Nikolai Shapiro

Lecturer

Center for Imaging the Earth's Interior

Department of Physics

University of Colorado at Boulder

Boulder, CO 80309-0390

Phone: (303) 735-1850

FAX: (303) 492-7935

Vita

Publications

 

Main research Interests

My principal research interests are related to the physics of the seismic wave propagation and to its application for studies of the EarthÕs interior. I have worked and I am interested to continue working on various aspects of observational and theoretical seismology including (but not limited to) the development of measurement and inversion techniques, the application of numerical simulations, the seismic tomography, and the quantitative interpretation of tomographic models. As a continuation of my research I would like to work on several projects, among them are:

Using measurements from random seismic wavefields to improve resolution of seismic tomographic models. In this project, I will explore a new approach that allows seismic measurements that are independent of earthquake occurrence. It is based on a recently demonstrated possibility to extract the deterministic surface-wave signals by cross-correlating the ambient seismic noise (Shapiro and Campillo, GRL, 2004). The dispersion measurements made from cross-correlations of the random noise have significant advantages over traditional measurements made from the ballistic waves. The new method will be particularly useful in the context of dense array of seismometers such as USArray as demonstrated by our recent results in California.

Group speed maps constructed by cross-correlating one month of ambient noise between Californian USArray stations (Shapiro et al., submitted to Science). (a) 7.5 s period Rayleigh waves. (b) 15 s period Rayleigh waves. Black solid lines show known active faults. White triangles show locations of USArray stations used in this study.

 

Studying structure and dynamics of the oceanic lithosphere. My previous studies helped to image the age-dependent structure of the Pacific lithosphere. A particularly exciting result is the observation of the punctuated cooling history of the Pacific that can be attributed to the effects of the small-scale lithospheric convection (Ritzwoller, et al., EPSL 2004). Future directions are: (1) extending of those results to other oceans, (2) studying relation between the seismic anisotropy and the dynamics of the oceanic lithosphere (Smith et al., JGR, 2004), and (3) combining seismic tomography and the geodynamical modeling to constrain the upper-mantle rheology (van Junen et al., submitted to Science).

Average temperature profile of the Pacific upper mantle versus lithospheric age (from Ritzwoller et al. EPSL 2004). Upper mantle temperature from the inversion based on the temperature parameterization averaged across the Pacific plotted versus lithospheric age. The green lines are isotherms from the Half-Space Cooling (HSC) model. An average perturbation of more than 100 8C develops between the observed and HSC temperature profiles at a depth of about 100 km due to processes of reheating that occur between 70 and 100 Ma in the Central Pacific.

Studying the Tibetan crust and upper mantle. Intermediate-period Rayleigh and Love waves propagating across Tibet indicate marked radial anisotropy within the mid-to-lower crust, consistent with a thinning of the middle crust by about 30% that appears to be more than the thinning of the upper crust, consistent with deformation of a mechanically weak layer in the mid-to-lower Tibetan crust that flows as if confined to a channel (Shapiro et al., Science, 2004). Now I am working on imaging and interpretation of the upper mantle structure beneath the Tibetan plateau to understand what geodynamic processes can account for the pattern of velocity anomalies observed at large scales in the upper mantle beneath Tibet.

Upper mantle seismic structure beneath Tibet. (1) The high velocities in western Tibet appear to be connected, as if derived from subducted Indian lithosphere. (2) Perhaps a similar connection in the east can be made with subducted Asian lithosphere. (3) In central Tibet, however, the high-speed zone appears to be detached from both the Indian and Asian lithosphere. (4) Overlying this feature is a low speed anomaly coincident with high attenuation and the blockage of regional phases.

Developing seismic inversion methods that estimate uncertainties in resulting models. A method based on a Monte-Carlo sampling of the model has been already applied to produce a global 3D shear-velocity model of the crust and the uppermost mantle with meaningful uncertainty estimates (Shapiro and Ritzwoller, GJI, 2002). My current and future work is aimed to use the estimated uncertainties in quantitative applications of the tomographic models such as inferring physical parameters (e.g., temperature, density, viscosity) or predicting 3D travel times to improve earthquake location. In both cases, the uncertainties in the obtained predictions must be estimated with the error propagation methods to guide the interpretation the results.

Developing techniques allowing application of physical constraints on seismic inversions which is required to resolve the trade-offs and non-uniquenesses inherent for inversions based on the seismic data alone. Currently, I am working on physical constraints derived from thermodynamics such as: incorporation of the heat-flow measurements in seismic inversions that helps to improve seismic models beneath cratons and continental platforms and replacing ad-hoc seismic parameterizations with physical parameters that describe the thermal state and evolution of the upper mantle (Shapiro and Ritzwoller, GJI, 2004). The inverse problem is, therefore, recast as a hypothesis test to determine if the data are consistent with the thermodynamic model.

Studying seismic and thermal structure of the old continental lithosphere. The new seismic and thermal model of the South-Eastern Canadian shield obtained with the method where the thermally constrained inversion of seismic surface waves (Shapiro et al., Geol. Soc. Lond. Spec. Publ., in press) showed that variations in lithospheric temperature and shear velocity are not well correlated with geological province or surface tectonic history and that the nearly inversely proportional relation between lithospheric thickness and mantle heat flow is consistent with the dynamical model when the lithosphere and asthenosphere beneath the Canadian Shield are in thermal equilibrium and heat flux into the deep lithosphere is governed by small scale sublithospheric convection. In the future, I plan to produce thermally constrained seismic models for other cratons and to see if similar relation between the lithospheric thickness and the mantle component of the heat flow can be found in these areas.

Studying the upper-mantle structure in the North-Western Pacific and the North-Eastern Asia. Seismic tomographic model of the Kamchatka-Aleutian region combined with other geophysical and geological data resulted in discovery of episodes of the slab loss beneath the Northern Kamchatka indicating that the process of the lithospheric detachment that iscommonly discussed in the context of a continental collision is also an important agent in the evolution of oceanic convergent margins (Levin et al., Nature, 2002). Another result is the imaging of a slab portal beneath the western Aleutians that has implications for the origin of ÒadakitesÓ or "high Mg # andesites" found in the western Aleutian arc (Levin et al., submitted to Geology). These rocks are interesting because their composition is similar to average continental crust, and consequently they have been hypothesized as examples of how to create continental crust without involvement of pre-existing continental material. In the future, I am planning to work on the interpretation of the tomographic model below the North-Eastern Asia with particular emphasis on the understanding of the interaction between the North-American and Asian plates and the origin of the Sea of Okhotsk.