摘要:

Linear inversion of τ(p) composites for three different profiles suggests that the lower crust beneath eastern Pennsylvania consists of material with high averaged P wave velocities (6.8–7.5 km/s) compared with 6.4–7.1 km/s for most crustal models derived for surrounding areas within the Appalachians and the Grenville Province of the Canadian Shield. Interpretations of these velocities in terms of rock type are limited by the low resolving power of the data and by the nonuniqueness in inferring rock type from P wave velocity alone. The high averaged P wave velocities could be associated with substantial volumes of mafic rock, perhaps emplaced by underplating, or with high‐grade metamorphic assemblages of intermediate composition and sedimentary origin, similar to those found in the Ivrea Zone in northern Italy. Because of the sparseness of the S wave data, averaged shear wave velocities are not well enough resolved over most depth ranges to distinguish among the different compositional models. Computations of gravity anomalies for two‐dimensional models using various linear velocity‐density relations, however, suggest that at least part of the crust beneath the Scranton Gravity High is mafic. Moreover, if the entire basement section beneath this anomaly consists of mafic material, then the gravity studies indicate that the lower crust beneath the Great Valley must be mafic as well. Migrated shot gathers show a concentration of reflectors at depths between 35 and 45 km, a result that is controlled in part by the limited range of offsets used, but that is also consistent with recent observations of a highly reflective lower crust. Reflections that are migrated to depths within the uppermost mantle could be multiples or primary reflections from upper mantle layering. Although the data are rather sparse, the interpretation of these later arrivals as primary reflections is supported by their small ray parameters, which suggests that they are not converted phases, and by their large relative amplitudes, which suggests that they are not multiples. Layering within the upper mantle could be in the form of alternating high‐ and low‐velocity zones, perhaps related to emplacement of eclogites or garnet granulites in the uppermost mantle during the process o

ISSN:0148-0227    

DOI:10.1029/91JB02229

年代:1992

数据来源: WILEY

摘要:

Seismic waves are used to explore large‐scale radial structure and lateral heterogeneity of shear velocity in the D″ region, the chemical and thermal boundary layer at the base of the mantle. We analyze long‐period, horizontally polarized (SH) shear waveforms from earthquakes in the northwest Pacific recorded in Europe and the Middle East for evidence of proposed laterally varying shear velocity stratification in D″ beneath Eurasia. Specifically, our data sample the lower mantle extending from beneath the Arctic Ocean near Novaya Zemlya, USSR, to under northern India. Using a reflectivity technique to model theSHwaveforms, we determine a locally stratified shear velocity model, SGLE, that provides a good average travel time and waveform fit to over 80% of the data sensitive to D″ structure. SGLE is characterized by a rapid 2.75% increase in shear velocity approximately 290 km above the core‐mantle boundary, similar to previously proposed models for this region. Unlike for other regions analyzed using similar waveform modeling techniques, the percentage of data inconsistent with this average radial model is fairly substantial, indicating that laterally heterogeneous D″ velocity structure is important in this region. Some waveforms lack the reflected arrival expected for a stratified model, while others show evidence of two arrivals, both possibly originating within D″. While our data are insufficient for direct inversion of the three‐dimensional structure, we utilize several simple parameterizations of the waveforms to gain some understanding of the region. Both the relative amplitude and differential travel time variations of the waveforms indicate that D″ beneath Eurasia is characterized by (1) broad regions in which the structure has large‐scale stratification giving coherent long‐period reflections, with superimposed velocity variations that scatter shorter‐wavelength waves; and (2) smaller regions which are sufficiently heterogeneous to scatter even the long‐period energy. The scale length of the lateral heterogeneity in D″ that produces these variations is inferred to be somewhat less than 500 km, which contrasts with weaker lateral heterogeneity that is observed in the central region of the lower mantle, where heterogeneity scale lengths of approximately 500–1000 km appear to dominate. These observations are consistent with a model of the lower mantle in which distributed heterogeneity is concentrated in D″, generating a locally stratified layer in some regions. The effects of a diffuse or topographically variable D″ discontinuity must be further explored using two‐ and three‐dimensional synthetic calculations before inversions for t

ISSN:0148-0227    

DOI:10.1029/91JB02347

年代:1992

数据来源: WILEY

摘要:

Earthquakes at depths of 20–30 km occur beneath the Ventura Basin in southern California. The epicentral distribution of deep seismicity outlines an east trending ellipse which corresponds closely with the mapped Santa Clara Syncline, the main structural element of the Ventura basin with 15 km thickness of sediments. ClearPnandPgphases are seen for even the deepest earthquakes which demonstrate that these earthquakes are occurring within the crust. Travel time curves from shallow and deep earthquakes in the western Transverse Ranges require that the depth to the Moho under the Ventura basin must be depressed by 7–10 km relative to the surrounding area, indicating crustal thickening of 20–35%. Other researchers have shown that the Ventura basin has the lowest heat flow in western California. Thus, low heat flow, exceptionally thick (15 km) sediments, rapid shortening of the crust, the deep earthquakes, and a depressed Moho all coincide within a small area, strongly suggesting a common cause for all five phenomena. The confinement of the deep earthquakes and depressed Moho to the Ventura region suggests that the rapid shortening of the Ventura basin is anomalous and not representative of the rate of shortening across all of the western Transverse R

ISSN:0148-0227    

DOI:10.1029/91JB02286

年代:1992

数据来源: WILEY

摘要:

The Makran subduction zone of Iran and Pakistan exhibits strong variation in seismicity between its eastern and western segments and has one of the world's largest forearcs. We determine the source parameters for 14 earthquakes at Makran including the great (Mw8.1) earthquake of 1945 (the only instrumentally recorded great earthquake at Makran); we determine the loci of seismic and aseismic slip along the plate boundary, and we assess the effects of the large forearc and accretionary wedge on the style of plate boundary slip. We apply body waveform inversions and, for small‐magnitude events, use first motions ofPwaves to estimate earthquake source parameters. For the 1945 event we also employ dislocation modeling of uplift data. We find that the earthquake of 1945 in eastern Makran is an interplate thrust event that ruptured approximately one‐fifth the length of the subduction zone. Nine smaller events in eastern Makran that are also located at or close to the plate interface have thrust mechanisms similar to that of the 1945 shock. Seaward of these thrust earthquakes lies the shallowest 70–80 km of the plate boundary; we find that this segment and the overlying accretionary wedge remain aseismic both during and between great earthquakes. This aseismic zone, as in other subduction zones, lies within that part of the accretionary wedge that consists of largely uconsolidated sediments (seismic velocities less than 4.0 km/s). The existence of thrust earthquakes indicates that either the sediments along the plate boundary in eastern Makran become sufficiently well consolidated and de watered about 70 km from the deformation front or older, lithified rocks are present within the forearc so that stick‐slip sliding behavior becomes possible. This study shows that a large quantity of unconsolidated sediment does not necessarily indicate a low potential for great thrust earthquakes. In contrast to the east, the plate boundary in western Makran has no clear record of historic great events, nor has modem instrumentation detected any shallow thrust events for at least the past 25 years. Most earthquakes in western Makran occur within the downgoing plate at intermediate depths. The large change in seismicity between eastern and western Makran along with two shallow events that exhibit right‐lateral strike‐slip motion in central Makran suggest segmentation of the subduction zone. Two Paleozoic continental blocks dominate the overriding plate. The boundary between them is approximately coincident with the transition in seismicity. Although relative motion between these blocks may account for some of the differing seismic behavior, the continuity of the deformation front and of other tectonic features along the subduction zone suggests that the rate of subduction does not change appreciably from east to west. The absence of plate boundary events in western Makran indicates either that entirely aseismic subduction occurs or that the plate boundary is currently locked and experiences great earthquakes with long repeat times. Evidence is presently inconclusive concerning which of these two hypotheses is most correct. The presence of well‐defined late Holocene marine terraces along portions of the coasts of eastern and western Makran could be interpreted as evidence that both sections of the arc are capable of generating large plate boundary earthquakes. If that hypothesis is correct, then western Makran could produce a great earthquake or it could rupture as a number of segments in somewhat smaller‐magnitude events. Alternatively, it is possible that western Makran is significantly different from eastern Makran and experiences largely aseismic slip at all times. A knowledge of the velocity structure and nature of the state of consolidation or lithification of rocks at depth in the interior portion of the forearc of western Makran should help to ascertain whether that portion of the plate boundary moves aseismically or ruptures in large to great earthquakes. A resolution of this question has important implications for seismic hazard not only for western Makran but also for other margins, such as the Cascadia subduction zone of western North America, where historical thrust events ha

ISSN:0148-0227    

DOI:10.1029/91JB02165

年代:1992

数据来源: WILEY

摘要:

We analyze the patterns of seismic activity which precede large events in a mechanical model of a fault. The model generates a statistical distribution of events similar to that observed for a single fault, with a scaling region consistent with the Gutenberg‐Richter law at small and moderate magnitudes, and an excess of events at large magnitudes. We find only slight variation in the scaling behavior during a loading cycle. However, we do observe systematic variations in space and time of the overall rate of activity. In the model, the activity accelerates dramatically preceding a large event and is usually a maximum hi the neighborhood of the future epicenter. These results are compared to California seismicity data, where we find that activity patterns vary regionally. Looking at patterns of activity in the San Francisco Bay Area since 1948, we find an increase of activity on the Calaveras fault near San Jose beginning in the 1980s which, if our model is relevant, would forecast a large earthquake in that region. The 1989 Loma Prieta earthquake occurred on the San Andreas fault within 30 km of the section of the Calaveras fault showing increased activit

ISSN:0148-0227    

DOI:10.1029/91JB01796

年代:1992

数据来源: WILEY

摘要:

The recent acquisition of high‐density and improved seismic data by the Kanto‐Tokai (K‐T) Observational Network of the National Research Institute of Earth Science and Disaster Prevention requires the revision of conventional plate configuration models for the K‐T district. We propose a new interpretation of the configuration and relative motion of the Philippine Sea (PHS) plate, Pacific (PAC) plate, and Eurasian (EUR) plate on the basis of the distribution of hypocenters, velocity structure, and focal mechanisms in the K‐T district. The new model for the configuration of the PHS and PAC slabs clearly delineates the PHS slab subducting northwestward beneath the K‐T district from the Sagami and Suruga troughs and the PAC slab subducting westward beneath the PHS slab from the Japan trench. The thickness of the descending PHS slab is estimated to be 30 ± 5 km. The proposed configuration of the PHS slab is significantly different from those obtained by other researchers, including our own previous study. The new model is fully consistent with global plate motion data. The new model leads to an interpretation of the tectonic processes taking place beneath the K‐T district. For the Sagami trough wing of the PHS slab, the northeastern portion of the slab is dipping northwestward in direct contact with the upper surface of the PAC slab; the southwestern portion carrying the Izu Peninsula has been bent upward, because of its buoyancy, producing the observed contorted shape. For the Suruga trough wing, the dip direction of the PHS slab gradually changes from northwestward at the eastern portion to northward at the western portion. The new model also indicates the presence of offset within the PHS slab between the Sagami and Suruga trough wings. This area is characterized by a low level of seismic activity, the absence of a high‐velocity structure, and the presence of Quaternary volcanoes. It seems reasonable to speculate that the distribution of Quaternary volcanic edifice construction is controlled by the location of the offset within the PHS slab. In view of this new model, the tectonic processes of large earthquakes which occurred recently in the studied area are

ISSN:0148-0227    

DOI:10.1029/91JB02567

年代:1992

数据来源: WILEY

摘要:

We discuss an approximate analytical model for the hydrodynamic evolution of the shock front produced by a spherically symmetric explosion in a homogeneous medium. The model assumes a particular relation between the energy of the explosion, the density of the medium into which the shock wave is expanding, and the particle speed immediately behind the shock front. The assumed relation is exact for shock waves that are strong and self‐similar. Comparison with numerical simulations indicates that the relation is also approximately valid for shock waves that are neither strong nor self‐similar. Using the assumed relation and the Hugoniot of the ambient medium expressed as a relation between the shock speed and the postshock particle speed, one can calculate the radius and other properties of the shock front as a function of time. The model also allows one to investigate how the evolution of the shock wave is influenced by the properties of the ambient medium and how these properties affect the characteristic radius at which the shock wave becomes a low‐pressure plastic wave. The shock front radius versus time curves predicted by the model agree well with numerical simulations of explosions in quartz and wet tuff and with data from four underground nuclear tests conducted in granite, basalt, and wet tuff when the official yields are assumed. When the model is used instead to fit radius versus time data from the hydrodynamic phases of these tests, it gives yields that are within 8% of the official yields when piecewise‐linear approximations to the Hugoniots are used. This accuracy is comparable to the accuracy of other

ISSN:0148-0227    

DOI:10.1029/91JB02348

年代:1992

数据来源: WILEY

摘要:

We present a global analysis of the depth distribution of moment release along the thrust contact at subduction zones using data from Harvard's centroid moment tensor catalogue. Substantial regional variations in the depth distribution of moment release are observed. A significant correlation exists between the depth of maximum moment release and the convergence rate of subducting lithosphere and the sediment thickness at the trench. Subduction zones with fast convergence rates and thick sediment accumulations concentrate moment release at greater depths than subduction zones with slow convergence rates and little sediment accumulation. The correlation between the convergence rate of oceanic lithosphere and the depth distribution of moment release may be best explained by differences in the thermal structure of the plate interface. Subduction zones with fast convergence velocities transport cold material to greater depths than subduction zones with slow convergence velocities. The colder temperature of the deep portions of the thrust contact at subduction zones with fast convergence velocities allows seismic failure to occur at greater depth than possible in subduction zones with slow convergence velocities. As has been demonstrated in intraplate environments, our observations in interplate thrust settings support the idea that temperature is the most important factor controlling the maximum depth of seismic failure. In subduction zones with thick sediment accumulations, the stable sliding properties of unconsolidated sediments may be responsible for the low levels of shallow moment release.

ISSN:0148-0227    

DOI:10.1029/91JB02345

年代:1992

数据来源: WILEY

摘要:

The 1987 Crustal Deformation Survey of the Yellowstone‐Hebgen Lake region produced accurate geodetic measurements in an area unique for its rugged topography and volcanic processes. Using the integrated geodesy approach, Global Positioning System (GPS) baseline vectors, geodetic leveling, and gravity measurements were combined in a simultaneous least squares adjustment. Using 30″ × 30″ digital terrain data, and an Airy model, isostatically corrected complete Bouguer gravity disturbances highlight both the Yellowstone Caldera and a second gravity low near Hebgen Lake. A linear relationship between these disturbances and the departures of geoid height from the Airy isostatic model have been found (r= 0.84). Adjusting the covariance function while incorporating the GPS contribution yields a function whose correlation length is significantly longer than that predicted by autocorrelation statistics of the gravity disturbances alone. The integrated adjustment produced a geoid whose low‐frequency components are in good agreement with OSU86F. Formal statistics indicate precision of 2–5 cm. GPS residuals for 35 leveled points in the adjustment have a root‐mean‐square value of 2.9 cm and a standard error estimate of 4.6 cm. In comparison, a free adjustment of 152 GPS vectors yields a differential height standard e

ISSN:0148-0227    

DOI:10.1029/91JB02027

年代:1992

数据来源: WILEY

摘要:

To generate the highly precise ephemerides of Global Positioning System (GPS) satellites necessary for modern geodetic applications, one must have an accurate force model that includes the pressure of solar radiation and spacecraft thermal emission. We present the dimensions and optical parameters of Block I and Block II GPS satellites, show how they are used to form the models of the solar force, and compare predictions of these models with values estimated from tracking data. Simple approximating functions are given for the solar/thermal radiation pressure, and the problem of estimating a smaller, unmodeled force calledYbias is discussed. A simple model is given for the effect of earthshine on GPS spacecraft. Two kinds of discrepancy are noted between model predictions and actual performance. For Block II satellites, beginning soon after launch, a transient force decaying exponentially with time, perhaps due to outgassing, aliases into estimates of the solar force. Also, at the beginning of eclipse season and for at least some satellites,Ybias estimates vary rapidly for reasons not presently understood. Therefore the geodetic worker should not only use the best available force models but also should avoid using a satellite in those critical time periods when unexplained model errors are present.

ISSN:0148-0227    

DOI:10.1029/91JB02564

年代:1992

数据来源: WILEY