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Seismic Wave Propagation

Propagation of seismic shear waves in Earth's mantle from a deep earthquake (red star at lower left). Part of a computer animation done with the summation of Earth's normal modes of oscillation (Image by M. E. Wysession and G. Caras). For access to digital animations of seismic wave propagation, and information on how to request VHS versions, go to Shear Wave Propagation Animations.


MACOMO (Madagascar, the Comores, and Mozambique: A Broadband IRIS PASSCAL Deployment)

An array of 32 seismic stations was installed over 2011-2012.We plan to investigate the occurrence of hot spot intraplate volcanism in Madagascar and Comores Islands and its connection to the African superplume with an array of broadband seismometers. The African superplume is the largest seismic anomaly within the earth, and as an enormous thermochemical boundary layer between the mantle and core, it plays a vitally important role in controlling the nature of vertical mass flux in the mantle, and therefore both mantle convection and its manifestation at the surface as plate tectonics. The African superplume is thought to play an important role in the abundant occurrence of hot spot volcanism on the African plate, but the mechanism by which this occurs is still not known due to the extremely limited data sampling available at this location in the Southern Hemisphere. A large array on the world’s 4th-largest island (the first of its kind) will provide u nprecedented data that will allow us to seismically map (using velocity and attenuation seismic tomography, shear-wave splitting, and receiver function analysis for topography variations on mantle boundaries) the connection between surface hot spot volcanism and the massive lower mantle African superplume. This project will be closely tied to complementary activities being carried out as part of AfricaArray.

Michael Wysession and Douglas Wiens, Department of Earth and Planetary Sciences, Washington University, St. Louis, MO
Andrew Nyblade, Department of Geosciences, Penn State University, University Park, PA


SPREE (Superior Province Rifting EarthScope Experiment)

We installed a group of 84 broadband seismic stations from the EarthScope Flexible Array pool to explore the deep structure of the Mid-Continent Rift System (MCRS), the remnant of an extensive rifting event that failed to split the North American craton during the Mid-Proterozoic. How such rifts form and die, rather than developing into oceanic spreading centers, is unclear despite their key role in shaping the fabric of the continents.

Understanding the structure of the deep crust and mantle beneath the MCRS will provide new constraints on key issues about the rifting event and its cessation. The role of the mantle should be reflected in velocity anomalies associated with melt depletion and seismic anisotropy. Velocity structure across the rift will constrain the across-strike extent of crustal and lithospheric thinning. The change in seismic velocity structure across the rift's northern end should give insight into what controlled its along-strike geometry. Because the MCRS is seismically inactive, in contrast to younger failed rifts, comparison of seismic velocities will give insight as to how the crust "heals" mechanically.

Suzan van der Lee, Donna Jurdy, Seth Stein (Dept Geological Sci., Northwestern University)
Andrew Frederiksen (Dept Geological Sci, University of Manitoba, Winnipeg)
Justin Revenaugh (Dept Geology and Geophysics, University of Minnesota, Minneapolis)
Fiona Darbyshire (Centre GEOTOP, University of Quebec, Montreal)
Douglas Wiens, Michael Wysession (Dept EPS, Washington University, St. Louis)


Whole-Mantle Seismic Attenuation

We have generated the first whole-mantle model of seismic shear-wave attenuation, and continue to develop means of looking at earth structure using seismic attenuation. The model VQM3D was made using 90,000 differential shear-wave attenuation measurements. A velocity model was simultaneously generated using the same data, and values were computed independently from radial and transverse data. The model shows a very strong correlation with the large-scale whole-mantle process of subduction. The Pacific and African megaplumes also show up as very broad regions of low Q. [Lawrence and Wysession, 2006a,b]

Here is an IRIS "1-Pager" on the subject.


Evidence for Water in the Lower Mantle

One of the most dramatic features in our global mantle shear-wave attenuation model is a very low-Q anomaly at the top of the lower mantle beneath eastern Asia. We believe that this is due to water that has been pumped into the lower mantle via the long history of the subduction of oceanic lithosphere in this region. This could result from the dehydration of hydrous phase D from cold lithosphere that has been subducted into the lower mantle. We are very interested in further pursuing the effects of water on seismic attenuation within the mantle. [Lawrence and Wysession, 2006a,b]

Go here for a press release on Water in the Mantle

Go here for a write-up on it in Popular Mechanics


Missouri-to-Massachusetts (MOMA) IRIS PASSCAL Seismic Array

The Missouri-to-Massachusetts (MOMA) IRIS PASSCAL Broadband Deployment was jointly supervised by Karen Fischer, Michael Wysession, and Timothy Clarke, and involved the deployment of 18 broadband seismometers during 1/95 - 4/96 [Fischer et al., 1996; Wysession et al., 1996 ]. These stations were evenly spaced between permanent sites CCM (Cathedral Caves, Missouri) and HRV (Harvard, Massachusetts), providing a 20-station array spanning 1740 km with a 91.5 km average interstation distance. The scientific goal of the deployment was to closely examine at high resolution several different regions within the Earth using a single linear array of stations. This geometry provided excellent data yielding exciting new information about the structure of the core-mantle boundary, the seismic structure of the eastern U.S. crust and upper mantle, and the occurrence of seismic anisotropy at the top and bottom of the mantle. The complete 20 sps data are publicly available in SEED format through the IRIS DMC.

For more information about the MOMA project, go to the WashU MOMA Page.

Go here for information about collaborative MOMA projects at other institutions.


Florida-to-Edmonton (FLED) IRIS PASSCAL Seismic Array

During the Florida to Edmonton Broadband Seismometer Experiment (FLED), 28 broadband IRIS/PASSCAL STS-2 seismometers were deployed from central Florida to Alberta, Canada. Together with adjacent permanent stations from the IRIS/GSN, the USNSN the CNSN, and the New Madrid Seismic Network, the roughly linear array crossed diverse tectonic provinces and features, including the rifted continental margin, the Appalachian orogen, the Proterozoic and Archaean provinces of the continental interior, the Mid-Continent Rift and the Williston Basin. The portable stations were in the field from May, 2001 to October, 2002, and data quality was in general very good.

Go here for an IRIS "1-Pager" on the deployment.


Diffracted Wave Dispersion

We have developed a method to use the dispersion of core-diffracted Pdiff and Sdiff waves as a means of identifying the vertical velocity structure of the D′′ layer at the base of the mantle. When compared to synthetic seismograms, the method not only gives a measure of the mean velocity of the region of the CMB sampled, but also a measure of the vertical velocity structure of D′′, which is a very important and difficult-to-measure quantity [Euler et al., 2005]. Here is an IRIS "1-Pager" on the subject.


Variations in Vp/Vs at the Base of the Mantle

We are interested in the variations in the Vp/Vs velocity ratio at the base of the mantle, as an inticator of structure and composition there. MOMA slownesses of Pdiff and Sdiff phases showed large-scale D'' lateral variations for both P (2%) and S (3%) waves. The sub-Alaskan core-mantle boundary region is unusual, with the study's fastest S velocities (Vs) but slowest P velocities (Vp). The map shows the inferred variations in D'' Vp/Vs found from the combination of Pdiff and Sdiff slowness residuals from western Pacific earthquakes recorded at MOMA. The huge drop in Vp/Vs beneath Alaska requires a non-thermal contribution such as from chemical heterogeneity or anisotropy. [Wysession et al., 1999]


Radial Lower Mantle Shear-Wave Attenuation Model

We have obtained a radial model for shear-wave attenuation in the mantle. We employed a niching genetic algorithm to invert roughly 30,000 differential ScS/S attenuation values for a new radial quality factor (Q) model with high sensitivity to the lower mantle. The new radial Q model, QLM, possesses greater sensitivity to Q at great depths than previous studies. The model has two high-Q regions at ~1000 and ~2500 km depth, roughly corresponding to the high viscosity regions observed by Forte and Mitrovica [1996]. The best-fit radial model also has a lower-Q layer at the lowermost mantle. By relating Q to homologous temperature we infer a divergence of the lower mantle solidus and geotherm and a convergence within D''. While large regional variations in Q certainly occur in the mantle, for applications such as geodynamic mantle flow models, radial models are still useful. [Lawrence and Wysession, 2005]

Here is an IRIS "1-Pager" on the subject.


Diffracted Wave Amplitude-Frequency Modeling

We have been able to analyze the amplitude decay (as a function of frequency) of core-diffracted waves using PASSCAL array data as a means of examining the vertical velocity structure of the lowermost mantle. The frequency-dependent amplitude decay of MOMA Sdiff waves from a Kermadec earthquake provided evidence for a very thick (~200 km) thermal boundary layer above the eastern Pacific CMB. Modeling was also compatible with a region of lower seismic velocities lying directly above the thermal boundary layer. While this model of discontinuity is usually identified with SdS and PdP waves, it also provided the best fit to the Sdiff amplitudes. [Valenzuela and Wysession, 1998]


Using ScS waves to Image the Lowermost Mantle

We have long been interested in using the reflections of ScS and PcP waves to map out lateral variations in the velocity structure of the D'' layer at the base of the mantle. (1) In the example shown here, South American earthquakes are recorded at the MOMA array and other North American stations, with the ScS bounce points beneath the western Caribbean Ocean. (2) Top: The differential ScS-S travel times suggest a region of slow seismic shear velocities in the center of our region of sampling. Middle: The differential attenuation anomalies also suggest a region of high attenuation in the same location. Bottom: When the travel times are corrected using the differential delta-t* values, the region of low-velocity and low-Q becomes narrowed and focused. (3) There is a log relationship between the attenuation and velocity, suggesting a thermal origin to the anomaly at the CMB. (4) This kind of a feature has been modeled by Tan and Gurnis [2002] as a small incipient plume. [Fisher et al., 2003]


Seismic Source Discrimination

We have used the amplitudes of Lg waves as a means of discriminating earthquakes from nuclear bombs. A genetic algorithm is used to determine both the Lg path attenuation and the source moment. As the diagram shows, the method clearly distinguishes Western U.S. earthquakes from Nevada nuclear tests. [Aleqabi et al., 2001]


Lg Wave Attenuation From Earthquakes and Bombs

The Lg paths used in the previous study for nuclear source discrimination were also inverted to constrain the crustal attenuation in the Western United States. [Aleqabi and Wysession, 2006]


The D'' Discontinuity

We are using array processing techniques, like this example of a slant-stack, or vespagram, in order to identify the locations of the D'' discontinuity using ScS precursors recorded at PASSCAL arrays like MOMA and FLED. In this case, a very small D'' discontinuity is identified on FLED array data for SdS waves reflecting off of the core in the vicinity of the Western Caribbean Ocean. [Wysession et al., 2005]

Department of Earth & Planetary Sciences Washington University in St. Louis