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Hyndman, RD, Shearer PM.  1989.  Water in the Lower Continental-Cruse: Modelling Magnetotelluric and Seismic-Reflection Results. Geophysical Journal International. 98:343-365.   10.1111/j.1365-246X.1989.tb03357.x   Website
Whitmarsh, RB, Orcutt JA, Jordan TH, Adair RG, Shearer PM.  1987.  Velocity Bounds on the Seismic Structure of Mesozoic Crust and Uppr Mantle in the Southwest Pacific Basin from Downhole Observations at Deep-Sea Drilling Project Hole-595B. Initial Reports of the Deep Sea Drilling Project. 88:437-444.Website
Kaneko, Y, Shearer PM.  2015.  Variability of seismic source spectra, estimated stress drop, and radiated energy, derived fromcohesive-zonemodels of symmetrical and asymmetrical circular and elliptical ruptures. Journal of Geophysical Research-Solid Earth. 120:1053-1079.   10.1002/2014jb011642   AbstractWebsite

Large variability of earthquake stress drops and scaled energy has been commonly reported in the literature, but it is difficult to assess how much of this variability is caused by underlying physical source processes rather than simply observational uncertainties. Here we examine a variety of dynamically realistic rupture scenarios for circular and elliptical faults and investigate to what extent the variability in seismically estimated stress drops and scaled energy comes from differences in source geometry, rupture directivity, and rupture speeds. We numerically simulate earthquake source scenarios using a cohesive-zone model with the small-scale yielding limit, where the solution approaches a singular crack model with spontaneous healing of slip. Compared to symmetrical circular source models, asymmetrical models result in larger variability of estimated corner frequencies and scaled energy over the focal sphere. The general behavior of the spherical averages of corner frequencies and scaled energy in the subshear regime extends to the supershear regime, although shear Mach waves generated by the propagation of supershear rupture lead to much higher corner frequency and scaled energy estimates locally. Our results suggest that at least a factor of 2 difference in the spherical average of corner frequencies is expected in observational studies simply from variability in source characteristics almost independent of the actual stress drops, translating into a factor of 8 difference in estimated stress drops. Furthermore, radiation efficiency estimates derived from observed seismic spectra should not be directly interpreted as describing rupture properties unless there are independent constraints on rupture speed and geometry.

Warren, LM, Shearer PM.  2005.  Using the effects of depth phases on P-wave spectra to determine earthquake depths. Bulletin of the Seismological Society of America. 95:173-184.   10.1785/0120030251   AbstractWebsite

For shallow earthquakes, the surface-reflected depth phases (pP and sP) arrive shortly after the primary arrival, and the time separation among the three phases can be used to determine the origin depth of the earthquake. To model the relative arrival times and amplitudes of these phases, and the core reflections and water-column reverberations for a given earthquake, we construct stick seismograms using the IASPE191 velocity model and the Harvard CMT focal mechanisms at the distances and azimuths of the recording seismometers. While the differing arrival times and amplitudes are features observable in the time series, they also affect the spectrum, and we compute the spectrum for a time window that includes the P wave and subsequent arrivals. We quantify the effects of variations in these properties over the focal sphere in terms of differences in the slope of the log spectrum at different stations. To determine the depth of an earthquake, we compare our observed spectral variations with the predicted spectral variations for earthquakes originating at depths within 30 km of the PDE depth and identify the depth with the smallest L1 misfit as the true earthquake depth. We demonstrate the effectiveness of this method by applying it to a group of 35 thrust earthquakes in the Aleutian arc near the Andreanof Islands, but we also describe some complications introduced by strongly directive ruptures, as illustrated for the 1995 Jalisco, Mexico, event.

Hardebeck, JL, Shearer PM.  2003.  Using S/P amplitude ratios to constrain the focal mechanisms of small earthquakes. Bulletin of the Seismological Society of America. 93:2434-2444.   10.1785/0120020236   AbstractWebsite

We test whether S-wave/P-wave amplitude ratio data can improve the computed focal mechanisms of small earthquakes, using events from two southern California aftershock sequences. The observed S/P ratios are generally consistent with the expected mechanisms, implying that S/P ratios can in fact be useful in constraining the focal mechanisms of small events. However, we also find that noise in the observations leads to scatter in the S/P ratios of factors of 2, and sometimes higher. This scatter limits the usefulness of the S/P ratios in two ways: (1) the focal mechanism cannot simply be fit to S/P amplitude data alone without accounting for the noise in a more sophisticated focal mechanism inversion process; (2) while the amplitude ratios may improve poorly constrained mechanisms, they are less useful in refining solutions that are already relatively well constrained.

Wang, W, Shearer PM.  2017.  Using direct and coda wave envelopes to resolve the scattering and intrinsic attenuation structure of Southern California. Journal of Geophysical Research-Solid Earth. 122:7236-7251.   10.1002/2016jb013810   AbstractWebsite

Characterizing scattering and absorbing properties and the power spectrum of crustal heterogeneity is a fundamental problem for informing strong ground motion estimates at high frequencies, where scattering and attenuation effects are critical. We perform a comprehensive study of local earthquake coda waves in Southern California to constrain scattering and intrinsic attenuation structure. We analyze data from 1195 spatially distributed earthquakes from 1981 to 2013 at source depths of 10 to 15 km and epicentral distances from 0 to 250 km with magnitudes larger than 1.8. We stack envelope functions from 28,127 vertical component and 27,521 transverse component seismograms, filtered from 2 to 4 Hz. We model these observations using a particle-based Monte Carlo algorithm that includes intrinsic attenuation as well as both P and S wave scattering and both single and multiple scattering events. We find that spatially averaged coda wave behavior for Southern California can be explained only with models containing an increase in scattering strength and intrinsic attenuation within the uppermost crust, i.e., they are poorly fit with half-space models of constant scattering strength. A reasonable fit to our data is obtained with a two-layer model, composed of a shallow crustal layer with strong wide-angle scattering and high P and S intrinsic attenuation and a deeper layer with weaker scattering and lower intrinsic attenuation (top 5.5 km: (alpha)Q(I) = 250, (beta)Q(I) = 125, heterogeneity correlation length a = 50 m, fractional velocity heterogeneity epsilon = 0.4; lower crust: (alpha)Q(I) = 900, (beta)Q(I) = 400, a = 2 km, epsilon = 0.05). Plain Language Summary In summary, we have built a one-dimensional depth-dependent intrinsic and scattering attenuation model of Southern California to resolve the characteristics of the high-frequency (2-4 Hz) wavefield for events from 1981 to 2013. The envelope function stacking method provides the spatially averaged coda energy, including both the P and S wave coda decay and their relative amplitude information. We model the data stacks using energy-conserving and multiscattering regional Monte Carlo simulations. The synthetic results show the Southern California region can be reasonably fit with a two-layered model composed of a shallow crustal layer with strong wide-angle scattering and high intrinsic attenuation and a deeper layer with weaker scattering and lower intrinsic attenuation.

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.

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.
Schulte-Pelkum, V, Masters G, Shearer PM.  2001.  Upper mantle anisotropy from long-period P polarization. Journal of Geophysical Research-Solid Earth. 106:21917-21934.   10.1029/2001jb000346   AbstractWebsite

We introduce a method to infer upper mantle azimuthal anisotropy from the polarization, i.e., the direction of particle motion, of teleseismic long-period P onsets. The horizontal polarization of the initial P particle motion can deviate by > 10 degrees from the great circle azimuth from station to source despite a high degree of linearity of motion. Recent global isotropic three-dimensional mantle models predict effects that are an order of magnitude smaller than our observations. Stations within regional distances of each other show consistent azimuthal deviation patterns, while the deviations seem to be independent of source depth and near-source structure. We demonstrate that despite this receiver-side spatial coherence, our polarization data cannot be fit by a large-scale joint inversion for whole mantle structure. However, they can be reproduced by azimuthal anisotropy in the upper mantle and crust. Modeling with an anisotropic reflectivity code provides bounds on the magnitude and depth range of the anisotropy manifested in our data. Our method senses anisotropy within one wavelength (250 km) under the receiver. We compare our inferred fast directions of anisotropy to those obtained from P-n travel times and SKS splitting. The results of the comparison are consistent, with azimuthal anisotropy situated in the uppermost mantle, with SKS results deviating from P,, and Pp., in some regions with probable additional deeper anisotropy. Generally, our fast directions are consistent with anisotropic alignment due to lithospheric deformation in tectonically active regions and to absolute plate motion in shield areas. Our data provide valuable additional constraints in regions where discrepancies between results from different methods exist since the effect we observe is local rather than cumulative as in the case of travel time anisotropy and shear wave splitting. Additionally, our measurements allow us to identify stations with incorrectly oriented horizontal components.

Prieto, GA, Parker RL, Vernon FL, Shearer PM, Thomson DJ.  2006.  Uncertainties in earthquake source spectrum estimation using empirical Green functions. Earthquakes; radiated energy and the physics of faulting. 170( Abercrombie RE, McGarr A, Kanamori H, Di Toro G, Eds.).:69-74., Washington: American Geophysical Union   10.1029/170gm08   Abstract

We analyze the problem of reliably estimating uncertainties of the earthquake source spectrum and related source parameters using Empirical Green Functions (EGF). We take advantage of the large dataset available from 10 seismic stations at hypocentral distances (10 km < d <50 km) to average spectral ratios of the 2001 M5.1 Anza earthquake and 160 nearby aftershocks. We estimate the uncertainty of the average source spectrum of the M5.1 target earthquake by performing propagation of errors, which, due to the large number of EGFs used, is significantly smaller than that obtained using a single EGF. Our approach provides estimates of both the earthquake source spectrum and its uncertainties, plus confidence intervals on related source parameters such as radiated seismic energy or apparent stress, allowing the assessment of statistical significance. This is of paramount importance when comparing different sized earthquakes and analyzing source scaling of the earthquake rupture process. Our best estimate of radiated energy for the target earthquake is 1.24×1011 Joules with 95% confidence intervals (0.73×1011, 2.28×1011). The estimated apparent stress of 0.33 (0.19, 0.59) MPa is relatively low compared to previous estimates from smaller earthquakes (1MPa) in the same region.

Shearer, PM.  1996.  Transition zone velocity gradients and the 520-km discontinuity. Journal of Geophysical Research-Solid Earth. 101:3053-3066.   10.1029/95jb02812   AbstractWebsite

Stacks of long-period Global Digital Seismograph Network (GDSN) seismograms at 110 degrees to 180 degrees epicentral distance reveal precursors to SS that result from underside reflections off upper mantle seismic discontinuities. The 410- and 660-km discontinuities are obvious in these stacks, but identification and modeling of other transition zone discontinuities are complicated by sidelobes from the 410- and 660-km reflections. These sidelobes result from the limited bandwidth of the GDSN instrument responses and the effect of crustal reverberations on the SS reference phase. The crustal effects can be minimized by restricting the records to oceanic bounce points where the NG-km-thick crust has little effect on the long-period waveforms. Over 2000 long-period, transverse-component seismograms with oceanic SS bounce points recorded by the GDSN from 1976 to 1991 are manually edited, aligned on SS, and then stacked using a new procedure that weights the records by data quality. The resulting image shows a clear reflection from a 520-km discontinuity that cannot be explained as a sidelobe artifact, confirming earlier results of Shearer [1990, 1991] and Revenaugh and Jordan. [1991]. By stacking along the expected travel time curves for discontinuity phases, the time versus range image of the precursor wave field is reduced to a single trace that measures upper mantle reflectivity versus time. The features in this reflectivity profile are sensitive to the brightness and depth of the transition zone discontinuities and to the steepness of the velocity gradients between the interfaces. Using geometrical ray theory and assuming a constant velocity versus density scaling relationship, I fit this reflectivity profile with velocity models of the upper mantle using both forward modeling and direct inversion. The inverse problem. is addressed by performing a deconvolution of the profile with the SS reference phase (after a correction for attenuation), followed by a direct mapping of reflectivity versus time into velocity versus depth. Velocity-depth profiles resulting from these procedures are roughly in agreement with standard upper mantle velocity models, except that the SS precursor data require a minor discontinuity near 520 km and a steeper gradient just below the 660-km discontinuity. Estimated discontinuity shear impedance changes are 6.7 +/- 0.1% at 420 km, 2.9 +/- 0.7% at 519 km, and 9.9 +/- 1.5% at 663 km. The impedance change near 520 km is consistent with current mineral physics results for the olivine beta to gamma phase change and places constraints on the fraction of olivine in the transition zone.

Minster, BJ, Day SM, Shearer PM.  1991.  The transition to the elastic regime in the vicinity of an underground explosion. Explosion Source Phenomenology. :229–238.: American Geophysical Union Abstract
Flanagan, MP, Shearer PM.  1998.  Topography on the 410-km seismic velocity discontinuity near subduction zones from stacking of sS, sP, and pP precursors. Journal of Geophysical Research-Solid Earth. 103:21165-21182.   10.1029/98jb00595   AbstractWebsite

We stack the teleseismic depth phases sS, sP, and pP produced by deep focus earthquakes to image precursory arrivals that result from near-source, underside reflections off the 410-km seismic velocity discontinuity (hereinafter referred to as the 410) and use differential time measurements between these phases and their precursors to compute discontinuity depths near seven subduction zones around the Pacific Ocean margin. We begin by selecting seismograms with high-quality depth phase arrivals recorded by, several long-period networks between the years 1976 and 1996. Filtering the waveforms and stacking them along theoretical travel-time curves reveals dear precursors which vary in shape and timing. We compute confidence levels to evaluate the reliability of the observed precursory features using a bootstrap method that randomly resamples the seismograms prior to stacking. We measure the differential travel time between the reference pulse and the precursor using a cross-correlation technique and convert this time to an apparent discontinuity depth using the isotropic Preliminary Reference Earth Model (PREM) at 25-s period corrected to an oceanic crustal thickness. The lateral resolution of our long-period stacks for 410 topography is limited compared to that sometimes achieved in short-period analyses but is much higher than that obtained from global SS precursor studies. For most subduction zones the results indicate little change in the average depth to the 410-km discontinuity in the local areas sampled by the precursor bounce points compared td broad regional depths inferred from SS precursor results. This implies that any large variations in depth to the 410-km discontinuity neat subduction zones are limited to a narrow zone within the slab itself where they may be difficult to resolve with long-period data. Coverage for the Tonga and Peru-Chile subduction zones is sufficiently dense that we can observe lateral variations in 410 depths. In Tonga the results suggest depth variations perpendicular to the slab of up to 33 km, after correcting for probable lateral heterogeneity in velocity above 400 km depth, and variations parallel to the Slab orientation as large as 13 km. The cross-slab variation is consistent with the elevation of olivine phase transformations in cold regions; the variation along strike suggests a more complex thermal heterogeneity that may be related to the subduction history of this region. We see evidence for additional reflectors above the 410 in some of the waveform stacks, but the inconsistency and weak amplitude of these features preclude definitive interpretations.

Lin, GQ, Shearer PM, Matoza RS, Okubo PG, Amelung F.  2014.  Three-dimensional seismic velocity structure of Mauna Loa and Kilauea volcanoes in Hawaii from local seismic tomography. Journal of Geophysical Research-Solid Earth. 119:4377-4392.   10.1002/2013jb010820   AbstractWebsite

We present a new three-dimensional seismic velocity model of the crustal and upper mantle structure for Mauna Loa and Kilauea volcanoes in Hawaii. Our model is derived from the first-arrival times of the compressional and shear waves from about 53,000 events on and near the Island of Hawaii between 1992 and 2009 recorded by the Hawaiian Volcano Observatory stations. The V-p model generally agrees with previous studies, showing high-velocity anomalies near the calderas and rift zones and low-velocity anomalies in the fault systems. The most significant difference from previous models is in V-p/V-s structure. The high-V-p and high-V-p/V-s anomalies below Mauna Loa caldera are interpreted as mafic magmatic cumulates. The observed low-V-p and high-V-p/V-s bodies in the Kaoiki seismic zone between 5 and 15 km depth are attributed to the underlying volcaniclastic sediments. The high-V-p and moderate- to low-V-p/V-s anomalies beneath Kilauea caldera can be explained by a combination of different mafic compositions, likely to be olivine-rich gabbro and dunite. The systematically low-V-p and low-V-p/V-s bodies in the southeast flank of Kilauea may be caused by the presence of volatiles. Another difference between this study and previous ones is the improved V-p model resolution in deeper layers, owing to the inclusion of events with large epicentral distances. The new velocity model is used to relocate the seismicity of Mauna Loa and Kilauea for improved absolute locations and ultimately to develop a high-precision earthquake catalog using waveform cross-correlation data.

Lin, G, Shearer PM, Hauksson E, Thurber CH.  2007.  A three-dimensional crustal seismic velocity model for southern California from a composite event method. Journal of Geophysical Research-Solid Earth. 112   10.1029/2007jb004977   AbstractWebsite

[1] We present a new crustal seismic velocity model for southern California derived from P and S arrival times from local earthquakes and explosions. To reduce the volume of data and ensure a more uniform source distribution, we compute "composite event" picks for 2597 distributed master events that include pick information for other events within spheres of 2 km radius. The approach reduces random picking error and maximizes the number of S wave picks. To constrain absolute event locations and shallow velocity structure, we also use times from controlled sources, including both refraction shots and quarries. We implement the SIMULPS tomography algorithm to obtain three-dimensional dimensional (3-D) V-p and V-p/V-s structure and hypocenter locations of the composite events. Our new velocity model in general agrees with previous studies, resolving low-velocity features at shallow depths in the basins and some high-velocity features in the midcrust. Using our velocity model and 3-D ray tracing, we relocate about 450,000 earthquakes from 1981 to 2005. We observe a weak correlation between seismic velocities and earthquake occurrence, with shallow earthquakes mostly occurring in high P velocity regions and midcrustal earthquakes occurring in low P velocity regions. In addition, most seismicity occurs in regions with relatively low V-p/V-s ratios, although aftershock sequences following large earthquakes are often an exception to this pattern.

Lin, GQ, Shearer P.  2005.  Tests of relative earthquake location techniques using synthetic data. Journal of Geophysical Research-Solid Earth. 110   10.1029/2004jb003380   AbstractWebsite

We compare three relative earthquake location techniques using tests on synthetic data that simulate many of the statistical properties of real travel time data. The methods are (1) the hypocentroidal decomposition method of Jordan and Sverdrup (1981), (2) the source-specific station term method (SSST) of Richards-Dinger and Shearer (2000), and (3) the modified double-difference method (DD) of Waldhauser and Ellsworth (2000). We generate a set of synthetic earthquakes, stations, and arrival time picks in half-space velocity models. We simulate the effect of travel time variations caused by random picking errors, station terms, and general three-dimensional velocity structure. We implement the algorithms with a common linearized approach and solve the systems using a conjugate gradient method. We constrain the mean location shift to be zero for the hypocentroidal decomposition and double-difference locations. For a single compact cluster of events, these three methods yield very similar improvements in relative location accuracy. For distributed seismicity, the DD and SSST algorithms both provide improved relative locations of comparable accuracy. We also present a new location technique, termed the shrinking box SSST method, which provides some improvement in absolute location accuracy compared to the SSST method. In our implementation of these algorithms, the SSST method runs significantly faster than the DD method.

Sumiejski, LE, Shearer PM.  2012.  Temporal Stability of Coda Q(-1) in Southern California. Bulletin of the Seismological Society of America. 102:873-877.   10.1785/0120110181   AbstractWebsite

Some studies of coda Q(-1) have found temporal changes that may be associated with earthquake activity, but these analyses are subject to biases due to differences in source locations and other nonstationary behavior in earthquake catalogs. These biases can be greatly reduced by using clusters of repeating earthquakes; studies using this approach have generally found no resolvable changes in coda Q(-1). We examine coda Q(-1) variations across southern California using 22 similar event clusters identified from a recent large-scale waveform cross-correlation project to improve earthquake locations. These clusters are found across the region and span the time period between 1981 and 2005. We apply the method of Beroza et al. (1995) to compute differential coda Q(-1) using waveforms from similar earthquake pairs and analyze the results to constrain any possible temporal variations. Results from individual event pairs show a great deal of scatter in differential coda Q(-1), but exhibit no clear temporal variations or changes associated with large earthquakes. Application of a median filter to smooth the results shows that any persistent large-scale changes in coda Q(-1) during this time period are less than about 30%.

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.

Buehler, JS, Shearer PM.  2015.  T phase observations in global seismogram stacks. Geophysical Research Letters. 42:6607-6613.   10.1002/2015gl064721   AbstractWebsite

The T phase, conversion of acoustic to seismic energy, is typically observed as a high-frequency wave train at hydrophones or coastal seismic stations. Here we show that the T phase can be observed in broadband waveform stacks of similar to 5200 earthquakes recorded by the Global Seismic Network. To enhance the phase arrivals in stacks, we apply short-time window average over long-time window average filtering to individual traces before stacking. Although the T phase arrival is visible in stacks from seismograms filtered at 0.5-5 Hz, it appears much stronger at higher frequencies (2-8 Hz) and is further enhanced by only stacking seismograms from oceanic paths. Stacking only subsets of the data depending on continental path lengths on the receiver side shows that the T phase can be observed at stations up to 4 degrees inland from the coast, and changes in the T phase arrival time correspond to reasonable crustal velocities.

Matoza, RS, Shearer PM, Lin GQ, Wolfe CJ, Okubo PG.  2013.  Systematic relocation of seismicity on Hawaii Island from 1992 to 2009 using waveform cross correlation and cluster analysis. Journal of Geophysical Research-Solid Earth. 118:2275-2288.   10.1002/jgrb.50189   AbstractWebsite

The analysis and interpretation of seismicity from mantle depths to the surface play a key role in understanding how Hawaiian volcanoes work. We present results from a comprehensive and systematic re-analysis of waveforms from 130,902 seismic events recorded by the U.S. Geological Survey Hawaiian Volcano Observatory permanent seismic network from January 1992 to March 2009. We compute high-precision relative relocations for 101,390 events (77% of all events considered) using waveform cross correlation and cluster analysis, resulting in a multiyear systematically processed catalog of seismicity for all of Hawaii Island. The 17 years of relocated seismicity exhibit a dramatic sharpening of earthquake clustering along faults, streaks, and magmatic features, permitting a more detailed understanding of fault geometries and volcanic and tectonic processes. Our relocation results are generally consistent with previous studies that have focused on more specific regions of Hawaii. The relocated catalog includes crustal seismicity at Kilauea and its rift zones, seismicity delineating crustal detachment faults separating volcanic pile and old oceanic crust on the flanks of Kilauea and Mauna Loa, events along inferred magma conduits, and events along inferred mantle fault zones. The relocated catalog is available for download in the supporting information.

Warren, LM, Shearer PM.  2006.  Systematic determination of earthquake rupture directivity and fault planes from analysis of long-period P-wave spectra. Geophysical Journal International. 164:46-62.   10.1111/j.1365-246X.2005.02769   AbstractWebsite

If an earthquake has a primarily unilateral rupture, the pulse width observed on seismograms will vary depending on the angle between the rupture direction and the takeoff vector to the station. We have developed a method to estimate the amount of pulse broadening from the spectrum and apply it to a long-period database of large, globally distributed earthquakes that occurred between 1988 and 2000. We select vertical-component P-waves at epicentral distances of 20 degrees-98 degrees. We compute the spectrum from a 64-s-long window around each P-wave arrival. Each spectrum is the product of source, receiver and propagation response functions as well as local source- and receiver-side effects. Since there are multiple receivers for each source and multiple sources for each receiver, we can estimate and remove the source- and receiver-side terms by stacking the appropriate P log spectra. For earthquakes deeper than similar to 200 km, source effects dominate the residual spectra. We use our pulse-width estimates to determine the best rupture direction and to identify which nodal plane of the Harvard centroid moment tensor (CMT) solution is most consistent with this rupture direction for 66 events. In about 30 per cent of the cases, one of the two nodal planes produces a much better fit to the data and can be identified as the true fault plane. When results from previous studies are available for comparison, our rupture directions are usually consistent with their results, particularly for earthquakes with simple rupture histories.

Vidale, JE, Shearer PM.  2006.  A survey of 71 earthquake bursts across southern California: Exploring the role of pore fluid pressure fluctuations and aseismic slip as drivers. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb004034   AbstractWebsite

[ 1] We investigate the cause of seismicity bursts by examining a waveform-relocated catalog for southern California between 1984 and 2002 and systematically identifying 71 isolated sequences of 40 or more earthquakes occurring within a 2-km-radius volume and a 4-week interval. Fifty-seven of the 71 bursts are difficult to interpret as primarily a main shock and its Omori-law-abiding foreshocks and aftershocks because they exhibit a more complicated evolution in space, time, and magnitude; we identify 18 of these sequences as particularly swarm-like. Evidence against a simple cascade of elastic stress triggering includes the presence of an interval of steady seismicity rate, the tendency of the largest event to strike later in the sequence, the large spatial extent of some of the swarms compared to their cumulative moment, and the weak correlation between the number of events in each burst and the magnitude of the largest event in each burst. Shallow sequences and normal faulting mechanism sequences are most likely to be swarm-like. The tendencies of the hypocenters in the swarm-like sequences to occur on vertical planes and expand over time suggest pore fluid pressure fluctuations as the most likely mechanism driving the swarm-like seismicity bursts. However, episodic aseismic slip could also be at least partly responsible and might provide a more compelling explanation for the steady rate of seismicity during swarms, whereas fluid pressure perturbations might be expected to diminish more rapidly with time. Both aftershock-like and swarm-like seismicity bursts are distributed across the entire study region, indicating that they are a general feature of tectonic faulting, rather than limited to a few geological conditions such as volcanic or geothermal areas.

Shearer, PM, Orcutt JA.  1987.  Surface and Near-Surface Effects on Seismic-Waves - Theory and Borehole Seismometer Results. Bulletin of the Seismological Society of America. 77:1168-1196.Website