Seismic Velocity Discontinuities Research Articles

Low velocity zone of upper mantle and its effect on PdSwr phase related to 670 km discontinuity

This paper studied some properties of PdSwr phase related to 670 km discontinuities in detail, and theoretically processed a preliminary analysis to this phase. We discussed the relationships between the incident angle i h of PdSwr phase with its path, epicentral distance, travel-time and relative amplitude due to low velocity zone (LVZ) of upper mantle, and preliminarily pointed out the main characters of PdSwr phase recorded in seismogram. The PdSwr phase is concentrated in range of 13.5°–96.5°. When epicentral distance is greater than 33°, the start point of PdSwr phase is relatively well distinguishable and could thus be determined more easily. When the epicentral distance is between 13.5° to 33°, the triplication of PdSwr’s travel-time curve could be slightly distinguished due to the low velocity zone and 220 km seismic velocity discontinuity of upper mantle. The relevant observed PdSwr phase should be in a more complex pattern and it should be more difficult to determine its start point.

J‐Array Project seismic data now on CD‐ROM

Two CD‐ROMs containing seismic waveform data obtained from the J‐array Project are now available.The J‐array was a large‐aperture seismic array extending over all of Japan. It consisted of 14 regional seismic networks and more than 200 stations. The project was concerned with detecting small‐amplitude waves that are refracted, reflected, converted, or scattered from seismic velocity discontinuities existing deep within the Earth, and with studying the detailed structure and regional variations of the core, lower mantle, and upper mantle.Continuous digital seismic waveform data were recorded at the rate of 20 Hz with short period seismometers developed under Japan's national program for earthquake prediction. The J‐array project began in April 1991 and continued for 3 years at most of the networks. Many studies have been done with the data, including investigations of the structure of the upper mantle, lower mantle discontinuities, D″ layer, and core. Anyone interested in using the data can refer to J‐array Group [1993] for more information.

Seismic evidence for an inner core transition zone

Seismic waves that traverse Earth's inner core along north-south paths produce unusually broad pulse shapes at long periods (compared with waves along east-west paths) and reflections from below the inner core boundary at short periods. The observations provide compelling evidence for a seismic velocity discontinuity along north-south paths about 200 kilometers below the inner core boundary separating an isotropic upper inner core from an anisotropic lower inner core. The triplication associated with such a structure might be responsible for reported waveform complexity of short-period inner core arrivals along north-south paths and, if the depth of the boundary is laterally variable, their large travel-time variation.

Topography on the 410‐km seismic velocity discontinuity near subduction zones from stacking of sS, sP, and pP precursors

We stack the teleseismic depth phases sS, sP, and pP produced by deep focus earthquakes to image precursory arrivals that result from near‐source, underside reflections off the 410‐km seismic velocity discontinuity (hereinafter referred to as the 410) and use differential time measurements between these phases and their precursors to compute discontinuity depths near seven subduction zones around the Pacific Ocean margin. We begin by selecting seismograms with high‐quality depth phase arrivals recorded by several long‐period networks between the years 1976 and 1996. Filtering the waveforms and stacking them along theoretical travel‐time curves reveals clear precursors which vary in shape and timing. We compute confidence levels to evaluate the reliability of the observed precursory features using a bootstrap method that randomly resamples the seismograms prior to stacking. We measure the differential travel time between the reference pulse and the precursor using a cross‐correlation technique and convert this time to an apparent discontinuity depth using the isotropic Preliminary Reference Earth Model (PREM) at 25‐s period, corrected to an oceanic crustal thickness. The lateral resolution of our long‐period stacks for 410 topography is limited compared to that sometimes achieved in short‐period analyses but is much higher than that obtained from global SS precursor studies. For most subduction zones the results indicate little change in the average depth to the 410‐km discontinuity in the local areas sampled by the precursor bounce points compared to broad regional depths inferred from SS precursor results. This implies that any large variations in depth to the 410‐km discontinuity near subduction zones are limited to a narrow zone within the slab itself where they may be difficult to resolve with long‐period data. Coverage for the Tonga and Peru‐Chile subduction zones is sufficiently dense that we can observe lateral variations in 410 depths. In Tonga the results suggest depth variations perpendicular to the slab of up to 33 km, after correcting for probable lateral heterogeneity in velocity above 400 km depth, and variations parallel to the slab orientation as large as 13 km. The cross‐slab variation is consistent with the elevation of olivine phase transformations in cold regions; the variation along strike suggests a more complex thermal heterogeneity that may be related to the subduction history of this region. We see evidence for additional reflectors above the 410 in some of the waveform stacks, but the inconsistency and weak amplitude of these features preclude definitive interpretations.

Global mapping of topography on transition zone velocity discontinuities by stacking SS precursors

We stack long‐period, transverse‐component seismograms recorded by the Global Digital Seismograph Network (GDSN) (1976–1996), Incorporated Research Institutions for Seismology‐International Deployment of Accelerometers (IRIS‐IDA) (1988–1996), and Geoscope (1988–1996) networks to map large‐scale topography on the 410‐ and 660‐km seismic velocity discontinuities. Underside reflections from these discontinuities arrive as precursors to the SS phase, and their timing can be used to obtain global variations of the depth to the reflectors. We analyze over 13,000 records from events mb>5.5, focal depth <75 km, and range 110° to 180° by picking and aligning on SS, then stacking the records along the theoretical travel time curves for the discontinuity reflections. Separate stacks are obtained for 416 equally spaced caps of 10° radius; clear 410‐ and 660‐km reflections are visible for almost all of the caps while 520‐km reflections are seen in about half of the caps. The differential travel times between the precursors and the SS arrival are measured on each stack, with uncertainty estimates obtained using a bootstrap resampling method. We then compute discontinuity depths relative to the isotropic Preliminary Reference Earth Model (PREM) at 40‐s period, correcting for surface topography and crustal thickness variations using the CRUST5.0 model of Mooney et al. [1995], and for upper mantle S velocity heterogeneity using model S16B30 of Masters et al. [1996]. The resulting maps of discontinuity topography have more complete coverage than previous studies; observed depths are highly correlated between adjacent caps and appear dominated by large‐scale topography variations. The 660‐km discontinuity exhibits peak‐to‐peak topography of about 38 km, with regional depressions that correlate with areas of current and past subduction around the Pacific Ocean. Large‐scale topography on the 410‐km discontinuity is lower in amplitude and largely uncorrelated with the topography on the 660‐km interface. The width of the transition zone, WTZ, as measured by the separation between the 410‐ and 660‐km discontinuities, appears thickest in areas of active subduction (e.g., Kurils, Philippines, and Tonga) and thins beneath Antarctica and much of the central Pacific Ocean. Spatial variations in WTZ appear unrelated to ocean‐continent differences but do roughly correlate with the S16B30 velocities in the transition zone, consistent with a common thermal origin for both patterns. The lower‐amplitude 520‐km reflector is more difficult to resolve but appears to be a global feature as it is observed preferentially for those bounce point caps with the most data.

Spectrum Analysis of the Reflected Waves Observed in the Nikko Region, Central Japan

—The Nikko region, in northern Kanto, central Japan, which is a high seismicity region, is located near the volcanic front. Many seismic observations have been carried out around the Nikko region because the seismograms of the shallow earthquakes occurring in this region show an S-wave reflected phase (SxS) and an S-to-P mode-converted and reflected phase (SxP) from a mid-crustal seismic velocity discontinuity. The depth of the mid-crustal reflector was estimated to be about 9–16 km. From the amplitude analyses of SxS and SxP phases, the material of the mid-crustal layer just beneath the reflector was expected to be a low rigidity layer, which was interpreted as partially molten rocks. No evidence has been shown for the internal properties of the material of the reflector. The Vp/Vs ratio is a good measure to distinguish between the two candidates for seismic reflectors; i.e., partially molten rocks and rocks containing water. We investigate the Vp/Vs ratio of the reflector using the spectrum analyses of the data observed by a seismic array. From spectrum analysis of the data, we indicate that the Vp/Vs ratio of 2.5 is better than that of 1.8 to explain the observed data, which shows that the material beneath the reflector consists of partially molten rocks.

Distinct S wave reflector in the midcrust beneath Nikko‐Shirane volcano in the northeastern Japan arc

Distinct S waves reflected from a midcrustal seismic velocity discontinuity are detected beneath Nikko‐Shirane volcano in the southernmost part of the northeastern Japan arc. A detailed travel time analysis of the reflected S waves by using data acquired through a dense seismic network temporarily set up in this region shows that this unusual S wave reflector is distributed over an area of 15 × 15 km2 at depths ranging from 8 to 15 km. The reflector has a conical shape becoming shallow toward the summit of Nikko‐Shirane volcano. Observed amplitude spectral ratios of reflected S waves to direct S waves show that the reflector body has a strong velocity contrast to the surrounding medium and its thickness is of the order of 100 m at most. The reflector body is approximated by two thin layers probably filled with partially molten materials. Cutoff depth for shallow seismicity in this area is 3–5 km above the reflector and becomes shallow toward Nikko‐Shirane volcano, nearly parallel to the reflector. The depth to brittle‐ductile transition zone seems to be prescribed by the existence of the reflector body, which is perhaps a thin magma body.

Elasticity of forsterite to 16 GPa and the composition of the upper mantle

NEARLY 60 years ago, Bernal1 proposed that a polymorphic phase transformation in olivine might be responsible for the seismic velocity discontinuity near 410 km depth in the mantle. Phase equilibria experiments2,3 have since shown that the olivine (α) to wadsleyite (β) transition in (Mg,Fe)2SiO4 occurs at the appropriate pressure (13.8 GPa) under mantle conditions. Comparison of laboratory measurements of the acoustic velocity contrast in the α–β system to the magnitude of the seismically observed discontinuity at 410 km provides a way to constrain the olivine content of the mantle at this depth. Here we report measurements of the full set of elastic moduli of single-crystal forsterite (α-Mg2SiO4) at pressures between 3 and 16 GPa, using Brillouin scattering in a diamond anvil cell. At 13.8 GPa, the aggregate compressional-and shear-wave velocities of α-Mg2SiO4 are 2.7 ± 0.7% lower than predicted from earlier low-pressure data4,5. From our data, and assuming a homogeneous mantle composition, the seismic velocity contrast at 410 km depth can be satisfied only by a mantle containing less than ~40% olivine. This is well below the olivine abundance assumed in peridotite-based upper-mantle models.

Thermodynamic coupling of phase and chemical boundaries in planetary interiors

Mass-transfer equilibrium in multi-component closed systems at thermal and mechanical equilibrium requires that the chemical potential of each component be uniform over all phases in the system, yielding the classical Gibbs phase rule relating the number of independent components (i.e. chemical species), the number of externally unconstrained state variables (i.e. ‘degrees of freedom’), and the number of coexisting phases at equilibrium. More general closed systems, however, may be subject to thermal or mechanical disequilibrium (e.g. non-zero temperature gradients), giving rise to non-zero chemical potential gradients. Such systems require ‘intra-phase’ equilibrium conditions upon the chemical potential gradients in addition to the usual ‘interphase’ conditions upon the chemical potentials themselves, yielding a modified phase rule restricting the stability of regions of multi-phase coexistence. The attainment of mass-transfer equilibrium under such conditions requires that isochemical multi-phase systems break down into chemically distinct juxtaposed single-phase regions, thermodynamically coupling multivariant phase transitions to chemical discontinuities. Hence, for example, phase change and chemical change hypotheses for seismic velocity discontinuities in the Earth's interior need not be mutually exclusive. The kinetic hindrance by slow diffusion of phase-transition-coupled chemical differentiation in the Earth may be alleviated by high temperatures, the presence of fluids or melts, convective mass flow, negative Clapeyron slopes, or stratification due to earlier episodes of differentiation. Radial mass-transfer equilibrium across the spinel-perovskite transition for an upper mantle with X Mg ≈ 0.89, for example, yields a stable contrast in Fe/Mg composition with X Mg ≈ 0.86 in the lower mantle.

The J-Array Program: System and Present Status

J-Array is a large-aperture short-period seismic array extending over the whole Japan with its dimension. being 3000 km in length and 300-500 km in width. It counts 218 stations. The recording system and the present status of J-Array program are presented in detail. Array analyses of a large volume of seismic waveform data recorded at J-Array enable us to detect even small-amplitude seismic waves refracted, reflected, converted or scattered at seismic velocity discontinuities or inhomogeneities existing in deep portions of the Earth, which convey information on the detailed structure and the regional variation of the central core, the lower mantle and also the upper mantle.

Tomographic imaging of P and S wave velocity structure beneath northeastern Japan

The seismic body wave tomography method has been improved and extended to adapt to a general velocity structure with a number of complexly shaped seismic velocity discontinuities (SVDs) and with three‐dimensional variations in the velocities in the modeling space. An efficient three dimensional ray tracing algorithm which iteratively uses the pseudobending technique and Snell's law is developed. The large and sparse system of observation equations is solved by using the LSQR algorithm. This method is applied to 18,679 arrival times from 470 shallow and intermediate‐depth earthquakes in order to study P and S wave tomographic images beneath northeastern Japan. In addition to first P and S wave arrivals, clear later arrivals of SP waves converted at the Moho and PS and SP waves converted at the upper boundary of the subducted Pacific plate (UBPP) are also used in the inversion. The UBPP, Conrad and Moho are taken as three SVDs, and their depth distributions obtained by previous studies are used. High‐resolution P and S wave tomographic images down to a depth of 200 km have been determined. Large velocity variations amounting to 6% for P wave and 10% for S wave are revealed in the crust and upper mantle. In the crust low‐velocity (low‐V) zones exist beneath active volcanoes. In the upper mantle the low‐V zones dip toward the west from the volcanic front. A high‐velocity (high‐ V) zone corresponding to the subducted Pacific plate is clearly delineated. Most earthquakes in the lower plane of the double‐planed deep seismic zone are found to occur in relatively high‐V areas. The obtained tomographic images are also found to explain other seismological observations well.

The Spatial Correlation Between a Lateral Seismic Velocity Discontinuity in the Precambrian Basement Rock and the Sharpsburg, Kentucky Earthquake of July 27, 1980

Seismic refraction data from a series of profiles in northeastern Kentucky near the epicenter of the 1980 Sharpsburg earthquake, show that the earthquake occurred within the immediate vicinity of a seismic velocity discontinuity in the Precambrian basement rock. The P-wave velocity of the basement rock to the west of the discontinuity is 6.15 km/s, while the P-wave velocity of the basement rock to the east of the discontinuity is 6.9 km/s. It is suggested that the geologic feature responsible for the localization of the Sharpsburg, Kentucky earthquake is the seismic velocity discontinuity in the basement.

Geologic and seismic velocity structure of the crust/mantle transition in the Bay of Islands Ophiolite Complex

Geological investigations of the Bay of Islands Ophiolite Complex show that while a typical layered ophiolite suite is present, the thickness of major lithologic units is extremely variable from place to place. The composition and internal structure of map‐scale (kilometers across) lithologic units as well as the contacts that bound them are laterally variable. Inferred velocity‐depth functions of the crust/mantle (“Moho”) transition reconstructed for this terrane as oceanic lithosphere suggest an extremely complex internal seismic structure. The crust/mantle transition from mafic to ultramafic lithologies varies across the ophiolite from a sharp geologic and seismic velocity discontinuity to a complexly interlayered transition zone as much as 3 km thick. The mafic/ultramafic transition is characterized by laterally discontinuous lithologic units on the order of hundreds of meters to a few kilometers thick and up to several kilometers long. These units (megalenses) may be composed of lithologies with higher or lower seismic velocities than those of the surrounding units. Solid‐state deformation has produced seismically anisotropic materials in the upper mantle and lower crustal units. Substantial relief and slopes of up to at least 12° over lateral distances of about 10 km occur on the top of the crust/mantle transition zone as well as other geologic unit contacts. Although these types of geological features are striking to the field geologist working in ophiolites and are significant to the understanding of crustal accretion processes, many of them occur on such a small scale that they might go undetected in different types of seismic experiments in contemporary oceanic lithosphere.

The moho as a phase change: A test of the hypothesis

Heat flow, seismic, and gravity data are used to determine the pressure and temperature at the crust/mantle boundary and at the Conrad seismic velocity discontinuity (where the compressional wave velocity jumps to 6.5 km/s or more) for 12 sites located in the central United States or in the High Plains region. If these discontinuities correspond to phase changes in thermodynamic equilibrium, the pressure and temperature are related by the integrated Clausius‐Clapeyron equation. An ‘empirical Clausius‐Clapeyron’ equation is determined as the best fitting line to the pressures and temperatures obtained at the different sites. Two thermal models of the crust corresponding to different assumptions on the variation of the thermal conductivity, yield as an integrated Claussius‐Clapeyron equation P = (0.085T + 6.2) kbar and P = (0.095T + 4.8) kbar for the Moho (where T is the temperature in degrees Celsius). The correlation coefficient between the temperatures and pressures is too low to give strong support to the hypothesis that the Moho in this region corresponds to a phase change in equilibrium; however, the authors favor the hypothesis because the pressures and temperatures are within the range experimentally determined by Ito and Kennedy [1970] for the transformation from garnet‐granulite to plagioclase‐eclogite.

Calculations of high-pressure phase transitions in the system MgOSiO 2 and implications for mantle discontinuities

By incorporating the knowledge of the observed high-pressure phase transformations, the measured equilibrium phase boundaries for certain transitions, and the measured thermochemical data for certain phases in the system MgOSiO 2, the equilibrium phase boundaries for all the phase transitions in this system at 1000°C have been calculated on the basis of an assumption that volume change across a phase boundary is independent of temperature. Three major seismic velocity discontinuities in the mantle (420, 570, and 650 km) have been chosen for comparison with the measured and calculated equilibrium phase boundaries for the high-pressure phases observed in the system MgOSiO 2. Complications of phase changes due to the addition of FeO and Al 2O 3 to the system have also been accounted for. It is suggested by the results of this study that the 420- and 570-km discontinuities are probably related to phase transitions observed in the system MgOSiO 2, but that the 650-km discontinuity is not likely to be associated with any of the equilibrium phase boundaries observed in olivine, pyroxene, and garnet, but may instead be a chemical change.