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Patagonia Transantarctic Mtns

3D V & Q Model North America

TZ Thickness TZ Structure
As a seismologist, my job is to use seismic energy that radiates from earthquakes to image the Earth. One way to image the Earth is to measure changes in the depth of the crust-mantle boundary using receiver functions. Lateral variations in seismic wave speed can illuminate mantle heterogeneity.

Most seismologists focus on the elastic properties of the Earth like seimic wave speed. I prefer to examine seismic attenuation (a form of anelastic energy loss) and compare it to seismic velocity, which can help distinquish the cause of seismic anomalies in the Earth.

The mantle transition zone is a region in the Earth where seismic velocities and density increase rapidly with depth/pressure. Discontinuities at about 410, 520, and 660 km depth are measured with energy that reflects and converts at depth. Better constraints on these discontinuities can help improve our understanding of how Earth cools itself. If the density contrast is too high, it will block material from flowing past this interface.

Receiver Functions: Receiver functions are waveforms that characterize the Earth's shear response to teleseismic P-waves. By modeling receiver functions with velocity profiles, we can measure crustal and mantle layer thicknesses and velocity underlying a broadband seismometer. Typically, seismologists use a non-unique calculus based, linearized inversion to determine the most appropriate model for a receiver function. By inverting for structure with both receiver functions and surface wave group/phase velocities, the non-uniqueness is reduced. I applied a non-linear method, called the niching genetic algorithm, to invert both phase velocities and receiver functions for velocity models. At the cost of increased computation time, this method imposes fewer a priori constraints, and yields excellent results without over-parameterizing.

Chilean Patagonia: The Seismic Experiment in Patagonia and Antarctica deployed five of ten broadband seismometers in Chilean Patagonia from 1997 to 1999. Using the niching genetic algorithm method I was able to determine that the crust thinned from ~32 km to ~24 and then infilled with sediment to restore crustal thickness to ~28 km at present day [Lawrence and Wiens, 2003]. The region appears to be in current mass balance.

This region has anomalously low topography relative to the rest of the Andes. In fact, the region underwent extension, which thinned the crust rather than compression which thickens the crust elsewhere. The southern end of the South America has rotated to the east as a result of Scotia Plate motion. The stress and strain related to this rotation may have resulted in the extension which thinned crust here.

The Transantarctic Mountains: The Transantarctic Mountains are the largest non-compressional mountain range in the World. They span over 4000 kilometers, marking a 200-kilometer boundary between East Antarctica and West Antarctica. Over the years, much debate has centered over what caused the Transantarctic Mountains to form. Due to the extreme cold and ice cover, it is difficult to directly examine most parts of Antarctica. Therefore, it is crutial to use remote sensing.

From 2000 to 2003 the Transantarctic Mountain Seismic Array (TAMSEIS) deployed 41 broadband seismometers from the Ross Sea to the Vostok Subglacial Heighlands. Using surface wave phase velocities[Lawrence et al., 2005a], S-wave attenuation, and receiver functions I was able to map out mantle temperatures [Lawrence et al., 2005b] and crustal thickness. The crust thins from ~35km in East Antarctica to ~20 km in West Antarctica. Beneath the Transantarctic Mountains, in the Dry Vallies region [Lawrence et al., 2005c].

There is an anomalously small crustal root considering the large topography (4 kilometers). Using gravity and topogrophy with the known crustal thickness we are able to better constrain the geodynamic processes that caused the mountain uplift. First, the temperature difference calculated from seismic anomalies (~300 degrees K) is necessary to produce make East Antarctic mantle dense enough to account for the observed topography[Lawrence et al., 2005c]. Second, the rapid change in temperature between East Antarctica and West Antarctica likely caused the Mountains to uplift.

Differential Measurements: By comparing two teleseismic body waves from the same earthquake, it is possible to both localize regions with seismic anomalies and improve the measurements themselves. The method uses waves with similar ray path geometries to reduce the influence of structure along similar ray paths, and accentuate effects of structure in locations where the paths diverge most. The cross-correlation method produces travel-time residuals, which correspond to isolated anomalous velocities. Spectral division yields a differential attenuation (or dampening) measurement (dt*).

D" Attenuation:The lowermost mantle, called D", is a region of extreme heterogeneity on lateral scales from 100s to 1000s of kilometers. The core-mantle boundary is a chemical, thermal, and dynamic boundary layer between the iron core and the silicate mantle. Consequently, measureable anamalous structures occur. For example, beneath Central America, I measured a ~250 km wide velocity and quality factor anomaly which may be characterized as thermal in nature [Fisher et al., 2003].

Radial Quality Factor: (QLM9) Over 30,000 differential ScS-S attenuation measurements were used to calculate the first radial quality factor model with high sensitivity in the lower mantle. The structure is unique, and shows that the Earth is more anelastic with greater radius. The model shows that attenuation is high just above the core-mantle-boundary, where high temperatures (due to heating from the core) likely increase the anelasticity.[Lawrence and Wysession., 2005a].

Global Quality Factor: (VQM3DA) Using over 70,000 differential measurements we invert for the 3D quality factor structure of the Earth. By analyzing both velocity and quality factor on the same scale, we can more easily understand the source of the heterogeneity. Quality factor is highly dependent upon temperature and water content, so observation of large anomalies indicates one or the other. Depending on how, velocity changes, it is possible to infer which one [Lawrence and Wysession., 2006b].

Transition Zone Topography: Seismic interfaces at about 410 and 660 kilometers depth are known to have significant topography [Flanagan and Shearer, 1998]. However, previous evidence lead scientists to believe that different measurement techniques (SdS underside reflections and Pds - P-to-S converted waves) resulted in different estimates of global topography. We demonstrated that the two techniques do actually yield the same average and laterally variations in transition zone thickness [Lawrence and Shearer., 2005].

North American Structure: Seismic attenuation and travel times are useful in-situe measurements of the mantle. Using them together we can delineate difference between thermal and water concentration anomalies. In the southern half of the United States attenuation and travel times are correlated. In the northern part they are not. The southern positive correlation likely indicates colder mantle temperatures beneath the old and stable eastern US in comparison to warm temperatures beneath the recent tectonic activity in the west. The presence of water may play a large role in the dynamics of the North America. In the far northwest, active subduction likely transports a large amount of water into the mantle, where it weakens the mantle causing elevated attenuation. In the surrounding Cascade Mountains/Volcanoes and the Yellowstone/Whyoming area recent volcanism may have cause melt to suck up all the mantle hyrates resulting in lower attenuation [Lawrence et al., 2006].

Ambient Noise Tomography: The common coherent signal of "noise" sources recorded on a pair of seismoeters can be used to image the structure between the two seismometers. By adding up the many coherent noise sources from microseisms and wind on our shores, the coherent signal is recovered, and the incoherent signal cancels out. The resulting signal contains information pertaining to the structure between the two seismometers such as seismic velocity and seismic amplitude. By calculating many inter-station Green's functions, we can then invert for high-resolution maps of subsurface structure.

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