more westward than the trench. Between parts IIA and IIB of segment II the subducted plate seems to be continuous, bending smoothly to accommodate the change in geometry. Local network data from Costa Rica suggest there may be a tear between segments I and II. Between segments II and III there is a gap in the hypocenters which makes it difficult to define the boundary. The change in geometry between these two segments indicates that there may be a tear, and two strike‐slip focal mechanisms in the region support this conclusion. We find no convincing evidence supporting the existence of segments smaller than the three described above. If there is smaller‐scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths. There is no evidence for any major tear in the subducted plate associated directly with either the Tehuantepec Ridge or the Orozco Fracture zone. The shallow subduction at the northwestern end of segment II may be related to the bouyancy of the Tehuantepec Ridge. The Cocos Ridge is probably directly responsible for the change in geometry between segments I and II and may even be slowing or stopping subduction in segment I. The structure of the subducted plate in segment II and the changes in the character of volcanism along the arc can be related to the relative motion of the North American and Caribbean Plates. The present geometry of part IIB of segment II is more consistent with the probable configuration of the trench about 7 Ma ago than with the present configuration, indicating that the North American plate is overriding the subduction zone.Appendices 2, 3, and 4 are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B84‐00

ISSN:0148-0227    

DOI:10.1029/JB089iB09p07719

年代:1984

数据来源: WILEY