Megan P. Flanagan

Global mapping of topography on transition zone velocity discontinuities by stacking SS precursors

We stack long-period, transverse-component seismograms recorded by the Global Digital Seismograph Network (GDSN) (1976–1996), Incorporated Research Institutions for Seismology-International Deployment of Accelerometers (IRIS-IDA) (1988–1996), and Geoscope (1988–1996) networks to map large-scale topography on the 410- and 660-km seismic velocity discontinuities. Underside reflections from these discontinuities arrive as precursors to the SS phase, and their timing can be used to obtain global variations of the depth to the reflectors. We analyze over 13,000 records from events mb>5.5, focal depth <75 km, and range 110° to 180° by picking and aligning on SS, then stacking the records along the theoretical travel time curves for the discontinuity reflections. Separate stacks are obtained for 416 equally spaced caps of 10° radius; clear 410- and 660-km reflections are visible for almost all of the caps while 520-km reflections are seen in about half of the caps. The differential travel times between the precursors and the SS arrival are measured on each stack, with uncertainty estimates obtained using a bootstrap resampling method. We then compute discontinuity depths relative to the isotropic Preliminary Reference Earth Model (PREM) at 40-s period, correcting for surface topography and crustal thickness variations using the CRUST5.0 model of Mooney et al. [1995], and for upper mantle S velocity heterogeneity using model S16B30 of Masters et al. [1996]. The resulting maps of discontinuity topography have more complete coverage than previous studies; observed depths are highly correlated between adjacent caps and appear dominated by large-scale topography variations. The 660-km discontinuity exhibits peak-to-peak topography of about 38 km, with regional depressions that correlate with areas of current and past subduction around the Pacific Ocean. Large-scale topography on the 410-km discontinuity is lower in amplitude and largely uncorrelated with the topography on the 660-km interface. The width of the transition zone, WTZ, as measured by the separation between the 410- and 660-km discontinuities, appears thickest in areas of active subduction (e.g., Kurils, Philippines, and Tonga) and thins beneath Antarctica and much of the central Pacific Ocean. Spatial variations in WTZ appear unrelated to ocean-continent differences but do roughly correlate with the S16B30 velocities in the transition zone, consistent with a common thermal origin for both patterns. The lower-amplitude 520-km reflector is more difficult to resolve but appears to be a global feature as it is observed preferentially for those bounce point caps with the most data.

A map of topography on the 410‐km discontinuity from PP precursors

We derive a new map of global topography on the 410-km discontinuity from observations of precursors to PP obtained by stacking almost 25,000 long-period seismograms. The inferred ‘410’ topography exhibits average peak-to-peak amplitude of about 30 km, has a strong degree-one component, and is highly correlated with previous results obtained from SS precursors [Flanagan and Shearer, 1998]. Spatial variations in ‘410’ topography appear unrelated to ocean-continent differences, suggesting that continental roots are not a significant factor in observed global temperature variations at 410 km depth.

Experiments in migration processing of SS precursor data to image upper mantle discontinuity structure

Long-period SS precursors result from underside reflections off upper mantle discontinuities. By grouping and stacking global seismic data by SS bounce point location it is possible to map lateral variations in depths to the 410- and 660-km discontinuities, a process analogous to common midpoint (CMP) stacking in reflection seismology. Because this method assumes horizontal reflectors, energy arriving from dipping or intermittent reflectors may not be correctly imaged. To address this possibility, we experiment with techniques based on migration processing of shallow seismic reflection data. The problem is complicated by the uneven distribution of sources and receivers for the SS precursor observations, but the data are sufficiently dense beneath the northwest Pacific Ocean that reasonably good coverage can be obtained for this region. We parameterize the model as a grid of point scatterers in latitude, longitude, and depth (from the surface to 1000 km depth) and compute travel times from each grid point to the source and receiver locations. These times are used to construct a matrix equation that yields predicted SS precursor waveforms from the assumed scatterers. To recover the model, we experiment with both simple back projection and full inversions using a conjugate gradient method. Tests on noise-free synthetic data (generated using the same source-receiver distribution as the actual data) suggest that detailed resolution of discontinuity structure is possible, at horizontal scales much smaller than the Fresnel zone. However, the real data do not produce coherent results unless some degree of horizontal smoothing is imposed, at least partially defeating the purpose of this approach. Results for the northwest Pacific find structure on the 410- and 660-km discontinuities and hints of intermittent reflectors at other depths. Random resampling tests, however, suggest that most of these features are not reliably resolved, with the exception of a depression on the 660-km discontinuity seen in the northwest Pacific. Our experiments show that it is unlikely that small-scale structure on the 660-km discontinuity near subducting slabs causes significant bias in maps of the large-scale 660-km topography derived from long-period SS precursor observations.

Joint inversion for three-dimensional S velocity mantle structure along the Tethyan margin

Abstract : For purposes of studying the lateral heterogeneity as well as for ultimately predicting seismograms for this region, we construct a new 3-D S-velocity model by jointly inverting a variety of different seismic data. We jointly invert regional waveforms, surface wave group velocity measurements, teleseismic S arrival times, and crustal thickness estimates from receiver functions, refraction lines, and gravity surveys. These data types have complementary resolving power for crust and mantle structures, vertical and lateral variations, shallow and deep mantle features, local and global structure. Therefore, a joint inversion of these data sets might help unravel the complexity of this tectonically diverse area. These measurements are made from a combination of mantle investigation of the deep suture between Europe and Africa (MIDSEA), Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL), GeoScope, Geofon, Global Seismographic Network (GSN), International Deployment of Accelerometeres (IDA), MedNet, national networks, and local deployments throughout the study region which extends from the western Mediterranean region to the Hindu Kush and encompasses northeastern Africa, the Arabian peninsula, the Middle East, and part of the Atlantic Ocean for reference. We have fitted the waveforms of regional S and Rayleigh waves from over 3800 seismograms using Partitioned Waveform Inversion. We include over 3000 crustal thickness estimates from receiver functions, gravity measurements, and refraction profiles. We include Rayleigh wave group velocities for hundred thousands of paths transecting the region. We have over 3000 teleseismic S arrival times measured through cross correlation and over 170000 from picks originally reported to the International Seismological Centre (ISC).

Radial upper mantle attenuation structure of inactive back arc basins from differential shear wave measurements

We measure differential attenuation between sS-S and sScS-ScS phase pairs to characterize the variation of attenuation with depth in the upper mantle of five inactive back arc basins: the Kuril Basin, Sea of Japan, Banda Sea, the Celebes and Sulu Seas, and the Shikoku Basin. A spectral ratio technique is used to measure the differential attenuation operator of the transversely polarized waveforms over a frequency band of 10 to 83 mHz. Two algorithms are employed to compute the vertically averaged attenuation structure: a spectral stacking procedure and a least squares inversion. In the spectral stacking method, the individual spectra are corrected for the elastic structure at the sS or sScS bounce point, and the differential attenuation operator is computed by spectral division. The attenuation operators are then normalized and stacked by source depth to obtain more stable spectra, and an average δt* for sources within a restricted depth range is obtained from the slope of the log-amplitude spectrum. A model for the depth dependent Q structure is then calculated from the δt* measurements assuming Q is frequency independent. Alternatively, δt* measurements for individual phase pairs are made using a similar technique and analyzed by ray tracing and a least squares inversion to obtain the Q−1 estimates. The Q results obtained from the stacking and inversion methods are generally in good agreement. The Q−1 structures for the various back arc regions are similar to each other within the uncertainties of the derived Q models. In addition to computing Q models for each individual region, we partitioned the data into three tectonic provinces based upon bounce point locations. The resulting average radial Qβ structure for an inactive basin shows a Q of 54 in the uppermost mantle, 115 at intermediate depths, and 173 in the transition zone; we also find a low Q zone of 36 beneath active island arcs. The Q values for inactive back arc basins are lower than the global averages derived from normal modes but are generally consistent with previous body and surface wave studies of other young oceanic regions. The most striking feature of this study is the observation of very strong attenuation concentrated at shallow depths (<160 km) in the upper mantle beneath these basins.

Regional Travel-Time Uncertainty and Seismic Location Improvement Using a Three-Dimensional a priori Velocity Model

We demonstrate our ability to improve regional travel-time prediction and seismic event location accuracy using an a priori 3D velocity model of Western Eurasia and North Africa (WENA1.0). Travel-time residuals are assessed relative to the iasp91 model for approximately 6000 Pg, Pn , and P arrivals, from seismic events having 2 σ epicenter accuracy between 1 km and 25 km (ground truth 1 [GT1] and GT25, respectively), recorded at 39 stations throughout the model region. Ray paths range in length between 0° and 40° (local, regional, and near teleseismic) providing depth sounding that spans the crust and upper mantle. The dataset also provides representative geographic sampling across Eurasia and North Africa including aseismic areas. The WENA1.0 model markedly improves travel-time predictions for most stations with an average variance reduction of 29% for all ray paths from the GT25 events; when we consider GT5 and better events alone, the variance reduction is 49%. For location tests we use 196 geographically distributed GT5 and better events. In 134 cases (68% of the events), locations are improved, and average mislocation is reduced from 24.9 km to 17.7 km. We develop a travel-time uncertainty model that is used to calculate location coverage ellipses. The coverage ellipses for WENA1.0 are validated to be representative of epicenter error and are smaller than those for iasp91 by 37%. We conclude that a priori models are directly applicable where data coverage limits tomographic and empirical approaches, and the development of the uncertainty model enables merging of a priori and data-driven approaches using Bayesian techniques. Online material: Correction surfaces and histograms of travel-time residuals for 40 stations.