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Rychert, CA, Shearer PM.  2009.  A Global View of the Lithosphere-Asthenosphere Boundary. Science. 324:495-498.   10.1126/science.1169754   AbstractWebsite

The lithosphere-asthenosphere boundary divides the rigid lid from the weaker mantle and is fundamental in plate tectonics. However, its depth and defining mechanism are not well known. We analyzed 15 years of global seismic data using P-to-S (Ps) converted phases and imaged an interface that correlates with tectonic environment, varying from 95 +/- 4 kilometers beneath Precambrian shields and platforms to 81 +/- 2 kilometers beneath tectonically altered regions and 70 +/- 4 kilometers at oceanic island stations. High-frequency Ps observations require a sharp discontinuity; therefore, this interface likely represents a boundary in composition, melting, or anisotropy, not temperature alone. It likely represents the lithosphere-asthenosphere boundary under oceans and tectonically altered regions, but it may constitute another boundary in cratonic regions where the lithosphere-asthenosphere boundary is thought to be much deeper.

Allmann, BP, Shearer PM.  2009.  Global variations of stress drop for moderate to large earthquakes. Journal of Geophysical Research-Solid Earth. 114   10.1029/2008jb005821   AbstractWebsite

We investigate the global variation of earthquake stress drops using spectra of about 2000 events of m(b) >= 5.5 between 1990 and 2007. We use an iterative least squares method to isolate source displacement spectra from travel path and receiver contributions, based on a convolutional model. The observed P wave source spectra are corrected with a globally averaged empirical correction spectrum and estimates of near-source attenuation. Assuming a Brune-type source model, we estimate corner frequencies and compute stress drops. Stress drop estimates for individual earthquakes range from about 0.3 to 50 MPa, but the median stress drop of about 4 MPa does not vary with moment, implying earthquake self-similarity over the M(w) = 5.2 to 8.3 range of our data. A comparison of our results with previous studies confirms this observation over most of the instrumentally observable magnitude range. While the absolute values of our estimated stress drops depend upon the assumed source model, we identify relative regional variations of stress drop that are robust with respect to the processing parameters and modeling assumptions, which includes an inherent assumption of constant rupture velocity. We find a dependence of median stress drop on focal mechanism, with a factor of 3-5 times higher stress drops for strike-slip earthquakes and also find a factor of 2 times higher stress drops for intraplate earthquakes compared to interplate earthquakes.

Lawrence, JF, Shearer PM.  2006.  A global study of transition zone thickness using receiver functions. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb003973   AbstractWebsite

[ 1] Systematic stacks of P wave receiver functions ( Pds) for 118 global seismic stations yield new transition zone thickness (W-TZ) estimates, as measured by the difference in depths between the 410- and 660-km mantle discontinuities. The receiver functions are computed from high signal-to-noise records of earthquakes between 1976 and 2002 recorded at distances of 30 degrees to 90 degrees. We obtain a globally averaged transition zone thickness of 242 +/- 2 km, in good agreement with SS precursor (SdS) results. Previously noted differences in average W-TZ between Pds and SdS studies are caused by both geographic bias and the constant ray parameter approximation used in many prior Pds studies, which causes a similar to 4 km overestimation of W-TZ. Pds observations suggest lateral variations in W-TZ of +/- 20 km with maximal variations of +/- 35 km and a long-wavelength topography pattern that agrees with SS precursor results showing thick W-TZ beneath cold subduction zones and thin W-TZ beneath warmer regions.

Astiz, L, Earle PS, Shearer P.  1996.  Global Stacking of Broadband Seismograms. Seismological Research Letters. 67:8-18. Abstract


Shearer, PM, Earle PS.  2004.  The global short-period wavefield modelled with a Monte Carlo seismic phonon method. Geophysical Journal International. 158:1103-1117.   10.1111/j.1365-246X.2004.02378.x   AbstractWebsite

At high frequencies (similar to1 Hz), much of the seismic energy arriving at teleseismic distances is not found in the main phases (e.g. P, PP, S, etc.) but is contained in the extended coda that follows these arrivals. This coda results from scattering off small-scale velocity and density perturbations within the crust and mantle and contains valuable information regarding the depth dependence and strength of this heterogeneity as well as the relative importance of intrinsic versus scattering attenuation. Most analyses of seismic coda to date have concentrated on S-wave coda generated from lithospheric scattering for events recorded at local and regional distances. Here, we examine the globally averaged vertical-component, 1-Hz wavefield (>10degrees range) for earthquakes recorded in the IRIS FARM archive from 1990 to 1999. We apply an envelope-function stacking technique to image the average time-distance behavior of the wavefield for both shallow (less than or equal to50 km) and deep (greater than or equal to500 km) earthquakes. Unlike regional records, our images are dominated by P and P coda owing to the large effect of attenuation on PP and S at high frequencies. Modelling our results is complicated by the need to include a variety of ray paths, the likely contributions of multiple scattering and the possible importance of P-to-S and S-to-P scattering. We adopt a stochastic, particle-based approach in which millions of seismic phonons are randomly sprayed from the source and tracked through the Earth. Each phonon represents an energy packet that travels along the appropriate ray path until it is affected by a discontinuity or a scatterer. Discontinuities are modelled by treating the energy normalized reflection and transmission coefficients as probabilities. Scattering probabilities and scattering angles are computed in a similar fashion, assuming random velocity and density perturbations characterized by an exponential autocorrelation function. Intrinsic attenuation is included by reducing the energy contained in each particle as an appropriate function of traveltime. We find that most scattering occurs in the lithosphere and upper mantle, as previous results have indicated, but that some lower-mantle scattering is likely also required. A model with 3 to 4 per cent rms velocity heterogeneity at 4-km scale length in the upper mantle and 0.5 per cent rms velocity heterogeneity at 8-km scale length in the lower mantle (with intrinsic attenuation of Q(alpha)= 450 above 200 km depth and Q(alpha)= 2500 below 200 km) provides a reasonable fit to both the shallow- and deep-earthquake observations, although many trade-offs exist between the scale length, depth extent and strength of the heterogeneity.

Shearer, PM.  1994.  Global Seismic Event Detection Using a Matched-Filter on Long-Period Seismograms. Journal of Geophysical Research-Solid Earth. 99:13713-13725.   10.1029/94jb00498   AbstractWebsite

An image derived by stacking long-period seismograms is used as an empirical matched filter to detect and locate earthquakes. Records from 564 events (m(b) greater-than-or-equal-to 6) recorded at very long periods (T greater-than-or-equal-to 60 s) by the 15 to 20 stations of the International Deployment of Accelerometers (IDA) network are stacked using a method that emphasizes the surface wave arrivals. The first 3 hours of this time versus range image are used to construct a matched filter for continuous application to the IDA data. The output of this filter contains spatial and temporal peaks that define the location and origin time of probable seismic events. Implementation of this technique to 11 years of IDA data from 1981 to 1991 identifies 4061 events. These include 65% of cataloged events Of m(b) greater-than-or-equal-to 5.5. Earthquakes which appear anomalously strong relative to their surface wave magnitudes are mainly located on oceanic transform faults and probably represent unusually slow ruptures. The method successfully detects two eruptions of the El Chichon volcano in southern Mexico, from the anomalous low-frequency energy radiated during the eruptions. In addition, 32 earthquakes are detected which are not in the standard global catalogs. These events appear to be about M(s) = 5 and are mainly located in the southern oceans, where there are gaps in the coverage of the high-frequency networks. Although these earthquakes are probably ''slow'' since they occur mostly on oceanic transform faults, they are not ''silent'' as they also can be observed in higher-frequency data.

Shearer, PM, Stark PB.  2012.  Global risk of big earthquakes has not recently increased. Proceedings of the National Academy of Sciences of the United States of America. 109:717-721.   10.1073/pnas.1118525109   AbstractWebsite

The recent elevated rate of large earthquakes has fueled concern that the underlying global rate of earthquake activity has increased, which would have important implications for assessments of seismic hazard and our understanding of how faults interact. We examine the timing of large (magnitude M >= 7) earthquakes from 1900 to the present, after removing local clustering related to aftershocks. The global rate of M >= 8 earthquakes has been at a record high roughly since 2004, but rates have been almost as high before, and the rate of smaller earthquakes is close to its historical average. Some features of the global catalog are improbable in retrospect, but so are some features of most random sequences-if the features are selected after looking at the data. For a variety of magnitude cutoffs and three statistical tests, the global catalog, with local clusters removed, is not distinguishable from a homogeneous Poisson process. Moreover, no plausible physical mechanism predicts real changes in the underlying global rate of large events. Together these facts suggest that the global risk of large earthquakes is no higher today than it has been in the past.

Gerstoft, P, Shearer PM, Harmon N, Zhang J.  2008.  Global P, PP, and PKP wave microseisms observed from distant storms. Geophysical Research Letters. 35   10.1029/2008gl036111   AbstractWebsite

Microseisms are the continuous background vibrations of the Earth observed between earthquakes. Most microseism studies have focused on low frequency energy (0.05-0.5 Hz) propagating as surface waves, but in the microseism spectrum there is also energy that propagates as body waves (P-waves). Using array analysis on southern California stations we show that these body waves are generated in the ocean from distant storms and propagate deep within the Earth's mantle and core as P, PP and PKP phases. Comparisons with ocean wave hindcast data identify several distinct source regions in both the northern and southern hemispheres. Analyses of these body waves demonstrate that microseisms often have a strong P-wave component originating from distant locations.

Shearer, PM.  1993.  Global Mapping of Upper-Mantle Reflections from Long-Period SS Precursors. Geophysical Journal International. 115:878-904.   10.1111/j.1365-246X.1993.tb01499.x   AbstractWebsite

Long-period precursors to SS resulting from underside reflections off upper mantle discontinuities (SdS where d is the discontinuity depth) can be used to map the global distribution and depth of these reflectors. We analyse 5,884 long-period seismograms from the Global Digital Seismograph Network (1976-1987, shallow sources, transverse component) in order to identify SdS arrivals. Corrections for velocity dispersion, topography and crustal thickness at the SS bounce point, and lateral variation in mantle velocity are critical for obtaining accurate estimates of discontinuity depths. The 410 and 660 km discontinuities are observed at average depths of 413 and 653 km, and exhibit large-scale coherent patterns of topography with depth variations up to 40 km. These patterns are roughly correlated with recent tomographic models, with fast anomalies in the transition zone associated with highs in the 410 km discontinuity and lows in the 660 km discontinuity, a result consistent with laboratory measurements of Clapeyron slopes for the appropriate phase changes. The best resolved feature in these maps is a trough in the 660 km discontinuity in the northwest Pacific, which appears to be associated with the subduction zones in this region. Amplitude variations in SdS arrivals are not correlated with discontinuity depths and probably result from focusing and defocusing effects along the ray paths. The SdS arrivals suggest the presence of regional reflectors in the upper mantle above 400 km. However, only the strongest of these features are above probable noise levels due to sampling inadequacies.

Flanagan, MP, Shearer PM.  1998.  Global mapping of topography on transition zone velocity discontinuities by stacking SS precursors. Journal of Geophysical Research-Solid Earth. 103:2673-2692.   10.1029/97jb03212   AbstractWebsite

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 m(b) > 5.5, focal depth < 75 km, and range 110 degrees to 180 degrees 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 degrees 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, W-TZ, 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 W,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.

Shearer, PM, Masters TG.  1992.  Global Mapping of Topography on the 660-KM Discontinuity. Nature. 355:791-796.   10.1038/355791a0   AbstractWebsite

Long-period precursors to the SS seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle. These maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660-km discontinuity and subduction zones - a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle.

Bhattacharyya, J, Masters G, Shearer P.  1996.  Global lateral variations of shear wave attenuation in the upper mantle. Journal of Geophysical Research-Solid Earth. 101:22273-22289.   10.1029/96jb01782   AbstractWebsite

We analyze several thousand high-quality, globally recorded SS-S differential waveforms to constrain the lateral variation of shear wave attenuation (Q beta) in the upper mantle. We use a multitaper frequency domain technique to measure attenuation, parameterized by a t* operator, and implement a robust estimation technique to compute t* and its variance. The differential waveform technique minimizes the effect of factors such as finite source duration and structural complexity near the source and receiver so the differential SS-S waveforms are mainly sensitive to the shear attenuation in the upper mantle under the SS bounce point. We use seismograms recorded at ranges of 45 degrees to 100 degrees and compute the SS-S differential t* from the broadening of the SS waveform relative to the Hilbert transform of the S waveform. A careful choice of fitting windows allows us to reduce the biasing effects of interfering phases which can affect t* by up to 0.5 s. The t* residuals (with respect to preliminary reference Earth model (PREM)) vary by +/-1.5 s with an average of approximate to 0.24 s. Our study suggests an average Q(beta) value of 112 (most of the lateral variations of Q(beta) are within 30% of this value) in the top 400 km of the mantle, slightly lower than the PREM value of 128. There is a qualitative correlation of t* residual with tectonic region with distinctly higher attenuation observed under young oceans compared to platforms and shields. Also, the lateral variations of the residuals are similar in trend to those observed in studies of the attenuation of ScS multiples. At long wavelengths, the Q(beta) map shows a modest correlation with shear wave attenuation maps computed from surface wave analyses and with the patterns of lateral variations of shear velocities at certain upper-mantle depths predicted by the modelS1GB30. The correlation with the velocity model is highest at 300-500 km depth indicating that there may be a contribution to long-wavelength attenuation from relatively deep regions. Formal inversion for an upper mantle Q(beta) model shows that while lateral resolution is quite good, depth resolution is poor as might be expected. Better depth resolution must await combined body wave and surface wave inversions.