摘要:

One or more earthquakes with abnormally large numbers of aftershocks at the beginning of their aftershock sequences are possible long‐term precursors of a stronger earthquake. This precursor, named pattern B (bursts of aftershocks), was tested together with two other possible premonitory patterns, S and Σ, described previously. Pattern S (swarm) consists of the spatial clustering of earthquakes at a time when the seismicity of the region is above average. Pattern Σ consists of an increase in the sum of earthquake energies to the 2/3 (roughly) power, in a sliding time window. These three patterns were tested by retrospective long‐term prediction of earthquakes of Southern California, 1932–1977, with magnitudes ≥6.5. The total duration of identified periods of elevated probability of occurrence is about 14 years. Five out of six earthquakes occurred during these intervals. However, the patterns do not indicate the exact location of future strong earthquakes within the region. All three patterns seem to represent different projections of the same general pattern, of ‘bursts of seismicity,’ i.e., an abnormal clustering of earthquakes in the time‐space‐energy domain. Pattern B also precedes strong earthquakes in New Zealand and Italy; Pattern Σ precedes strong earthqua

ISSN:0148-0227    

DOI:10.1029/JB085iB02p00803

年代:1980

数据来源: WILEY

摘要:

An attempt is made to identify seismicity patterns precursory to great earthquakes in most of Tibet as well as the central and eastern Himalayas. The region has considerable tectonic homogeneity and encompasses parts of China, India, Nepal, Bhutan, Bangladesh, and Burma. Two seismicity patterns previously described were used: (1) pattern Σ is a peak in the sum of earthquake energies raised to the power of about 2/3, taken over a sliding time window and within a magnitude range less than that of events we are trying to predict; and (2) pattern S (swarms) consists of the spatial clustering of earthquakes during a time interval when the seismicity is above average. Within the test region, distinct peaks in pattern Σ have occurred twice during the 78‐year‐long test period: in 1948–49, prior to the great 1950 Assam‐Tibet earthquake (M= 8.6), and in 1976. Peaks in pattern S have occurred three times; in 1932–1933, prior to the great 1934 Bihar‐Nepal earthquake (M= 8.3), in 1946, and in 1978. The 1934 and 1950 earthquakes were the only events in the region that exceededM= 8.0 during the test period. On the basis of experience here and elsewhere, the current peaks in both Σ and S suggest the likelihood of anM= 8.0 event within 6 years or anM= 8.5 event within 14 years. Such a prognostication should be viewed more as an experimental long‐term enhancement of the probability that a large earthquake will occur than as an actual prediction, in view of the exceedingly large area encompassed and the very lengthy time window. Furthermore, the chances of a randomly occurring event as large asM= 8.0 in the region are perhaps 21% within the next 6 years, and the present state of the art is such that we can place only limited confidence in such forecasts. The primary impact of the study, in our opinion, should be to stimulate the search for medium‐ and short‐term precursors in the region and to search for similar long‐ter

ISSN:0148-0227    

DOI:10.1029/JB085iB02p00813

年代:1980

数据来源: WILEY

摘要:

A reference point equalization method is presented for the determination of source and propagation effects of surface waves. The method works on seismic events located in a small source region, allowing the assumption that all events share the same path effects to a given receiver. Two steps in the method are initialization and iteration. Initialization obtains the first reference events in order to compute initial estimates of phase velocity and attenuation coefficient. Iteration simultaneously refines the propagation parameters and determines source parameters of new earthquakes. This method was applied to nine earthquakes in the Pamir Mountains, Central Asia (reference point: 39.58°N, 73.55°E). Source parameters were determined using the linear moment tensor inversion on 26 to 60‐s Rayleigh wave complex spectra. It was necessary to modify the straight least squares inversion method because of its sensitivity to even just a few bad data points in the data set. Residuals obtained from repeated application of the moment tensor inversion over trial focal depths showed two minima: one at depths less than 20 km and the other at depths greater than 70 km, with values of the residuals at these minima close enough to cast doubt on the determination of focal depth. The inversion generally gave three‐couple force systems having significant nonzero intermediate component. These features of source parameter inversions are discussed in light of errors in the data set. Propagation parameters, in particular, the phase velocities, show good qualitative correlation with geologic and geographic features on the Eurasian cont

ISSN:0148-0227    

DOI:10.1029/JB085iB02p00821

年代:1980

数据来源: WILEY

摘要:

Eight variable‐angle seismic reflection stations in the Arabian Fan, Northwestern Indian Ocean, provided 40 determinations of sound velocity in sediment and sedimentary rock. Sound velocity in the homogeneous, largely terrigenous fan increases smoothly with depth. Regression analysis yielded the velocity‐time relationshipV(km/s) = 1.510 + 1.863t, whereVis instantaneous velocity andtis one‐way travel time below the sea floor to 1 s. The velocity‐depth function isV(km/s) = 1.510 + 1.200h; − 0.253h2+ 0.034h3, wherehis subbottom de

ISSN:0148-0227    

DOI:10.1029/JB085iB02p00849

年代:1980

数据来源: WILEY

摘要:

Multiple ScS travel times have been obtained by wave form cross correlation from seismograms digitally recorded by the High Gain Long Period (HGLP) and Seismic Research Observatory (SRO) networks. The surface projections of the paths corresponding to these data cross the western Pacific on oceanic crust greater than 100 m.y. old or traverse continental regions. The difference between the median ScSn‐ScSn−1residuals for all western Pacific paths and all continental paths is +5.2 s, in agreement with our World Wide Standardized Seismograph Network (WWSSN) data (Sipkin and Jordan, 1976). These results support the hypothesis that the average mantle shear velocity of old ocean basins is significantly less than that of old continental nuclei. The medians of both the oceanic and continental residuals for the HGLP and SRO data are more positive than those for the higher‐frequency WWSSN data by amounts consistent with attenuative dispersion, which we take to be direct evidence for such dispersion. The residuals for paths crossing China have a median 2 s greater than the median for all continental paths, supporting the inference from dispersion studies that the upper mantle beneath China is characterized by anomalously low shear velocities. The residuals for western Pacific paths show lateral variations of 5 s or more not correlated in any systematic way with crustal ages along the paths. An analysis of these variations suggests that for horizontal scale lengths of the order of 103km the amplitude of lateral variability is greater along a SW‐NE axis than along a SE‐NW axis. Mesoscale heterogeneity in the western Pacific may thus consist of predominantly NW trending s

ISSN:0148-0227    

DOI:10.1029/JB085iB02p00853

年代:1980

数据来源: WILEY

摘要:

Six refraction lines were shot parallel to structure, from the Sunda Trench axis to the west coast of Sumatra, near Nias Island. Seaward of the Nias Island ridge, 2.4–3.8 km of low‐velocity sediment (2.0–3.6 km/s) is underlain by a wedge of 4.7‐ to 4.9‐km/s material, probably dewatered sediment and oceanic basement. The wedge thickens from 1.2 km beneath the trench axis to 13.5 km immediately seaward of the ridge. Layer 3, 6.6–7.5 km/s and 3.7–23 km thick, dips 5° landward beneath this wedge. The large variation in layer 3 thickness may be evidence that imbricate thrust faulting, responsible for the thickening which forms the 4.7‐ to 4.9‐km/s wedge, also extends downward into layer 3. The forearc basin, landward of Nias Island, has 3.0 km of low‐velocity sediment overlying 19 km of 6.5‐km/s material and 8.1‐km/s Moho. Near the coast of Sumatra, layers of 2.1, 4.3, 5.3, and 6.0 km/s, whose thicknesses total 8.1 km, are underlain by a large, undetermined thickness of 6.8‐km/s material. This 6.5‐ to 6.8‐km/s layer may be either lower continental (granitic) crus

ISSN:0148-0227    

DOI:10.1029/JB085iB02p00863

年代:1980

数据来源: WILEY

摘要:

Experiments were conducted to determine the ‘quasi‐static’ mechanical behavior of rock salt in support of the structural design of a waste isolation pilot plant. The salt studied was from the Salado formation in the Delaware Basin, southeastern New Mexico. Three groups of tests are discussed to identify the relative and site‐specific importance of deviator stress, confining pressure (mean stress), temperature, time (loading rate), and stress path. The three groups of experiments consist of (1) hydrostatic loading, (2) triaxial compression tests (σ1>σ2= σ3= const), and (3) variable stress path tests including experiments at approximately constant σ1and at constant mean stress. The range of parameters considered was 0 ≤p(hydrostatic pressure) ≤ 34.5 MPa, 0 ≤ σ2= σ3≤ 20.7 MPa, 0 ≤ (σ1− σ3) ≤ 62 MPa, 23 ≤T(temperature) ≤ 200°C, and 1.7 ≤d(σ1− σ3)/dt≤ 3.5 kPa/s. All data were generated on 100‐mm‐diameter specimens. The rock salt exhibited nonlinear response under all loading coditions, a low initial elastic limit (σ1− σ3) ≤ 0.7 MPa, and an apparent inseparability of permanent deformations into time‐independent and time‐dependent components. Pressure and temperature did not alter the elastic constants over the range 29.6 ≤E≤ 36.5 GPa and 0.17 ≤v≤ 0.26, depending on core lithology. Pressure and temperature affected the principal strain ratio, the ratio of volumetric to shear strain, rock salt ductility, and the ultimate stress. In particular, low pressure and temperature permitted pronounced dilatancy and loss in load bearing ability. Under such conditions the volumetric strains reached sizeable fractions of the shear strains, ∣∈/(∈1− ∈3)∣ ≥ 0.2. Pressure remained important even at 200°C temperature because it influenced the rate of shearing at constant loading rates. Load path and stress history may be signi

ISSN:0148-0227    

DOI:10.1029/JB085iB02p00891

年代:1980

数据来源: WILEY

摘要:

The flexure profile of the Kurile trench‐Hokkaido rise system has been fitted numerically to observational accuracy by an elastic, time‐dependent plastic plate model. The elastic part of the constitutive relation is derived from seismology (relaxed moduli = 56% of the seismic values), while the strain‐rate‐dependent plastic part is derived from dunite deformation data extrapolated to the appropriate loading rates. The numerical fit of the flexure profile depends on a number of parameters such as the rock flow law parameters, the temperature distribution within the lithosphere, and the state of prestress of the plate before it enters the subduction zone. Two examples of fitting, one with Kirby‐Raleigh flow law parameters and one with Carter‐Avé Lallement flow law parameters, are given. The oceanic crust is considered to be made of strong and competent rocks in the former example, and it is considered to be made of fractured rocks with negligible strength in the latter. Because the solution is nonunique, conclusions regarding the values of the individual parameters cannot be drawn from the flexure profile alone. The present calculation, however, demonstrates the consistency between the rock deformation data and the observed plate fle

ISSN:0148-0227    

DOI:10.1029/JB085iB02p00901

年代:1980

数据来源: WILEY

摘要:

In terms of shock‐compression characteristics, minerals of geophysical interest are placed in a class of materials which, because of some rather unique Hugoniot and shock‐compression properties, are classified as brittle solids. The physical processes responsible for these shock‐wave properties are not yet well established. However, recent experimental observations and theories based on localized, nonhomogeneous shear deformation and a transient nonuniform thermal state appear to provide a reasonable qualitative picture. In the present work, some of the terminal and transient shock‐compression features observed in brittle solids are reviewed with particular emphasis on Hugoniot release‐wave measurements. The possibility of instabilities in the laws governing shear deformation leading to observed heterogeneous deformation is considered, and although an elementary model is treated, the method shows promise of predicting the material and kinematic properties governing occurrence, growth, and frequency of localized deformation features. Transient stress wave calculations in crystalline quartz demonstrate how the kinematic environment governing instability growth is established under shock‐wave compression. In addition, the transient nonuniform thermal state resulting from heterogeneous deformation is shown to provide a possible explanation for the observation of both ‘fluidlike’ and ‘solidlike’ shock release waves depending on the competing properties of thermal diffusion, melting temperature, and degree of thermal localization. The analysis shows a striking difference between those minerals which do, and do not, undergo a shock‐induced phase transition and leads to speculated similarities between the kinetics of shock‐induced phase transformation in brittle solids and the kinetics of thermal d

ISSN:0148-0227    

DOI:10.1029/JB085iB02p00913

年代:1980

数据来源: WILEY

摘要:

UltrasonicPandSwaveQvalues as functions of hydrostatic pressure are presented for the Berea and Navajo sandstones, Bedford limestone, and Colorado oil shale. In most cases the attenuation was obtained for both dry and water‐saturated samples.Qis determined by measuring the attenuation of the rock sample relative to a highQstandard of identical geometry using Fourier spectral ratios. The data show thatQincreases with pressure, the rate of increase being dependent on rock type and crack density and distribution.Qsis about equal to or larger thanQpfor dry rocks and less thanQpfor saturated rocks. In all cases,Qfor saturated rocks are lower than for dry samples. The attenuation in the oil shale shows extreme anisotrop

ISSN:0148-0227    

DOI:10.1029/JB085iB02p00925

年代:1980

数据来源: WILEY