摘要:

An analytic model has been developed for the prediction of the three‐dimensional deformation field generated by the withdrawal of magma from a sill‐like storage compartment during an intrusion or eruption cycle. The model is based on the work of Berry and Sales (1961, 1962) and predicts the vertical displacement components over the areal plane. Model parameters are the depth of burialh, the intrusion half widtha, the intrusion half lengthb, the thickness of the magmatic interior at the moment of melt withdrawaltm, and the planform aspect ratio ξ =a/b. The products of the model include areal deformation maps. Systematic variation in model parameters within the context of Kilauea Volcano, Hawaii, have revealed that circular and elliptical deformation patterns result from the collapse of draining rectilinear intrusions at depth. Moreover, the geometric parameters of a storage compartment may interact in complex ways to produce similar deformation patterns. The model has been applied to Kilauea Volcano for three periods of pronounced summit subsidence: (1) 1921–1927 (bracketing the steamblast eruptive phases of 1924); (2) June 1972 to December 1972, and (3) December 1972 to May 1973. Application of the model requires the simultaneous optimization of five predicted deformation features with respect to field measurements and the derivative deformation maps: (1) the vertical displacement maxima; (2) the vertical displacement gradients over the areal plane, (3) the lateral extent of the deformation field, (4) the aspect ratio of the subsidence pattern, and (5) the strike of the major axis of the deformation field. The constrained geometries and volumes of the inferred collapsed storage cavities for each period are (1) 1921–1927: depth ≅ 3 km,a≅ 1500 m,b≅ 4500 m,tm≅ 20 m,V540×106m3, (2) June 1972 to December 1972: depth ≅ 3.3 km,a≅ 600 m,b≅ 2000 m,tm≅ 1 m,V≅ 4.8×106m3, and (3) December 1972 to May 1973: depth ≅ 2.2 km,a≅ 500 m,b≅ 1612 m,tm≅ 1 m,V≅ 3.2×106m3. For 2 and 3, calculated magmatic thicknesses tmhappen to be in the range (3.48–0.15 m) of measurements for sill‐like bodies in deeply dissected Hawaiian shield volcanoes. The fits obtained between calculated and observed deformation patterns allow quantification of the location, overall dimensions, orientation, and volume of the discrete, still molten, interior of sill‐like compartments from which

ISSN:0148-0227    

DOI:10.1029/JB088iB05p04147

年代:1983

数据来源: WILEY

摘要:

We investigate the distribution of focal depths for earthquakes that do not appear to be associated with zones of recent subduction, using both new results from analyses of individual events recorded at teleseismic distances and published data for both microearthquakes and larger events. The deepest events in oceanic regions occur in old lithosphere (≥100 Ma), and excluding earthquakes in active mountain belts, the deepest crustal events occur in old cratons (tectonic age ≥800 Ma). Therefore, the temperature at the source region is likely to be an important factor determining whether deformation occurs seismically or not. From estimates of the temperatures at depths of the deepest events, we conclude that those limiting temperatures are about 250°–450°C and 600°–800°C for crustal and mantle materials, respectively. In several regions of recent continental convergence, in addition to shallow crustal seismicity, there is seismic activity in the uppermost mantle. The lower crust, however, is essentially aseismic. We infer that both the upper crustal and the mantle seismic regions correspond to zones of relatively high strength and that they are separated by a zone of lower strength in the lower crust where aseismic, ductile deformation predominates. This simple interpretation is qualitatively in agreement with extrapolated values of brittle and ductile strengths of geologic materials studied under appropriate pressure and temperature conditions in the laboratory. A low‐strength zone in the lower crust might allow detachment of crystalline nappes from the underlying mantle (and lower crustal) lithosphere. The apparently greater strength of mantle materials than crustal materials at the same temperature implies that oceanic lithosphere is much stronger than continental lithosphere, and this difference may account for why plate tectonics works well in oceanic regions but not

ISSN:0148-0227    

DOI:10.1029/JB088iB05p04183

年代:1983

数据来源: WILEY

摘要:

The regional variation of seismicity along the northern Honshu arc, Japan, is studied using accurate focal depths and focal mechanism types. We use focal depths determined from pP‐P time intervals reported in the ISC bulletins. For submarine earthquakes, depths are corrected by considering the pP phase reported in the bulletins as the pwP phase (the reflection from the ocean surface). Out of more than 600 well‐located earthquakes selected from the ISC bulletins, we determine the types of the focal mechanisms of 184 events using P wave first motion data. Based on historical seismicity of great and large earthquakes, we divide the zone of thrust type earthquakes at the plate interface into two regions: the shallow thrust zone (0–40km), where great earthquakes (MS∼8.0) occur, and the deep thrust zone (40–60km), where large earthquakes (MS∼7.4) occur. The activity of great or large earthquakes shows a variation along the arc; in some regions, both the shallow and deep thrust zones are active, and in other regions, only one of the thrust zones is active. The seismicity of recent moderate size earthquakes (mb>4) combined with the focal mechanism type shows a variation along the arc, which reflects the variation of the activity of great or large earthquakes. Where large earthquakes do not occur in the deep thrust zone, neither thrust type nor down‐dip compression/tension type events occur in and beneath the deep thrust zone. Where large earthquakes do occur in the deep thrust zone, we find a number of thrust type earthquakes. Further, in the latter case, in some regions, the down‐dip compression and tension type events of the double seismic zones extend seaward just beneath the deep thrust zone and form a triple‐planed structure of seismicity (the triple seismic zone). This study confirms the hypothesis of previous workers (Seno and Pongsawat, 1981) on the causal relation between the strong seismic coupling of two converging plates at the deep thrust zone and extension of the double seismic zone; i.e., the presence or absence of activity within the slab beneath the deep seismic zone occurs when the deep thrust zone has a strong or weak coupling, respectively. Here, the weak coupling could be interpreted as either aseismic slip or as low stress buildup since the last large event occurred at the deep thrust zone. Triple seismic zones are found offshore of Miyagi prefecture, where the deep thrust zone has been broken recently in 1978, and offshore of Fukushima prefecture. We expect a future large earthquake at the deep thrust zone offshore of Fukushima prefecture because the presence of the triple seismic zone suggests stress has been accumulating and 40 years have passed since the deep thrust zone was

ISSN:0148-0227    

DOI:10.1029/JB088iB05p04215

年代:1983

数据来源: WILEY

摘要:

We present a procedure for modeling the initially quasi‐static upward progression of a zone of slip from some depth in the lithosphere toward the earth's surface, along a transform plate margin, culminating in a great crustal earthquake. Stress transmission in the lithosphere is analyzed with a generalized Elsasser model, in which elastic lithospheric plates undergo plane stress deformation and are coupled by an elementary foundation model to a Maxwellian viscoelastic asthenosphere. Upward progression of rupture over a finite length of plate boundary, corresponding to a seismic gap along strike, is analyzed by a method based on the ‘line‐spring’ concept, whereby a two‐dimensional antiplane analysis of the upward progression provides the relation between lithospheric thickness‐averaged stress and slip used as a boundary condition in the generalized Elsasser plate model. The formulation results in a nonlinear integral equation for the rupture progression as a function of time and distance along strike. A simpler approximate single degree of freedom analysis procedure is described and shown to lead to instability results that can be formulated in terms of the slip‐softening slope at the boundary falling below the elastic unloading stiffness of the surroundings. The results also indicate a delay of ultimate (seismic) instability due to the stiffer short versus long time asthenospheric response and predict a final period of self‐driven creep toward instability. The procedures for prediction of rupture progression and instability are illustrated in detail for an elastic‐brittle crack model of slip zone advance, and parameters of the model are chosen consistently with great earthquake slips and stress drops. For example, an effective crack fracture energy of the order 4×106J/m2at the peak, 7 to 10 km below surface, of a Gaussian bell‐shaped distribution of fracture energy with depth, with variance of the order 5 km, simulating strength build‐up in a seismogenic layer, leads to prediction of nominal seismic stress drops of 30 to 60 bars and slips of 2 to 5 m in great strike slip earthquake ruptures breaking 100 to 400 km along strike. Precursory surface straining in the self‐driven stage is predicted to proceed at a distinctly higher rate over time intervals beginning 3 to 10 months before such an earthquake, this interval being greater for longer distances along strike over which the preseismic upward rupture

ISSN:0148-0227    

DOI:10.1029/JB088iB05p04231

年代:1983

数据来源: WILEY

摘要:

Local seismicity surrounding the epicenter of the March 14, 1979, Petatlan, Mexico earthquake was monitored by a network of portable seismographs of the Hawaii Institute of Geophysics from 6 weeks before to 4 weeks after the main shock. Prior to the main shock, the recorded local seismic activity was shallow and restricted within the continental plate above the Benioff zone. The relocated main shock hypocenter also lay above the Benioff zone, suggesting an initial failure within the continental lithosphere. Four zones can be recognized that showed relatively higher seismic activity than the background. Activity within these zones has followed a number of moderate earthquakes that occurred before or after the initial deployment of the network. Three of these moderate earthquakes were near the Mexican coastline and occurred sequentially from southeast to northwest during the three months before the Petatlan earthquake. The Petatlan event occurred along the northwestern extension of this trend. We infer a possible connection between this observed earthquake migration pattern and the subduction of a fracture zone because the 200‐km segment that includes the aftershock zones of the Petatlan earthquake and the three preceding moderate earthquakes matches the intersection of the southeastern limb of the Orozco Fracture Zone and the Middle America Trench. The Petatlan earthquake source region includes the region of the last of the three near‐coast seismic activities (zone A). Earthquakes of zone A migrated toward the Petatlan main shock epicenter and were separated from it by an aseismic zone about 10 km wide. We designate this group of earthquakes as the foreshocks of the Petatlan earthquake. These foreshocks occurred within the continental lithosphere and their observed characteristics are interpreted as due to the high‐stress environment before the main shock. Pre‐main shock seismicity of the Petatlan earthquake source region shows a good correlation with the aftershocks in their spatial distribution. This suggests that an asperity existing along the Benioff zone may have affected both the pre‐main shock activity in the continental lithosphere and the aftershocks along the Benioff zone. Although major thrust earthquakes at trenches occur along Benioff zones, in the present study we find little activity on this interplate boundary before the Petatlan earthquake. The overlying continental block, on the contrary, is very active seismically. Our data suggest that the activity is probably governed by the stress transmitted from below due to coupling between two plates and the heterogeneity within the continental lithosphere. The continental material is probably the more likely place for p

ISSN:0148-0227    

DOI:10.1029/JB088iB05p04247

年代:1983

数据来源: WILEY

摘要:

The spatio‐temporal patterns of seismic activity for events with body wave magnitude mb≥ 4.0 are investigated for the three most recent large earthquakes in the Oaxaca region of southern Mexico (August 23, 1965, MS= 7.6, MW= 7.5; August 2, 1968, MS= 7.1, MW= 7.3; November 29, 1978, MS= 7.8, MW= 7.6). A master catalogue of earthquakes is compiled for the analyses, using the International Seismological Center (ISC) catalogue supplemented by the National Earthquake Information Service (NEIS) catalogue; the events are then relocated with the Joint Hypocenter Determination (JHD) method. At this magnitude threshold, we find that the aftershock zone of the 1965 earthquake was seismically quiescent for at least 20 months prior to the main event; the 1968 earthquake was preceded by 1 year of foreshock type activity clustered in the aftershock zone; the 1978 earthquake was preceded by 43 months of quiescence and one event (mb= 4.7) within the aftershock zone 4 months prior to the mainshock. We also find that a segment of the subduction zone in Oaxaca remains unbroken by these earthquakes. In addition, some catalogue problems are pointed out which are important to interpretations of seismicity patte

ISSN:0148-0227    

DOI:10.1029/JB088iB05p04263

年代:1983

数据来源: WILEY

摘要:

The problem of spontaneous shear rupture of a single circular asperity on an infinite fault plane is studied. Initially, the fault plane is broken everywhere except at a circular asperity. An applied displacement at infinity results in a stress concentration along the bounding edge of the asperity. The frictional stress on the broken part of the fault plane is taken to be a constant. Once a point on the asperity breaks, the stress there drops to the same value as on the ‘main’ fault surface. The rupture is started by relaxing the shear stress at a point on the asperity edge and is then allowed to propagate spontaneously, using a critical stress level fracture criterion. The rupture process is calculated numerically. It is found that for asperities of constant strength, the rupture first propagates around the edge of the asperity and then inward, a phenomenon best described by the well‐known term of classic military maneuver: ‘the double encircling pincer movement.’ In the appendix, the expressions for the far‐field seismic radiation due to the rupture of such an asperity are derived. It is shown that thenth Cartesian component of the far‐field displacement at (x,t) forP,SV, andSHwaves, using the notation of Aki and Richards (1980), is given byun(x,t) = (Dni/4πρc2R)∫∫s0τi3{ξ,t− [(R− ξ · γ)/c]}dS(ξ). Thus the far‐field pulses can be directly found from the stress drops on the fault plane. This formula is also true for ‘crack’ or ‘dislocation’ problems. The directivity functionDnifor displacement for the asperity problem is found to be that for the double couple, modified by some factor. In particular, the fault plane is a nodal plane forSVwaves. For the rupturing of asperities on a finite fault, these directivity functions are applicable only to the initial part of observed pulses at a receiver, provided the receiver is not located on the fault plane outside the broken part of the main crack edge, in whi

ISSN:0148-0227    

DOI:10.1029/JB088iB05p04277

年代:1983

数据来源: WILEY

摘要:

Pn and Sn phases from 25 selected earthquakes recorded since July of 1979 on ocean bottom hydrophones near Wake Island are used to complement and extend prior investigations of high‐frequency Pn, Sn spectra in the Western Pacific. At a distance of about 18° (≃2000 km), frequencies for Pn and Sn are as high as 30 and 35 Hz, respectively; at a distance of about 30° ( ≃3300 km), as high as 15 and 20 Hz, respectively. Pn phases lose their high‐frequency energy more rapidly than Sn phases do, yet Pn wavetrains are much longer than Sn wavetrains, Pn wavetrains of longer duration, more energy, and higher frequencies are found for travel paths primarily in the Northwestern Pacific Basin than for travel paths across the transition zone from the shallow Ontong‐Java Plateau to the deep Northwestern Pacific Basin. Sn phases are extremely weak or absent for travel paths crossing this transition zone from the shallower Ontong‐Java Plateau to the deeper Northwestern Pacific Basin, whereas Sn phases are well recorded for travel paths crossing the transition zone in the opposite direction. Although normal, mantle‐refracted P phases are well recorded beyond about 21° ( ≃2300 km), available data indicate that detectable normal, mantle‐refracted P phases may not exist at distances f

ISSN:0148-0227    

DOI:10.1029/JB088iB05p04289

年代:1983

数据来源: WILEY

摘要:

It is shown that the density and bulk modulus profiles of a spherically symmetric fluid earth model are uniquely determined from the surface particle acceleration and pressure at two arbitrary frequencies. The excitation is a harmonically oscillating point source which is located on the surface of the earth.

ISSN:0148-0227    

DOI:10.1029/JB088iB05p04299

年代:1983

数据来源: WILEY

摘要:

Experimental techniques have been developed to study the propagation of large‐amplitude, one‐dimensional shear and compression waves in shocked solids of geophysical interest. Shear wave velocities and amplitudes can be directly measured along with the usual measurements on longitudinal waves. Although shear wave amplitude measurements do not approach the quality of the compression wave data, shear wave velocities can be measured to a precision of better than two percent. Experimental measurements to 90 kbar compressive shock stresses are reported in Al2O3. Within experimental scatter (±2%), our shear wave velocity data are in good agreement with the extrapolation of the ultrasonic measurements. The longitudinal measurements are in good agreement with earlier work. The experimental developments reported here are expected to be important to high pressure geophysics applications because they provide a direct measure of the shear modulus in the shocked state. The shear modulus is not only a more sensitive indicator of the solid state than the longitudinal modulus but in conjunction with the longitudinal wave data can provide the mean stress‐volume relations for comparison with stati

ISSN:0148-0227    

DOI:10.1029/JB088iB05p04304

年代:1983

数据来源: WILEY