Publications

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2017
Buehler, JS, Shearer PM.  2017.  Uppermost mantle seismic velocity structure beneath USArray. Journal of Geophysical Research-Solid Earth. 122:436-448.   10.1002/2016jb013265   AbstractWebsite

We apply Pn tomography beneath the entire USArray footprint to image uppermost mantle velocity structure and anisotropy, as well as crustal thickness constraints, beneath the United States. The sparse source distribution in the eastern United States and the resulting longer raypaths provide new challenges and justify the inclusion of additional parameters that account for the velocity gradient in the mantle lid. At large scale, Pn velocities are higher in the eastern United States compared to the west, but we observe patches of lower velocities around the New Madrid seismic zone and below the eastern Appalachians. For much of the mantle lid below the central and eastern United States we find a moderate positive velocity gradient. In the western United States, we observe a moderate gradient in the region of the Juan de Fuca subduction zone, but no significant gradient to the south and east of this region. In terms of anisotropy, we find that the Pn fast axes generally do not agree with SKS splitting orientations, suggesting significant vertical changes in anisotropy in the upper mantle. In particular the circular pattern of the fast polarization direction of SKS in the western United States is much less pronounced in the Pn results, and in the eastern US the dominant Pn fast direction is approximately north-south, whereas the SKS fast polarizations are oriented roughly parallel to the absolute plate motion direction.

2016
Buehler, JS, Shearer PM.  2016.  Characterizing earthquake location uncertainty in North America using source-receiver reciprocity and USArray. Bulletin of the Seismological Society of America. 106:2395-2401.   10.1785/0120150173   AbstractWebsite

The Comprehensive Nuclear-Test-Ban Treaty community often uses calibration events with well-determined origins to improve absolute locations of nearby seismic events by accounting for the biasing effects of unknown velocity structure, but the number of these ground-truth events is limited. To provide additional constraints, source-receiver reciprocity allows us to use seismic stations as calibration events with known locations. The dense and uniform spacing of the USArray transportable array stations makes them ideal to measure the spatial coherence of mislocation vectors across North America and hence to assess how close calibration events (or stations) need to be to target events to improve locations for a given region. We use a gridsearch approach for the station"relocations," using both teleseismic earthquakes and simulated regional events. Our results show that the mislocation vectors are spatially coherent for scales up to 500 km in many regions, but that in some places, such as regions that can be associated with strong velocity anomalies in the upper mantle, mislocation vectors exhibit large changes over short distances.

2013
Buehler, JS, Shearer PM.  2013.  Sn propagation in the Western United States from common midpoint stacks of USArray data. Geophysical Research Letters. 40:6106-6111.   10.1002/2013gl057680   AbstractWebsite

The regional seismic phase Sn propagates horizontally in the uppermost mantle and is sensitive to lateral variations in mantle lid thickness, temperature, and melt. Sn is therefore often used as an indicator for physical properties of the lithosphere. It has previously been noticed that Sn is not observed at many stations in the Western United States and Sn seems especially highly attenuated for paths across the Basin and Range. Here we apply stacking methods to USArray data to identify highly attenuating regions in the uppermost mantle and increase the spatial resolution of the Sn propagation image. We find evidence for Sn propagation at short ranges in the central Great Basin and the northeastern part of the Colorado Plateau, both regions where lithospheric stability and thickness is debated, and observe strong Sn attenuation around the perimeter of the central Great Basin.

2012
Buehler, JS, Shearer PM.  2012.  Localized imaging of the uppermost mantle with USArray Pn data. Journal of Geophysical Research-Solid Earth. 117   10.1029/2012jb009433   AbstractWebsite

USArray has now provided several years of high-quality seismic data and improved ray coverage for much of the western United States. This allows increased resolution for regional studies of the lithosphere and deeper structure of the North American continent. In this study, we use Pn phases in the USArray data set to solve for velocity structure in the uppermost mantle in the western United States. This article focuses on localized imaging techniques that complement traditional Pn tomography analysis. We apply waveform cross-correlation to obtain inter-station travel times between the closely and uniformly spaced USArray stations. This allows us to use traces without phase picks and reduces errors associated with the picking. We obtain differential times that can directly be used to fit locally for slowness and, depending on the approach, for the direction and curvature of the incoming wavefront. The various measurements of incoming wavefronts at different sub-arrays provide constraints on azimuthal variations in velocity. The traditional tomography approach and the local fitting method reveal similar large-scale features. No regularization is applied with the local method, and the resulting velocity maps reveal smaller-scale structures than the tomographic images.

Traer, J, Gerstoft P, Bromirski PD, Shearer PM.  2012.  Microseisms and hum from ocean surface gravity waves. Journal of Geophysical Research-Solid Earth. 117   10.1029/2012jb009550   AbstractWebsite

Ocean waves incident on coasts generate seismic surface waves in three frequency bands via three pathways: direct pressure on the seafloor (primary microseisms, PM), standing waves from interaction of incident and reflected waves (double-frequency microseisms, DF), and swell-transformed infragravity wave interactions (the Earth's seismic hum). Beamforming of USArray seismic data shows that the source azimuths of the generation regions of hum, PM and DF microseisms vary seasonally, consistent with hemispheric storm patterns. The correlation of beam power with wave height over all azimuths is highest in near-coastal waters. Seismic signals generated by waves from Hurricane Irene and from a storm in the Southern Ocean have good spatial and temporal correlation with nearshore wave height and peak period for all three wave-induced seismic signals, suggesting that ocean waves in shallow water commonly excite hum (via infragravity waves), PM, and DF microseisms concurrently.

2011
Rychert, CA, Shearer PM.  2011.  Imaging the lithosphere-asthenosphere boundary beneath the Pacific using SS waveform modeling. Journal of Geophysical Research-Solid Earth. 116   10.1029/2010jb008070   AbstractWebsite

Oceanic lithosphere constitutes the bulk of Earth's tectonic plates and also likely represents the building blocks of the continental lithosphere. The depth and nature of the oceanic lithosphere-asthenosphere boundary are central to our understanding of the definition of the tectonic plates and lithospheric evolution. Although it is well established that oceanic lithosphere cools, thickens, and subsides as it ages according to conductive cooling models, this relatively simple realization of the tectonic plates is not completely understood. Old (> 70 Ma) ocean depths are shallower than predicted. Furthermore, precise imaging of the lower boundary of the oceanic lithosphere has proven challenging. Here we directly map the depth and nature of a seismic discontinuity that is likely the lithosphere-asthenosphere boundary across the Pacific plate using a new method that models variations in the shapes of stacked SS waveforms from 17 years of seismic data. The depth to the discontinuity varies from 25 to 130 km and correlates with distance from the ridge along mantle flow lines. This implies that the depth of the oceanic lithosphere-asthenosphere boundary depends on the temperature of the underlying asthenosphere, defined by a best fitting isotherm at 930 degrees C with a 95% confidence region of 820-1020 degrees C, although the sharpness of the observations in some locations implies a mechanism besides temperature may also be required.

2010
Zhang, J, Gerstoft P, Shearer PM.  2010.  Resolving P-wave travel-time anomalies using seismic array observations of oceanic storms. Earth and Planetary Science Letters. 292:419-427.   10.1016/j.epsl.2010.02.014   AbstractWebsite

Array analysis of seismic noise has the potential to be very useful in improving body-wave tomography of Earth structure, just as noise cross-correlation methods have recently proven successful in surface-wave tomography. Beamforming of seismic noise recorded in southern California reveals P-wave arrivals from distant storms in open oceans. In this case, the noise can be processed using cross-correlation among different station pairs and optimal P-wave relative arrival times can be estimated using the same approach traditionally used to analyze earthquake arrival times. Using three storms in the Gulf of Mexico, the Western Pacific (near Japan), and the South Pacific (near Fiji) respectively, we demonstrate that travel-time anomalies can be obtained from P waves generated by a distant storm, and that they are similar to those obtained from using an earthquake close to the storm. Our results suggest using oceanic storms as additional seismic sources for resolving P-wave travel-time anomalies. (C) 2010 Elsevier B.V. All rights reserved.

2009
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.

2008
Lawrence, JF, Shearer PM.  2008.  Imaging mantle transition zone thickness with SdS-SS finite-frequency sensitivity kernels. Geophysical Journal International. 174:143-158.   10.1111/j.1365-246X.2007.03673.x   AbstractWebsite

We invert differential SdS-SS traveltime residuals measured from stacked waveforms and finite-frequency sensitivity kernels for topography on the 410- and 660-km discontinuities. This approach yields higher resolution images of transition zone thickness than previous stacking methods, which simply average/smooth over topographic features. Apparent structure measured using simple stacking is highly dependent upon the bin size of each stack. By inverting for discontinuity topography with a variety of bin sizes, we can more accurately calculate the true structure. The inverted transition zone model is similar to simple stack models with an average thickness of 242 km, but the lateral variations in thickness are larger in amplitude and smaller in scale. Fast seismic velocities in 3-D mantle models such as SB4L18 correlate with areas of thicker transition zone. The elongated curvilinear regions of thickened transition zone that occur near subduction zones are narrow and high amplitude, which suggests relatively little lateral spreading and warming of subducted lithosphere within the transition zone. The anomalously thin transition zone regions are laterally narrow, and not broadly continuous. If these variations in transition zone thickness are interpreted as thermal in nature, then this model suggests significant temperature variations on small lateral scales.

Houser, C, Masters G, Shearer P, Laske G.  2008.  Shear and compressional velocity models of the mantle from cluster analysis of long-period waveforms. Geophysical Journal International. 174:195-212.   10.1111/j.1365-246X.2008.03763.x   AbstractWebsite

We present a new technique for the efficient measurement of the traveltimes of long period body wave phases. The technique is based on the fact that all arrivals of a particular seismic phase are remarkably similar in shape for a single event. This allows the application of cross-correlation techniques that are usually used in a regional context to measure precise global differential times. The analysis is enhanced by the inclusion of a clustering algorithm that automatically clusters waveforms by their degree of similarity. This allows the algorithm to discriminate against unusual or distorted waveforms and makes for an extremely efficient measurement technique. This technique can be applied to any seismic phase that is observed over a reasonably large distance range. Here, we present the results of applying the algorithm to the long-period channels of all data archived at the IRIS DMC from 1976 to 2005 for the seismic phases S and P (from 23 degrees to 100 degrees) and SS and PP (from 50 degrees to 170 degrees). The resulting large data sets are inverted along with existing surface wave and updated differential traveltime measurements for new mantle models of S and P velocity. The resolution of the new model is enhanced, particularly, in the mid-mantle where SS and PP turn. We find that slow anomalies in the central Pacific and Africa extend from the core-mantle boundary to the upper mantle, but their direct connection to surface hotspots is beyond our resolution. Furthermore, we find that fast anomalies that are likely associated with subducting slabs disappear between 1700 and 2500 km, and thus are not continuous features from the upper to lower mantle despite our extensive coverage and high resolution of the mid-mantle.

2007
Lin, GQ, Shearer PM, Hauksson E.  2007.  Applying a three-dimensional velocity model, waveform cross correlation, and cluster analysis to locate southern California seismicity from 1981 to 2005. Journal of Geophysical Research-Solid Earth. 112   10.1029/2007jb004986   AbstractWebsite

[1] We compute high-precision earthquake locations using southern California pick and waveform data from 1981 to 2005. Our latest results are significantly improved compared to our previous catalog by the following: (1) We locate events with respect to a new crustal P and S velocity model using three-dimensional ray tracing, (2) we examine six more years of waveform data and compute cross-correlation results for many more pairs than our last analysis, and (3) we compute locations within similar event clusters using a new method that applies a robust fitting method to obtain the best locations satisfying all the differential time constraints from the waveform cross correlation. These results build on the relocated catalogs of Hauksson and Shearer (2005) and Shearer et al. (2005) and provide additional insight regarding the fine-scale fault structure in southern California and the relationship between the San Andreas Fault (SAF) and nearby seismicity. In particular, we present results for two regions in which the seismicity near the southern SAF seems to align on dipping faults.

Lin, GQ, Shearer P.  2007.  Estimating local V-P/V-s ratios within similar earthquake clusters. Bulletin of the Seismological Society of America. 97:379-388.   10.1785/0120060115   AbstractWebsite

We develop and test a method to estimate local VP/V, ratios for compact similar earthquake clusters using the precise P and S differential times obtained using waveform cross-correlation. We demonstrate how our technique works using synthetic data and evaluate likely errors arising from near-source takeoff angle differences between P and S waves. We use a robust misfit function method to compute VPA, ratios for both synthetic data sets and several similar event clusters in southern California, and use a bootstrap resampling approach to estimate standard errors for real data. Our technique has higher resolution for near-source VP/V, ratios than typical tomographic inversion methods and provides constraints on near-fault rock properties.

2006
Lawrence, JF, Shearer PM, Masters G.  2006.  Mapping attenuation beneath North America using waveform cross-correlation and cluster analysis. Geophysical Research Letters. 33   10.1029/2006gl025813   AbstractWebsite

We measure seismic attenuation beneath North America using waveform cross-correlation and cluster analysis, and obtain images of the laterally varying anelastic structure of the upper mantle. Cluster analysis improves attenuation measurements by systematically comparing only highly similar waveforms, which reduces bias from scattering, directional differences in source functions, and source-side structure. While lacking station coverage in many areas, the P- and S-wave results are correlated (R-2 >= 0.5) in both travel time and attenuation. Much weaker correlations are observed between travel-time and attenuation measurements. Similarities and differences between attenuation and travel times may be used to infer the source of the observed anomalies. The observed anelastic structure has a long-wavelength pattern crudely similar to that of seismic velocity, which likely indicates higher temperatures beneath western North America than in the east. Shorter-wavelength structure suggests complex variations requiring alternate explanations such as variable water content.

2002
Warren, LM, Shearer PM.  2002.  Mapping lateral variations in upper mantle attenuation by stacking P and PP spectra. Journal of Geophysical Research-Solid Earth. 107   10.1029/2001jb001195   AbstractWebsite

[1] We study the lateral variations in P wave attenuation in the upper mantle at frequencies between 0.16 and 0.86 Hz by analyzing the spectra from >18,000 P and >14,000 PP arrivals. We select seismograms from shallow earthquakes at epicentral distances of 40degrees-80degrees for P waves and 80degrees-160degrees for PP waves. Each spectrum is the product of source, receiver, and propagation response functions as well as local source- and receiver-side effects. We correct each spectrum for average source and attenuation models. Since there are multiple receivers for each source and multiple sources for each receiver, we can approximate the source- and receiver-side terms by stacking the appropriate P log spectra. The resulting source- specific response functions include any remaining source spectrum and near-source Q structure; the receiver stacks include the site response and near-receiver Q structure. We correct the PP log spectra for the appropriate source- and receiver-side stacks. Since attenuation in the lower mantle is small, the residual log spectrum approximates attenuation in the upper mantle near the PP bounce point and is used to estimate delta(t*) over bar. We constrain the anomalies to the top 220 km of the mantle, as suggested by previous Q studies, and translate the delta(t*) over bar measurements to variations in 1000/Q(alpha). The patterns of more and less attenuating regions generally correlate with previously published shear attenuation models and surface tectonics. Continents are usually less attenuating than the global average, whereas oceanic regions tend to be more attenuating. There are interesting exceptions to this tectonic pattern, such as an attenuating region beneath southern Africa.

2000
Shearer, PM.  2000.  Upper mantle seismic discontinuities. Earth's deep interior; mineral physics and tomography from the atomic to the global scale. 117:115-131.