摘要:

PKP(Cdiff), the PKP diffraction waves after PKP‐C cut‐off, was identified in the PKIKP (or PKP(DF)) wave coda of teleseismic seismograms from a deep earthquake in western Brazil. An array stacking technique was applied to the short‐period waveforms from a regional seismic network in Taiwan to retrieve the weak PKP(Cdiff) waves between 159° and 162°. The differential travel times and slowness of PKP(Cdiff) and PKP(DF) were used to constrain the P‐velocity structure near the inner‐outer core boundary (ICB). The results of this study demonstrate that in some cases the ICB structure can be constrained by data from a single event using presently available short‐period regional seismic data and applying suitable array techniques. With a correction for axi‐symmetric anisotropy applied to the inner core, the results of this study agree with the PKP properties of model VMOI, a regional ICB model revised from PREM covering the ICB sampled area of this study, and model AK135, a new proposed earth model. The C‐cusp position was reported at 151.4° from this event. Theoretical predictions for another new proposed earth model SP6 is in significant disagreement with the observed differential travel times between PKP(Cdiff) and PKP(DF). At a distance of 159°, the residuals (observation‐prediction) included the observation errors (±0.2 sec) are approximately +0.17 sec, −0.31 sec and +0.69 sec for VMOI, AK135 and SP6, respectively. Neither model predicts a significant differential slowness residual. This misfit along this path may be the result of the lateral heterogeneity of the sampled region near the ICB relative to both spherically symmetric models or the complications imposed by the non‐axi‐symmetric

ISSN:0094-8276    

DOI:10.1029/96GL00087

年代:1996

数据来源: WILEY

摘要:

Viscosity measurements of Fe73S27(wt%) liquid at pressures between 2.0 and 5.0 GPa and at temperatures in the range 1100–1300°C have been made using Stokes' method of rising spheres in conjunction with a newly developed electro‐detection technique. The viscosities range from 43.6±7.0 Pa‐s at 3.9 GPa and 1100°C to 1.6±0.5 Pa‐s at 3.0 GPa and 1300°C. These high P viscosity values are three orders of magnitude higher than the 1 atm values and are tentatively attributed to the effects of S on the size of viscous flow units. From the T and P dependences of viscosity, activation energy and volume, respectively, have been determined. Activation energy decreases with increasing P, from 405±77 kJ/mol at 2.0 GPa to 240±72 kJ/mol at 5.0 GPa, and activation volume increases with increasing T, from 1.51±0.01 cm³/mol at 1100°C to 3.61±0

ISSN:0094-8276    

DOI:10.1029/96GL00216

年代:1996

数据来源: WILEY

摘要:

Pulsating phenomena can control continental breakup and the formation and closing of oceanic basins. Using a two‐dimensional time‐dependent convection model with a non‐linear, temperature‐ and depth‐dependent rheology, we have obtained self‐consistently such types of pulsating behavior. Very hot mantle plumes ascend to the surface, where they can quickly thin the lithosphere down to a few kilometers. An episode of intense shear heating then ensues, which is comparable to the length of the main episode for flood volcanism, lasting a couple of Myrs. The highest plate velocities occur during this period, when both the lithosphere and slab are profusely lubricated by the impingement of th

ISSN:0094-8276    

DOI:10.1029/95GL03661

年代:1996

数据来源: WILEY

摘要:

Volcanic rocks collected on flowline traverses on the flanks of the East Pacific Rise (EPR) document changes in axial magma chemistry with time, providing a record of the thermal history of axial magma chambers (AMC). We present data for a sample set of closely spaced (1–2 km) samples along EPR flowlines at three localities out to ∼800 ka showing both steady (constant average temperature) and non‐steady state behavior of magma chambers on time scales of 200–500 ka. Though based on only three symmetrical traverses so far, it appears that magmatically robust ridge locations (11°20′N and 9°30′N) have steady state chambers, whereas a magmatically starved axis (10°30′N) shows large temperature changes with time. These observations provide a new petrologic perspective to the ongoing debate regarding the significance and causes of morphologic variations along the axis of the E

ISSN:0094-8276    

DOI:10.1029/95GL00016

年代:1996

数据来源: WILEY

摘要:

Coronae are typically circular features, 100–600 km in diameter, characterized by a deformed annular ring that is often topographically high. The central region may be raised or depressed relative to the ambient elevation. Previous studies have proposed an evolutionary progression beginning with dome‐shaped features which have radiating extensional rifts, followed by plateau‐shaped features which have both concentric deformation and radial extension, ending with the classic corona as described above. Previous dynamical calculations of a rising and spreading diapir simulated the evolution from dome to plateau‐shaped topography, but could not account for the raised rim or depressed central topography of some coronae. We demonstrate that these features can be modeled by a diapir that spreads laterally at a depth of neutral b

ISSN:0094-8276    

DOI:10.1029/95GL03776

年代:1996

数据来源: WILEY

摘要:

The great 1857 Fort Tejon earthquake is the largest earthquake to have hit southern California during the historic period. We investigated if seismicity patterns following 1857 could be due to static stress changes generated by the 1857 earthquake. When post‐1857 earthquakes with unknown focal mechanisms were assigned strike‐slip mechanisms with strike and rake determined by the nearest active fault, 13 of the 13 southern California M≥5.5 earthquakes between 1857 and 1907 were encouraged by the 1857 rupture. When post‐1857 earthquakes in the Transverse Ranges with unknown focal mechanisms were assigned reverse mechanisms and all other events were assumed strike‐slip, 11 of the 13 earthquakes were encouraged by the 1857 earthquake. These results show significant correlations between static stress changes and seismicity patterns. The correlation disappears around 1907, suggesting that tectonic loading began to overwhelm the effect of the 1857 earthquake early in the 20t

ISSN:0094-8276    

DOI:10.1029/96GL00015

年代:1996

数据来源: WILEY

摘要:

Stress‐drops for small to moderately sized earthquakes in Southern California are found to vary systematically with source‐depth and location (tectonic environment). We determine high‐quality fault‐plane solutions, plus depth and source duration, for 17 significant (M>3.9) aftershocks associated with the June 28, 1992 Big Bear sequence, including the more recent April 4, 1994 19:04 GMTMw4.6 Lake Arrowhead aftershock, and aMw4.2 Banning Pass event which occurred on May 31, 1993 at 08:55 GMT. Given source durations and moments obtained from long‐period source estimations, and assuming a circular fault model, we estimate stress‐drop for each event. Big Bear aftershocks are moderate to high (>100 bars) stress‐drop. Events deeper than 12 km are generally high stress‐drop (>100 bars), while shallower events exhibit moderate to high stress‐drops. These results are compared with a similar analysis of Landers aftershocks in the Mojave block. For the Big Bear region, stress drops appear to correlate with depth, with the deepest events yielding the highest stress‐drops. In general, events in this region yield higher stress‐drops than events occuring in the Mojave block and those associated with the Landers and Joshua Tree sequences. Comparisons ofMLtoMOare consistent with the stress‐drop results: deep, high stress‐drop events

ISSN:0094-8276    

DOI:10.1029/96GL00012

年代:1996

数据来源: WILEY

摘要:

Major near‐vertical strike‐slip faults should reflect seismic energy due to velocity contrasts within and across the fault zone, analogous to sub‐horizontal faults commonly imaged by both multichannel and wide‐angle reflection surveys. An increase of velocity with depth in the upper crust allows such reflections to be detected in surface seismic surveys. We have observed fault reflection events from both sides of the San Andreas Fault in long‐offset common‐receiver gathers acquired near San Francisco. The events can be traced to the known surface position of the fault and provide constraints on the subsurface position to 3–7 km depth. Such reflections can be used to find the geometry of major near‐vertical faults, even in areas without active seismicity, providing information crucial to the understanding of the tectonics and seismic hazards of strik

ISSN:0094-8276    

DOI:10.1029/96GL00019

年代:1996

数据来源: WILEY

摘要:

We report the results of measurements of ultra‐low‐frequency magnetic noise during a large earthquake (Ms=7.1) at Guam of 8 August, 1993 (depth ∼60 km). The ULF observing system is located in the Guam Island, about 65 km from the epicenter. Several distinct features of this analysis are summarized. (1) We have proposed rather sophisticated statistical analyses (monthly mean, standard deviation) in order to estimate the wave intensity and polarization (i.e. ratio Z/H). (2) A comparison between the ULF wave activity and ΣKp, is useful in distinguishing between the space geomagnetic pulsations and non‐space emissions. (3) Then, the use of the ratio (Z/H) is found to be of essential importance in discrimating the emissions presumably of seismic origin from space plasma waves. (4) The statistical analysis of the temporal evolution of this ratio, has yielded that it shows a broad maximum only about one month before the earthquake, and this suggests that the emissions during this period are very likely to be magnetic precursors. (5) The temporal variation of Z component is similar to that for the Loma Prieta earthquake such that it shows a broad maximum ten days ∼ two weeks before the earthquake and another increase a few days before the earthquake. (6) The emissions presumably associated with the earthquake are of noise‐like nature, and their main frequency is 0.02 ∼ 0.05 Hz (with maximum inte

ISSN:0094-8276    

DOI:10.1029/95GL02863

年代:1996

数据来源: WILEY

摘要:

Conventional intercomparisons between absolute gravity meters are limited in accuracy by the difficulty of removing environmental signals, including effects such as Earth tides, ocean loading, and barometric pressure loading. These error sources can be eliminated completely through the use of exactly simultaneous measurements. Differencing techniques have been developed and tested which are capable of determining both the differential meter bias and the gravity difference between two nearby piers to an accuracy that is limited only by the instrumental noise. Preliminary tests show that a comparison accuracy of 0.3 µgal may be obtained in less than 400 drops, independent of drop interval (1 µgal = 10 nanometer/sec

ISSN:0094-8276    

DOI:10.1029/96GL00014

年代:1996

数据来源: WILEY