摘要:

TheMw6.7 Northridge earthquake occurred on January 17, 1994, beneath the San Fernando Valley. Two seismicity clusters, located 25 km to the south and 35 km to the north‐northwest, preceded the mainshock by 7 days and 16 hours, respectively. The mainshock hypocenter was relatively deep, at 19 km depth in the lower crust. It had a thrust faulting focal mechanism with a rake of 100° on a fault plane dipping 35° to the south‐southwest and striking N75°W. Because the mainshock did not rupture the surface, its association with surficial geological features remains difficult to resolve. Nonetheless, its occurrence reemphasized the seismic hazard of concealed faults associated with the contractional deformation of the Transverse Ranges. The Northridge earthquake is part of the temporal increase in earthquake activity in the Los Angeles area since 1970. The mainshock was followed by an energetic aftershock sequence. Eight aftershocks ofM≥ 5.0 and 48 aftershocks of 4 ≤M≤ 5 occurred between January 17 and September 30, 1994. The aftershocks extend over most of the western San Fernando Valley and Santa Susana Mountains. They form a diffuse spatial distribution around the mainshock rupture plane, illuminating a previously unmapped thrust ramp, extending from 7–10 km depth into the lower crust to a depth of 23 km. No flattening of the aftershock distribution is observed near its bottom. At shallow depths, above 7–10 km, the thrust ramp is topped by a dense distribution of aftershock hypocenters bounded by some of the surficial faults. The dip of the ramp increases from east to west. The west side of the aftershock zoae is characterized by a dense, steeply dipping, and north‐northeast striking planar cluster of aftershocks that exhibited mostly thrust faulting. These events coincided with the Gillibrand Canyon lateral ramp. Along the east side of the aftershock zone the aftershocks also exhibited primarily thrust faulting focal mechanisms. The focal mechanisms of the aftershocks were dominated by thrust faulting in the large aftershocks, with some strike‐slip and normal faulting in the smaller aftershocks. The 1971 San Fernando and the 1994 Northridge earthquakes ruptured partially abutting fault surfaces on opposite sides of a ridge. Both earthquakes accommodated north‐south contractional deformation of the Transverse Ranges. The two earthquakes differ primarily in the dip direction of the faults and the depth of faulting. The 1971 north‐northeast trend of left‐lateral faulting (Chatsworth trend)

ISSN:0148-0227    

DOI:10.1029/95JB00865

年代:1995

数据来源: WILEY

摘要:

The geometry of the subducted Rivera and Cocos plates beneath the North American plate in southern Mexico was determined based on the accurately located hypocenters of local and teleseismic earthquakes. The hypocenters of the teleseisms were relocated, and the focal depths of 21 events were constrained using a body wave inversion scheme. The suduction in southern Mexico may be approximated as a subhorizontal slab bounded at the edges by the steep subduction geometry of the Cocos plate beneath the Caribbean plate to the east and of the Rivera plate beneath North America to the west. The dip of the interplate contact geometry is constant to a depth of 30 km, and lateral changes in the dip of the subducted plate are only observed once it is decoupled from the overriding plate. On the basis of the seismicity, the focal mechanisms, and the geometry of the downgoing slab, southern Mexico may be segmented into four regions: (1) the Jalisco region to the west, where the Rivera plate subducts at a steep angle that resembles the geometry of the Cocos plate beneath the Caribbean plate in Central America; (2) the Michoacan region, where the dip angle of the Cocos plate decreases gradually toward the southeast, (3) the Guerrero‐Oaxaca region, bounded approximately by the onshore projection of the Orozco and O'Gorman fracture zones, where the subducted slab is almost subhorizontal and underplates the upper continental plate for about 250 km, and (4) the southern Oaxaca and Chiapas region, in southeastern Mexico, where the dip of the subduction gradually increases to a steeper subduction in Central America. These drastic changes in dip do not appear to take place on tear faults, suggesting that smooth contortions accommodate these changes in geometry. The inferred 80 and 100 km depth contours of the subducted slab lie beneath the southern front of the Trans‐Mexican Volcanic Belt, suggesting it is directly related to the subduction. Thus the observed nonparallelism with the Middle American Trench is apparently due to the changing geometry of the Rivera and Cocos plates beneath the North American plate in southern Mexico, and not to zones of weakness in the crust of the North American plate as some authors have sugges

ISSN:0148-0227    

DOI:10.1029/95JB00919

年代:1995

数据来源: WILEY

摘要:

We present a global three‐dimensional model of shear attenuation in the upper mantle, based on the measurement of amplitudes of low‐frequency (100–300s) Rayleigh waves observed at stations of the Geoscope and Iris networks. Attenuation coefficients are measured on R1 and R2 paths using a method which minimizes the effects of focussing due to propagation in a three‐dimensional elastic Earth. Through a series of tests which, in particular, involve the computation of synthetic models of attenuation and focussing, we demonstrate that long wavelength lateral variations in attenuation in the first 400–500 km of the mantle can indeed be resolved. The model is obtained in a two‐step procedure. The first step consists in the computation of maps of Rayleigh wave attenuation at different periods, using an inversion method without a priori parametrisation, which involves the introduction of a correlation length, chosen here at 3000 km to optimize the trade‐off between resolution and variance in the model. In the second step, after corrections for shallow structure, an inversion with depth is performed, assuming lateral heterogeneity is confined to depths between 80 and 650 km. The resulting model presents lateral variations in Qβthat are correlated with tectonic features, in particular ridges and shields in the first 250 km of the upper mantle. Below that depth the pattern shifts and becomes correlated with the hotspot distribution, particularly so if the buoyancy strength of hotspots is taken into account. Two major low‐velocity zones appear to be located in the central pacific and beneath northern Africa, in the depth range 300–500 km. This pattern seems to continue at greater depth, but resolution becomes insufficient below 500 km to draw definitive conclusions. The smooth lateral variations retrieved are on the order of ±50% down to 400 km. We propose an interpretation in terms of plume/lithosphere/ridge interaction in the upper mantle, arguing for deflection of the bulk of hot upwelling material from plumes towards ridges, which may be occurring between 2

ISSN:0148-0227    

DOI:10.1029/95JB00957

年代:1995

数据来源: WILEY

摘要:

UsingS‐to‐Pamplitude ratios measured from three‐component seismograms we tomographically imaged spatial variations inSwave attenuation in parts of Long Valley caldera. Data from seismograms of local earthquakes in the Long Valley caldera region that occurred during 1984 were employed with theS‐to‐Pamplitude ratio technique with corrections for source radiation pattern to determineQS. Resolution is limited throughout most of the model; however, significant resolution and acceptable error in parts of the model allow meaningful interpretation of the results. The primary result is the image of a low‐QSanomaly at 7–8 km depth beneath the resurgent dome. This is independent evidence for high temperatures and possible magma accumulation at this location. Other regions of anomalously lowQSare marginally significant or poorly resolved; of these lowQSat 4–5 km depth beneath the Inyo Domes has the most potential significance. The similarity of the results from the three‐component data and from the vertical‐component data implies that vertical‐component data, which are generally more abundant, can be

ISSN:0148-0227    

DOI:10.1029/95JB00962

年代:1995

数据来源: WILEY

摘要:

Compressional velocity structure of the crust and the upper mantle in south‐eastern Europe (broader Aegean area) is studied by inverting residuals of the firstParrivals from earthquakes in this region (16°E–31°E, 34°N–43°N). The data used are from regional events recorded by the permanent network of stations during the period 1971–1987, enriched with data from experiments with portable seismographs in four regions of this broad area. This study confirms the strong variations of crustal thickness in this area as well as the subduction of the eastern Mediterranean lithosphere under the southern Aegean and gives further detailed information on the crustal and upper mantle structure of the area. Important new information is the existence of a low‐velocity crustal layer in western Greece and Albania and that the velocity anomaly in the mantle under the southern Aegean extends much farther and deeper to the northeast than the Benioff zone of the intermediate depth earthquakes indicates. Furthermore, evidence is presented about the possible existence of older subduction in the northern Aegean and about the influence of the tectonic regime on the v

ISSN:0148-0227    

DOI:10.1029/95JB00669

年代:1995

数据来源: WILEY

摘要:

We have constructed a dynamical model of seismicity, wherein a fault is embedded in an infinite, continuous, elastic medium. Therefore the influence of the energy dissipated in seismic wave radiation oil the sequential history of model earthquakes is fully taken into account. In this model, the drop in friction at rupture takes place gradually, thus introducing a relaxation dimension. As an example, we consider a finite homogeneous fault that is terminated by infinitely strong barriers at the ends. The features of the seismicity are dominated by the stresses that are reflected from the unbreakable barriers. There is a strong dependence of the patterns of seismicity on the ratio of the relaxation dimension to the distance between the barriers at the ends. For large values of this parameter, we find that periodicity begins after a short transient interval and a dominance of the statistical distributions by large events that break completely through the fault from end to end. For small values of this parameter, smaller‐scale seismicity is interspersed with the large events, and no periodicity is observed within the time spanned by the computations. We conclude that major unquenched heterogeneities, such as those found at barriers, which we suppose arise in nature due to the nonuniform geometry of faults, are vital ingredients for generating complex seismic histories as well as giving rise to the complex features of individual earthquake source‐time functions in models of seismicity as a dynamical proc

ISSN:0148-0227    

DOI:10.1029/95JB00714

年代:1995

数据来源: WILEY

摘要:

Probabilistic formulation of inverse problems leads to the definition of a probability distribution in the model space. This probability distribution combines a priori information with new information obtained by measuring some observable parameters (data). As, in the general case, the theory linking data with model parameters is nonlinear, the a posteriori probability in the model space may not be easy to describe (it may be multimodal, some moments may not be defined, etc.). When analysing an inverse problem, obtaining a maximum likelihood model is usually not sufficient, as we normally also wish to have information on the resolution power of the data. In the general case we may have a large number of model parameters, and an inspection of the marginal probability densities of interest may be impractical, or even useless. But it is possible to pseudorandomly generate a large collection of models according to the posterior probability distribution and to analyse and display the models in such a way that information on the relative likelihoods of model properties is conveyed to the spectator. This can be accomplished by means of an efficient Monte Carlo method, even in cases where no explicit formula for the a priori distribution is available. The most well known importance sampling method, the Metropolis algorithm, can be generalized, and this gives a method that allows analysis of (possibly highly nonlinear) inverse problems with complex a priori information and data with an arbitrary noise distribution.

ISSN:0148-0227    

DOI:10.1029/94JB03097

年代:1995

数据来源: WILEY

摘要:

Seismic reflection tomography obtains an estimate of the subsurface slowness field and the location of strong reflectors by minimizing the difference between measured travel times from seismic reflection events and the corresponding travel times computed from a model of the subsurface. We present some theoretical results for the undiscretized problem regarding the possible ambiguity between slowness and depth. These results indicate that the depths of the reflectors are determined in theory except for edge effects, but a sufficiently large aperture at the reflector is necessary to resolve this ambiguity in practice. The slowness field, however, does have some undetermined features. These results have strong implications for how the tomography problem should be discretized and regularized to compute solutions which are accurate in the features of the model which are well determined from the travel time data. In particular, the slowness model should not be discretized much more coarsely than the reflectors as a way of regularizing the problem because that may force the computed reflector depths to try to match aspects of the travel time data which are caused by features in the slowness field.

ISSN:0148-0227    

DOI:10.1029/95JB00920

年代:1995

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/94JB02560

年代:1995

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/95JB00666

年代:1995

数据来源: WILEY