摘要:

This paper is a discussion of the short‐wavelength (25–100 km) undulations of the geoid recovered from Seasat altimeter data in the South Atlantic. These undulations are known to be highly correlated to seafloor topography, and they considerably improve our knowledge about global bathymetry of the South Atlantic, which remains poorly surveyed. The orientation of fracture zones (FZ) with respect to the Seasat profiles ensures an efficient recovery of the FZ pattern in the younger (<65 Ma) parts of the South Atlantic between 20°N and 50°S. We used 22 well‐marked FZs regularly distributed between 12°N and 47°S to compute Eulerian poles describing the finite relative movement between Africa and South America. From the present back to 35 Ma, FZ trends south of the equator are well accounted for with a single rotation, and standard statistical tests do not indicate a need for distinct and successive rotations. However, these tests indicate an improvement of the fit when distinct poles are adjusted to separate subsets of our 22 FZs (either FZs north and south of the equator, or FZs north and south of Tristan da Cunha). This points out the possibility of the existence of either “corridors” in which slight relative movements occur, or diffuse elastic deformation over the entire South Atlantic, although this is really a second‐order effect. We also present and discuss a map of the roughness of the geoid (for the 25‐ to 110‐km wave band). Strong disagreements between the roughness map and the GEBCO maps exist in regions of sparse ship track coverage; for example, the Rio Grande Rise has an associated roughness feature which appears elongated and more symmetric with respect to the Walvis Ridge than what is displayed on the GEBCO maps. The roughness map also clearly documents a long‐wavelength (∼400 km) segmentation of the Mid‐Atlantic Ridge bet

ISSN:0148-0227    

DOI:10.1029/JB094iB05p05545

年代:1989

数据来源: WILEY

摘要:

All plate motion data available for describing the present‐day motion between the Arabian and Indian plates are analyzed to understand the present‐day tectonic role of the presumed plate boundary: the Owen fracture zone and Dalrymple trough in the northern Arabian Sea. All prior plate motion models are shown to be inconsistent with some of the data along the presumed boundary. In particular, published global plate motion models predict some convergence across the Owen fracture zone and Dalrymple trough. Convergence along the Owen fracture zone is contradicted by earthquake focal mechanisms, which show right‐lateral strike slip. Convergence along the Dalrymple trough is contradicted by observed normal faulting in the trough, and by normal and strike‐slip focal mechanisms for earthquakes occurring along or near the trough. The sense and rate of motion is further constrained by spreading rates along the Sheba Ridge (northwest of the Owen fracture zone) and along the Carlsberg Ridge (southeast of the Owen fracture zone). To estimate the present‐day motion, we determine 43 3‐m.y.‐average spreading rates from Sheba Ridge, Carlsberg Ridge, and Central Indian Ridge magnetic profiles and have combined them with eight transform azimuths and 15 earthquake slip vectors along these ridges and the Arabia‐India boundary. We determine an Arabia‐India Euler vector that predicts 2 mm/yr of right‐lateral strike‐slip motion along the Owen fracture zone, with a 95% confidence interval of 0–7 mm/yr, which excludes the faster rates of slips predicted by several prior models. The direction of motion is 35°–50° counterclockwise from that predicted by published global plate motion models. If we omit the data along the Arabia‐India boundary, the motion differs insignificantly from zero, suggesting that magnetic profiles from the Carlsberg and Sheba ridges give spreading rates too imprecise to detect the slow Arabia‐India motion. If the Owen fracture zone and Dalrymple trough are regarded as a plate boundary, it is the slowest slipping plate boundary known on Earth, with motion about 100 times slower than the fastest slipping fau

ISSN:0148-0227    

DOI:10.1029/JB094iB05p05560

年代:1989

数据来源: WILEY

摘要:

The evolution of midplate hotspot swells on an oceanic plate moving over a hot, upwelling mantle plume is amenable to numerical simulation. In our model, the plume supplies to the lithosphere a Gaussian‐shaped thermal perturbation and dynamic support dominated by thermal buoyancy. We consider the two fundamental mechanisms of transferring heat, conduction and convection, during the interaction of the lithosphere with the mantle plume. The transient heat transfer equations, with boundary conditions varying in both time and space, are solved in cylindrical coordinates using the alternating direction implicit finite difference method on a 100×100 grid. The topography, geoid anomaly, and heat flow anomaly of the Hawaiian swell and the Bermuda rise are used to constrain the models. Our results confirm the conclusion from previous work that the Hawaiian swell can not be explained by conductive heating alone, even if an extremely high thermal perturbation is allowed. On the other hand, the model of convective thinning predicts successfully the topography, geoid anomaly, and the heat flow anomaly around the Hawaiian islands, as well as the changes in the topography and anomalous heat flow along the Hawaiian volcanic chain. The model constrains the Hawaiian plume to have a half wavelength of about 500 km, a center heat flux 5–6 times the background value, and a convective current velocity 3–10 times that of the background convective current. Comparatively, the mantle plume for the Bermuda rise is much weaker. Conductive heating is probably the dominant mechanism for the evolution of the Bermud

ISSN:0148-0227    

DOI:10.1029/JB094iB05p05571

年代:1989

数据来源: WILEY

摘要:

We examine the closure of the current plate motion circuit between the African, North American, and Eurasian plates to test whether these plates are rigid and whether the Gloria fault is an active transform fault. We also investigate the possible existence of microplates that have been previously proposed to lie along these plate boundaries, and compare the predicted direction of motion along the African‐Eurasian plate boundary in the Mediterranean with the direction of slip observed in earthquakes. From marine geophysical data we obtain 13 transform fault azimuths and 40 3‐m.y.‐average spreading rates, 34 of which are determined from comparison of synthetic magnetic anomaly profiles to ∼140 observed profiles. Slip vectors from 32 earthquake focal mechanisms further describe plate motion. Detailed magnetic surveys north of Iceland provide 11 rates in a region where prior plate motion models had few data. Magnetic profiles north of the Azores triple junction record a rate of 24 mm/yr, 4 mm/yr slower than used by prior models. Gloria and Sea Beam surveys accurately measure the azimuths of seven transform faults; our plate motion model fits six of the seven within 2°. Two transform faults surveyed by Gloria side scan sonar lie near FAMOUS area transform faults A and B and give azimuths 13° clockwise of them. Because recent studies show that short‐offset transforms, such as transforms A and B, are in many places oblique to the direction of plate motion, we exclude azimuths from transforms with less than 35‐km offset. The best fitting and closure‐enforced vectors fit the data well, except for a small systematic misfit to the slip vectors: On right‐lateral slipping transforms, slip vectors tend to be a few degrees clockwise of plate motion and mapped fault azimuths, whereas on left‐lateral slipping transforms, slip vectors tend to be a few degrees counterclockwise of plate motion and mapped fault azimuths. We search the long Eurasia‐North America boundary for evidence of an additional plate, but find no systematic misfits to the data. In particular, if a Spitsbergen plate exists and moves relative to Eurasia, its motion is less than 3 mm/yr. An Africa‐Eurasia Euler vector determined by adding the Eurasia‐North America and Africa‐North America Euler vectors is consistent with the Gloria fault trend and with slip vectors from eastern Azores‐Gibraltar Ridge focal mechanisms. A small circle, centered at the Africa‐Eurasia closure‐enforced pole, fits the trace of the Gloria fault. The model in which closure was enforced predicts ∼4 mm/yr slip across the Azores‐Gibraltar Ridge, and west‐northwest convergence near Gibraltar, ∼45° more oblique than suggested by a recent model based on compressive axes of focal mechanisms. Moreover, our model predicts directions of plate motion that agree well with northwest trending slip vectors from thrust earthquakes between Gibraltar and Sicily. Because closure‐enforced vectors fit the data nearly as well as the best fitting vectors, we conclude that the data are consistent with a rigid plate model and w

ISSN:0148-0227    

DOI:10.1029/JB094iB05p05585

年代:1989

数据来源: WILEY

摘要:

Long‐period teleseismicPandSwaveforms for the largest four earthquakes in the Serdo sequence are compared with synthetic seismograms to infer mechanisms, focal depths, source time functions, and dynamic energy release. For all four events studied, the mechanisms are predominantly strike slip. Depth estimates obtained for the four earthquakes by matching synthetic seismograms to the observations are found to lie between 5 and 8 km. The azimuthal variation of observed body wave duration for the main event indicates that the rupture propagated from NE to SW. Average seismic moments obtained from body wave synthetics vary from 4.6×1017to 2.5×1018N m. Average final displacements, stress drops, and lengths of the fault are found to lie between 0.16 and 0.56 m, 0.45 and 1.56 MPa, and 10 and 16 km, respectively. The low stress drop value obtained may indicate the presence of softer materials near the source. The energy release shows that the mode of energy release during the sequence took place in two steps, mainly by the two largest sho

ISSN:0148-0227    

DOI:10.1029/JB094iB05p05603

年代:1989

数据来源: WILEY

摘要:

Previous workers have pioneered statistical techniques to study the spatial distribution of aftershocks with respect to the focal mechanism of the main shock. Application of these techniques to deep focus earthquakes failed to show clustering of aftershocks near the nodal planes of the main shocks. To better understand the behavior of these statistics, this study applies them to the aftershocks of six large shallow focus earthquakes in California (August 6, 1979, Coyote Lake; May 2, 1983, Coalinga; April 24, 1984, Morgan Hill; August 4, 1985, Kettleman Hills; July 8, 1986, North Palm Springs; and October 1, 1987, Whittier Narrows). The large number of aftershocks accurately located by dense local networks allows us to treat these aftershock sequences individually instead of combining them, as was done for the deep earthquakes. The results for individual sequences show significant clustering about the closest nodal plane and the strike direction for five of the sequences and about the presumed fault plane for all six sequences. This implies that the previously developed method does work properly. Nonrandom behavior was also found about the slip directions, thePaxis, theTaxis, and theBaxis, but this is probably caused by the lack of independence between these axes and the previously mentioned features of the focal mechanisms. Given that the method does work and that deep aftershocks were not shown to cluster about the main shock nodal planes, the shallow focus data were used to simulate the deep focus study. The goal is to determine if there are artificial factors that make clustering in the deep focus data unobservable. To more closely mimic the work on deep earthquakes, the largest aftershocks from each of the six sequences were combined and studied with respect to their respective main shock focal mechanisms. This reduced the significance of the clustering about the focal mechanism parameters, but not below 95% confidence. Gaussian noise was then added to the aftershock hypocenters in order to determine if the larger hypocentral and focal mechanism errors in the deep focus data could account for the previous negative result. The conclusion is that the following reasons are sufficient to explain the lack of clustering about the main shock nodal planes for the deep focus aftershocks: the need to combine aftershocks from several sequences, the size of the hypocentral location and focal mechanism errors, and the alignment of distant aftershocks with the Wadati‐Benioff zon

ISSN:0148-0227    

DOI:10.1029/JB094iB05p05615

年代:1989

数据来源: WILEY

摘要:

Waveform inversion of tsunami data provides independent and reliable information on earthquake source processes. The propagation effects of tsunamis are accounted for by computing Green's functions using a finite difference method and using the known bathymetry, which is much better constrained than the seismic velocity structure. The observed tsunami waveforms are corrected for the response of the tide gage system by using the results of in situ measurements. Using Green's functions computed from bathymetric data on a 2.5‐km grid and the observed waveforms digitized at 1‐min intervals, the spatial resolution of the faulting heterogeneity by the present method is as small as 30 km. This method is applied to the 1968 Tokachi‐oki and 1983 Japan Sea earthquakes. The slip is concentrated at the northwestern part of the fault plane for the 1968 event. For the 1983 event the slip on the fault just north of the epicenter is twice that of the other parts. These asperity distributions are similar to those inferred from seismic wave analysis. However, the recovered total seismic moments are smaller. The present method is shown to be a useful technique for estimating heterogeneous fault motion, especially for historical events with no or poor seismic records, and tsunami earthquakes in which the excitations of seismic and tsunami waves are diff

ISSN:0148-0227    

DOI:10.1029/JB094iB05p05627

年代:1989

数据来源: WILEY

摘要:

The seismic behavior in the southern Kurile Islands Arc is strongly influenced by spatial heterogeneity in the mechanical properties of the plate interface. Maps showing sites of maximum seismic moment release for the great underthrusting earthquakes of 1958, 1963, 1969, and 1973 provide a first‐order image of the major regions of enhanced strength, or asperities. Hypocenters of these and an additional 83 smaller earthquakes withmb≥ 5.3 and known underthrusting mechanisms were recomputed using the method of joint hypocenter determination in order to assess better the influence that the strength distribution exerts on the interplate seismicity patterns. Nearly all of the smaller‐magnitude earthquakes locate outside of the principal asperities of the great earthquakes. This observation is consistent with substantial release of accumulated strain energy in asperity regions during the great earthquakes and a redistribution of stress in adjacent areas following their rupture. An along‐dip, depth dependent component of strength heterogeneity appears to influence the seismicity patterns. The asperities of the 1958 and 1973 great earthquakes and hypocenters of most of theMs≥ 7 earthquakes occur near the downdip edge of the 100‐km‐wide seismically active plate interface. This would be expected if shear resistance on the interface increases monotonically with lithostatic pressure up to the brittle‐ductile or stick slip‐stable slip transition that defines the base of the seismogenic zone. The asperities of the 1963 and 1969 earthquakes, however, appear to be located well above the downdip edge of the coupled plate interface. The occurrence of large asperities in middepth ranges of the seismogenic interface suggests the influence of along‐dip changes in other parameters, besides lithostatic pressure, that control shear resistance. Along‐strike segmentation of the Kurile Islands thrust zone may be a consequence of along‐strike alternation in the location of the largest asperities between the deep interfa

ISSN:0148-0227    

DOI:10.1029/JB094iB05p05637

年代:1989

数据来源: WILEY

摘要:

For the SE Palmdale network in California, triangulation data for the years 1938 and 1958 and trilateration data at about 1–year intervals from 1971 to 1980 are the basis for a strain analysis. A uniform shear strain accumulation over the 40–year period appears to be evident from this data set ( γ = +0.34 ± 0.18 µstrain/yr, ψ = 119° ± 10°). Previous computations of this coseismic strain change needed further qualification due to the corrupting influence of uncertainties in the network height differences because height differences are critical when triangulation data are compared with trilateration observations. As a result, the height differences of the SE Palmdale network were resurveyed by trigonometric heighting in March 1981. Using these new data, a significant coseismic strain jump related to the 1971 San Fernando earthquake is postulated in the observations. The strain jump can be identified with engineering shearing strains of Δ γ1= +3.3 ± 1.7 µstrain and Δ γ2= +4.9 ± 1.7 μstrain or a total shear strain jump of +(5.9±2.4) µstrain with an azimuth (ψ) of 107°±6° of a line across which right–lateral shear is a maximum. These values are in excellent agreement with those predicted by eart

ISSN:0148-0227    

DOI:10.1029/JB094iB05p05651

年代:1989

数据来源: WILEY

摘要:

From May 1987 to June 1988 the National Geodetic Survey (NGS) made approximately 50 observations at 30 sites with one of the six absolute gravimeters built by the Joint Institute for Laboratory Astrophysics (JILA) between 1983 and 1986. Of the 10 sites where two to three observations were made, the scatter about the mean site values ranged from under ±1 μGal to ±4.0 μGal. This high degree of repeatability is the result of three factors: the improvements made to the instrument at JILA in 1986, methods of observation and quality control of the data sets, and independent monitoring of and correcting for the environmental effects on gravity. The data correction methods now employed at NGS allow the establishment of high‐precision reference gravity stations in the United States and abroad for monitoring the temporal variations of gravity and studying vertical ground mo

ISSN:0148-0227    

DOI:10.1029/JB094iB05p05659

年代:1989

数据来源: WILEY