Attenuation of compressional waves in peridotite measured as a function of temperature at 200 MPa   总被引：3，自引：0，他引：3  

Hiroki Sato  I. Selwyn Sacks  Tsutomu Murase  Gregory Muncill  Hiroyuki Fukuyama 《Pure and Applied Geophysics》1988,128(1-2):433-447

A technique has been developed to determine attenuation in rocks at high temperature using a gas-media, high-pressure apparatus. A pulse transmission technique and a spectral ratio method are used to study compressional seismic properties of rocks. Seismic waves are transmitted to and from the sample through buffer rods of mullite. The effect of seismic wave reflections within the sample assembly are cancelled out by taking ratios of the spectra measured at different temperatures. In order to obtain good signal-to-noise ratio for resolving the attenuation at high pressure and temperature, special care is taken in the sample assembly and the ultrasonic coupling between the sample, buffer rods and transducers. A very tight connection of the sample-buffer rod-transducer is essential for obtaining high frequency signals (>300 kHz) at high temperature. A small mass is attached to each outside end of the transducer to drive low frequency signals (<250 kHz) into the sample. Before attenuation measurements, the sample and the buffer rods are tightly compacted in a platinum tube at high pressure and room temperature to ensure pressure seal of the sample assembly. The frequency range of measurement covers 50 to 450 kHz for the sample. Attenuation is very small in the buffer rod compared to the sample for the entire temperature range of the study. Because of the small attenuation, a wide frequency band of 50 kHz to 3.2 MHz can be covered for investigating the attenuation in the buffer rod. The technique has been used to measure attenuation at high confining pressure, and temperatures including sub- and hyper-solidus of upper mantle rocks. Therefore, effects of partial melting on attenuation can be studied.The method is applied to the attenuation measurement in a peridotite as a function of temperature to 1225°C at 200 MPa confining pressure. At high temperature, signal amplitude decays more rapidly at high frequency than at low frequency, from which attenuation (andQ) can be determined using a spectral ratio method. No frequency dependence ofQ is resolved for both the sample and the buffer rod over the entire temperature and frequency ranges of the measurement. The results show thatQ decreases rapidly with increasing temperature even in the temperature range below the solidus of peridotites. Such temperature sensitivity ofQ is probably more useful to probe thermal structure in the upper mantle than that of conductivity at temperatures below the solidus. The results in this study are compared with available seismic velocity, electrical conductivity and solidus data for peridotites, suggesting that there is no discontinuous change in both mechanical and electrical properties of peridotites at the solidus temperature. Even at hypersolidus temperatures, it appears that velocity drops and conductivity increases continuously (not abruptly) with increasing melt fraction. This implies that mechanical and electrical properties of the upper mantle will gradually change at the boundary where the geotherm crosses the solidus.  相似文献   

Aseismio fault slip and block deformation in North China   总被引：1，自引：0，他引：1  

Lanbo Liu  Alan T. Linde  I. Selwyn Sacks  Shihai He 《Pure and Applied Geophysics》1996,146(3-4):717-740

In North China, the tectonic fault-block system enables us to use the Discontinuous Deformation Analysis (DDA) method to simulate the long-term cross-fault survey and other geodetic data related to aseismic tectonic deformation. By the simulation we have found that: (1) Slips on faults with different orientation are generally in agreement with the ENE-WSW tectonic stress field, but the slip pattern of faulting can vary from nearly orthogonal, to pure shear along the strike of the faults, this pattern cannot be explained by simple geometric relation between the strike of the fault and the direction of the tectonic shortening. This phenomenon has been observed at many sites of cross-fault geodetic surveys, and might be caused by the interactions between different blocks and faults. (2) According to the DDA model, if the average aseismic slip rate along major active faults is at the order of several tenths of millimeter per year as observed by the cross-fault geodetic surveys, the typical strain rate inside a block is at the order of 10^–8 year^–1 or less, so that the rate of 10^–6 year^–1, as reported by observations in smaller areas, cannot be the representative deformation rate in this region. (3) Between the slips caused by regional compression and block rotation, there is a possibility that the sense of slip caused by rigid body rotation in two adjacent blocks is opposite to the slip caused by the tectonic compression. But the magnitude of slip resulting from the tectonic compression is much larger than that due to the block rotation. Thus, in general, the slip pattern on faults as a whole agrees with the sense of tectonic compression in this region. That is to say, the slip caused by regional compression dominates the entire slip budget. (4) Based on (3), some observed slips in contradiction to ENE tectonic stress field may be caused by more localized sources, and have no tectonic significance.  相似文献   

Coseismic and other short-term strain changes recorded with Sacks-Evertson strainmeters in a deep mine, South Africa   总被引：2，自引：0，他引：2  

A. McGarr  I. S. Sacks  A. T. Linde  S. M. Spottiswoode  R. W. E. Green 《Geophysical Journal International》1982,70(3):717-740

Summary. During 1977 March and April, three Sacks-Evertson borehole dilatometers were installed at the ends of boreholes drilled into the sidewall of an experimental tunnel at a depth of 3.1 km in the ERPM gold mine near Johannesburg. In the following year coseismic strain changes ranging from 5 ± 10⁻¹⁰ to values exceeding 5 ± 10⁻⁶ were recorded for hundreds of mine tremors in the magnitude range -1 to 3.7 and at hypocentral distances of 50 m to about 2 km. Hypocentral coordinates and magnitudes were determined from seismograms recorded from an underground array of geophones. Amplitudes and polarities of the coseismic strain steps are generally in excellent agreement with theoretical expectations based on point-source dislocation theory; specifically, the strain steps are proportional to the seismic moment divided by the cube of hypocentral distance. At a strain level of 5 ± 10⁻⁹ or greater the tremors do not appear to be preceded by any short-term indications of instability even for tremors producing coseismic steps greater than 5 ± 10⁻⁶ and for which the strainmeters were within a source radius of the hypocentre. Continuous strain changes observed at the times when the mine excavation, at a distance of about 100 m, is extended are in good agreement with calculated changes based on the theory of elasticity. A similar calculation is consistent with post-seismic strain changes observed to follow some of the closer tremors. These post-seismic strains show a logarithmic dependence on time following the tremor and appear to be due to the interaction of a tremor with the adjacent mine excavation rather than to deformation within the actual seismic source region.  相似文献   

A model not requiring continuous lithosphere for anomalous high-frequency arrivals from deep-focus South American earthquakes     

J.Arthur Snoke  I.Selwyn Sacks  Hiromu Okada 《Physics of the Earth and Planetary Interiors》1974,9(3):199-206

Various workers have constructed models to explain a class of anomalous arrivals at Peruvian and Chilean stations from deep-focus South American earthquakes. These arrivals are shear waves with a later arrival time, a higher frequency content, a longer duration and a lower apparent velocity than direct S. Our models assume that there is a sufficiently sharp discontinuity at the upper interface of the descending lithospheric slab between depths of 80 and 250 km to provide efficient reflection (≈0.1) for S-waves incident from below. The observed travel times require a single S-to-S reflection at this interface if the J-B velocity-depth model is modified to allow for 7% higher velocities down to a depth of 300 km (excluding the crust). The locus of required reflection points correlates well with the upper boundary of the observed seismicity (strike and dip angles within 5°) and Q for the proposed path is consistent with the frequency content of the anomalous arrivals. Thus the existence of these arrivals requires a dipping interface down to about 250 km, but, contrary to the wave-guide model of Isacks and Barazangi, cannot be used to infer a continuous lithospheric slab down to the deep-focus earthquakes (h #62; 500 km).  相似文献   

Geochronology and geochemistry of lavas from the 1996 North Gorda Ridge eruption     

K.H. Rubin  M.C. Smith  M.R. Perfit  D.M. Christie  L.F. Sacks 《Deep Sea Research Part II: Topical Studies in Oceanography》1998,45(12):2571-2597

Radiometric dating of three North Gorda Ridge lavas by the ²¹⁰Po–²¹⁰Pb method confirms that an eruption occurred during a period of increased seismic activity along the ridge during late February/early March 1996. These lavas were collected following detection of enhanced T-phase seismicity and subsequent ocean bottom photographs documented the existence of a large pillow mound of fresh-appearing lavas. ²¹⁰Po–²¹⁰Pb dating of these lavas indicates that an eruption coinciding with this seismicity did occur (within analytical error) and that followup efforts to sample the recent lava flows were successful. Compositions of the three confirmed young lavas and eleven other samples of this contiguous “new flow” sequence are distinct from older lavas from this area but are variable at a level outside analytical uncertainty. These intraflow variations can not easily be related to a single, common parent magma. Compositional variability within the new flow is compared to that of other recently documented individual flow sequences, and this comparison reveals a strong positive correlation of compositional variance with flow volumes spanning a range of >2 orders of magnitude. The geochemical heterogeneity in the North Gorda new flow probably reflects incomplete mixing of magmas generated from a heterogeneous mantle source or from slightly different melting conditions of a single source. The compositional variability, range in sample ages (up to 6 weeks) and range in active seismicity (4 weeks) imply that this relatively large flow was erupted over an interval of several weeks.  相似文献   

Asthenospheric Viscosity and Stress Diffusion: A Mechanism to Explain Correlated Earthquakes and Surface Deformations In Ne Japan   总被引：1，自引：0，他引：1  

P. A. Rydelek  I. S. Sacks 《Geophysical Journal International》1990,100(1):39-58

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Recovery of the elastic parameters of a layered half-space     

Paul Sacks  Wolliam Symes 《Geophysical Journal International》1987,88(3):593-620

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Subduction beneath western South America: evidence from converted phases     

J. Arthur Snoke  I. Selwyn Sacks  David E. James 《Geophysical Journal International》1979,59(1):219-225

Summary. There are two conflicting models for the dip of the subduction zone beneath central Peru and beneath central Chile. One model, based primarily on the distribution of hypocentres thought to be most reliably located, postulates a shallow (∼ 10°) dip for the downgoing plate. The second model, re-examined in this paper and based chiefly on an analysis of ScSp converted phases, postulates a normal (∼ 30°) dip for the subduction.
A detailed examination of ScSp data for central Peru and central Chile shows that ScSp arrivals can be identified on all seismograms on which the predicted signal/noise ratio is greater than 1, and the measured amplitude ratios of ScSp to ScS imply a normal dip for the conversion interfaces. The characteristics of the ScSp arrivals in these regions are virtually identical to those for ScSp arrivals in southern Peru, where a well-defined Benioff zone and the calculated ScSp -conversion region both imply a normal dip for the subduction. Hence ScSp observations in South America support the model of an approximately constant dip of subduction from central Peru to central Chile to a depth of at least 120 km, and impose a constraint that must be satisfied by any satisfactory tectonic model for these regions.  相似文献   

A tertiary fold and thrust belt in the Valle del Cauca Basin, Colombian Andes     

C.A. Alfonso  P.E. Sacks  D.T. Secor  J. Rine  V. Perez 《Journal of South American Earth Sciences》1994,7(3-4)

Surface geology and heophysical data, supplemented by regional structural interpretations, indicate that the Valle del Cauca basin and adjacent areas in west-central Colombia form a west-vergent, basement-involved fold and thrust belt. This belt is part of a Cenozoic orogen developed along the west side of the Romeral fault system. Structural analysis and geometrical constraints show that the Mesozoic ophiolitic basement and its Cenozoic sedimentary cover are involved in a “thick-skinned” west-vergent foreland style deformation. The rocks are transported and shortened by deeply rooted thrust faults and stacked in imbricate fashion. The faults have a NE---SW regional trend, are listric in shape, developed as splay faults which are interpreted as joining a common detachment at over 10 km depth. The faults carry Paleogene sedimentary strata and Cretaceous basement rocks westward over Miocene strata of the Valle del Cauca Basin. Fold axes trend parallel or sub parallel to the thrust faults. The folds are westwardly asymmetrical with parallel to kink geometry, and are interpreted to be fault-propagation folds stacked in an imbricate thrust system. Stratigraphic evidence suggests that the Valle del Cauca basin was deformed between Oligocene and upper Miocene time. The kinematic history outlined above is consistent with an oblique convergence between the Panama and South American plates during the Cenozoic.A negative residual Bouguer anomaly of 20–70 mgls in the central part of the Valle del Cauca basin indicates that a substantial volume of low density sedimentary rocks is concealed beneath the thrust sheets exposed at the land surface. The hydrocarbon potential of the Valle del Cauca should be reevaluated in light of the structural interpretations presented in this paper.  相似文献   

Slow earthquakes and great earthquakes along the Nankai trough   总被引：2，自引：0，他引：2  

Alan T Linde  I.Selwyn Sacks 《Earth and Planetary Science Letters》2002,203(1):265-275

We have reexamined reports indicating that slow deformation occurred before the great Japan earthquakes of 1944 (Tonankai) and 1946 (Nankaido) and find that the observations are well founded. Although no quantitative models have previously been proposed to explain all of the relevant data we show that they are satisfied by a simple model for both earthquakes. The model, based on known properties of subduction zones, has slow slip on the subduction interface in an area deeper than the seismic rupture. If this model is correct and a similar physical situation holds for an anticipated Tokai earthquake, existing instruments will be able to reveal the pre-slip in real time. While differences among the deformation time series at different sites will provide strong constraints on the slow rupture propagation, these differences could result in delaying the recognition of a coherent event.  相似文献