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.

Fan, WY, Shearer PM, Ji C, Bassett D.  2016.  Multiple branching rupture of the 2009 Tonga-Samoa earthquake. Journal of Geophysical Research-Solid Earth. 121:5809-5827.   10.1002/2016jb012945   AbstractWebsite

Several source models have been proposed to explain the enigmatic 2009 Tonga-Samoa earthquake. The long-period data require a composite source model and can be fit with a normal-faulting subevent followed by one or more reverse-faulting subevents. The short-period data, in contrast, indicate a more compact rupture pattern around the epicenter. The lack of a unified source model reflects the complexity of the event. We analyze the spatiotemporal evolution of this earthquake with P wave back-projection from globally distributed stations in different frequency bands (low frequency: 0.05-0.2Hz, high frequency: 0.2-2Hz) and a multiple moment tensor inversion. The rupture propagation revealed by back-projection exhibits frequency-dependent behavior, with two branches of high-frequency-enriched bilateral rupture around the epicenter and a high-frequency-deficient rupture branch at the subduction interface. A composite source model with one M(w)8.0 normal-faulting earthquake east of the trench axis (seaward) followed by one M(w)8.1 reverse-faulting earthquake along the subduction interface west of the trench axis (landward) can explain the very long period data (200 approximate to 500s). Combined with high-resolution swath bathymetry data, the back-projection images show that the azimuth of rupture branches east of the trench axis were controlled by the geometry of bending-related faults on the Pacific plate and that the rupture branch west of the trench axis may correlate with the along-strike fore-arc segmentation. The rupture along the subduction interface was triggered by the seaward rupture and a partially subducted normal fault may have played a key role in facilitating the triggering. The apparent normal-reverse faulting interactions pose a higher seismic risk to this region than their individual strands at the northernmost corner of the Tonga subduction zone.

Trugman, DT, Shearer PM, Borsa AA, Fialko Y.  2016.  A comparison of long-term changes in seismicity at The Geysers, Salton Sea, and Coso geothermal fields. Journal of Geophysical Research-Solid Earth. 121:225-247.   10.1002/2015jb012510   AbstractWebsite

Geothermal energy is an important source of renewable energy, yet its production is known to induce seismicity. Here we analyze seismicity at the three largest geothermal fields in California: The Geysers, Salton Sea, and Coso. We focus on resolving the temporal evolution of seismicity rates, which provides important observational constraints on how geothermal fields respond to natural and anthropogenic loading. We develop an iterative, regularized inversion procedure to partition the observed seismicity rate into two components: (1) the interaction rate due to earthquake-earthquake triggering and (2) the smoothly varying background rate controlled by other time-dependent stresses, including anthropogenic forcing. We apply our methodology to compare long-term changes in seismicity to monthly records of fluid injection and withdrawal. At The Geysers, we find that the background seismicity rate is highly correlated with fluid injection, with the mean rate increasing by approximately 50% and exhibiting strong seasonal fluctuations following construction of the Santa Rosa pipeline in 2003. In contrast, at both Salton Sea and Coso, the background seismicity rate has remained relatively stable since 1990, though both experience short-term rate fluctuations that are not obviously modulated by geothermal plant operation. We also observe significant temporal variations in Gutenberg-Richter b value, earthquake magnitude distribution, and earthquake depth distribution, providing further evidence for the dynamic evolution of stresses within these fields. The differing field-wide responses to fluid injection and withdrawal may reflect differences in in situ reservoir conditions and local tectonics, suggesting that a complex interplay of natural and anthropogenic stressing controls seismicity within California's geothermal fields.

2010
Rychert, CA, Shearer PM.  2010.  Resolving crustal thickness using SS waveform stacks. Geophysical Journal International. 180:1128-1137.   10.1111/j.1365-246X.2009.04497.x   AbstractWebsite

We image lithospheric interfaces using variations in the character of SS waveform stacks, a method we term SS Lithospheric Interface Profiling (SSLIP). The variations are caused by reflected phases, that is, underside reflections (SS precursors) and topside multiples (SS reverberations), created at velocity discontinuities near the midpoint of the SS ray path. Stacks from continental versus oceanic bounce point regions produce distinctly different SS waveforms, consistent with the large continent/ocean difference in crustal thickness. To investigate the potential of SS waveform stacks to constrain Moho depths under continents, we develop a method to fit continental bounce point stacks with a reference SS waveform convolved with a crustal operator. The SSLIP inferred Moho depths agree with the CRUST 2.0 model in Asia for those regions where the SS bounce point density is the highest. The SSLIP depths are correlated (correlation coefficient 0.82) with the CRUST 2.0 values averaged over sample bins of 10 degrees radius. The SSLIP method has broad lateral resolution in comparison to most other methods for resolving crustal thickness, but has the potential to sample regions where station coverage may be sparse.

2008
Peng, ZG, Koper KD, Vidale JE, Leyton F, Shearer P.  2008.  Inner-core fine-scale structure from scattered waves recorded by LASA. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb005412   AbstractWebsite

Recent observations of inner-core scattering (ICS) waves provide evidence that the outermost 300 km of the inner-core has strong heterogeneities with a length scale of a few kilometers. These waves follow a path similar to that of the inner-core-reflected waves PKiKP and were originally observed in data from 16 events in the distance range 58 degrees to 73 degrees recorded by the Large Aperture Seismic Array (LASA). Here we present additional observations of the ICS waves from a total of 78 events recorded by LASA at distances from 18 degrees to 98 degrees. We use a modified version of the Generic Array Processing software package to identify ICS waves on the basis of travel time, back azimuth, ray parameter, amplitude, and coherence. There are 44 events that produce clear ICS waves. We then perform forward modeling of the observed ICS waves using a Monte Carlo seismic phonon method that allows for multiple scattering along the raypath. Most of the ICS waves appear without a visible PKiKP phase, initially grow in time, and have a spindle-shaped envelope. The duration, risetime, and decay rates of the observed ICS waves can be best explained by small-scale volumetric heterogeneities in the outermost few hundred kilometers of the inner core. The average Qc value for the 44 events is similar to 600. Most clear ICS waves are found for raypaths sampling the Pacific Ocean and Asia, and relatively few observations are from the Atlantic Ocean, roughly consistent with the recently observed hemispheric difference in the inner-core structure.

Oki, S, Shearer PM.  2008.  Mantle Q structure from S-P differential attenuation measurements. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb005567   AbstractWebsite

We describe a new one-dimensional Q model for short-period body waves derived from a data set of 15,000 differential t* measurements of teleseismic P and S waves recorded in broadband seismograms. Measured t* values are little affected by the source time function or instrument response since the P and S waves are recorded at the same station from the same event. We process the data using a waveform cross-correlation method applied to the first half cycle of the waveforms to avoid reflection and conversion effects. We invert our t* measurements for a two-layer Q(S) model. Our new Q model has about the same attenuation in the upper mantle and less attenuation in the lower mantle than models derived from longer period data sets. This implies that the frequency dependence of Q is more apparent in the lower mantle and that the effects of attenuation in the upper mantle are approximately constant at frequencies below about 1 Hz. We also observe lateral variations of attenuation in the uppermost mantle by solving for station and event terms, which exhibit correlations with regional tectonics.

2007
Bulow, RC, Johnson CL, Bills BG, Shearer PM.  2007.  Temporal and spatial properties of some deep moonquake clusters. Journal of Geophysical Research-Planets. 112   10.1029/2006je002847   AbstractWebsite

Using the event search method of Bulow et al. ( 2005), we have found 503 new deep moonquakes among the eight largest ( in terms of total number) nearside source regions, increasing the number of identified events for each cluster an average of 36% over the existing catalog. These new events provide an improved deep event catalog, with which we explore some temporal and spatial aspects of deep moonquakes. First, we examine the spectra of moonquake occurrence times at each deep source region, and observe known tidal periodicities, notably those at similar to 27 days and 206 days. Application of spectral methods for the analyses of point processes ( discrete events) allows us to resolve closely spaced tidal periods not previously seen in moonquake data. Second, we pick seismic phase arrival times from optimized stacks of events from each source region. We use these picks, along with published velocity models, to relocate the nine source regions. Source regions A1 and A18 are the best located, with 95% confidence bounds of less than +/- 5 degrees in latitude and longitude, and consistent with estimates from different studies. The locations of source regions A8 and A9 are poorly constrained, with uncertainties in latitude of up to +/- 28 degrees resulting from the absence of clear phase arrivals at station 15. Large trade-offs exist between relocation estimates and choice of velocity model, and the lack of reliable seismic phase arrivals severely affects location error.

Ishii, M, Shearer PM, Houston H, Vidale JE.  2007.  Teleseismic P wave imaging of the 26 December 2004 Sumatra-Andaman and 28 March 2005 Sumatra earthquake ruptures using the Hi-net array. Journal of Geophysical Research-Solid Earth. 112   10.1029/2006jb004700   AbstractWebsite

Seismograms from a dense, high-quality seismic network in Japan are used to investigate the characteristics of the 26 December 2004 Sumatra-Andaman and the 28 March 2005 Sumatran earthquakes. The onset of the P waveforms are aligned through cross correlation, and a simple concept of back-projecting seismic energy to a grid of potential source locations is applied. The waveform alignment removes the effects due to lateral variations in wave speed between the hypocenter and each station. To better approximate the effects of three-dimensional heterogeneity for paths originating from grid points away from the hypocenter, cross-correlation results of the P waveforms from aftershocks are introduced. This additional information leads to improved resolution of smaller-scale features near many of the aftershocks by reducing wavefront distortion. The back-projection analysis provides a quick assessment of the spatiotemporal extent and variability of relative high-frequency energy release, which can be translated into an estimate of the moment magnitude, as well as an unparalleled view of high-frequency rupture propagation. The results are, in general, consistent with those obtained from more involved source inversion methods. The 2004 Sumatra-Andaman earthquake released most energy in a region northwest of the Sumatra island and the rupture extended to the northern Andaman islands, about 1300 km from the epicenter. This northern portion of the rupture radiated a considerable amount of energy, but there is little evidence of slow slip. The 2005 event is imaged to have bilateral rupture with northwestern slip occurring for about 50 s before it moved to the southeast of the epicenter.

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.

2005
Bulow, RC, Johnson CL, Shearer PM.  2005.  New events discovered in the Apollo lunar seismic data. Journal of Geophysical Research-Planets. 110   10.1029/2005je002414   AbstractWebsite

We use modern seismological data processing tools to revisit the Apollo lunar seismic data set with the goal of extending and further characterizing the existing catalog of deep moonquakes. Our studies focus on the long-period data and include filtering and despiking noisy data, event classification, cluster identification, and robust methods for amplitude estimation. We perform cross-correlation analyses for known groups of deep events, confirming earlier visual classifications. By combining the cross-correlation approach with a robust median despiking algorithm, we produce improved differential times and amplitudes, enabling us to construct cleaner stacks. Each event group, represented by a single waveform stack of its constituent members, is cross correlated with the continuous time series. We focus on the A1 cluster because it has more cataloged events than any other cluster and is generally well characterized. Using this approach, we identify additional events that can be associated with previously defined deep clusters. For the deep event group A1 we have found 123 new events, which show phase behavior similar to the 323 previously cataloged events. Our new event search allows us to create optimized event stacks with improved signal to noise from which revised travel time picks (and thus location estimates) can be made. Application of our methods to other deep clusters should form a more complete event catalog and improve our understanding of the spatial and temporal distribution of deep lunar events.

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.

1999
Shearer, PM, Flanagan MP.  1999.  Seismic velocity and density jumps across the 410- and 660-kilometer discontinuities. Science. 285:1545-1548.   10.1126/science.285.5433.1545   AbstractWebsite

The average seismic velocity and density jumps across the 410- and 660-kilometer discontinuities in the upper mantle were determined by modeling the observed range dependence in Long-period seismic wave arrivals that reflect off of these interfaces. The preliminary reference Earth model (PREM) is within the computed 95 percent confidence ellipse for the 410-km discontinuity but outside the allowed jumps across the 660-kilometer discontinuity. Current pyrolite mantle models appear consistent with the constraints for the 410-kilometer discontinuity but overpredict amplitudes for the 660-kilometer reflections. The density jump across the 660-kilometer discontinuity is between 4 and 6 percent, below the PREM value of 9.3 percent commonly used in mantle convection calculations.

Flanagan, MP, Shearer PM.  1999.  A map of topography on the 410-km discontinuity from PP precursors. Geophysical Research Letters. 26:549-552.   10.1029/1999gl900036   AbstractWebsite

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.

Shearer, PM, Flanagan MP, Hedlin MAH.  1999.  Experiments in migration processing of SS precursor data to image upper mantle discontinuity structure. Journal of Geophysical Research-Solid Earth. 104:7229-7242.   10.1029/1998jb900119   AbstractWebsite

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.

1997
Hedlin, MAH, Shearer PM, Earle PS.  1997.  Seismic evidence for small-scale heterogeneity throughout the Earth's mantle. Nature. 387:145-150.   10.1038/387145a0   AbstractWebsite

Details in the amplitude structure of short-period precursors to the seismic core phase PKP can be used to constrain the depth extent of mantle scattering. The simplest model consistent with the observations invokes small (similar to 8 km), weak (r.m.s. velocity perturbations of 1%), random heterogeneities uniformly distributed throughout the mantle. The data do not support previously proposed models that place an increase in the concentration of scattering sites near the base of the mantle, and instead place an upper limit on the amplitude of any short-wavelength topography at the core-mantle boundary.

1993
Bhattacharyya, J, Shearer P, Masters G.  1993.  Inner-Core Attenuation from Short-Period PKP(BC) Versus PKP(DF) Wave-Forms. Geophysical Journal International. 114:1-11.   10.1111/j.1365-246X.1993.tb01461.x   AbstractWebsite

Differential waveform analysis provides an excellent tool for studying the attenuation properties of the top of the inner core. We analyse 108 PKP(BC) versus PKP(DF) waveforms from Global Digital Seismograph Network (GDSN) vertical-component seismograms to constrain the frequency and depth dependency of Q(alpha) in this region. We use both frequency- and time-domain techniques. In the time-domain method, the BC phase is mapped onto the DF phase using an attenuation band operator. The mapping operator is parameterized by the upper and lower cut-off frequencies of the absorption band, the time shift required to align these two phases, and t*, the integrated effect of Q(alpha)-1 in the top of the inner core. In the frequency-domain analysis, multitaper spectral estimation is used to compute the complex spectrum of the two phases. The shape of the amplitude spectrum of the spectral ratio between these two phases gives an estimate of Q(alpha). Similar results are obtained from frequency- and time-domain analysis but the Q(alpha) obtained from frequency-domain analysis is approximately 20 per cent greater than the value obtained from time-domain analysis. We prefer the frequency-domain results since they are not affected by the presence of noise at higher frequencies. Apparent Q(alpha) values exhibit considerable scatter with no clear frequency or depth dependence. We find that the average value of Q(alpha) in the top of the inner core is about 360 which is consistent with previous body wave studies but differs by a factor of two from values obtained from studies of the decay of free oscillations.