摘要:

The Manila Trench subduction zone is an active convergent plate margin between the South China Sea and the northern Philippines. The trench trends northerly and is associated with a volcanic arc, an east dipping Benioff zone beneath Luzon, and a well‐developed fore arc basin system. The Luzon Trough fore arc basins lie landward of the Manila Trench and contain up to 4.5 km of Cenozoic sediments. Offscraping and accretion of turbidite sediments deposited in the Manila Trench have resulted in the upbuilding and outbuilding of an accretionary prism which forms the seaward margin of the fore arc basin system. The uplifted Zambales ophiolite and its offshore extension form the landward side, of the fore arc basin. Multichannel seismic reflection profiles reveal that folding and thrust faulting of trench strata occur at the base of the trench slope. The major structural décollement at the Manila Trench usually forms near the unconformity separating the hemipelagic sediments from the turbidite sediments. Subduction and perhaps underplating of the basal hemipelagic trench sediments accompany the deformation and accretion of the overlying clastic deposits. Faulting and uplift occur within a narrow zone of the fore arc where seamounts associated with the relict spreading center of the South China Sea basin have presumably been subducted. North of Lingayan Gulf the fore arc is disrupted by active fault systems that trend north and northwest offshore from the Luzon Central Valley and from the southern Cordillera Central. Based on regional geological and tectonic observations, we infer that subduction probably began at the Manila Trench in late Oligocene to middle Miocene time. The long‐term average convergence rate at the Manila Trench is estimated to be between 10 and 20 mm/yr and may be slowing in the north owing to the collision of Taiwan with Eur

ISSN:0148-0227    

DOI:10.1029/JB089iB11p09171

年代:1984

数据来源: WILEY

摘要:

The pattern of sediment distribution within the Manila Trench subduction zone west of Luzon exerts a strong influence on the tectonic evolution of the accretionary prism and the fore arc basins of this convergent plate margin. Multichannel common depth point seismic reflection data reveal up to 2600 m of turbidite sediments overlying a thin pelagic/hemipelagic sediment layer and oceanic basement in the northern part of the Manila Trench. These turbidites were transported from the uplifted collision zone of Taiwan, southward along the Manila Trench by gravity‐controlled processes. Similarly, thinner turbidite deposits in the southern end of the trench were transported northward from the collision zone involving the North Palawan block and the central Philippines. Sedimentation within the Manila Trench is dominated by sources at the ends of the trench with longitudinal transport along the trench axis. In contrast, the fore arc basin system of the Manila Trench receives sediment through submarine canyons incised into the West Luzon continental slope. The rate of sediment supply to the fore arc basin system is not directly related to the rate of sediment supply to the trench. The size of the accretionary prism varies significantly along strike of the trench and it strongly correlated with the local thickness of turbidite sediments within the trench. A thickness of more than 200 m of trench turbidites is probably required before accretion and uplift become the dominant process affecting the fore arc region. This generalization may apply to other convergent plate margins as well. The uplift rate of the accretionary prism probably varies dramatically along the trench and is largely independent of variations in the local sediment accumulation rates in the fore arc basins. This variability results in the formation of several different types of fore arc morphology along the Manila Trench. The overall seismic stratigraphy of the West Luzon Trough fore arc basin is remarkably uniform, with seismic sequences continuous for up to 110 km along strike. The unconformities that bound these seismic sequences are probably the result of a complex interplay of relatively long‐term (approximately 10 Ma) uplift of the accretionary prism and relatively short‐term (approximately 1–2 Ma) variations in sediment supply to the fore arc basin, modulated by eustatic sea level

ISSN:0148-0227    

DOI:10.1029/JB089iB11p09196

年代:1984

数据来源: WILEY

摘要:

We present a revised model of the tectonic evolution of the Juan de Fuca ridge by propagating rifting. The new model has three different relative rotation poles, covering the time intervals 17.0–8.5 Ma, 8.5–5.0 Ma, and 5.0 Ma to the present. The rotation pole shifts at 8.5 and 5.0 Ma imply clockwise shifts in the direction of relative motion of 10° to 15°. At each of these shifts, the pattern of propagation reorganizes, and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges. The model, containing a total of seven propagation sequences, shows excellent agreement with the isochrons inferred from the magnetic anomaly data, except in areas complicated by the separate Explorer and Gorda plates. The agreement between model and data near the Explorer plate breaks down abruptly at an age of about 5 Ma, indicating that the probable cause of the rotation pole shift at that time was the separation of the Explorer plate from the Juan de Fuca

ISSN:0148-0227    

DOI:10.1029/JB089iB11p09215

年代:1984

数据来源: WILEY

摘要:

We have constructed a thermomechanical model for upper mantle convection such that the thickness and structure of the lithosphere are determined self‐consistently by the heat transported by convection. In this study of the interaction between the lithosphere and upper mantle, strongly temperature‐ and pressure‐dependent rheologies for both Newtonian and non‐Newtonian creep mechanisms are employed. For a strictly temperature‐dependent rheology an insignificant amount of heat, less than 12.5 mW/m2, can be transported convectively for an interior viscosity, 0(1021Pas), compatible with postglacial rebound. On the other hand, for similar values of the interior viscosity, steady heat fluxes between 20 and 40 mW/m2are produced by introducing pressure dependence into the rheology. For the temperature‐ and pressure‐dependent flow law the horizontally averaged interior temperature displays very little variation with the amount of heat evacuated, once all of the rheological parameters are fixed. This finding may have important ramifications for parameterized convection. We employ both the single‐mode, mean field approximation and the complete two‐dimensional equations, using finite elements, to obtain solutions for the various types of rheologies. From evaluating the geophysically relevant observables, such as topography, free air gravity anomalies, surface heat flow, and stress fields in the lithosphere, we find that the lateral variations of these quantities predicted by a non‐Newtonian rheology are much smaller than those derived from a linear rheology. These results suggest that surface variations of geophysical observables are more compatible with a non‐Newtonian rheolog

ISSN:0148-0227    

DOI:10.1029/JB089iB11p09227

年代:1984

数据来源: WILEY

摘要:

The principal motivation for the development of multicomponent ocean bottom seismometers (OBS's) was the hope that measurement of vector particle velocities would permit sharper interpretation of seismic data, since wave types (PandS) could be better indentified with this knowledge. The practical results of OBS use have not entirely borne out this expectation. Instead, the OBS‐seafloor coupling system characteristically shows strong resonances, resulting in a ringy appearance of almost all OBS data. Previous attempts to deal with this problem have used seriously oversimplified one‐dimensional analyses. In the present work we first consider an ideal model of an OBS as a Hamiltonian system with three spatial degrees of freedom. This is generalized to a linear system havingndegrees of freedom with three inputs and three outputs, and we find that the OBS site system is an all‐pole minimum delay multichannel filter whose properties may be estimated from in situ seismic data. We apply the inverse of this filter to noise series obtained with a three‐component OBS used in the Rivera Ocean Seismic Experiment. Resonance peaks in the noise power spectra are almost completely removed but at the cost of the gain obtained through resonant detection. The most significant implication of this work is that accurate ground motion can be recovered from almost any three‐component OBS. Nevertheless, poor performance of OBS's will lead to degraded recovery of ground motion, so that design considerations which reduce the resonances in OBS‐seafloor coupling remai

ISSN:0148-0227    

DOI:10.1029/JB089iB11p09245

年代:1984

数据来源: WILEY

摘要:

Analysis of pulse rise time data produced from repetitive piezoelectric and magnetostrictive seismic sources shows that the change in rise time as a function of distance is source dependent. Thus a linear rise time law of the form τ = τ0+CT/Q, withCsource independent, cannot be used to assess the attenuation of rock in situ. In fact, application of such a law to the data results in a significant overestimate in the value ofQ. A numerical example is also presented to show that in terms of filter theory a linear rise time law is ill founded. An alternative, source independent method of assessing the attenuation is employed for both constant‐Qand non‐constant‐Qmodels, and it is found that either model may be made to give a reasonable fit to the data. However, it is shown that if a non‐constant‐Qmechanism is operating, then there is probably less than a 5% change inQover the frequency range 0

ISSN:0148-0227    

DOI:10.1029/JB089iB11p09253

年代:1984

数据来源: WILEY

摘要:

This study examines the effects of dilatant hardening on the development of concentrated shear deformation. Specifically, the analysis considers the shear of an inelastic ally deforming rock mass containing a weakened layer of thicknessh. The presence of the weakened layer causes localization instability, characterized by an unbounded ratio of a strain increment in the weakened layer to that in the far field, to occur earlier than it would be predicted from the response of the material surrounding the embedded layer. The development of instability in time depends on the ratio of the rate of imposed shear strainto that for fluid mass exchange between the layers,c/h2, wherecis an effective diffusivity. In the limit, the pressure in the weakened layer is equal to that in the surrounding material and localization instability occurs at the peak of the drained stress‐strain curve. For finite, instability is delayed until the material in the weakened layer has been driven to the peak of its undrained, dilatantly hardened stress‐strain curve. The time delay between final instability and the time at which the weakened layer passes the peak of its drained stress‐strain curve is called the precursor timetprbecause rapid straining of the weakened layer occurs during this period. For small, as appropriate for tectonic applications and most laboratory experiments, a nonlinear asymptotic analysis predicts that, where λ is the half width of the peak of the stress‐strain curve, Δ is the difference in the peak stresses of the rock mass and the weak layer divided by λ times the elastic shear modulus, and α is a nondimensional measure of the strength of dilatant hardening. For a wide range of numerical values the precursor times are very short: less than a few hours for tectonic strain rates and less than a few tens of seconds for typical laboratory s

ISSN:0148-0227    

DOI:10.1029/JB089iB11p09259

年代:1984

数据来源: WILEY

摘要:

Frictional slippage on material interfaces is pervasive in mechanical problems of all types, but it can be particularly important in geomechanics. Extant models of cracked or jointed rock usually take frictional resistance along rock interfaces to be described by uniform Coulomb friction. A simple theory is presented which incorporates nonuniform friction. The description is by means of continuous distributions of infinitesimal dislocations. The resulting stress‐plastic strain behavior is nonlinear and stress history dependent and is a result of the generation, interaction, and annihilation of slip zones of unlike sign. The theory provides an explanation for the discrete memory effect observed during cyclic stressing of rock and also indicates potential difficulties in the design and interpretation of experiments on jointed roc

ISSN:0148-0227    

DOI:10.1029/JB089iB11p09271

年代:1984

数据来源: WILEY

摘要:

We have determined the asperity distribution in the segment of the Ecuador‐Colombia subduction zone that was ruptured by the 1979 (Mw= 8.2) and 1958 (Mw= 7.7) earthquakes. For the 1979 earthquake, we are able to document directivity in theP‐wave source time functions and consequently, have spatially located the moment release. By combining results from bothP‐wave and surface wave directivity, we have defined three regions: (1) from 0 to 56 km northeast of the epicenter, with low moment release and a small seismic displacement, (2) the asperity, from 56 to 116 km northeast of the epicenter, with most of the moment release and a displacement of ∼6 m, and (3) a small‐displacement region from 116 to 180–240 km northeast of the epicenter. Thus the 1979 rupture zone is composed of an asperity 60 km in length, bounded by two weak (small displacement) regions. The displacement at the asperity is equal to the cumulative tectonic displacement between 1979 and 1906 (the year of the previous great earthquake in this segment). The 1958 earthquake is characterized by the rupture of a small asperity (length scale approximately 25 km) in the epicentral region. The 1958 event continued to rupture past this small epicentral asperity and stopped near the epicenter of the following 1979 event. This detailed study of the spatial variation in moment release offers strong support for the asperity model of large earthquake

ISSN:0148-0227    

DOI:10.1029/JB089iB11p09281

年代:1984

数据来源: WILEY

摘要:

Highly significant temporal and spatial seismicity rate changes were found along the TongasKermadec plate boundary. The standard deviate (z) test was used to compare systematically the rates in different periods and different volumes, and to test for uniqueness of anomalies found. The NOAA hypocenter data file showed a pronounced decrease in the number of small‐magnitude earthquakes reported after 1969 at the same time as such a reporting decline exists in the worldwide data. Therefore only earthquakes withmb≥4.9 were studied. A search for quiescence before recent mainshocks yielded two uniquely significant precursory anomalies, one missed mainshock and one false alarm. Quiescence started 63 months before the January 1976,M= 8.0, Kermadec; and 25 months before the June 1977, Tonga,M= 7.2, earthquakes. The December 1975,M= 7.5, shock was not preceded by a seismicity rate change. The seismic gaps in the Tonga‐Kermadec arc with sufficient seismicity for analysis (17°–22° and 31°–34°) show constant rates up to the present. Therefore, on the basis of the quiescence hypothesis, earthquakes are not forecast for these segments. The stress drops, Δσ, of 380 interface events were estimated using themb/Msmethod. The average Δσ was uniform along the island arc with the exception of two segments with average Δσ of less than half the normal values. These segments coincided with the two largest recent interface ruptures; however, in the northern Tonga the Δσ was low before the 1975 rupture, whereas in the Kermadec the 1976 aftershocks showed low Δσ. Asperities could not be identified on the basis of high stress drops. A systematic study of the seismicity rate as a function of space defined large variations along the arc. The areas of highest and lowest seismicity rate correlate with the northern end of the trench and with the Louisville ridge intersection, respectively. However, other large rate contrasts exists between arc segments without obvious tectonic differences. This implies that seismicity doughnut patterns do not reliably define precursors. One segment of outstandingly high seismicity rate (30°–31°S) is identified as a major asperity because it contains a large number ofm>6 events and a large number of recent mainshock‐aftershock sequences and because it stopped the great 1976 rupture. We propose that the only other segment of high seismicity rate (near 20.5 °S) al

ISSN:0148-0227    

DOI:10.1029/JB089iB11p09293

年代:1984

数据来源: WILEY