摘要:

Analytical solutions are presented for the deformation and stress state of a horizontally stratified earth subject to normal loads of size and wavelength that are characteristic of seamounts. The models investigated include layered elastic plates, homogeneous viscoelastic plates, and elastic over viscoelastic models. In all cases the composite models overlie an inviscid half space. The principal advantage of the laminated elastic models over homogeneous elastic plates is that they can result in a substantial reduction in the maximum stress differences in those parts of the plate least capable of supporting large stress differences. A number of geophysical observations point to stress relaxation when the lithosphere is loaded on geological time scales, and this has been modeled with viscoelastic and elastic‐viscoelastic models. The viscoelastic models explain many of these observations, but the stresses remain unrepresentative of those in the real lithosphere. The elastic‐viscoelastic models, in which the parameters defining each layer are considered as effective moduli or viscosities, represent better the rheological zonation of the earth with the crust being modeled by one or several elastic layers, the subcrustal lithosphere by a viscoelastic layer, and the mantle by an inviscid fluid. Observations of gravity or geoid height over the seamounts cannot readily discern between the viscoelastic and elastic‐viscoelastic models unless detailed observations over the moat and arch are available. A summary of some estimates of effective flexural rigidity indicates that the stress relaxation is an important factor in determining the response of the lithosphere to lo

ISSN:0148-0227    

DOI:10.1029/JB086iB08p06961

年代:1981

数据来源: WILEY

摘要:

A comparison is made of the errors in determining the geoid undulation according to the conventional Molodenskii truncation theory and its modifications as suggested by Molodenskii (1958) and Meissl (1971). The undulation is assumed to be computed from gravity anomalies in a spherical cap and a finite set of harmonic potential coefficients (e.g., GEM 9 or GEM 10B). Only the effects of errors in the coefficients, as well as the neglect of higher degree coefficients, are considered. It is found that the error estimates (rms values) are significantly lower for any of the modified methods than for the conventional method. For example, errorless and continuous gravity data in a cap with 10° radius and the GEM 9 coefficients together yield undulations with an average error of 109 cm if the unmodified truncation theory is applied and 41 cm and 33 cm with Meissl's method and a Molodenskii type of modification, respectively

ISSN:0148-0227    

DOI:10.1029/JB086iB08p06985

年代:1981

数据来源: WILEY

摘要:

Frequent surveys of seven trilateration networks in southern California over the interval 1973–1980 suggest that a regional increment in strain may have occurred in 1978–1979. Prior to 1978 and after late 1979 the strain accumulation has been predominantly a uniaxial north‐south compression. This secular trend was interrupted sometime in 1978–1979 by an increment in both north‐south and east‐west extension in five of the seven networks. The onset of this change appears to have occurred first in the networks farthest south. The changes occurred without any unusual seismicity within the networks, but the overall seismicity in southern California was unusually low prior to and has been unusually high since the occurrence. The average principal strain rates for the seven networks in the 1973–1980 interval are 0.17 μstrain/yr north‐south contraction and 0.08 μstrain/yr east‐west extension. Although the observed increment in strain could be related to unidentified systematic error in the measuring system, a careful review of the measurements and comparisons with three other measuring systems reveal no appreciable cumulati

ISSN:0148-0227    

DOI:10.1029/JB086iB08p06991

年代:1981

数据来源: WILEY

摘要:

Free‐air gravity anomalies near Nias Island include a −100 mGal low centered 10–20 km landward of the trench axis and a +80 mGal high over the outer arc ridge but also show a few large anomalies unrelated to topography. A −90 mGal low is found on the structural high between the Banyak Islands and Sumatra, north of Nias. An 80 mGal rise (at a gradient of 8 mGal/km) is modeled by a near‐surface body of high‐density material (oceanic crust?) 4 km wide and tens of kilometers long. This slab may be exposed on the southwest coast of Nias, where ultramafic bodies have been mapped, and its northerly trend (compared with northwesterly bathymetric trends) may be related to other obliquely trending structures near here, to a proposed strike slip fault which crosses Nias, and/or to the basement high topped by the Batu Islands. A −30 mGal free‐air low over the forearc basin can be modeled best if pre‐Miocene mélange or continental crust underlies the basin, and it seems unlikely that the basin can be underlain by a large thicknes

ISSN:0148-0227    

DOI:10.1029/JB086iB08p07003

年代:1981

数据来源: WILEY

摘要:

A double zone of seismicity has been found beneath the northern Marianas in the depth range 80–120 km. The Benioff zone undergoes a pronounced bend, with radius of curvature about 200 km, before straightening and descending nearly vertically to a depth of more than 600 km. Thus this is an ideal area in which to search for evidence of stresses associated with unbending of the subducted oceanic plate. We relocated all teleseismic events from 1963 through 1975 by using a regional, composite master event technique, which significantly decreased the scatter of the hypocenters. The three events of the lower seismic zone are separated from the upper zone by about 30–35 km. The true thickness of the upper seismic zone is probably 20 km or less. The limited focal mechanism data indicate that there is downdip tension in the lower zone and downdip compression in the upper zone. The seismicity patterns could be generated either by thermal stresses or by unbending a plate which was originally deformed anelastically. We show that an elastic‐perfectly plastic model of the rheology of the plate can account for the separation of the two seismic zones, the focal mechanisms, and the greater activity of the upper seismic

ISSN:0148-0227    

DOI:10.1029/JB086iB08p07013

年代:1981

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/JB086iB08p07023

年代:1981

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/JB086iB08p07025

年代:1981

数据来源: WILEY

摘要:

The Bermuda earthquake (M∼ 6) occurred near the westerly extension of the Kane Fracture Zone roughly 370 km southwest of the island of Bermuda. It is one of the largest oceanic intraplate earthquakes to occur off the eastern coast of North America. Because of its size and location, it has provided an excellent set of WWSSN body waves. They can be used to infer its depth and faulting parameters by waveform modeling techniques. The results indicate a north‐northwest striking thrust mechanism (strike = N20°W, dip = 42°NE, rake = 90°) with the hypocenter located at a depth of 11 km, which for an oceanic crust places it predominantly in the mantle. The event had a seismic moment of 3.4 × 1025dyne cm, and its time history was modeled with a symmetric trapezoidal time function 3 s in duration. The north‐northwest strike of the event is in good agreement with the bathymetry of the area, the epicenter being close to the southwestern edge of the Bermuda Rise. The strike of the event is also close to that of the inferred extensions of the present ridge fracture zones in the region. The presence of such fracture zones is indicative of local weak zones in the lithosphere. The Bermuda earthquake most likely is associated with one of these zones of weakness and is the result of the application of present day stress imposed on the region by the North American plate in the direction of its absolute motion. This is an important event in terms of understanding and estimating seismic hazard on the eastern seaboard of Nort

ISSN:0148-0227    

DOI:10.1029/JB086iB08p07027

年代:1981

数据来源: WILEY

摘要:

Discrete samples of geothermal water have been collected from a network of nine stations for analysis of radon (222Rn) content. The sampling network consisted of wells that range in wellhead temperature from 48°C to 100°C, and the depths range from 38 m to 1338 m. The sampling frequency at most stations was about once per week and twice per week at the station Fludir. The wells are either artesian or pumped more or less continuously. The network covered two regions of transform faulting in Iceland with seven stations in the Southern Iceland Seismic Zone (SISZ) and two stations in northern Iceland in the Tjornes Fracture Zone (TFZ). During 1978 and 1979 several anomalous changes in radon content were observed to precede some of the local earthquakes. Criteria based on tectonic regimes, a magnitude‐distance relationship and time clustering were applied to select a set of 23 earthquakes that could be expected to be preceded by a radon anomaly. The magnitude of these earthquakes ranged between 1.0 and 4.3. Each of the 23 earthquakes was within the distance range of one or more stations such that altogether 57 potential observations of possible anomalies were available. The method of analysis applied to the radon and earthquake data consisted of identifying radon anomalies in retrospect, and resulted in nine precursory anomalies, 48 cases of failure to observe an anomaly, and seven false alarms. The probability of observing radon anomalies before earthquakes with magnitudes between 2.0 and 4.3 (the largest event observed) was found to be approximately 65% based on a weekly sampling rate. In the SISZ, five out of eight earthquakes (M>2) were preceded by an anomaly. In two cases, anomalies were observed at two different stations prior to the same earthquake. The anomalies appeared to occur farther away for larger earthquake magnitude. An aquiclude that divides the SISZ did not seem to affect the occurrence of radon anomalies, but the amplitude of anomalies on the east side were larger than the ones on the west side. The duration times of the anomalies ranged from 17 to 37 days. The large number of failures to observe an anomaly indicated that the occurrence of an anomaly was more strongly dependent on local conditions and earthquake parameters than distance between epicenter and radon station. Some stations also appeared to be less sensitive than others and three stations never showed an anomaly. During periods of bursts in local seismicity, some stations reflected distinctively larger background fluctuations, which hampered the correlation of the seismicity with the radon data. Most of the false alarms were related to disturbances caused by changes in well operation. In the SISZ the high rate of failure to observe an anomaly and the occurrence of false alarms was partly compensated for by operating a dense network of sampling stations. Wells that are utilized at moderate flow rates are less likely to show false alarms than wells utilized at high flow rates. Wells situated close to rock formations rich in radioactive minerals may be more likely to show anomalies than oth

ISSN:0148-0227    

DOI:10.1029/JB086iB08p07037

年代:1981

数据来源: WILEY

摘要:

A network of seismograph stations has operated in north‐central New Mexico since 1975. The network is approximately 200 by 300 km in size and encompasses the Rio Grande rift there. Several seismic refraction experiments have been reported in the literature for the region of the network and adjacent areas. Because all of the seismic refraction lines are unreversed,Pnvelocities reported were mainly of the inverse travel time slope for the direction of the corresponding line. The values of the inverse slope for those studies range from 7.6 to 8.2 km/s. The purpose of our study is to estimate thePwave velocity of the uppermost mantle by using the time term method. First, we timed thePnwaves of strong signals from five explosions and eight shallow earthquakes recorded by the network. The main data set, which contains 87 time‐distance pairs, was processed by using the time term method. ThePnvelocity estimated by this method is 8.0 ± 0.1 km/s. To corroborate this estimate, we then processed 10 subsets of the main data set in the same way. Almost all of the solutions show velocities of 7.9–8.1 km/s, in agreement with the velocity determined for the main data set. The station time terms of the main data set also are substantiated, and they suggest that the base of the crust dips northward by a few degrees in the region of the survey. The smallest value reported by other investigators for the inverse slope (7.6 km/s) appears to be related to the dip. The normalPwave velocity of the uppermost mantle of north‐central New Mexico places restrictions on thermal models of the rift. For instance, the results exclude the likelihood of a wide zone of asthenosphere at the base of the crust beneath the rift, but they do not exclude a narrow s

ISSN:0148-0227    

DOI:10.1029/JB086iB08p07055

年代:1981

数据来源: WILEY