摘要:

Procedures of the theory of extremes give unacceptably large probable errors in determinations of return times, b values, and maximum magnitudes of large‐ and intermediate‐magnitude earthquakes using limited runs of data. On all accounts, methods which utilize all available data give superior estimates of the parameters of seismicity than do extreme value methods and provide for a procedure for handling inhomogeneous catalogs as w

ISSN:0148-0227    

DOI:10.1029/JB082i036p05647

年代:1977

数据来源: WILEY

摘要:

Shear cracks with finite cohesive forces can propagate by skipping past barriers. The barriers left behind may remain unbroken or may eventually break because of subsequent increase in dynamic stress depending on the ratio of barrier strength to tectonic stress. This model can explain a variety of observations on rupture in the earth, including (1) segmentation of the fault or ruptured zone in earthquakes and rock bursts, (2) ripples in seismograms which cannot be explained by path effect, and (3) departure of the scaling law of the seismic spectrum from that based upon the similarity assumption. The model also explains why the simple uniform dislocation model sometimes works better than the crack model without barriers. It also predicts, contrary to common belief, that an earthquake with low average stress drop may generate relatively greater amounts of high‐frequency waves than an earthquake with high average stress drop. One important consequence of our barrier model is the possibility of predicting the occurrence of aftershocks by analyzing the source spectrum of the main shoc

ISSN:0148-0227    

DOI:10.1029/JB082i036p05658

年代:1977

数据来源: WILEY

摘要:

Relative teleseismic P wave residuals between Alaskan‐Aleutian stations from deep focus (>500 km) Fiji‐Tonga earthquakes are examined in detail. At large station separations the effects of hypocenter mislocation become important. When both sources and stations are widely separated, we observe effects produced by differences between travel time tables, by plate structure beneath the stations, by deep expression of the descending plate at the source, and/or by lateral variations in lower mantle structure. When either source or station separations are small, these effects are considerably reduced, so that relative residuals between stations can be resolved to within ±0.1 s. Average levels of relative residuals between Alaskan stations can be explained to first order by a plate structure beneath Al

ISSN:0148-0227    

DOI:10.1029/JB082i036p05671

年代:1977

数据来源: WILEY

摘要:

Continuous strain measurements from 3 three‐component invar wire strainmeters installed 1200, 1500, and 1700 m from the San Andreas fault indicate no observable strain change at the instrument resolution (<10−8) during 10 episodic creep events on the fault. These strain observations indicate that the slip area responsible for the creep observations is near surface and of quite limited extent. The episodic creep character probably results from the failure properties of near‐surface materials rather than general fault behavior, which is better indicated perhaps by averaged creep. Deeper slower slip apparently loads the surface material. Longer‐term strain changes (∼10−7) do occur, but the form of the signal is not what would be expected from simple models, nor is it consistent, for successive events. The amplitude does not increase with creep event amplitude, and similar changes occur without creep events. Deeper slip on the San Andreas fault apparently is smoother than would be inferred from the duration of episodic creep observations. Unfortunately, signal discrimination capability gets worse at longer periods and needs improvements if slow deformation waves are to be detected at strain levels

ISSN:0148-0227    

DOI:10.1029/JB082i036p05683

年代:1977

数据来源: WILEY

摘要:

Local earthquakes with magnitudes of ≳2.5 and within 20–50 km of tiltmeters along the San Andreas fault typically generate offsets in tilt, tilt seismograms, and impulsive tilt behavior at the time of the earthquake. The amplitudes and azimuths of the coseismic tilt offsets from local earthquakes observed at a small array of four instruments approximately 30 km south of Hollister, California, are compared to the amplitudes and azimuths predicted by a rectangular source, elastic half‐space dislocation model. Almost all observed coseismic offset amplitudes are 1–3 orders of magnitude larger than the predicted amplitudes. The predicted offset azimuths also are not in agreement with the observed azimuths. There is neither a uniform method of scaling the predicted amplitudes nor a constant rotation that may be applied to the predicted azimuths that will consistently reproduce the observed offsets. Errors in hypocenter location and fault plane orientation are not sufficient to explain the discrepancies between observations and predictions. Similar results occur for teleseismic earthquakes. A lack of agreement in the observed offset amplitudes across the array indicates that tilt changes are triggered at or near the instrument site by the passage of seismic waves. No significant agreement was obtained between the direction of coseismic tilts and either the secular tilt trends or local geologic features. Triggered movement on near‐surface cracks, fractures, and minor faults appears the most viable physical explanation for the observed offsets. Short‐base‐line near‐surface tiltmeters appear inadequate for measuring tilt displacement fields generated by local earthquakes. Deep borehole installations appear necessary for this measurement. Coseismic tilt transients appear to be caused by seismically induced water table perturbations near the

ISSN:0148-0227    

DOI:10.1029/JB082i036p05692

年代:1977

数据来源: WILEY

摘要:

The effects of temperature variations on the tilt measured by long fluid tube tiltmeters are examined in detail. Temperature effects are important both in instruments which measure tilt as changes in fluid level between the ends of a fluid‐filled tube and in instruments which measure tilt as changes in fluid pressure. Methods are outlined for reducing temperature effects in both types of tiltmeters. A new temperature‐compensating method for fluid pressure tiltmeters is demonstrated both in theory and in practice. The method requires that the fluid‐filled tube(s) connecting the pressure transducer to the end reservoir(s) be laid at a constant small angle to the horizontal. Tiltmeters which use a horizontal half‐filled fluid tube are recommended on the grounds of long‐term stability and freedom from temperatur

ISSN:0148-0227    

DOI:10.1029/JB082i036p05699

年代:1977

数据来源: WILEY

摘要:

The deformation behavior of quartz and feldspar has been studied in Westerly granite deformed dry at a constant strain rate of 10−6/s, confining pressures of 1.5–15 kbar, and temperatures of 25°–1000°C. Samples deformed at lower temperatures and pressures show throughgoing faults; those deformed at intermediate conditions show a combination of grain‐scale faults and plastic deformation; and those deformed at higher temperatures and pressures show plastic deformation with no faults of any scale. On a grain scale the deformation is inhomogeneous at all conditions because of the polyphase nature of the material. Detailed petrographic and transmission electron microscope (TEM) observations have been made of the deformed specimens. The fault gouge consists of very fine grained material which verges on being amorphous, but no evidence of melt was seen. In the regions away from fault zones, there is a transition from dominantly microcracking to dominantly dislocation glide and climb; this transition is primarily a function of temperature. Dislocation motion is thermally activated and is probably almost unaffected by pressure over the range investigated. Thus at low temperature the strain rate that can be produced by dislocation motion is limited, and the difference between this and the imposed strain rate must occur by microcracking. The way in which the microcracking accomplishes the deformation depends on pressure. At low pressures (<5 kbar) the microcracks link up to form a throughgoing fault after very low strain; at higher pressures (7.5–15 kbar) they produce only grain‐scale faults, ‘deformation bands,’ and undulatory extinction, and no throughgoing faults are formed after 15–20% shortening. At the laboratory strain rate of 10−6/s the transition from dominantly microcracking to dominantly dislocation motion occurs at approximately 300°–400°C for quartz and 550°–650°C for feldspar. When one allows for the slower natural strain rates and the presence of water, this grain‐scale brittle‐ductile transition may correspond to the limiting depth of

ISSN:0148-0227    

DOI:10.1029/JB082i036p05705

年代:1977

数据来源: WILEY

摘要:

The effective elastic moduli of a fluid‐saturated solid containing thin cracks depend on the degree of interconnection between the cracks. Three separate regimes may be identified: (1) dry (drained), in which fluid in cracks can flow out of bulk regions of compression, (2) saturated isobaric, in which fluid may flow from one crack to another but no bulk flow takes place, and (3) saturated isolated, in which there is no communication of fluid between cracks. Transitions between these cases involve fluid flow, resulting in dissipation of energy. Relaxation of shear stresses in viscous fluid inclusions also results in dissipation. Viscoelastic moduli are derived, by using a self‐consistent approximation, that describe the complete range of behavior. There are two characteristic frequencies near which dissipation is largest and the moduli change rapidly with frequency. The first corresponds to fluid flow between cracks, and its value can be estimated from the crack geometry or permeability. The second corresponds to the relaxation of shear stress in an isolated viscous fluid inclusion; its value may also be estimated. Variations of crack geometry result in a distribution of characteristic frequencies and causeQto be relatively constant over many decades of frequency. Fluid flow between cracks accounts for attenuation of seismic waves in water‐saturated rocks and attenuation observed in laboratory measurements on water‐saturated rocks and partially molten aggregates. Attenuation in a partially molten upper mantle is probably due to fluid flow between cracks, although grain boundary relaxation in an unmelted upper mantle could also account for the seismic low‐velocity zone. Grain boundary relaxation in the mantle may cause the long‐term shear modulus to be around 20% less than that measured from seismic o

ISSN:0148-0227    

DOI:10.1029/JB082i036p05719

年代:1977

数据来源: WILEY

摘要:

A total of 41 selectively oriented single crystals of olivine (Fo92) were deformed under uniaxial stresses of 100–1800 bars in the temperature range 1150°–1600°C. Under uniform stress no strain inhomogeneities were observed. For all orientations both strain rate and dislocation structure stabilized after 1–2% strain and remained stable to the highest strains achieved (40%). For all orientations, creep could be represented by a power law of the form= Aσ3.6where A varied with orientation by a factor of 50. Shape change and crystallographic rotation data for some orientations could only be accounted for by a substantial dislocation climb contribution to the strain rate. All specimens were decorated to show the dislocation structure.Supplement is available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document J77‐009. Payment must accom

ISSN:0148-0227    

DOI:10.1029/JB082i036p05737

年代:1977

数据来源: WILEY

摘要:

In a companion paper we have reported on high‐temperature creep data from 41 selectively oriented single crystals of olivine. We report here on a study of the dislocation structures in these specimens by a decoration technique. Forty‐two additional specimens were deformed to explore the dislocation multiplication process and glide mobility in the temperature range 1080°–1575°C and differential stress range 80 bars to 2.4 kbars. We report here on these tests as well. All specimens achieved what appeared to be steady state dislocation structures after only a few percent strain. These structures were distinct, repeatable, and diagnostic of particular orientations and slip systems. Under steady state conditions, statistical data for dislocation density, dislocation curvature, and subboundary spacing correlated strongly with differential stress and provided the basis for paleostress estimates of natural specimens. In the range of temperature and stress studied here, climb and glide mobilities were found to be comparable, and creep appeared to be under glide, or joint glide and climb

ISSN:0148-0227    

DOI:10.1029/JB082i036p05755

年代:1977

数据来源: WILEY