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Velocity structure and anisotropy of the western United States.

Daphné-Anne Griot, Barbara Romanowicz, Michael E. Pasyanos

Subsections


Dynamics of the western United States ?

The geology of California illustrates the complex dynamics in this region which is the meeting point of Pacific, North American and Juan de Fuca plates. The interaction between these plates led to the formation of the Mendocino Triple Junction (MTJ) and to the development of San Andreas fault system. The MTJ is constrained to migrate northward to accommodate these relative motions between plates. Because of this specific geometry, and in particular, due to the migration of MTJ, a "slab window", zone filled with hot asthenospheric mantle, has probably been formed in the south of this MTJ (Atwater, 1970; Zandt and Furlong, 1982). Hence, this "slab window" could explain the presence of volcanism in the Coast Ranges. Understanding these complex motions between plates requires a perfect knowledge of lithospheric and asthenospheric structures.

So far, some previous studies revealed a strong correlation between geological features and crustal velocities (Benz et al., 1992), such as the volcanism associated to slow velocities. The most important results, highlighted by Zandt and Furlong (1982), Benz et al. (1992), or also Pasyanos and Romanowicz (1998), is that high velocities are observed in the north of MTJ whereas, in the south of MTJ, beneath the Coast Ranges, the structure is composed of low velocities. These patterns can be interpreted as evidence of the subduction of the Juan de Fuca plate and of a "slab window", respectively.

Nevertheless, the interaction between these different structures must again be investigated. To address these issues, we performed an anisotropic tomography using surface waves (Montagner and Nataf, 1986). With this powerful technique, both velocity and anisotropy can be inferred and then provide information both on the inside structure and on the motions of this structure, which enables us to map the dynamics (Griot et al., 1998). The data used in this study are phase velocity measurements, assessed from regional surface wave data (Pasyanos and Romanowicz, 1998).

Velocity anomalies, radial and azimuthal anisotropies

At 47 km depth (Figure 10.1), high shear velocities are observed in the North of the MTJ, which is due to the subduction of the Juan de Fuca plate. At this shallow depth, the observation of low velocities beneath the Cascade region and the Mammoth Lake zone is related to the location of volcanism. At 120 km depths, the subduction of Juan de Fuca plate remains visible and is characterized by high velocity. To the south of the MTJ, low velocities have been inferred from the inversion beneath the Coast Ranges and the Sierra Nevada. All around this large zone of low velocity, high velocity zones are observed. This specific observation probably relates to the presence of the past Farallon subduction surrounding a zone of low velocity which could be interpreted as a "slab window".

The radial anisotropy (Figure 10.2) displays two different patterns for shallow and deep depths. At shallow depths, the region is characterized, as a whole, by horizontal motions, except beneath the Cascade zone where vertical motions are observed and can then be related to volcanism. These horizontal motions could be explained by the horizontal motion of the subduction zone in the north of MTJ and by the horizontal motions related to the San Andreas fault system. On the opposite, for deeper depths, the radial anisotropy displays essentially vertical motions, which are in agreement, especially in the south of the MTJ, with the presence of a "slab window" because in this case, a hot asthenospheric upwelling is attempted.

Two patterns, radically different are also observed for the azimuthal anisotropy (Figure 10.3). At shallow depth (47 km), the pattern seems in a relative agreement with the surface motions because the azimuthal anisotropies are more or less parallel to the San Andreas fault, in the south of MTJ. On the contrary, for deeper depths (120 km), in this same region, the azimuthal anisotropy displays a direction perpendicular to the SAF, which could be explained by a flow motion due to the slab window.

The observation of both radial and azimuthal anisotropies highlights two kinds of dynamical behavior: one in the shallow depths, where the influence of surface motions is important, and one at deeper depths where anisotropies seem to be in agreement with the assumption of a "slab window" in the south of MTJ. Consequently, the dynamics of this region is characterized by two anisotropic layers, which had already been observed from other seismic data by Ozalaybey and Savage (1994) and by Silver and Savage (1994). These results on velocities and anisotropies seem to show that in the north of the MTJ, the dynamics is essentially related to the subduction of Juan de Fuca plate, whereas in the south of the MTJ, the dynamics is carried out by two mechanisms. At shallow depths, motions are horizontal and related with the surface to the San Andreas fault system. For deeper depths, motions are more or less vertical and probably due to a "slab window", which invokes dynamical motions, such as an asthenospheric upwelling.


  
Figure 10.1: Shear velocity anomalies at 47 and 120 km depth. The color scale is expressed in per cent, relative to continental velocities.
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\epsfig{file=figs/bsl98_griot_fig1.eps, width=8cm}\end{center}\end{figure}


  
Figure:10.2 Radial anisotropy at 47 and 120 km depth. The color scale illustrates the ratio between VSH and VSV, ie. $\xi $ factor: when $\xi $ is lower than zero, flow motions are vertical, and when $\xi $ is higher than zero, flow motions are horizontal.
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\epsfig{file=figs/bsl98_griot_fig2.eps, width=7.33cm}\end{center}\end{figure}


  
Figure 10.3: Maps of azimuthal anisotropy (parameter G) at 47 and 120 km depth. G is reflecting the azimuthal variation of SV wave velocity: the maximum anisotropy is 1.5%.
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\epsfig{file=figs/bsl98_griot_fig3.eps, width=6.5cm}\end{center}\end{figure}

References

Atwater, T., Implications of plate tectonics for the Cenozoic tectonic evolution of wsetern North America, geol. soc. am. bull., 81, 3513-3536, 1970.

Benz, H.M., G. Zandt and D.H. Oppenheimer, Lithospheric structure of northern California from teleseismic images of the upper mantle, J. Geophys. Res., 97, 4791-4807, 1992.

Griot, D.A., J.P. Montagner and P. Tapponnier, Confrontation of mantellic anisotropy with two extreme models of strain, in Central Asia, Geophys. Res. Lett., 25, 1447-1450, 1998.

Montagner, J.P., and H.C. Nataf, A simple method for inverting the azimuthal anisotropy of surface waves, J. Geophys. Res., 91, 511-520, 1986.

Ozalaybey, and M.K. Savage, Double layer anisotropy resolved from S phases, Geophys. J. Int., 117, 653-, 1994.

Pasyanos, M. and B. Romanowicz, Velocity structure of the western United States from phase velocity measurements: Part I, J. Geophys. Res., In press, 1998.

Silver, P.G., and M.K. Savage, The interpretation of shear wave splitting parameters in the presence of two anisotropic layers, Geophys. J. Int., 119, 949- , 1994.

Zandt, G., and K. P. Furlong, Evolution and thickness of the lithosphere beneath coastal California, Geology, 10, 376-381, 1982.


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Next: Seismological Studies at Parkfield, Up: Ongoing Research Previous: 3D Wave Propagation Studies

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