Search

Elastic anisotropy is present where the speed of a seismic wave depends on its direction. In Earth's mantle, elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation. Earthquakes generate two seismic wave types: compressional (P) and shear (S) waves, whose coupling in anisotropic rocks leads to scattering, birefringence, and waves with hybrid polarizations. This varied behavior is helping geophysicists explore rock textures within Earth's mantle and crust, map present-day upper-mantle convection, and study the formation of lithospheric plates and the accretion of continents in Earth history., The rocks of Earth's upper mantle deform in the slow convective flows of our planet's hot interior. Deformation preferentially aligns mineral crystals within rock (1-3). This alignment induces elastic anisotropy, [...]

Data from earthquakes in the Tonga-Fiji region recorded at a seismic array in northern Australia show evidence for rigid zones at the top of the outer core. The ScP waveforms can be modeled by thin (0.12 to 0.18 kilometer) zones of molten iron mixed with solid material with a small, but positive, S-wave velocity (0.6 to 0.8 kilometer per second) that enables the propagation of S-waves in the outermost core. The zones may be topographic highs of the core-mantle boundary filled by light core sediments and might be important for variation of Earth's nutation and for convection of the outer core., The core-mantle boundary (CMB) region, where the molten iron outer core meets the solid silicate mantle, represents the largest compositional and theological contrast in Earth's interior. Strong lateral variations of [...]

The time it takes seismic waves to propagate from South Atlantic earthquakes through the inner core to station COL in Alaska has decreased systematically over the past 30 years. Travel times from three earthquakes in 1991 to an array of 37 seismometers in Alaska suggest that lateral gradients in seismic wavespeeds are steep in this part of the inner core. This combination of observations can be explained by postulating that the inner core is rotating 0.2 ° to 0.3 ° per year faster than the mantle., Numerical modeling of fluid flow in Earth's outer core has produced realistic models for the geodynamo that generates Earth's magnetic field (1-4). Rotation of the inner core should be coupled [...]

Long-period surface-wave ([R.sub.1], [G.sub.1]), body-wave (S, SS, SSS), and ScS-reverberation data have been inverted to obtain anisotropic structures along seismic corridors that sample Australia and the western Pacific. These models support the proposal that the Lehmann discontinuity beneath stable continents represents a transition from an anisotropic lithosphere to a more isotropic material in the lower part of the continental tectosphere., Lehmann (1) suggested that the behavior of refracted arrivals from North America and Europe at epicentral distances between 10° and 20° could be modeled by an increase in P-wave and [...]

When deformed, many rocks develop anisotropic elastic properties. On many seismic records, a long-period (100 to 250 seconds), 'quasi-Love' wave with elliptical polarization arrives slightly after the Love wave but [...]

Analysis of seismograph network data, earthquake catalogs from 1727 to 1982, and paleoseismic data for the central and eastern United States indicate that the Poisson probability of a damaging earthquake [...]

Seismic Trapped Modes in the Oroville and San Andreas Fault Zones SEISMIC TRAPPED MODES IN CRUSTAL fault zones illustrate the coherent interference phenomenon of wave propagation in a highly fractured [...]

The attenuation of upper crustal seismic waves that are refracted with a velocity of about 6 kilometers per second varies greatly among profiles in the area of the New Madrid seismic zone in the central Mississippi Valley. The waves that have the strongest attenuation pass through the seismic trend along the axis of the Reelfoot rift in the area of the Blytheville arch. Defocusing of the waves in a low-velocity zone and/or seismic scattering and absorption could cause the attenuation; these effects are most likely associated with the highly deformed rocks along the arch. Consequently, strong seismic-wave attenuation may be a useful criterion for identifying seismogenic fault zones., UNDERSTANDING THE CAUSES OF intraplate seismicity is one of the major problems in earthquake hazard assessment, particularly in the United States. Intraplate seismicity is commonly attributed to zones of crustal [...]

EMERGENT FIRST ARRIVALS IN SEIS-mic records of earthquake monitoring networks often confuse efforts to interpret the initial ground motion caused by an earthquake. Emergent phases have been ascribed to various [...]

The Southern California Earthquake Center (SCEC) was initiated in 1991 to try to understand the physical processes of earthquakes in southern California. In its third phase, the SCEC was to [...]

Heat is constantly transferred from Earth's hot interior to the surface through convection of the mantle. At Earth's surface, mantle convection drives plate tectonics, as manifested by earthquakes, volcanoes, and [...]

There's probably no stranger place in all Earth's layered interior than the intersection of the molten iron of the core with the bottom of the rocky mantle, 2900 kilometers down. [...]

With the use of a large collection of free-oscillation data and additional constraints imposed by the free-air gravity anomaly, lateral variations in shear velocity, compressional velocity, and density within the mantle; dynamic topography on the free surface; and topography on the 660-km discontinuity and the core-mantle boundary were determined. The velocity models are consistent with existing models based on travel-time and waveform inversions. In the lowermost mantle, near the core-mantle boundary, denser than average material is found beneath regions of upwellings centered on the Pacific Ocean and Africa that are characterized by slow shear velocities. These anomalies suggest the existence of compositional heterogeneity near the core-mantle boundary., Seismologists have produced numerous shear (S) and compressional (P) velocity models of the mantle based mainly on body-wave data (1-4). Unfortunately, they have been unable to develop a whole-mantle density [...]

After a large earthquake, seismic waves travel around the planet many times and eventually set up oscillations with typical periods of 3 to 54 min (see figure). Recently, several reports, […]

Observations of core-diffracted P ([P.sub.diff]) and SH ([SH.sub.diff]) waves recorded by the Missouri-to-Massachusetts (MOMA) seismic array show that the ratio of compressional (P) seismic velocities to horizontal shear (SH) velocities at the base of the mantle changes abruptly from beneath the mid-Pacific ([V.sub.p]/[V.sub.s] = 1.88, also the value predicted by reference Earth models) to beneath Alaska ([V.sub.p]/[V.sub.s] = 1.83). This change signifies a sudden lateral variation in material properties that may have a mineralogical or textural origin. A textural change could be a result of shear stresses induced during the arrival at the core of ancient lithosphere from the northern Pacific paleotrench., The core-mantle boundary (CMB) is an important component of the global system of mantle convection (1), but direct observations of mantle flow at this depth have been frustratingly difficult to [...]

Earth's core consists of a solid inner core with a radius of 1220 km surrounded by a liquid iron-rich outer core with a radius of 3480 km. The inner core […]