Thermoelastic Properties of Ringwoodite [Fe_x,Mg_(1-x)]_2SiO_4: Its Relationship to the 520 km Seismic Discontinuity

We combine density functional theory (DFT) within the local density approximation (LDA), the quasiharmonic approximation (QHA), and a model vibrational density of states (VDoS) to calculate elastic moduli and sound velocities of gamma-[Fe_x,Mg_(1-x)]_2SiO_4 (ringwoodite), the most abundant mineral of the lower Earth's transition zone (TZ). Comparison with experimental values at room-temperature and high pressure or ambient-pressure and high temperature shows good agreement with our first-principles findings. Then, we investigate the contrasts associated with the beta-to-gamma-[Fe_x,Mg_(1-x)]_2SiO_4 transformation at pressures and temperatures relevant to the TZ. This information offers clearly defined reference values to advance the understanding of the nature of the 520 km seismic discontinuity.Comment: 29 pages, 6 figures, 2 tables. Under Revie

Distinct crustal structure of the North American Midcontinent Rift from P wave receiver functions

Eighty-two broadband seismic stations of the Superior Province Rifting Earthscope Experiment (SPREE) collected 2.5 years of continuous seismic data in the area of the high gravity anomaly associated with the Midcontinent Rift (MCR). Over 100 high-quality teleseismic earthquakes were used for crustal P wave receiver function analysis. Our analysis reveals that the base of the sedimentary layer is shallow outside the MCR, thickens near the flanks where gravity anomalies are low, and shallows again in the MCR's center where the gravity anomalies peak. This pattern is similar to that found from local geophysical studies and is consistent with reverse faulting having accompanied the cessation of rifting at 1.1 Ga. Intermittent intracrustal boundaries imaged by our analysis might represent the bottom of the MCR's mostly buried dense volcanic layers. Outside the MCR, the Moho is strong, sharp, and relatively flat, both beneath the Archean Superior Province and the Proterozoic terranes to its south. Inside the MCR, two weaker candidate Mohos are found at depths up to 25 km apart in the rift's center. The intermediate layer between these discontinuities tapers toward the edges of the MCR. The presence of this transitional layer is remarkably consistent along the strike of the MCR, including beneath its jog in southern Minnesota, near the Belle Plaine Fault. We interpret these results as evidence for extensive underplating as a defining characteristic of the rift, which remains continuous along the Minnesota jog, where due to its orientation, it is minimally affected by the reverse faulting that characterizes the NNE striking parts of the rift

GEOLOGIC APPLICATIONS OF SEISMIC SCATTERING

▪ Abstract  Once disregarded as noise, scattered seismic waves are finding increasing application in subsurface imaging. This sea change is driven by the increasing density and quality of seismic recordings and advances in waveform modeling which, together, are allowing seismologists to exploit their unique properties. In addition to extensive application in the energy exploration industry, seismic scattering is now used to characterize heterogeneity in the lower continental crust and subcrustal lithosphere, to examine the relationship between crustal structure and seismogenesis, and to probe the plumbing of active volcanoes. In each application, the study of seismic scattering brings wavelength-scale structure into sharper focus and characterizes the short scale-length fabric of geology. </jats:p

The nature of mantle layering from first-order reverberations

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1989.Includes bibliographical references (leaves 248-267).by Justin Scott Revenaugh.Ph.D

Mapping shallow heterogeneity with teleseismic <i>P</i> to <i>Rg</i> scattered waves

Abstract Over 9000 records of teleseismic P-wave coda from the Southern California Seismic Network are used to map lateral variations in P to Rg scattering strength and the group velocity of scattered Rg. Building on the results of Revenaugh (1995a), we use the correlation of scattering strength and topographic roughness as an indicator of model fit to estimate group-velocity dispersion over five frequency bands between 0 and 3 Hz. Rg group velocity displays normal dispersion, decreasing from 2.7 ± 0.3 km/sec for frequencies between 0 and 0.6 Hz to a low of 2.2 ± 0.4 km/sec for the 2.0 to 3.0 Hz band. Lateral heterogeneity is introduced by assuming Rg velocity is a function of local topographic roughness. Grid-search/migration analysis yields group velocities of 2.2 and 2.85 km/sec for “smooth” and “rough” terrain, respectively, and a 150-m threshold for the local standard deviation of topography distinguishing “smooth” from “rough”. The map of scattering strength approximates topographic roughness throughout southern California, implying that teleseismic P-wave coda duration and relative amplitude level depend strongly on near-surface (&amp;lt;3 km) geology.</jats:p

Seismic Detection of Rigid Zones at the Top of the Core

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. </jats:p

Observations of first-order mantle reverberations

AbstractWe have observed first-order mantle reverberations, specifically SH-polarized ScSn and sScSn phases reflected at near-normal incidence from upper mantle discontinuities, as discrete phases on long-period digital seismograms of the HGLP and SRO networks. Such arrivals correspond to an n or n + 1 member dynamic ray family denoted by {ScSn, Sd±S}, where n is the multiple number of the parent phase, “d+” signifies a reflection from the top of an internal discontinuity at depth d, and “d−” signifies a reflection from the bottom. The travel times and attenuation of these phases place important constraints on the nature of the transition zone. We have employed the phase equalization and stacking algorithm of Jordan and Sipkin (1977) to obtain the differential attenuation operators of {sScSn, S650+S} − sScSn and {ScSn, S650−S} − ScSn phase pairs from deep-focus Tonga events recorded by the HGLP station KIP on the island of Oahu. The apparent Q's of the zeroth-order and first-order reverberations over the frequency band of 10 to 30 mHz have been inverted for the average quality factors of the upper mantle (QUM) and lower mantle (QLM) and the reflection coefficient of the 650 km discontinuity (R650). At 20 mHz, the results are: QUM = 82 ± 18, QLM = 231 ± 60, and |R650| = 0.080 ± 0.004 (at normal incidence). The QLM estimate is significantly less than recent normal mode solutions, suggesting that lower mantle attenuation structure is frequency-dependent at very low frequencies. We have measured the differential travel times between the first-order reverberations {sScSn, S400+S} and {sScSn, S650+S}, which yields a direct estimate of the vertical shear-wave travel time between the discontinuities along the Tonga-to-KIP path. The observed two-way time, 92.2 ± 1.5 sec, is less than predicted by some recent oceanic upper mantle models, indicating either a smaller separation of the discontinuities and/or greater shear velocity in the transition zone. Measurements of differential travel times between first-order reverberations and multiple ScS phases suggest that the former is more likely and are consistent with depths of 400 and 650 km for the transition zone discontinuities. Observations of first-order mantle reverberations can potentially provide unique constraints on lateral heterogeneity of the earth's mantle, especially within the transition zone.</jats:p