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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.

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.

Walker, KT, Zumberge MA, Hedlin MAH, Shearer PM.  2008.  Methods for determining infrasound phase velocity direction with an array of line sensors. Journal of the Acoustical Society of America. 124:2090-2099.   10.1121/1.2968675   AbstractWebsite

Infrasound arrays typically consist of several microbarometers separated by distances that provide predictable signal time separations, forming the basis for processing techniques that estimate the phase velocity direction. The directional resolution depends on the noise level and is proportional to the number of these point sensors; additional sensors help attenuate noise and improve direction resolution. An alternative approach is to form an array of directional line sensors, each of which emulates a line of many microphones that instantaneously integrate pressure change. The instrument response is a function of the orientation of the line with respect to the signal wavefront. Real data recorded at the Pinon Flat Observatory in southern California and synthetic data show that this spectral property can be exploited with multiple line sensors to determine the phase velocity direction with a precision comparable to a larger aperture array of microbarometers. Three types of instrument-response-dependent beamforming and an array deconvolution technique are evaluated. The results imply that an array of five radial line sensors, with equal azimuthal separation and an aperture that depends on the frequency band of interest, provides directional resolution while requiring less space compared to an equally effective array of five microbarometers with rosette wind filters. (C) 2008 Acoustical Society of America. [DOI: 10.1121/1.2968675]

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.

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.

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.

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.

Wolfe, CJ, Okubo PG, Shearer PM.  2003.  Mantle fault zone beneath Kilauea volcano, Hawaii. Science. 300:478-480.   10.1126/science.1082205   AbstractWebsite

Relocations and focal mechanism analyses of deep earthquakes (greater than or equal to13 kilometers) at Kilauea volcano demonstrate that seismicity is focused on an active fault zone at 30-kilometer depth, with seaward slip on a low-angle plane, and other smaller, distinct fault zones. The earthquakes we have analyzed predominantly reflect tectonic faulting in the brittle lithosphere rather than magma movement associated with volcanic activity. The tectonic earthquakes may be induced on preexisting faults by stresses of magmatic origin, although background stresses from volcano loading and lithospheric flexure may also contribute.

Schulte-Pelkum, V, Monsalve G, Sheehan AF, Shearer P, Wu F, Rajaure S.  2019.  Mantle earthquakes in the Himalayan collision zone. Geology. 47:815-819.   10.1130/g46378.1   AbstractWebsite

Earthquakes are known to occur beneath southern Tibet at depths up to similar to 95 km. Whether these earthquakes occur within the lower crust thickened in the Himalayan collision or in the mantle is a matter of current debate. Here we compare vertical travel paths expressed as delay times between S and P arrivals for local events to delay times of P-to-S conversions from the Moho in receiver functions. The method removes most of the uncertainty introduced in standard analysis from using velocity models for depth location and migration. We show that deep seismicity in southern Tibet is unequivocally located beneath the Moho in the mantle. Deep seismicity in continental lithosphere occurs under normally ductile conditions and has therefore garnered interest in whether its occurrence is due to particularly cold temperatures or whether other factors are causing embrittlement of ductile material. Eclogitization in the subducting Indian crust has been proposed as a cause for the deep seismicity in this area. Our observation of seismicity in the mantle, falling below rather than within the crustal layer with proposed eclogitization, requires revisiting this concept and favors other embrittlement mechanisms that operate within mantle material.