摘要:

The northward migration of the Mendocino Triple Junction is associated with complex faulting within the Gorda plate and in the convergent zone between the Gorda and North American plates. This region has experienced substantial recent large earthquake activity, and quantification of these faulting processes is essential for understanding the evolution of the triple junction. Using an empirical Green function deconvolution method for teleseismic and regional surface waves and body waves, we obtain relative source time functions for the April 25, 1992, Cape Mendocino thrust earthquake (Mw= 7.2), its two large strike‐slip aftershocks on April 26, 1992 (Mw= 6.5, 6.6), and two large strike‐slip events in the Gorda plate that occurred July 13, 1991 (Mw= 6.8), and August 17, 1991 (Mw= 7.1). The removal of propagation effects using empirical Green functions provides unusually detailed source rupture information and indicates that all the ruptures are less than 16 s in duration. Analyzing the directivity effects observed in the source time functions, we resolve rupture directions and corresponding fault orientations for the four largest events and place some constraints on the smaller aftershock. Significant differences in the rupture duration and stress drop are observed between the Cape Mendocino mainshock and its two largest aftershocks. The mainshock, which ruptured westward on a shallow dipping plane either on or paralleling the interplate contact between the Gorda and North American plates, had a smooth 9‐s‐long rupture with a 0.9–4.6 MPa static stress drop. The two largest aftershocks occurred within the uppermost mantle of the Gorda plate and have more complex ruptures with total durations of 14–15 s and static stress drops of less than 0.3 MPa, perhaps reflecting a difference between mantle and crustal earthquakes in this region. The aftershocks appear to have ruptured conjugate strike‐slip faults, accounting for differences in their damage patterns. The August 17, 1991, Gorda plate event ruptured a southwest striking fault, while the July 13, 1991, event ruptured a southeast striking fault, with both events relieving north‐south compression within the deforming Gorda plate. The existence of multiple active faults in the region constitutes a significant earthquake hazard associated with the complex stress environment of the migrating

ISSN:0148-0227    

DOI:10.1029/93JB02390

年代:1994

数据来源: WILEY

摘要:

Two catalogs of earthquakes in the Mexican part of the Middle America trench are analyzed to elucidate principal relations between structure of the subducting Cocos plate and seismicity. A catalog of historical events that have occurred during the last two centuries with large magnitudes (Ms>6.0) is used to obtain cumulative seismic moment (Mo) and seismic moment release rate (M˙0) distributions along the Mexican subduction zone. The catalog of instrumentally observed earthquakes from 1963 to 1990 (International Seismological Center and U.S. Geological Survey) with 4.5 ≤mb<6.0 is applied to investigate background seismicity for the region. The strength of coupling between the Cocos and North American plates would be expected to grow gradually from the southeast to the northwest according to the variation of convergence rate (V) and age (A) of the Cocos plate. This correlates in general with a steady reduction in background seismicity and a slight average increase ofM˙0and seismic energy release rate (W˙). At the sites where the main fracture zones of the Cocos plate; Tehuantepec, O'Gorman, Orozco and Rivera, undergo the subduction the general correlation breaks down. The background seismicity increases at fracture zones whileM0andM˙0decrease significantly. This feature is interpreted as a drop of the coupling at the areas where transform faults are being subducted. Seismic slip rates along the trench obtained fromM˙0are lower then the values of plate convergence rates but the average seismic slip is in agreement with the estimates from the V model (interaction between lithospheric plates at convergent zones through the viscous layer of subducted sediments). Variability ofM˙0and seismic slip rate in relation with tectonics should be taken into account when the seismic gap model is being used for the prediction of strong earthquakes. An examination of space‐time plots for the historical catalogs supposes a probable tendency of northwest migration of strong events with a rate of

ISSN:0148-0227    

DOI:10.1029/93JB01556

年代:1994

数据来源: WILEY

摘要:

We have applied surface wave analysis to the Mojave Desert in southern California to extract the regional lithosphere velocity structure. Seismograms from teleseismic sources recorded by the TERRAscope stations, a recently established network of broadband instruments, were used to determine the phase velocity dispersion data of Rayleigh waves for wave paths crossing the Mojave Desert. An inversion analysis on the dispersion data for each path gave the shear wave velocity structure beneath the path. Inversion results show that there is a low‐velocity layer at the middle and lower crust, which is particularly pronounced for paths traversing the western part of the Mojave Desert. This low‐velocity layer at the midcrust may reflect the existence of a transition zone from the brittle to the ductile regimes, where there is an absence of earthquake occurre

ISSN:0148-0227    

DOI:10.1029/93JB02517

年代:1994

数据来源: WILEY

摘要:

In this paper a system of equations for wide‐angle one‐way elastic wave propagation in arbitrarily heterogeneous media is formulated in both the space and wavenumber domains using elastic Rayleigh integrals and local elastic Born scattering theory. The wavenumber domain formulation leads to compact solutions to one‐way propagation and scattering problems. It is shown that wide‐angle scattering in heterogeneous elastic media cannot be formulated as passage through regular phase‐screens, since the interaction between the incident wavefield and the heterogeneities is not local in both the space domain and the wavenumber domain. Our more generally valid formulation is called the “thin‐slab” formulation. After applying the small‐angle approximation, the thin‐slab effect degenerates to that of an elastic complex‐screen (or “generalized phase‐screen”). Compared with scalar phase‐screen, the elastic complex‐screen has the following features. (1) ForP‐Pscattering andS‐Sin‐plane scattering, the elastic complex‐screen acts as two separate scalar phase‐screens forPandSwaves respectively. The phase distortions are determined by thePandSwave velocity perturbations respectively. (2) ForP‐SandS‐Pconversions, the screen is no longer a pure phase‐screen and becomes complex (with both phase and amplitude terms); both conversions are determined by the shear wave velocity perturbation and the shear modulus perturbation. For Poisson solids theSwave velocity perturbation plays a major role. In the special case of α0= 2β0,Swave velocity perturbation becomes the only factor for both conversions. (3) For the cross‐coupling between in‐planeSwaves and off‐planeSwaves, only the shear modulus perturbation δμ has influence in the thin‐slab formulation. For the complex‐screen method the cross‐coupling term is neglected because it is a higher order small quantity for small‐angle scattering. Relative to prior derivations of vector phase‐screen method, our method can correctly treat the conversion betweenPandSwaves and the cross‐coupling between differently polarizedSwaves. A comparison with solutions from three‐dimensional finite difference and exact solutions using eigenfunction expansion is made for two special cases. One is for a solid sphere with onlyPvelocity perturbation; the other is with onlySvelocity perturbation. The Elastic complex‐screen method generally agrees well with the three‐dimensional finite difference method and the exact solutions. In the limiting case of scalar waves, the derivation in this paper leads to a more generally valid new method, namely, a scalar thin‐slab method. When making the small‐angle approximation to the interaction term while keeping the propagation term unchanged, the thin‐sla

ISSN:0148-0227    

DOI:10.1029/93JB02518

年代:1994

数据来源: WILEY

摘要:

Shales constitute about 75% of the clastic fill of sedimentary basins and have a decisive effect on fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure. The elastic stiffnesses of a shale with partially oriented clay particles is expressed in terms of the coefficients Wlmn in an expansion of the clay‐particle orientation distribution function in generalized Legendre functions. Application is made to the determination of the anellipticity of shales. For transverse isotropy the anellipticity quantifies the deviation of thePwave slowness curve from an ellipse and is shown to depend on a single coefficientW400in the expansion of the clay‐particle orientation distribution function. IfW400is small, the anellipticity may be neglected, as is apparently the case for a near‐surface late Tertiary shale studied by Winterstein and Paulson. Strongly aligned clay particles result in a positive value ofW400and a positive anellipticity, in agreement with the majority of field measurements. However, less well ordered shales could have a significantly positive second momentW200but only a small positive or even negative value ofW400For such shales the anellipticity would be small or negative despite a preferred alignment of clay particles in the bedding plane. Numerical examples of clay particle orientation distribution functions leading to zero or negative anellipticity are

ISSN:0148-0227    

DOI:10.1029/93JB02579

年代:1994

数据来源: WILEY