摘要:

We determine the deformation produced by the lunisolar tidal potential in a rotating, spheroidal model Earth. We proceed by decomposing the equations of motion into separate, though coupled, equations for the nutational and deformational parts of the Earth's response. Using this scheme, we derive a simpler set of equations for the deformational displacements, where the driving forces include not only the tidal terms but also inertial forces and gravitational perturbations associated with the nutational motions. We show that the deformations are affected only to a very small extent by the Earth's asphericity and rotation. This fact is exploited to set up a perturbative procedure, whereby the equation governing the deformation is separated into equations of zeroth and first orders in the perturbation. In the initial calculation (the zeroth order), the influences of the Earth's asphericity and the inertial forces associated with the deformation are neglected, while the forces arising from the nutational motions are taken into account. The resulting calculation for the quasi‐static deformation is equivalent to the so‐called spherical approximation used by Sasao et al. (1980), although the solutions obtained here are physically more insightful. This zeroth‐order calculation is used to determine the compliances defined in the work of Mathews et al. (1991a), which characterize the deformability of the Earth. In the second step of the calculation, the solutions obtained under the spherical approximation are used to determine corrections to the deformation for the omitted effects of ellipticity and inertia (including the Coriolis force). Corresponding corrections to the zeroth‐order compliances used by Mathews et al. (1991b) are found to be nominallyO(ϵ) smaller than the zeroth‐order compliances, where ϵ is the geometric ellipticity (surface flattening) of the Earth. As a consequence of these corrections to the compliance parameters, changes in the nutation amplitudes as computed by Mathews et al. (1991b) are produced, which amount to −0.18, 0.46, and 0.26 milliarcseconds, in the prograde semiannual, and the retrograde annual and 18.6‐year terms, respectively. Additional corrections are introduced if we require the theoretical value of the retrograde annual nutation to match the determination made using very long baseline interferometry. The procedure presented here to account for the effects of ellipticity and rotation could also be used to determine corrections to nutations for the effects of anelasticity in the mantle and inner core or for the effect; of lateral heterogeneity in the Earth's density and ela

ISSN:0148-0227    

DOI:10.1029/92JB01339

年代:1993

数据来源: WILEY

摘要:

We estimate the velocity field in central and southern California using Global Positioning System (GPS) observations from 1986 to 1992 and very long baseline interferometry (VLBI) observations from 1984 to 1991. Our core network includes 12 GPS sites spaced approximately 50 km apart, mostly in the western Transverse Ranges and the coastal Borderlands. The precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95% confidence ellipse with a semiminor axis of approximately 2 mm/yr oriented roughly north‐south, and a semimajor axis of approximately 3 mm/yr oriented east‐west. For other stations, occupied fewer than 5 times, or occupied during experiments with poor tracking geometries, the uncertainty is larger. These uncertainties are calibrated by analyzing the scatter in three types of comparisons: (1) multiple measurements of relative position (“repeatability”), (2) independent velocity estimates from separate analyses of the GPS and VLBI data, and (3) rates of change in baseline length estimated from the joint GPS+VLBI solution and from a comparison of GPS with trilateration. The dominant tectonic signature in the velocity field is shear deformation associated with the San Andreas and Garlock faults, which we model as resulting from slip below a given locking depth. Removing the effects of this simple model from the observed velocity field reveals residual deformation that is not attributable to the San Andreas fault. Baselines spanning the eastern Santa Barbara Channel, the Ventura basin, the Los Angeles basin, and the Santa Maria Fold and Thrust Belt are shortening at rates of up to 5 ± 1, 5 ± 1, 5 ± 1, and 2 ± 1 mm/yr, respectively. North of the Big Bend, some compression normal to the trace of the San Andreas fault can be resolved on both sides of the fault. The rates of rotation about vertical axes in the residual geodetic velocity field differ by up to a factor of 2 from those inferred from paleomagnetic declinations. Our estimates indicate that the “San Andreas discrepancy” can be resolved to within the 3 mm/yr uncertainties by accounting for deformation in California between Vandenberg (near Point Conception) and the westernmost Basin and Range. Strain accumulation of 1–2 mm/yr on structures offshore of Vandenberg is also allowed by the uncertainties. South of the Transverse Ranges, the deformation budget must include 5 mm/yr between the offshore islands

ISSN:0148-0227    

DOI:10.1029/93JB02405

年代:1993

数据来源: WILEY

摘要:

Five years of measurements from the Global Positioning System (GPS) satellites collected between 1986 and 1991 are used to investigate deformation in the offshore regions of southern California. GPS provides the first practical technique to make precise geodetic measurements in the region. The geodetic network is situated along the California coastline from Vandenberg (120.6°W, 34.6°N) to San Diego, with additional sites on Santa Cruz, San Nicolas, Santa Catalina, Santa Rosa, and San Clemente Islands. The precision of horizontal interstation vectors is subcentimeter, and the interstation vector rate between OVRO and Vandenberg agrees with the very long baseline interferometry derived rate to within one standard deviation. No significant motion is observed in the western Santa Barbara Channel between Vandenberg and Santa Rosa Island, 0.5 ± 1.6 mm/yr, where the quoted uncertainties are one standard deviation. Motions in the eastern Santa Barbara Channel are consistent with compressional deformation of 6 ± 1 mm/yr at N16 ± 3°E. This motion is in agreement with seismicity and an independent geodetic analysis for the period 1971–1987 (Larsen, 1991). San Clemente Island is moving relative to San Diego at the rate of 5.9 ± 1.8 mm/yr at a direction of N38 ± 20°W. The motion between San Nicolas Island and San Clemente Island, 0.8 ± 1.5 mm/yr, is in

ISSN:0148-0227    

DOI:10.1029/92JB02116

年代:1993

数据来源: WILEY

摘要:

We have measured the deformation in the Ventura basin region, southern California, with Global Positioning System (GPS) measurements carried out over 4.6 years between 1987 and 1992. The deformation within our network is spatially variable on scales of tens of kilometers, with strain rates reaching 0.6 ± 1 μrad/yr in the east‐central basin. Blocklike rotations are observed south and northwest of the basin where the maximum shear strain rates are an order of magnitude lower (0.06 ± 1 μrad/yr to the south). We also observed clockwise rotations of 1°–7°/m.y. Shear strain rates determined by comparing angle changes from historical triangulation spanning several decades and GPS measurements give consistent, though less precise, results. The geodetic rates of shortening across the basin and Western Transverse Ranges are lower than those estimated from geological observations, but the patterns of deformation from the two methods agree qual

ISSN:0148-0227    

DOI:10.1029/93JB02766

年代:1993

数据来源: WILEY

摘要:

Dramatic coastline changes demonstrate rapid Quaternary uplift of Calabria in southern Italy. Because most of the west (Tyrrhenian Sea) coast is normal fault bounded, previous work has asserted that its uplift is local footwall uplift related to extension. However, the east (Ionian Sea) coast is also uplifting but is not normal fault bounded. This reanalysis, based on original fieldwork and reinterpretation of the literature, reaches the following conclusions. First, although radiometric dating of only one marine terrace sequence in Calabria is available, on the east coast at Crotone, terraces occur at similar elevations, and can thus be correlated, throughout this coast and the west coast of northern and central Calabria. Uplift rate at both coasts and across the region in between is the same, 1.0 ± 0.1 mm yr−1, provided my correlations are correct. Second, the oldest raised shorelines date from the 0.9 Ma marine highstand and have uplifted ∼700 m since ∼0.7 Ma. Regional uplift rate earlier was minimal, possibly zero. Third, localities on this west coast show marginally (up to ∼5%) higher uplift rates, indicating some local footwall uplift (at ∼0.05 mm yr−) associated with slow extension (at ∼0.1 mm yr−1). Fourth, the 12‐m Holocene marine terrace formed around 7 ka. Allowing for a 3‐m range of wave and tidal action, it is interpreted as the result of 7 m of tectonic uplift and 2 m of eustatic sea level fall during ∼6–4 ka. Finally, the west coast of southern Calabria shows significant elevation changes caused by active normal faulting, as well as regional uplift. Footwall localities have uplifted up to 1300 m since 0.9 Ma. The estimated maximum present‐day uplift rate of 1.67 mm yr−1is interpreted as 1 mm yr of regional uplift plus 0.67 mm yr of local footwall uplift, indicating much faster extension than farther north. It is suggested that the Tyrrhenian Benioff zone detached from beneath Calabria shortly before 0.7 Ma. The regional uplift of the overlying landmass is thus explained as its transient isostatic response to removal of this load. Order‐of‐magnitude calculations suggest that 1 mm yr−1uplift rate is reasonable, with ∼2 km total uplift expected, indicating that uplift will continue for>∼1 Myr into the future. Extension of Calabria appears to have begun at ∼11 Ma, at the same time as formation of the Tyrrhenian Sea and the related subduction at its Benioff zone. Extension rate in southern Calabria abruptly increased from ∼0.1 to ∼1 mm yr−1around 0.9–0.7 Ma., when the slab detached and the regional uplift began; extension in northern and central Calabria

ISSN:0148-0227    

DOI:10.1029/93JB01566

年代:1993

数据来源: WILEY

摘要:

The northern piedmont of the western Kunlun mountains (Xinjiang, China) is marked at its easternmost extremity, south of the Hotan‐Qira oases, by a set of normal faults trending N50E for nearly 70 km. Conspicuous on Landsat and SPOT images, these faults follow the southeastern border of a deep flexural basin and may be related to the subsidence of the Tarim platform loaded by the western Kunlun northward overthrust. The Hotan‐Qira normal fault system vertically offsets the piedmont slope by 70 m. Highest fault scarps reach 20 m and often display evidence for recent reactivations about 2 m high. Successive stream entrenchments in uplifted footwalls have formed inset terraces. We have leveled topographic profiles across fault scarps and transverse abandoned terrace risers. The state of degradation of each terrace edge has been characterized by a degradation coefficient τ, derived by comparison with analytical erosion models. Edges of highest abandoned terraces yield a degradation coefficient of 33 ± 4 m2. Profiles of cumulative fault scarps have been analyzed in a similar way using synthetic profiles generated with a simple incremental fault scarp model. The analysis shows that (1) rate of fault slip remained essentially constant since the aggradation of the piedmont surface and (2) the occurrence of inset terraces was synchronous at all studied sites, suggesting a climate‐driven terrace formation. Observation of glacial and periglacial geomorphic features along the northern front of the western Kunlun range indicates that the Qira glaciofluvial fan emplaced after the last glacial maximum, during the retreat of the Kunlun glaciers (12–22 ka). The age of the most developed inset terrace in uplifted valleys is inferred to be 10 ± 3 ka, coeval with humid climate pulses of the last deglaciation. The mass diffusivity constant (k=τ/T, being time B.P.) in the Hotan region is determined to be 3.3 ± 1.4 m2/103years, consistent with other estimates in similar climatic and geologic environments of western China. These results imply a minimum rate for the Tarim subsidence of 3.5 ± 2 mm/yr. If Western Kunlun overthrusts the Tarim platform on a crustal ramp dipping 40°–45° to the south, it would absorb at least 4.5 ± 3 mm/yr of convergence between weste

ISSN:0148-0227    

DOI:10.1029/93JB02172

年代:1993

数据来源: WILEY

摘要:

A statistical physics model is used to simulate antiplane shear deformation and rupture of a tectonic plate with heterogeneous material properties. Rupture occurs when the chosen state variable reaches a threshold value. After rupture, broken elements are instantaneously healed and retain the original material properties. We document the spatiotemporal evolution of the rupture pattern in response to a constant velocity boundary condition. A fundamental feature of this model is that ruptures become strongly correlated in space and time, leading to the development of complex fractal structures. These structures, or “faults,” are simply defined by the loci where deformation accumulates. Repeated rupture of a fault occurs in events (“earthquakes”) which themselves exhibit both spatial and temporal clustering. Furthermore, we observe that a fault may be active for long periods of time until the locus of activity spontaneously switches to a different fault. The formation of the faults and the temporal variation of rupture activity is due to a complex interplay between the random small‐scale structure, long‐range elastic interactions, and the threshold nature of rupture physics. The characteristics of this scalar model suggest that spontaneous self‐organization of active tectonics does not result solely from the tensorial nature of crustal deformation; that is, kinematic compatibility is not required for complex fault pattern formation. Furthermore, the localization of the deformation is a dynamical effect rather than a consequence of preexisting structure or preferential weakening of faults compared to the surrounding medium. We present an analysis of scaling relationships exhibited by the fault pattern and the earthquakes

ISSN:0148-0227    

DOI:10.1029/93JB02223

年代:1993

数据来源: WILEY

摘要:

The 1989 Loma Prieta, California, earthquake is characterized by the lack of major, throughgoing, coseismic, right‐lateral faulting along strands of the San Andreas fault zone in the epicentral area. Instead, throughout the Summit Ridge area there are zones of tension cracks and left‐lateral fracture zones oriented about N45°W, that is, roughly parallel to the San Andreas fault in this area. The left‐lateral fractures zones are enigmatic because their left‐lateral slip is opposite to the right‐lateral sense of the relative motion between the Pacific and North American plates. We suggest that the enigmatic fractures can be understood if we assume that coseismic deformation was by right‐lateral shear across a broad zone, about 0.5 km wide and 4 km long, beneath Summit Ridge. The shear zone has an orientation of about N70° W and is oblique to the San Andreas fault zone, which has a trend of N45° to 50°W, so that right‐lateral shearing, along with some dilation, can account for the orientation of the fracture zones. Using an analog, kinematic model of the Summit Ridge shear zone and the observable geometric parameters (a shear zone about 0.5 km wide, an acute angle of 25° between the tension cracks and the shear‐zone walls, left‐lateral slip of 5 cm, and spacing of about 100 m in the tension cracks) we estimate that the amount of right‐lateral shift across the Summit Ridge shear zone was on the order of 1.4 m. This is comparable to the amount of slip for coseismic faulting at depth, 1.9 to 2.4 m, as computed by several investigators. Thus contrary to most previous reports on the Loma Prieta earthquake, which assert that coseismic, right‐lateral ground rupture was restricted to considerable (greater than 4 km) depths in the epicentral area, we find that nearly all the right‐lateral offset is represented at the ground surface by

ISSN:0148-0227    

DOI:10.1029/93JB02385

年代:1993

数据来源: WILEY

摘要:

Both volcanism and faulting contribute to the rugged topography that is created at the Mid‐Atlantic Ridge (MAR) and preserved off‐axis in Atlantic abyssal hill terrain. Distinguishing volcanic from fault‐generated topography is essential to understanding the variations in these processes and how these variations are affected by the three‐dimensional pattern of mantle upwelling, ridge segmentation, and offsets. Here we describe a new quantitative method for identifying fault‐generated topography in swath bathymetry data by measuring topographic curvature. The curvature method can distinguish large normal faults from volcanic features, whereas slope methods cannot because both faults and volcanic constructs can produce steep slopes. The combination of curvature and slope information allows inward and outward facing fault faces to be mapped. We apply the method to Sea Beam data collected along the MAR between 28° and 29°30′N. The fault styles mapped in this way are strongly correlated with their location within the ridge segmentation framework: long, linear, small‐throw faults occur toward segment centers, while shorter, larger‐throw, curved faults occur toward ends; these variations reflect those of active faults within the axial valley. We investigate two different physical mechanisms that could affect fault interactions and thus underlie variations in abyssal hill topography at the MAR. In the first model only one fault is active at a time on each side of the rift valley. Each fault grows while migrating away from the volcanic center due to dike injection; extension across the fault causes a flexural rotation of nearby inactive faults. The amount of stress necessary to displace the fault increases as the fault grows. When reaching a critical size the fault stops growing as fault activity jumps inward as a new fault starts its growth near the rift valley. This model yields a realistic terracelike morphology from the rift valley floor into the rift mountains; the relief is caused by the net rotation accumulated in the lithosphere from the active faults (e.g., 10° reached 20 km from the active fault). Fault spacing is controlled by lithospheric thickness, fault angle, and the ratio of amagmatic to magmatic extension. We hypothesize that this mechanism may be dominant toward ridge segment offsets. An alternative model considers multiple active faults; each fault relieves stresses as it grows and inhibits the growth of nearby faults, causing a characteristic fault spacing. Such fault interactions would occur in a region of necking instability involving deformation over an extended area. This mode of extension would drive a feedback mechanism that would act to regulate the size of nearby faults. We hypothesize that this mechanism may be active in the relatively weak regions of strong mantle upwelling near segment midpoints, causing the homogeneous abyssal hill fabri

ISSN:0148-0227    

DOI:10.1029/93JB01565

年代:1993

数据来源: WILEY

摘要:

Repeated geodetic measurements with the Global Positioning System (GPS) provide direct measurements of displacements due to plate motions and active crustal deformation in Central America and northern South America, an area of complex interaction of the Nazca, Cocos, Caribbean and South American plates. The displacement rates for the period 1988–1991, obtained from the results of the first three Central And South America (CASA) GPS campaigns, are in general agreement with the predictions of the NUVEL‐1 plate motion model, but there are differences in detail between the observations and the model. The Nazca‐North Andes convergence rate vector measured by GPS is different from the NUVEL‐1 vector at 95% confidence. The difference implies that the North Andes are moving northward relative to South America. The measured convergence between the Caribbean plate and the North Andes suggests that the southern margin of the Caribbean plate is located in the South Caribbean deformed belt. The April 1991 Costa Rica earthquake and the Cocos‐Caribbean convergence rate determined by GPS suggest the possibility of significant ongoing deformation between Central America and the stable interior of the Caribbean plate. Our GPS results are consistent with deformation of the overriding plates at the convergent margins of Central and South America and confirm that active convergence is occurring around much of the southern margin of the Caribbean plate, from Colombia west to Costa Rica. Costa Rica and Panama are not part of the stable Caribbean plate. Instead, the South Caribbean deformed belt and the North Panama fold belt probably represent the southern margin of the Caribb

ISSN:0148-0227    

DOI:10.1029/93JB00520

年代:1993

数据来源: WILEY