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Shearer, PM, Rychert CA, Liu QY.  2011.  On the visibility of the inner-core shear wave phase PKJKP at long periods. Geophysical Journal International. 185:1379-1383.   10.1111/j.1365-246X.2011.05011.x   AbstractWebsite

P>Stacks of over 90 000 long-period seismograms recorded by the global seismic networks resolve many seismic phases but fail to detect the inner-core shear phase PKJKP. We compare these results to synthetic seismograms computed for PREM using the spectral-element method and show that expected PKJKP amplitudes are over 10 times smaller than signal-generated 'noise' caused by high-order P surface multiples and reverberations from upper-mantle discontinuities. Indeed, PKJKP can only be seen in synthetic seismograms when differences are taken between two sets of synthetics generated from Earth models with slightly different inner-core shear velocities. These results suggest that routine observation of PKJKP is unlikely and that reported observations, if real, must have resulted from exceptional focusing effects or inner-core attenuation much less that current models.

Kendall, JM, Shearer PM.  1995.  On the Structure of the Lowermost Mantle beneath the Southwest Pacific, Southeast-Asia and Australasia. Physics of the Earth and Planetary Interiors. 92:85-98. AbstractWebsite

The region of the lowermost mantle beneath the southwest Pacific, Australasia and southeast Asia (50 degrees S-50 degrees N and 80-190 degrees E) has been studied using a wide variety of seismic techniques. We complement these studies with results obtained from long-period Global Digital Seismograph Network (GDSN) data using a recently developed phase-stripping technique that permits the isolation of D'' reflections from the stronger neighbouring S and ScS signals. We identify patches with D'' reflections and areas where we cannot confidently determine the presence or absence of a D'' reflection. A synthesis of our results with other studies suggests that D'' varies dramatically through the region, generally thickening from 100-150 km in a central zone to about 300 km at the eastern and western margins. However, there are irregularities in this overall pattern, including areas where there seems to be little evidence for a D'' discontinuity. Inspection of waveform amplitudes shows considerable scatter in not only the D'' reflected phases, but also core-mantle boundary reflected phases. Experiments with synthetic seismograms for a variety of D'' models and the observed lateral variability through the region suggest that this is to be expected. Furthermore, ray-theory calculations for large-scale 3D Earth models predict significant variations (+/-30%) in S-wave amplitudes of lowermost mantle turning rays. Finally, we investigate possible correlations between lower-mantle velocity, flow and D'' thickness. We find some correlation between D'' thickness and lower-mantle velocities obtained from tomographic inversions, with a thin D'' layer in high-velocity regions and a thickening of the layer toward slower regions. The relationship between predicted lower-mantle flow and D'' thickness is less clear. These results are qualitatively consistent with thermal boundary layer predictions for D'', but do not preclude the possibility of compositionally distinct material in the layer.

Mancinelli, N, Shearer P, Thomas C.  2016.  On the frequency dependence and spatial coherence of PKP precursor amplitudes. Journal of Geophysical Research-Solid Earth. 121:1873-1889.   10.1002/2015jb012768   AbstractWebsite

Studies now agree that small-scale (approximate to 10km) weak (approximate to 0.1%) velocity perturbations throughout the lowermost mantle generate the globally averaged amplitudes of 1Hz precursors to the core phase, . The possible frequency dependence and spatial coherence of this scattered phase, however, has been given less attention. Using a large global data set of approximate to 150,000 PKP precursor recordings, we characterize the frequency dependence of PKP precursors at central frequencies ranging from 0.5 to 4Hz. At greater frequencies, we observe more scattered energy (relative to the reference phase PKPdf), particularly at shorter ranges. We model this observation by invoking heterogeneity at length scales from 2 to 30km. Amplitudes at 0.5Hz, in particular, suggest the presence of more heterogeneity at scales >8km than present in previously published models. Using a regional bootstrap approach, we identify large (>20 degrees), spatially coherent regions of anomalously strong scattering beneath the West Pacific, Central/North America, andto a lesser extentEast Africa. Finally, as proof of concept, we use array processing techniques to locate the origin of scattered energy observed in Southern California by the Anza and Southern California Seismic Networks. The energy appears to come primarily from out-of-plane scattering on the receiver side. We suggest that such improvised arrays can increase global coverage and may reveal whether a majority of precursor energy comes from localized heterogeneity in the lowermost mantle.

Lin, GQ, Shearer P, Fialko Y.  2006.  Obtaining absolute locations for quarry seismicity using remote sensing data. Bulletin of the Seismological Society of America. 96:722-728.   10.1785/0120050146   AbstractWebsite

We obtain absolute locations for 19 clusters of mining-induced seismicity in southern California by identifying quarries using remote sensing data, including optical imagery and differential digital elevation models. These seismicity clusters contain 16,574 events from the Southern California Seismic Network from 1984 to 2002, which are flagged as quarry blasts but without any -round-truth location constraints. Using georeferenced airphotos and satellite radar topography data, we identify the likely sources of these events as quarries that are clearly visible within 1 to 2 km of the seismically determined locations. We then shift the clusters to align with the airphoto images, obtaining an estimated absolute location accuracy of similar to 200 m for the cluster centroids. The improved locations of these explosions should be helpful for constraining regional 3D velocity models.

Shearer, PM, Earle PS.  2008.  Observing and Modeling Elastic Scattering in the Deep Earth. Advances in Geophysics, Vol 50: Earth Heterogeneity and Scattering Effects on Seismic Waves. 50( Sato H, Fehler MC, Eds.).:167-193., San Diego: Elsevier Academic Press Inc   10.1016/s0065-2687(08)00006-x   Abstract

Seismic scattering in the deep Earth below the mantle transition zone is observed from precursors and codas to a number of body-wave arrivals, including P, P(diff), PKP, PKiKP, PKKP, and P'P'. Envelope-stacking methods applied to large teleseismic databases are useful for resolving the globally averaged time and amplitude dependence of these arrivals. Slacks of P coda near 1 Hz from shallow earthquakes exhibit significant variations among different source and receiver locations, indicating lateral variations in scattering strength. At least some deep-mantle, core-mantle boundary, and inner L-core scattering is indicated by the observations, bill the strength and scale length of the random velocity heterogeneity required to explain the data are not yet firmly established. Monte Carlo seismic "particle" algorithms, based on numerical evalualion of radiative transfer theory with Born scattering amplitudes for random elastic heterogeneity, provide a powerful tool for computer modeling of scattering in the whole Earth because they preserve energy and can handle multiple scattering through depth-varying heterogeneity models. Efficient implementation of these algorithms can fie achieved by precomputing ray tracing tables and discretized scattering probability functions.

Earle, PS, Shearer PM.  1997.  Observations of PKKP precursors used to estimate small-scale topography on the core-mantle boundary. Science. 277:667-670.   10.1126/science.277.5326.667   AbstractWebsite

Stacks of global seismic data reveal the time and distance dependence of high-frequency (1-hertz) precursors to the seismic phase PKKP. Synthetic seismogram modeling shows that scattering from random small-scale topography at the PKKP core-mantle-boundary reflection point generates precursory arrivals similar to those seen in the data. Models with a root mean square core-mantle-boundary topography of 250 to 350 meters and correlation length of 7 to 10 kilometers explain the main features of the data. However, a systematic range-dependent misfit between observed and predicted precursor power suggests that inner core scattering may contribute to the precursors.

Earle, PS, Shearer PM.  1998.  Observations of high-frequency scattered energy associated with the core phase PKKP. Geophysical Research Letters. 25:405-408.   10.1029/97gl53365   AbstractWebsite

Stacks of global seismic network data reveal emergent high-frequency (similar to 1 Hz) energy extending back from the PKKP c caustic at 72 degrees to a distance of about 60 degrees. The arrival's emergent onset, long duration (similar to 150 s), apparent slowness, and proximity to the core phase PKKP indicate a deep scattering origin. However, the amplitude and timing of the observations are difficult to explain with simple models of PKKP scattering.