摘要:

We solve the elastic wave equation in spherical coordinates {γ,φ,θ} by a high‐order finite‐difference (FD) scheme. The Earth model and the required fields are defined on a staggered grid, independent in φ, and thus rotationally symmetric with respect to the axis θ = 0,π. This scheme allows us to model P‐SV wave propagation in a heterogeneous two‐dimensional Earth model. Since a uniform grid spacing in γ and θ is used, the maximum depth and epicentral distance that can be modeled is limited. Comparison with seismograms obtained by the Reflectivity Method (RM), and the Direct Solution Method (DSM), demonstrates the accuracy of the FD scheme. We use this algorithm to study the effects of heterogeneities in the core‐mantle transition zone, the D″ layer, on long‐period P‐waves. Long‐wavelength topography of a reflector in D″ produces significant focusing and defocusing. Random fluctuations (maximum perturbation ±10%) in a D″ layer of 300 km thickness produce a wave‐field similar to that of a sharp discontinuity only 200 km above the coremantle boundary (CMB) at the dominant period considered (15 seconds). Maps of global variations of D″ thickness determined with long‐period d

ISSN:0094-8276    

DOI:10.1029/96GL00422

年代:1996

数据来源: WILEY

摘要:

P‐wave data from the short‐period (around 1 Hz) Southern California Seismic Network reveal pre‐critical reflections from the 410‐km discontinuity below the Gulf of California, appearing as a clear and continuous feature in the data to epicentral distances as small as 10°. Deconvolution and stacking techniques are used to extract these weak arrivals from the coda of earlier arrivingPwaves. The reflections are used to estimate the width of the gradient zone near 410 km depth, constraining the velocity jump across the discontinuity using critically refracted waves. The velocity jump across the 410‐km discontinuity is 6±2.5 %, which is in agreement with existing estimates of the velocity jump for this region. The clear and uninterrupted branch of reflected waves suggests laterally homogeneous discontinuity properties on a scale of several hundred km. Synthetic seismogram modelling of the reflected waves using realistic models of the phase transition occurring at this depth shows that the transition must be as thin as 10 km, with most of the velocity increase occurring over about 4 km or less, for the reflections to be visible in short‐period data. Applying this result to recent thermodynamic models of upper mantle composition suggests that the high temperatures associated with the spreading centre continue down to at least

ISSN:0094-8276    

DOI:10.1029/96GL00378

年代:1996

数据来源: WILEY

摘要:

Compressional wave velocities (Vp) have been determined up to 2350 K on CaSiO3, MgSiO3, CaMgSi2O6, CaAl2Si2O8and Ca3Al2Si3O12glasses and melts from Brillouin‐scattering experiments made with a 180° backscattering geometry. At the glass transition, the decrease ofVpwith increasing temperatures becomes much stronger and the width of the Brillouin lines begins to increase markedly. At the highest temperatures investigated,Vpis similar to the relaxed values determined in acoustic measurements for several of these melts. This indicates that the configurational degrees of freedom of these liquids have become accessible within the very short timescale of Brillouin scattering experiments: equilibrium compressibilities of silicate melts can thus be determined with this technique. Our measurements also suggest that the shear modulus at infinite frequency of silicate melts could vary with either temperature or composition more strongly than assumed current

ISSN:0094-8276    

DOI:10.1029/96GL00308

年代:1996

数据来源: WILEY

摘要:

Phase B synthesized from a natural peridotite has been studied to 30.5 (±1.5) GPa on compression and decompression using infrared spectroscopy. The mode shifts of the OH1 (3345 cm−1) and the OH2 (3404 cm−1) vibrations of phase B are extremely small, −0.8 cm−1/GPa and 0.9 cm−1/Gpa, respectively, indicating that the hydrogen bonding in this phase is extremely insensitive to pressure. The opposite signs of the two mode shifts indicate that the OH1 hydrogen bond becomes stronger on compression, while the OH2 hydrogen bond weakens. Because both hydroxyl units are hydrogen bound to the same oxygen, this behavior may reflect hydrogen‐hydrogen repulsion in these sites as the lattice undergoes pressure induced compaction. Our results indicate that the environment surrounding the hydroxyl units in this phase is significantly more rigid than that present in a range of other hydrous magnesian silicate phases. Systematics of vibrational spectra among hydrous phases in the MgO‐SiO2‐H2O system indicate that pressure enhances hydrogen bonding through polymorphic transitions; yet, as illustrated by our results, the effect of pressure on hydrogen bonding within any single phase is small. Therefore, variations in hydrogen bonding may play a primary role in the notable polymorphism of hydrous phases a

ISSN:0094-8276    

DOI:10.1029/96GL00417

年代:1996

数据来源: WILEY

摘要:

We investigate a possible isotropic component in the source of the Bolivian earthquake of 09 June 1994 through a study of the radial modespS0(p= 0, 1) excited by the event. Our study departs from previous similar attempts by making use of a time series of sufficient duration (over 116 days) to sample the shape of the spectral line of the fundamental radial mode0S0, and by combining measurements of two radial modes, thereby lifting the trade‐off between deviatoric and isotropic sources of excitation. Our result (an isotropic component of 0.5±1.3% of the main double‐couple) fails to identify a significant change in volume in the source of this, the largest deep earthquake ever reco

ISSN:0094-8276    

DOI:10.1029/96GL00375

年代:1996

数据来源: WILEY

摘要:

We report on three intermediate depth earthquakes in the India‐Tibet collision zone, two under the Himalayan Thrust Belt (HTB) and one beneath the Indus Zangbo suture. Thembmagnitudes of these three events are from 4.3 to 4.9, and are too small to be well located by conventional means. However, from modeling their broadband waveforms recorded at near‐regional distances on a temporary PASSCAL array, we can confidently confine the sources to be below the crust, between 70 and 80 km deep. The existence of these intermediate depth earthquakes in this area suggests relatively low temperatures in the mantle lithosphere. The two events under the HTB display strike‐slip mechanisms with some normal faulting component; this is quite different from the shallow thrust events typical of the same area. The principal P and T axes of all 3 fault plane solutions show roughly NS compression and EW extension, consistent with a regional stress field produced by the indenting of the India cont

ISSN:0094-8276    

DOI:10.1029/96GL00385

年代:1996

数据来源: WILEY

摘要:

We use a forward numerical model to analyse the intra‐plate stress field in an area of active extension of continental lithosphere: the Aegean region. Recent tomographic images of the Hellenic subduction zone offer the unique opportunity to address the relation between the evolving Aegean stress field and temporal variations in the subduction process. To this extent we apply our model to both the present‐day and the Late Pliocene situation. Regarding the current situation it is found that subduction‐related forces largely explain the observed pattern of tension. Results obtained for the Pliocene indicate that detachment of the slab subducted below western Greece may have controlled the observed temporal variation in the pattern of te

ISSN:0094-8276    

DOI:10.1029/96GL00380

年代:1996

数据来源: WILEY

摘要:

The Investigator Fracture Zone (IFZ) subducts beneath Toba Caldera, the Earth's largest Quaternary caldera, in northern Sumatra, suggesting a possible relationship between them. Locations of sub‐crustal earthquakes based on arrival times of P and S waves at a seismograph network surrounding Toba reveal the geometry of the subducted slab and the IFZ beneath Toba. A vertical tear of less than 20 km in the slab across the IFZ, as previously suggested, cannot be ruled out but the large‐scale geometry of the slab is dominated by a broad bend of slab contours parallel to the concaveseaward indentation of the trench. The slab shape is probably a response to the trench curvature, can explain the change in trend of the volcanic arc near Toba, and may cause shallowing of the forearc basin near Nias Island. The decrease in radius of curvature of the slab contours is not accompanied by an observable decrease in dip angle, possibly resulting in lateral compressive stress in the slab. The high rate of seismicity along the subducted Investigator Fracture Zone, that intersects the slab obliquely to its plunge direction, is uncommon at subducted fracture zones and is likely caused by such lateral stress in the s

ISSN:0094-8276    

DOI:10.1029/96GL00381

年代:1996

数据来源: WILEY

摘要:

An unusual pattern of recurring seismic quiescence is observed in the Kalapana, Hawaii region of Kilauea Volcano. Statistically significant intervals of quiescence preceded the Kalapana earthquakes of 1975 (M7.2) and 1989 (M6.1) and a third quiescence is presently underway. The sensitivity of the volcano flank to continuing magmatic activity in the nearby east rift zone complicates interpretation of these observations. The current quiescence episode may be caused by magmatic processes in the east rift zone or by changes within the flank of Kilauea. The latter possibility, if correct, may represent a precursor to another earthquake.

ISSN:0094-8276    

DOI:10.1029/96GL00009

年代:1996

数据来源: WILEY

摘要:

The 1991–93 Etna eruption, that represented the most important recent eruptive event both in terms of duration (472 days) and total volume of erupted lava (ca. 250 · 106m³), caused marked ground deformation measured by using different geodetic techniques such as EDM, GPS, levelling, and tiltmetry. An inversion, termed “dynamic” because it took into account the sequence of events considering both the separate and cumulative effects of two sources, (a shallow tensile crack and a deeper ellipsoidal deflating source), was performed. It was based on 163 data collected by the different techniques plus 2 geometrical conditions for constraining the geometry of the crack on the surface. The final solution furnished a good fit for all measurements, defining a shallow tensile crack located inside the volcanic edifice, which represents the effect of the intrusion, and a depressuring body with centre at ca. 3000 m b.s.l., which could represent the centre of a generalized and more complex deflation of the volcanic edifice during the course of the e

ISSN:0094-8276    

DOI:10.1029/96GL00418

年代:1996

数据来源: WILEY