Götz H. R. Bokelmann

Null Detection in Shear-Wave Splitting Measurements

Shear-wave splitting measurements are widely used to analyze orien- tations of anisotropy. We compare two different shear-wave splitting techniques, which are generally assumed to give similar results. Using a synthetic test, which covers the whole backazimuthal range, we find characteristic differences, however, in fast-axis and delay-time estimates near Null directions between the rotation cor- relation and the minimum energy method. We show how this difference can be used to identify Null measurements and to determine the quality of the result. This tech- nique is then applied to teleseismic events recorded at station LVZ in northern Scan- dinavia, for which our method constrains the fast-axis azimuth to be 15� and the delay time 1.1 sec. Online material: Additional comparisons between the RC and SC techniques.

On the relationship between extension and anisotropy: Constraints from shear wave splitting across the East African Plateau

Received 28 October 2003; revised 2 April 2004; accepted 21 May 2004; published 6 August 2004. [1] East Africa is a tectonically complex region owing to the presence of a rigid craton, paleothrust belts and shear zones, active magmatism and rifting, and possibly even a mantle plume. We present new splitting results of teleseismic shear phases recorded by 21 broadband seismic stations in Tanzania, seven broadband stations in Kenya, and three permanent broadband Global Seismic Network stations in Kenya and Uganda. Inconsistent apparent splitting is observed beneath the craton and along its southern and southeastern flank in Tanzania. Splitting at stations elsewhere in the rifts and orogenic belts is more consistent. We test between different models of anisotropy for stations with inconsistent splitting (single layer with horizontal fast axis, single layer with dipping fast axis, and two layers with horizontal fast axes). However, we show that these more complicated models do not reasonably explain the data. The data are explained better by a laterally (and/or in some places vertically) varying single-layer model with a horizontal fast axis. We arrive at a conceptual model of anisotropy in Tanzania and Kenya that is controlled by (1) active shear along the base of the plate associated with asthenospheric flow beneath and around the moving craton keel, (2) asthenospheric flow from a plume north of central Kenya, (3) fossilized anisotropy in the lithosphere due to past orogenic events, and possibly (4) aligned magma-filled lenses beneath the rifts. Our most robust conclusion is that we can rule out an extension-induced lattice preferred orientation of olivine as a dominant factor, which is surprising given the long history of extension in the region. This indicates that mantle-lithospheric extension in East Africa occurs via dikeintrusion and/or ductile thinning within narrow rift zones and is possibly facilitated by a mechanical lithospheric anisotropy imparted by fossilized north/south structural or mineralogical fabrics. INDEX TERMS: 7203 Seismology: Body wave propagation; 7218 Seismology: Lithosphere and upper mantle; 8109 Tectonophysics: Continental tectonics—extensional (0905); 8120 Tectonophysics: Dynamics of lithosphere and mantle—general; 8121 Tectonophysics: Dynamics, convection currents and mantle plumes; KEYWORDS: anisotropy, shear wave splitting, extension

Seismic waveform attributes before and after the Loma Prieta earthquake: Scattering change near the earthquake and temporal recovery

During 1987–1995 several clusters of nearly identical seismic events (multiplets) occurred near the Loma Prieta source region. These multiplets allow us to investigate and demonstrate spatial and temporal changes in seismic wave character associated with the 1989 Loma Prieta main shock. For seismogram pairs we use a moving window technique to compute coherencies depending on lapse time and frequency. Post-Loma Prieta events have reduced coherencies with pre-Loma Prieta events in a spatially limited region close to the Loma Prieta hypocenter, while other paths remain nearly unaffected. These changes gradually recover within a time interval of 5 years after the Loma Prieta earthquake. A possible explanation for the time dependence is coseismically opened cracks which cause scattering increase for wavefields after the Loma Prieta event. Postseismic relaxation processes such as crack healing, fluid diffusion, or after deformations lead to progressive closure of these cracks with time after the main shock. Thus the scattering properties of the local crust approach the pre-main shock state.

Which forces drive North America

Understanding the mechanism of plate tectonics is one of the most important problems in the geosciences. Are the tectonic plates pulled and pushed from the side, as the Orowan-Elsasser model suggests, or does mantle convection play an active role in driving the plates? This question can be addressed by studying the deformation of deep continental roots. The application to North America shown here indicates that the deeper mantle moves at a higher velocity than the North American plate and that the mantle plays an important role in driving the plates. I suggest that this finding (1) provides a natural explanation for why the motion of North America slowed down dramatically during the past 100 m.y. and (2) predicts that North American motion will eventually come to a halt.

Shear-wave splitting to test mantle deformation models around Hawaii

Seismic anisotropy allows us to study mantle deformation, and it can thus help to constrain mantle flow in the vicinity of hotspots. Hypotheses for the cause of seismic anisotropy in this environment include the “parabolic asthenospheric flow” (PAF) model: radial flow from a mantle plume impinging on a moving lithosphere is dragged by the plate in the direction of absolute plate motion. In map view, this gives a parabolic pattern of flow, opening in the direction of plate motion. We present new shear-wave splitting observations from land and ocean stations around the Hawaiian Islands that can be explained by the parabolic flow model. The observations suggest asthenospheric anisotropy under the Hawaiian islands, which may be explained if dislocation-creep persists to deeper depths there than in other regions, perhaps due to the higher temperatures near hotspots.

Mantle variation within the Canadian Shield: Travel times from the portable broadband Archean‐Proterozoic Transect 1989

We report travel times from the Archean-Proterozoic Transect 1989. This type of data set recorded by a transect of portable broadband instruments allows us to make inferences about mantle structure in the region between the Wyoming Craton and the Superior Province of the Canadian Shield. With station separations of 50 to 100 km and frequencies up to 5 Hz the resolution of lateral changes is increased by nearly an order of magnitude over previous studies to a scale that allows us to study the relation between velocity variation in the continental upper mantle, surface geology, tectonic features and age provinces. Travel times of direct and later phases are obtained from waveform matching. The values are corrected for crustal contribution and inverted for the vertical path upper mantle delay δtUM under each station as well as the azimuthal dependence of this quantity. The prominent feature in the upper mantle delays δtUM is the variation by at least 1.5 s for S, much of which occurs over a narrow zone of just a few hundred kilometers width. This suggests a major lateral upper mantle transition which does not coincide with the surface geological edge of the Canadian Shield but is located within the shield. This same transition is also observed in shear wave splitting delay times. Surprisingly, however, the P delays do not exhibit a corresponding variation. We address this apparent contradiction and show how it may be explained in conjunction with anisotropy in the subcontinental lithosphere. A simplified thermal model of the lithospheric transition zone, in which temperature controls the degree of crystallographic alignment and thus seismic anisotropy, predicts this phenomenon.

Upper mantle deformation beneath the North American-Pacific plate boundary in California from SKS splitting

In order to constrain the vertical and lateral extent of deformation and the interactions between lithosphere and asthenosphere in a context of a transpressional plate boundary, we performed teleseismic shear wave splitting measurements for 65 permanent and temporary broadband stations in central California. We present evidence for the presence of two anisotropic domains: (1) one with clear E–W trending fast directions and delay times in the range 1.5 to 2.0 s and (2) the other closely associated with the San Andreas Fault system with large azimuthal variations of the splitting parameters that can be modeled by two anisotropic layers. The upper of the two layers provides fast directions close to the strike of the main Californian faults and averaged delay times of 0.7 s; the lower layers show E–W directions and delay times in the range 1.5 to 2.5 s and thus can be compared to what is observed in stations that require a single layer. We propose the E–W trending anisotropic layer to be a 150 to 200 km thick asthenospheric layer explained by the shearing associated with the absolute plate motion of the North American lithosphere. The shallower anisotropic layer ought to be related to the dynamics of the San Andreas Fault system and thus characterized by a vertical foliation with lineation parallel to the strike of the faults localized in the lithosphere. We also propose that the anisotropic layer associated with each fault of the San Andreas Fault system is about 40 km wide at the base of the lithosphere.

Mantle structure under Gibraltar constrained by dispersion of body waves

[1] WestudytheAfrica-Iberiaplateboundaryinthevicinity of Gibraltar. Numerous models have been proposed for that region throughout the last decades, proposing mechanisms that range widely from continental delamination, convective removal, to subduction of oceanic lithosphere. To better constrain upper-mantle structure under the region, we study waveforms of P-waves that traverse the Alboran Sea region between Spain and Morocco. These show dispersive behavior, which, together with early arrival times, confirms the presence of an anomalous upper mantle structure under the Alboran Sea. The dispersion is consistent with that expected from subducted lithosphere. Waveforms of body waves therefore provide a way to better constrain the elusive mantle structure and dynamics of the Alboran Sea region. Citation: Bokelmann, G., and E. Maufroy (2007), Mantle structure under Gibraltar constrained by dispersion of body waves, Geophys. Res. Lett., 34, L22305, doi:10.1029/2007GL030964.

Shear wave splitting around hotspots: Evidence for upwelling-related mantle flow?

We review evidence for plumelike upwellings beneath the Eifel, Great Basin, Hawaii, Afar, and Iceland hotspots by using teleseismic shear wave splitting to resolve the anisotropy associated with mantle flow. An approximately parabolic pattern of fast polarization azimuths (f) is consistent with splitting observations around the Eifel, Great Basin, and Hawaii hotspots, and this pattern may be explained by a model of upwelling material that is being horizontally deflected or sheared in the direction of absolute plate motion (parabolic asthenospheric flow, PAF). The source of splitting beneath Iceland and the Afar is not clear, but the data are not inconsistent with a plumelike upwelling. The success of the upwelling model in explaining the splitting data for the Eifel and the Great Basin, regions far from plate boundaries, suggests that a mantle anisotropy pattern exists for at least some hotspots driven by plumelike upwellings and that splitting can be a useful diagnostic to differentiate between plumelike and alternative sources (e.g., propagating cracks, leaky transform faults) for mantle hotspots. Furthermore, the PAF pattern provides two useful tectonic and geodynamic parameters: the direction of relative motion between the lithosphere and asthenosphere and the stagnation distance, which is proportional to the strength of the upwelling and the speed of the moving plate. When this pattern is used with other types of geophysical data such as seismic velocity tomographic images, one can estimate the plate speed, upwelling volumetric flow rate, buoyancy flux, asthenospheric thickness, excess temperature, and viscosity.

Depth-dependent earthquake focal mechanism orientation : Evidence for a weak zone in the lower crust

The traction free boundary condition across the Earths surface provides an opportunity for studying the relationship between stress orientation and earthquake focal mechanisms because it requires alignment of principal stress axes with vertical and horizontal orientations. A survey of earthquake focal mechanisms in northern California shows that their principal axes are also closely aligned with the vertical and the horizontal in the upper few kilometers of Earths crust. Thus the signature of the free surface boundary condition on stress appears in focal mechanism orientations as well. The focal mechanism alignment can also be characterized by the relative magnitude of the off-diagonal elements, Mxz and Myz, of the seismic moment tensor. We find significant and systematic depth variations in the “horizontal moment tensor element” ms, which relates to the shear traction acting on a horizontal plane for the special case of perfect alignment between principal stress and focal mechanism axes. Values of ms near Earths surface are small but increase with depth to a maximum between 5 and 8 km. At greater depths, there is a gradual decrease, which suggests decreasing horizontal shear traction toward the base of the seismogenic zone. We interpret this tendency of axes to become oriented near the base of the seismogenic zone (and its expression in ms) as the signature of a weak zone in the lower crust. If correct, this observation would have important implications for the mechanics of lithospheric deformation.