John H. Woodhouse

On the robustness of global radially anisotropic surface wave tomography

A number of recent global tomographic studies have modeled three dimensional variations in the parameters of radial anisotropy. As yet there is limited agreement among such studies, suggesting significant uncertainties in the models, which could lead to divergent geodynamical interpretations. In this study we assess the robustness of lateral variations in radial anisotropy globally in the upper mantle and in the transition zone to determine the extent to which anisotropic parameters are constrained by a data set of over 10,000,000 fundamental and higher mode surface wave dispersion measurements. We carry out inversions for isotropic and radially anisotropic shear wave velocity, systematically changing regularization and using three different crustal models to remove the effects of the crust on the data. Using crustal corrections from different crustal models has an impact on the data fit comparable or larger than that obtained by including lateral variations of radial anisotropy in the modeling. Moreover, the use of crustal corrections from different a priori crustal models may lead to different images of radial anisotropy suggesting divergent geodynamical interpretations. This work suggests that the three‐dimensional determination of global radial anisotropy in the Earths mantle using surface wave dispersion data is still an ongoing experiment.

The great March 25, 1998, Antarctic Plate earthquake: Moment tensor and rupture history

We use broadband body and mantle wave data to study the 1998 Antarctic intraplate earthquake. The centroid moment tensor (CMT) has a large non-double-couple component. There exist two pure double-couple constrained solutions that fit the data almost equally well. The frequent practice of taking the “best double-couple” gives a far from optimal solution. We use P and SH body waves to determine the rupture parameters of the first and larger of the two observed subevents. The best rupture plane, with strike 96°, dip 69°, and rake −18°, is compatible with only one of the two CMT solutions: strike 96°, dip 64°, rake −23°, centroid location (63.1°S, 148.4°E, 10 km depth), centroid time 0313:02 UT, and M0 = 1.3 × 1021 N m (Mw = 8.0). The first subevent is a simple, primarily westward propagating ∼140-km rupture, of ∼45-s duration, with average velocity ≳3 km s−1; it has a seismic moment of 1.2×1021 N m (Mw = 8.0), with 75% of its moment released between 10 and 27 s, and a stress drop of ∼240 bars. The rupture is physically bounded by two fracture zones at 147.5°E and 150°E. The second subevent lasted from 70 to 90 s on a fault extending from 210 to 270 km west of the epicenter, with a moment of 0.3–0.6×1021 N m (Mw = 7.6–7.8). This is a spectacular example of dynamic stress triggering over a 100-km separation distance with a time delay of ∼40 s. The complex pattern of aftershocks is primarily controlled by preexisting fracture zones on the ocean floor.

Reflectivity of the 410-km discontinuity from PP and SS precursors

A number of studies have confirmed the global existence of a transition zone discontinuity at 410 km depth by aligning large numbers of long-period seismograms on a surface reflection phase before stacking. In particular, SS and PP precursors from the 410-km discontinuity (termed P410P and S410S) have revealed long-wavelength topography of this discontinuity. Here we extend these techniques to examine the reflection coefficient of the 410-km discontinuity. Using our measurements of P410P and S410S amplitudes, we constrain the impedance contrasts across the 410-km discontinuity. We also show lateral variations in the P wave impedance contrast at 410 km, which is typically low under North America and China and higher beneath the North Pacific. The S wave impedance contrast shows less variability on the regional scale. However, analysis of P410P and S410S amplitudes over smaller areas (by binning traces into spherical caps) shows that the S wave reflection coefficient varies over much shorter scale lengths than that for P waves. The different patterns of variation for P410P and S410S reflection amplitudes could be due to the presence of melt, water, or other chemical heterogeneities in the transition zone. Other factors such as temperature or mantle olivine content variations could also influence precursor amplitudes, but they would be expected to lead to correlated variations, and so they cannot explain all the variation that we observe.

Joint inversion for global isotropic and radially anisotropic mantle structure including crustal thickness perturbations

We present a new global whole-mantle model of isotropic and radially anisotropic S velocity structure (SGLOBE-rani) based on ~43,000,000 surface wave and ~420,000 body wave travel time measurements, which is expanded in spherical harmonic basis functions up to degree 35. We incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure. This is possible since we utilize short-period group-velocity data with a period range down to 16 s, which are strongly sensitive to the crust. The isotropic S velocity model shares common features with previous global S velocity models and shows excellent consistency with several high-resolution upper mantle models. Our anisotropic model also agrees well with previous regional studies. Anomalous features in our anisotropic model are faster SV velocity anomalies along subduction zones at transition zone depths and faster SH velocity beneath slabs in the lower mantle. The derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts, which has been difficult to constrain due to poor access compared with continental crusts.

Stability of earthquake moment tensor inversions: effect of the double-couple constraint

Abstract We show that spurious large non-double-couple components can be obtained in inversions for the full deviatoric moment tensor for shallow crustal earthquakes due to inaccurate Earth models. The traditional “best double-couple” solution does not in general provide an optimal estimate of a double-couple mechanism, and is only reliable when the non-double-couple component of the full deviatoric solution is small. The inverse problem for the moment tensors of the 1998 Antarctic Plate and 2000 Wharton Basin strike-slip earthquakes is shown in each case to have two well-fitting minima in the misfit function of pure double-couple solutions. Such pairs of solutions are most likely to exist for earthquakes which are close either to vertical strike-slip or to dip-slip on a fault plane dipping at 45°. It is shown theoretically that these pairs of solutions arise from the combination of the pure double-couple constraint and the instability of two elements of the moment tensor. No significant non-double-couple component is found for the shallow thrusting 1996 Biak, Indonesia earthquake.

Comparison of fluid tiltmeter data with long-period seismograms: Surface waves and Earth's free oscillations

We compare observations of long-period seismic surface waves and free oscillations recorded by high-resolution long-base fluid tube tiltmeters and by nearby broadband seismometers after large earthquakes. The quality of the tiltmeter data is comparable to that of the best horizontal component seismic data, recording some of the gravest free oscillations of the Earth, as well as successive passages of seismic surface waves circling the globe. We compare the observations with theoretical seismograms and with theoretical tilt. The predicted and observed surface wave tilt waveforms are very similar provided that we take into account horizontal acceleration effects on the tiltmeter. Phase and amplitude anomalies between the waveforms are well explained by the theoretical transfer function of the instrument. Likewise, observed horizontal seismograms converted into tilt match the tiltmeter data very well. Long-base fluid tube tiltmeters could potentially contribute to obtain high-quality measurements of the long-period seismic spectrum.

Comparison of P and S station corrections and their relationship to upper mantle structure

We use International Seismological Centre data between the years 1964 and 1994 to derive a data set of P and S station corrections. By restricting the raw P and S data to those which share the same source to receiver paths, we minimize any bias to the station corrections from differential P and S data distributions. We find that both static and azimuth dependent terms are highly correlated. In addition, we perform synthetic tests to determine the sensitivity of our data to mantle structure and show that static terms are sensitive to structure, at least throughout the upper mantle. In determining the regression slope of the static terms, we allow for regional offsets and find a best-fit global slope of S to P correction a=2.85±0.19. This is lower than that obtained in previous station correction studies but is in agreement with results in the upper mantle using differential travel times and with values derived for the upper parts of the lower mantle. Regionalizing the data by tectonics does not give statistically distinct ratios of S to P station correction. The result for shield areas, however, is significantly smaller than previously believed, probably because of our ability to derive station corrections from similar data sets. Variations in the second azimuthal terms are similar in both fast direction and relative magnitude. In another synthetic experiment we find that their general trends can be predicted well by isotropic models of heterogeneous velocity structure. From this result and also because they do not correlate with SKS splitting data, we conclude that station correction second azimuthal terms should not be interpreted in terms of anisotropy.

Constraints on lower mantle physical properties from seismology and mineral physics

The ratio, ν, of relative shear (S) to compressional (P) lateral seismic velocity variations is potentially an important constraint on the mineral physics of the Earths lower mantle. Recent seismic results show that there is an increase in this ratio from 1.7 to 2.6 in the lower mantle to 2000 km depth, which is significantly greater than published values in the mineral physics literature. We calculate ν in terms of δS = ∂ ln Ks∂ ln ϱ and a = ∂ ln ϱ∂ ln υS, and show that the seismic variation of ν within the depth range that P and S models appear proportional, is consistent with scalings computed using recent results from geodynamics and mineral physics studies. Furthermore we find that δS = 1 at 1900 ± 300 km depth irrespective of a.

Earthquake source parameters from GPS-measured static displacements with potential for real-time application

[1] We describe a method for determining an optimal centroid–moment tensor solution of an earthquake from a set of static displacements measured using a network of Global Positioning System receivers. Using static displacements observed after the 4 April 2010, MW 7.2 El Mayor-Cucapah, Mexico, earthquake, we perform an iterative inversion to obtain the source mechanism and location, which minimize the least-squares difference between data and synthetics. The efficiency of our algorithm for forward modeling static displacements in a layered elastic medium allows the inversion to be performed in real-time on a single processor without the need for precomputed libraries of excitation kernels; we present simulated real-time results for the El MayorCucapah earthquake. The only a priori information that our inversion scheme needs is a crustal model and approximate source location, so the method proposed here may represent an improvement on existing early warning approaches that rely on foreknowledge of fault locations and geometries. Citation: O’Toole, T. B., A. P. Valentine, and J. H. Woodhouse (2013), Earthquake source parameters from GPSmeasured static displacements with potential for real-time application, Geophys. Res. Lett., 40 ,6 0–65, doi:10.1029/2012GL054209.

Woodhouse receives 2001 Inge Lehmann Award

John Woodhouse received the Inge Lehmann Award at the 2001 Fall Meeting Honors Ceremony on 12 December, in San Francisco, California. The award is given for outstanding contributions to the understanding of the structure, composition, and dynamics of the Earths mantle and core.