USArray
A Continental-scale Seismic Observatory

Science

The Science and Discoveries of USArray

High-Resolution Tomography

Numerous studies are exploiting all facets of the rapidly increasing USArray data set and are resulting in much more refined pictures of crustal and upper mantle structure beneath western North America. By combining the thousands of seismic wave readings from the Transportable Array with observations from seismic stations around the globe, high-resolution images of the structure under western North America have been produced. Additional tomography methodologies that combine data from the Transportable and Flexible Arrays, as well as data from existing regional networks, have also produced more high-resolution images of the earth’s interior.

Burdick, S., et al. 2008. Upper mantle heterogeneity beneath North America from travel time tomography with global and USArray Transportable Array data. Seismological Research Letters 79(3):384–392.

 

 

 Ambient Noise Tomography

A recent innovation in seismic imaging reveals information about Earth structure without the occurrence of earthquakes. This technique, ambient noise tomography (ANT), is based on using long time series of noise that is present at all seismic stations. Application of ANT to data from ambitious new deployments of seismic arrays, such as USArray, has led to the development of large-scale seismic models of the earth’s crust and uppermost mantle at high resolution. In addition, new methods of data analysis and interpretation of ambient noise data that exploit the array nature of the Transportable Array are currently under development and have the potential to provide more reliable information about crustal and uppermost mantle anisotropy.

Yang, Y., et al. 2008. Structure of the crust and uppermost mantle beneath the western United States revealed by ambient noise and earthquake tomography. Journal of Geophysical Research 113, B12310, doi:10.1029/2008JB005833.

 

 

Lithospheric Drips

Regional mantle downwellings, referred to as lithospheric drips, are widely inferred to exist, based on surface expressions. However, direct observations of such downwellings have proved elusive, due to their small size and transient nature. Analyses of new seismic data recorded by the USArray Transportable Array and other regional broadband seismic stations combined with other non-seismic data provide clear evidence for a large lithospheric drip beneath the Great Basin in the western United States. Such analyses are revealing the importance of lithospheric drips in the framework of global tectonics.

Image courtesy of J.D. West and M.J. Fouch, Arizona State University. 

West, J.D., M.J. Fouch, J.B. Roth, and L.T. Elkins-Tanton. 2009. Vertical mantle flow associated with a lithospheric drip beneath the Great Basin. Nature Geoscience 2:439–444, doi:10.1038/NGEO526.

 

 

Episodic Tremor and Slip

There is significant earthquake hazard resulting from fault slip associated with subduction of an oceanic plate beneath the region straddling the US-Canadian border. This plate collision is accompanied by intervals of surprisingly regular seismic tremor and slow fault slip that are not generally felt by the local population. Existence of this phenomenon was discovered prior to deployment of EarthScope instruments. However, the combined information from USArray and Plate Boundary Observatory data sets have provided critical new constraints that have greatly improved our understanding of this complex phenomenon. The new picture of the fault zone provided by data analyses suggests that the probable region of strong ground motion during future earthquakes extends significantly further inland than had been thought, closer to the large population centers of Cascadia.

Image courtesy of K. Creager, University of Washington.

 

Probing the Lithosphere-Asthenosphere Boundary

Transportable Array data provide a means for probing boundaries within the earth beneath USArray seismic stations. Key boundaries include the Mohorovicic discontinuity, which marks the transition between the crust and mantle, and the lithosphere-asthenosphere boundary, which marks the depth extent of continental structures. The resultant seismically determined boundary-depth maps can be combined with results from mineral physics and geochemistry to better understand the tectonic development and evolution of North America.

Images courtesy of A. Levander, Rice University; Meghan Miller, University of Southern California; and F. Niu, Rice University.

 

3D Conductivity

Mantle conductivity models generated from magnetotelluric data complement the seismic tomography images of the structure beneath North America. In some cases, conductivity observations provide constraints that are difficult to obtain from seismic data. For example, conductivity is particularly sensitive to the water content of the mantle. Joint interpretation of conductivity, velocity, and attenuation is beginning to provide better constraints on composition and physiochemical state than analysis of any one property alone. There is significant anticipation of further development of joint electrical and seismic interpretation of mantle observations.

Image courtesy of G. Egbert, Oregon State University, and P. Patro, National Geophysical Research Institute, India.

Patro, P.K., and G.D. Egbert. 2008. Regional conductivity structure of Cascadia: Preliminary results from 3D inversion of USArray Transportable Array magnetotelluric data. Geophysical Research Letters 35, L20311, doi:10.1029/2008GL035326.