摘要:

This special section is a direct outgrowth of an AGU Chapman Conference on Fault Behavior and the Earthquake Generation Process that was convened in Snowbird, Utah, from October 11 to 15, 1982. The body of information concerning the behavior of faults in space and time, especially with regard to earthquake recurrence, fault zone geometry, and the mechanical and physical properties of fault zones that control rupture processes has been increasing at an enormous rate. And, perhaps more importantly, our understanding of these subjects has direct societal application in the mitigation of earthquake hazards, especially the identification of potential seismogenic sources and the characterization of these sources in terms of quantitative estimates of the location and probability of earthquake occurrence. The motivation behind the conference was the desire to pause and reflect on the recent progress in the field and to evaluate, and perhaps identify, potentially important new areas of research.

ISSN:0148-0227    

DOI:10.1029/JB089iB07p05669

年代:1984

数据来源: WILEY

摘要:

Geodetic and geologic observations from a handful of well‐studied great plate boundary earthquakes provide a basis for exploring those features of the deformation cycle having the strongest influence on recurrence estimation. In ideal circumstances the time‐predictable model requires knowledge of only the seismic slip (and time) of the last earthquake and the rate of relative plate motion or fault slip. Practically, indirect measurements must usually suffice, local coseismic strain changes rather than seismic slip and local deformation rate rather than slip rate. Permanent, nonrecoverable deformation, often roughly half the coseismic strain offset near great thrust earthquakes, complicates the inference of interplate seismic slip. Owing to short‐ and long‐term postseismic transients, commonly 20–40% of the coseismic strain drop, deformation rates are variable and not simply related to the plate motion rate. At strike‐slip plate boundaries these complications can sometimes be avoided and seismic slip and slip rate can be obtained more or less directly. When these favorable circumstances are absent and when knowledge of the deformation cycle is incomplete, recurrence accuracy can be improved by empirically correcting for unknown elements of the cycle. Systematic features in the spatial distribution of the transients and permanent deformation near subduction zones help identify the regions where these corrections are largest and where the shortcomings of the empirical approach are

ISSN:0148-0227    

DOI:10.1029/JB089iB07p05674

年代:1984

数据来源: WILEY

摘要:

Paleoseismological data for the Wasatch and San Andreas fault zones have led to the formulation of the characteristic earthquake model, which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum. Analysis of scarp‐derived colluvium in trench exposures across the Wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes. At all of the sites studied, the displacement per event has been consistently large; measured values range from 1.6 to 2.6 m, and the average is about 2 m. On the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry, six major segments are recognized along the Wasatch fault. On the basis of the most likely number of surface faulting events (18) that have occurred on segments of the Wasatch fault zone during the past 8000 years, an average recurrence interval of 400–666 years with a preferred average of 444 years is calculated for the entire zone. Geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south‐central segment of the San Andreas fault suggest that theM8 1857 earthquake is a characteristic earthquake for this segment. Comparisons of earthquake recurrence relationships on both the Wasatch and San Andreas faults based on historical seismicity data and geologic data show that a linear (constantbvalue) extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes. Only by assuming a lowbvalue in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes. The characteristic earthquake appears to be a fundamental aspect of the behavior of the Wasatch and San Andreas faults and may apply to many other faults as

ISSN:0148-0227    

DOI:10.1029/JB089iB07p05681

年代:1984

数据来源: WILEY

摘要:

The geometry of brittle shear fracture zones has been studied by the examination of some strike‐slip faults and shear joints in China. Pinnate and en echelon deformational patterns are two fundamental arrangements of fractures within brittle shear fracture zones. The pinnate angle is usually less than 25° and the en echelon is more than 25°. The deformation in the “rock bridge,” the area between two secondary fractures, depends on the arrangements of secondary fractures and the sense of shear. Some tensile ruptures are formed at the terminations of both primary shear fracture zones and secondary fractures. The direction of a tensile rupture is commonly parallel to the orientation of the principal compression. Along the direction of propagation of the shear fracture, the horizontal displacement is large, and the attenuation of displacements is slow. The maximum horizontal displacement of the shear zone may be located in the region of the initial fracture. Under certain conditions the shear fracture zones may possess pivotal movement. This type of movement may be expressed by the behavior of the fault or by the geomorphology of the fau

ISSN:0148-0227    

DOI:10.1029/JB089iB07p05699

年代:1984

数据来源: WILEY

摘要:

Within the boundary zone between the Caribbean‐South America plates, which includes the Boconó‐Morón‐El Pilar right‐lateral fault system that has been active since the late Tertiary, there are several sedimentary basins in the Venezuelan Andes and the Caribbean Mountains. These were formed in zones of crustal depression, where faults diverge, and as pull‐apart basins in zones of en echelon stepovers (or along releasing bends) in the faults. In addition, there are pull‐apart basins along the Avila and La Victoria faultzones in the Caribbean Mountains, which are associated with the plate boundary. The basins are filled with alluvial, lacustrine, fluvial, and marine sediments, mainly of Quaternary age. On the basis of model studies, dimensions of the basins, and age of their sedimentary fill, it is estimated that right‐lateral offset necessary to form them along the Boconó‐Morón‐El Pilar fault system varies from 2 to 125 km. Along the Avila and La Victoria fault zones, this right‐lateral offset is estimated to range from 2 to 30 km. On this basis, the maximum right‐lateral offset along the southern Caribbean plate boundary that can be documented along the Boconó‐Morón‐El Pilar fault system has not exceeded 100–125 km. If there has been larger offset, it has to be documented by other means, or it has taken place along ot

ISSN:0148-0227    

DOI:10.1029/JB089iB07p05711

年代:1984

数据来源: WILEY

摘要:

We present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet, or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates. We compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal. A moving‐window analysis of the entire seismograms, including the coda, gives δ(t), the difference in arrival times versus running time along the seismogram. The time resolution of the method is an order of magnitude better than the digitization interval. The δ(t) technique is illustrated with a pair of microearthquakes,M= 1.7 and 2.0, that occurred before and after the Coyote Lake, California, earthquake (M= 5.9) of August 6, 1979, and on the same segment of the Calaveras fault that ruptured during the earthquake. The coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet, so that its seismogram appears as a stretched version of the earlier event. We interpret this systematic variation in δ(t) along the coda as a change in the averageSvelocity in the upper crust in the time interval between the two doublets.Swave velocities appear to have decreased by 0.2% in an oblong region 5–10 km in radius at the south end of the aftershoc

ISSN:0148-0227    

DOI:10.1029/JB089iB07p05719

年代:1984

数据来源: WILEY

摘要:

The general lack of a correlation between earthquakes and surface faulting in the interior of the western U.S. cordillera has motivated our efforts to evaluate the geometry, structural style, and mechanism of normal faulting characteristic of this region of intraplate extension. To address the problem, we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust. Rheological models of the continental crust were calculated to examine the possibility of shallow, quasi‐plastic flow and its influence on faulting in the eastern Basin‐Range, the western Colorado Plateau, and the Middle Rocky Mountains. Our data and interpretations have revealed the following styles of Cenozoic deformation: (1) steep‐ to low‐angle dip, normal faulting along the Wasatch fault, (2) low‐angle dip and listric normal faulting possibly associated with movement on preexisting thrusts, (3) the occurrence of asymmetric, mostly eastward tilted Tertiary basins that are bounded by low‐ to moderate‐dipping planar and listric faults, and (4) at least three vertically stacked, en echelon low‐angle reflections in the mid to upper crust that dip gently westward from ∼3 km beneath the Wasatch Plateau to over ∼15 km at the Utah‐Nevada border; these reflections are interpreted as normal detachment faults. The structural style of the pervasive low‐ to moderate‐angle dipping faults cannot be easily reconciled with classic brittle failure theory, but the interpreted termination of normal faults at or above the deeper low‐angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip. An important observation, based on interpretations of seismic reflection profiles, is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits. Segment boundaries of the Wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the Precambrian Uinta aulacogen, suggesting that preexisting crustal structure partly controls the geometry of extension. To examine the influence of ductile deformation, quasi‐plastic flow was modeled for appropriate geotherms of the Basin‐Range and Colorado Plateau crusts. These models provide constraints on the depth to the frictional/quasi‐plastic transition that occurs as shallow as ∼8 km in the eastern Basin‐Range. This depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes (M5.5) appear to nucleate at 10–15 km in or near the brittle/ductile transition. The rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region. Large‐scale low‐angle reflections may represent decollements and normal faults at the top of upper crustal zones of r

ISSN:0148-0227    

DOI:10.1029/JB089iB07p05733

年代:1984

数据来源: WILEY

摘要:

Large‐scale surface faulting events, each tens of kilometers long and involving more than a meter of displacement, of late Quaternary age have not been uniformly distributed but have been concentrated in subprovincial and smaller belts and areas within the Great Basin province. Furthermore, faulting apparently has not been uniformly distributed over time. The average recurrence interval of large‐scale faulting events on individual faults in the province is measured in thousands or tens of thousands of years. In contrast to such long recurrence intervals, in the central Nevada and eastern California seismic belts, large‐scale faulting events have occurred during the last 111 years in an apparently coherent, sequential belt‐filling pulse of activity in which events were separated by only a few years or a few decades. The questions are, where are other pulses of large‐scale faulting likely to occur and what controls the localization of such belts and areas characterized by higher rates of faulting? Two examples of tectonic subprovinces that may play a role in delimiting smaller zones of faulting are the Black Rock‐Carson Sink zone of extension and the central Nevada downwarp. The central Nevada seismic belt lies along the dividing line between these two subprovinces, and the northwest margin of the Black Rock‐Carson Sink zone of extension coincides with the margin of a fingerlike belt of dense faulting in northw

ISSN:0148-0227    

DOI:10.1029/JB089iB07p05763

年代:1984

数据来源: WILEY

摘要:

From a casual observation that the form of degraded fault scarps resembles the error function, this investigation proceeds through an elementary diffusion equation representation of landform evolution to the application of the resulting equations to the modern topography of scarplike landforms. The morphologic observations can be analyzed either in the form of one or more cross‐strike elevation profiles or in the form of the slope‐offset plot, a point plot of maximum scarp slope versus scarp offset. Working with either or both of these data representations for nine geologic structures, which range in age from 3 to 400 ka B.P. and in offset from 1 to 50 m, we apply analytical solutions for the vertical initial value scarp, the vertical continuous offset scarp, and the finite slope, initial value scarp. The model calculations are intrinsically ambiguous, yielding as the final answer only the product κt(in the case of the initial value problem) or the product κA−1(in the case of the repeated faulting problem); heretis the age of a single scarp‐forming event, 2Ais the vertical slip rate, and κ is the “mass diffusivity.” A single profile across three sea cliffs along the Santa Cruz, California, coast is analyzed as three separate initial value problems. A reasonably constrained age for the sea cliff standing above the Highway 1 platform returns κ = 11 GKG (1 GKG = 1 m2/ka). With this κ, we can date the two older sea cliffs. In fact, we do the converse: age estimates for these two older sea cliffs based on a uniform rate of uplift both yield the same κ as for the lower sea cliff. We treat a single profile of the Raymond fault in Pasadena/San Marino in terms of the repeated faulting problem; for it the uplift rate of R. Crook and others yields κ = 16 GKG. The very substantial preexisting offset across the Raymond fault must have been buried/leveled some 230 ka B.P., when the modern topography began to form. Our analysis of the Lake Bonneville shoreline scarps reveals a dependence of κton 2a, suggestive of nonlinear modification processes. This appearance is treated with the finite slope initial value scarp model to determine κ=1.1 GKG for the Lake Bonneville shoreline scarps. The suggestion of M. N. Machette that approximately 100,000‐year‐old, meter‐high scarps are “unobservable” in weakly consolidated alluvial terranes of the Basin and Range and Rio Grande Rift Valley provinces can be formulated as κ ≳ 1 GKG. The coincidence between this inequality and the Lake Bonneville shoreline κ is striking, and it suggests that the value of κ = 1 GKG may be generally applicable, as a good first approximation, to the modification of alluvial terranes within the semiarid regions of the western United States. The Lake Bonneville shoreline κ is the basis for dating four sets of fault scarps in west‐central Utah. The Drum Mountains fault scarps can be modeled in several different circumstances, but the most likely interpretation is that these fault scarps formed as the result of a single episode of normal faulting 3.6 to 5.7 ka B.P. The younger age is associated with quite low initial slope angles (25°). The other three sets of fault scarps show no evidence for finite initial value slopes. Fault scarps along the eastern base of the Fish Springs Range are very young, 3 ka B.P. We estimate the age of fault scarps along the western flank of the Oquirrh Mountains to be 32 ka B.P., which meets the weak geologic constraint that they be older than the Lake Bonneville shoreline. Fault scarps along the northeastern margin of the Sheeprock Mountains are even older, 53 ka B.P. An intriguing consequence of our single‐event analysis of these scarps is that an 11.5

ISSN:0148-0227    

DOI:10.1029/JB089iB07p05771

年代:1984

数据来源: WILEY

摘要:

The cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300°C. It can generally be modeled as the transition from frictional to quasi‐plastic behavior in quartzofeldspathic crust. Shear resistance increases with depth through the frictional regime to peak at the transition, beneath which it falls off exponentially with increasing temperature. Larger earthquake ruptures (ML>5.5) nucleate around this transition depth where the highest concentrations of strain energy may accumulate. Varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone. Factors affecting the depth of the transition include crustal composition, geometry and mode of faulting, fluid pressure levels in the frictional regime, and water content in the quasi‐plastic regime, quasi‐plastic strain rate, and geothermal gradient. Evaluation of their relative importance is complicated because several are interdependent. However, compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance, while regional variations in heat flow look to be particularly effective in creating long‐wavelength heterogeneities in strain energy concentration affecting faulti

ISSN:0148-0227    

DOI:10.1029/JB089iB07p05791

年代:1984

数据来源: WILEY