Export 25 results:
Sort by: Author [ Title  (Desc)] Type Year
A B [C] D E F G H I J K L M N O P Q R S T U V W X Y Z   [Show ALL]
Vidale, JE, Boyle KL, Shearer PM.  2006.  Crustal earthquake bursts in California and Japan: Their patterns and relation to volcanoes. Geophysical Research Letters. 33   10.1029/2006gl027723   AbstractWebsite

[1] We analyze 153 bursts of earthquakes in southern California and Japan. The burst patterns are similar in southern California and Japan; they fill a spectrum between "swarm-like'' sequences without obvious mainshocks and mainshocks with Omori-law-abiding aftershocks. In agreement with our previous work, the "swarm-like'' sequences in Japan have more events, are more voluminous, and tend to expand with time, when compared to "mainshock-aftershock'' type sequences. In both regions, we find that the sequences starting with their largest events tend to be much shorter in duration. Bursts within 50 km of volcanoes are similar in character to those elsewhere except they tend to have longer duration. We hypothesize that swarminess is a proxy for fluid pressure redistribution and/or aseismic slip driving the seismicity bursts, and conversely, the mainshock-aftershock-style sequences have end-member behavior that results solely from a cascade of elastic failures. The complexity of the spatial seismicity distribution does not correlate with the style of swarm observed, indicating that fluid conditions and composition are likely more influential than geometry in determining the patterns we observe.

Shearer, PM.  1991.  Constraints on Upper Mantle Discontinuities From Observations of Long-Period Reflected and Converted Phases. Journal of Geophysical Research-Solid Earth. 96:18147-18182.   10.1029/91jb01592   AbstractWebsite

Stacked images combining over 5 years of long-period Global Digital Seismograph Network data reveal many phases associated with reflections and conversions from upper mantle discontinuities. These images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking. Results obtained for P, S, SS, and PP reference arrivals resolve numerous phases from discontinuities near 410 and 660 km, while some of the stacks also show evidence for a weaker discontinuity near 520 km. Phases of particular interest include P and SH multiples resulting from topside reflections, precursors to SS from underside reflections, and P-to-SV converted phases. These phases can be clearly seen both in the waveform stacks and in cross-correlation analysis of individual seismograms. Travel times for these arrivals are converted to discontinuity depths relative to velocities in the Preliminary Reference Earth Model, and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle. Average apparent depths to the discontinuities for the different phases agree to within +/- 3 km for the 410-km discontinuity, to within +/- 4 km for the 520-km discontinuity, and to within +/- 8 km for the 660-km discontinuity. The best global averages are obtained from the SS precursor data which indicate discontinuities at 415, 519, and 659 km. Discontinuity depths obtained from the P-to-SV converted phases at over 35 individual seismic stations exhibit variations of less than +/- 20 km. Apparent depths to the 660-km discontinuity consistently show greater variability than depths to the 410-km discontinuity, supporting recent laboratory results which indicate that the Clapeyron slope for the 660-km discontinuity is significantly larger in magnitude than the slope for the 410-km discontinuity. Precursors to SS (seen between 110-degrees and 180-degrees) are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point. Apparent discontinuity depths computed from SS precursors for different tectonic regions agree to within about +/- 5 km. The SS precursors have especially good coverage near the subducting slabs in the northwest Pacific. Analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000- to 1500-km-wide region near the slab in which the 660-km discontinuity is depressed by about 20 km. Measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking. Relative amplitude analysis suggests that the P and S wave impedance changes at 410 km are about 0.8-1.1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0.3 and 0.6 of the changes at 410 km.

Mancinelli, N, Shearer P, Liu QY.  2016.  Constraints on the heterogeneity spectrum of Earth's upper mantle. Journal of Geophysical Research-Solid Earth. 121:3703-3721.   10.1002/2015jb012641   AbstractWebsite

We constrain the heterogeneity spectrum of Earth's upper mantle at scales from a few kilometers to tens of thousands of kilometers using observations from high-frequency scattering, long-period scattering, and tomography. Tomography and high-frequency scattering constraints are drawn from previous studies, but constraints on mantle heterogeneity at intermediate scales (5-500 km) are lacking. To address this, we stack similar to 15,000 long-period P coda envelopes to characterize the globally averaged scattered wavefield at periods from 5 to 60 s and at ranges from 50 to 98 degrees. To fit these observations, we consider models of random mantle heterogeneity and compute the corresponding global wavefield using both a ray theoretical "seismic particle" approach and full spectral element simulations. Von Karman random media distributed throughout the uppermost 600 km of the mantle with a = 2000 km, epsilon = 10%, and kappa = 0.05 provide a good fit to the time, range, and frequency dependence of the stacks, although there is a trade-off between epsilon and the thickness of the assumed scattering layer. This random media model also fits previously published 1 Hz stacks of P coda and agrees with constraints on long-wavelength structure from tomography. Finally, we explore geodynamically plausible scenarios that might be responsible for the RMS and falloff rate of the proposed spectrum, including a self-similar mixture of basalt and harzburgite.

Haase, JS, Shearer PM, Aster RC.  1995.  Constraints on Temporal Variations in Velocity Near Anza, California, From Analysis of Similar Event Pairs. Bulletin of the Seismological Society of America. 85:194-206. AbstractWebsite

Similar earthquake pairs recorded by the Anza Seismic Network in southern California are used as repeatable sources to place an upper limit on temporal changes in seismic velocity which occurred in the vicinity of the Anza seismic gap in the last 9 yr. Relative arrival times for each pair of events are found using a cross-correlation method and relative locations are calculated to verify that the pairs have nearly identical hypocenters. The time separation between events in these pairs varies from less than a day to almost 7 yr. The longterm changes in seismic travel times, as measured from the pairs with the longest time separation, are not significantly greater than the noise level estimated from the short-time-separation event pairs. Almost all P-wave paths show less than 0.06% (0.007 sec) change in travel time and all S-wave paths have less than 0.03% (0.004 sec) change. Sensitivity tests place an upper bound on traveltime changes that could be compensated by hypocenter mislocation at 0.2%. There is no evidence that localized stress accumulation causes measurable changes in seismic velocity in the Anza region.

Shearer, PM.  1994.  Constraints on Inner-Core Anisotropy From PKP(F) Travel-Times. Journal of Geophysical Research-Solid Earth. 99:19647-19659.   10.1029/94jb01470   AbstractWebsite

PKP(DF) travel times reported by the International Seismological Centre (ISC) are analyzed to study the depth dependence of inner core anisotropy. Plots of PKP(DF) travel time residuals versus range for different subsets of the data clearly show large anomalies at ranges beyond about 149 degrees for those paths with inner core ray angles within 30 degrees of the rotation axis. On average, the times are about 3 to 5 s earlier for these paths than for rays at other angles, consistent with recent waveform measurements of anomalous PKP(DF) times by Creager (1992) and Song and Helmberger (1993), and indicative of inner core anisotropy in which the P velocity is about 3% faster for N-S paths than E-W paths. The size of these apparent anomalies in the ISC data are much larger than those inferred from several previous analyses of ISC PKP(DF) data which indicated 0.5 to 2 s anomalies at these ranges. The earlier studies applied binning and averaging schemes to the data using windows that were overly tight, thus excluding some of the most anomalous data points and biasing the results toward smaller anomalies. To avoid this problem, I apply a method for computing summary ray residuals that finds the maxima in smoothed histograms of the residuals and does not rely on a fixed interval to window the data points. Standard errors on each summary ray residual are estimated using a bootstrap technique that randomly resamples the data. Plots of the summary ray residuals for several range intervals reveal a small number of independent paths nearly parallel to the rotation axis for which the arrivals are 3 to 5 s early. Although the data are sparse for paths within 30 degrees of the rotation axis, anisotropy does not appear to diminish with depth within the inner core and may cause a much larger anomaly near 180 degrees than the 2-s value obtained in several prior studies of ISC data. The uniform anisotropy models of Creager (1992) and Song and Helmberger (1993) fit the bulk of the data reasonably well. However, the observed residuals at ranges between 132 degrees and 140 degrees are smaller than those predicted by most anisotropy models, suggesting that the anisotropy may diminish within the outermost 50 km of the inner core.

Lawrence, JF, Shearer PM.  2006.  Constraining seismic velocity and density for the mantle transition zone with reflected and transmitted waveforms. Geochemistry Geophysics Geosystems. 7   10.1029/2006gc001339   AbstractWebsite

[1] We examine stacks of several seismic phases having different sensitivities to mantle transition zone structure. When analyzed separately, underside P and S reflections (PdP and SdS) are suggestive of very different structures despite similar raypaths and data coverage. By stacking the radial component of PdP rather than the vertical PdP, we show that this difference does not result from interference from other more steeply inclined phases such as PKP and Ppdp(diff). In general, stacks of P-to-S converted phases (Pds) appear to lack evidence of a 520-km discontinuity when examined without other phases. When these phases and stacked topside P reflections (Ppdp) are analyzed jointly using a nonlinear inversion method, consistent but nonunique, seismological models emerge. These models show that a discontinuity at similar to 653 km depth has smaller contrasts in density and velocity than found in most previous studies. A sub-660 gradient can account for the majority of this difference. A 1.6 +/- 0.5% P-velocity contrast and a 2.2 +/- 0.3% density contrast at similar to 518 km depth without a S-velocity contrast can explain the lack of a P520s, together with robust Pp520p and S520S phases. For models parameterized with a finite thickness for each discontinuity, the 410-km discontinuity is consistently similar to 3 times thicker than the 660-km discontinuity.

Prieto, GA, Thomson DJ, Vernon FL, Shearer PM, Parker RL.  2007.  Confidence intervals for earthquake source parameters. Geophysical Journal International. 168:1227-1234.   10.1111/j.1365-246X.2006.03257.x   AbstractWebsite

We develop a method to obtain confidence intervals of earthquake source parameters, such as stress drop, seismic moment and corner frequency, from single station measurements. We use the idea of jackknife variance combined with a multitaper spectrum estimation to obtain the confidence regions. The approximately independent spectral estimates provide an ideal case to perform jackknife analysis. Given the particular properties of the problem to solve for source parameters, including high dynamic range, non-negativity, non-linearity, etc., a log transformation is necessary before performing the jackknife analysis. We use a Student's t distribution after transformation to obtain accurate confidence intervals. Even without the distribution assumption, we can generate typical standard deviation confidence regions. We apply this approach to four earthquakes recorded at 1.5 and 2.9 km depth at Cajon Pass, California. It is necessary to propagate the errors from all unknowns to obtain reliable confidence regions. From the example, it is shown that a 50 per cent error in stress drop is not unrealistic, and even higher errors are expected if velocity structure and location errors are present. An extension to multiple station measurement is discussed.

Yang, WZ, Hauksson E, Shearer PM.  2012.  Computing a Large Refined Catalog of Focal Mechanisms for Southern California (1981-2010): Temporal Stability of the Style of Faulting. Bulletin of the Seismological Society of America. 102:1179-1194.   10.1785/0120110311   AbstractWebsite

Using the method developed by Hardebeck and Shearer (2002, 2003) termed the HASH method, we calculate focal mechanisms for earthquakes that occurred in the southern California region from 1981 to 2010. When available, we use hypocenters refined with differential travel times from waveform cross correlation. Using both the P-wave first motion polarities and the S/P amplitude ratios computed from three-component seismograms, we determine mechanisms for more than 480,000 earthquakes and analyze the statistical features of the whole catalog. We filter the preliminary catalog with criteria associated with mean nodal plane uncertainty and azimuthal gap and obtain a high-quality catalog with approximately 179,000 focal mechanisms. As more S/P amplitude ratios become available after 2000, the average nodal plane uncertainty decreases significantly compared with mechanisms that include only P-wave polarities. In general the parameters of the focal mechanisms have been stable during the three decades. The dominant style of faulting is high angle strike-slip faulting with the most likely P axis centered at N5 degrees E. For earthquakes of M < 2.5, there are more normal-faulting events than reverse-faulting events, while the opposite holds for M > 2.5 events. Using the 210 moment-tensor solutions in Tape et al. (2010) as benchmarks, we compare the focal plane rotation angles of common events in the catalog. Seventy percent of common earthquakes match well with rotation angles less than the typical nodal plane uncertainty. The common events with relatively large rotation angles are either located around the edge of the (SCSN) network or poorly recorded.

Yao, HJ, Gerstoft P, Shearer PM, Mecklenbrauker C.  2011.  Compressive sensing of the Tohoku-Oki Mw 9.0 earthquake: Frequency-dependent rupture modes. Geophysical Research Letters. 38   10.1029/2011gl049223   AbstractWebsite

Compressive sensing (CS) is a technique for finding sparse signal representations to underdetermined linear measurement equations. We use CS to locate seismic sources during the rupture of the 2011 Tohoku-Oki Mw9.0 earthquake in Japan from teleseismic P waves recorded by an array of stations in the United States. The seismic sources are located by minimizing the l(2)-norm of the difference between the observed and modeled waveforms penalized by the l(1)-norm of the seismic source vector. The resulting minimization problem is convex and can be solved efficiently. Our results show clear frequency-dependent rupture modes with high-frequency energy radiation dominant in the down-dip region and low-frequency radiation in the updip region, which may be caused by differences in rupture behavior (more intermittent or continuous) at the slab interface due to heterogeneous frictional properties. Citation: Yao, H., P. Gerstoft, P. M. Shearer, and C. Mecklenbrauker (2011), Compressive sensing of the Tohoku-Oki Mw 9.0 earthquake: Frequency-dependent rupture modes, Geophys. Res. Lett., 38, L20310, doi: 10.1029/2011GL049223.

Yao, HJ, Shearer PM, Gerstoft P.  2013.  Compressive sensing of frequency-dependent seismic radiation from subduction zone megathrust ruptures. Proceedings of the National Academy of Sciences of the United States of America. 110:4512-4517.   10.1073/pnas.1212790110   AbstractWebsite

Megathrust earthquakes rupture a broad zone of the subducting plate interface in both along-strike and along-dip directions. The along-dip rupture characteristics of megathrust events, e.g., their slip and energy radiation distribution, reflect depth-varying frictional properties of the slab interface. Here, we report high-resolution frequency-dependent seismic radiation of the four largest megathrust earthquakes in the past 10 y using a compressive-sensing (sparse source recovery) technique, resolving generally low-frequency radiation closer to the trench at shallower depths and high-frequency radiation farther from the trench at greater depths. Together with coseismic slip models and early aftershock locations, our results suggest depth-varying frictional properties at the subducting plate interfaces. The shallower portion of the slab interface (above similar to 15 km) is frictionally stable or conditionally stable and is the source region for tsunami earthquakes with large coseismic slip, deficient high-frequency radiation, and few early aftershocks. The slab interface at intermediate depths (similar to 15-35 km) is the main unstable seismogenic zone for the nucleation of megathrust quakes, typically with large coseismic slip, abundant early aftershocks, and intermediate- to high-frequency radiation. The deeper portion of the slab interface (similar to 35-45 km) is seismically unstable, however with small coseismic slip, dominant high-frequency radiation, and relatively fewer aftershocks.

Shearer, PM, Orcutt JA.  1986.  Compressional and Shear-Wave Anisotropy in the Oceanic Lithosphere - The Ngendei Seismic Refraction Experiment. Geophysical Journal of the Royal Astronomical Society. 87:967-1003.   10.1111/j.1365-246X.1986.tb01979.x   Website
Shearer, PM, Prieto GA, Hauksson E.  2006.  Comprehensive analysis of earthquake source spectra in southern California. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb003979   AbstractWebsite

[ 1] We compute and analyze P wave spectra from earthquakes in southern California between 1989 and 2001 using a method that isolates source-, receiver-, and path-dependent terms. We correct observed source spectra for attenuation using both fixed and spatially varying empirical Green's function methods. Estimated Brune-type stress drops for over 60,000 M-L = 1.5 to 3.1 earthquakes range from 0.2 to 20 MPa with no dependence on moment or local b value. Median computed stress drop increases with depth in the upper crust, from about 0.6 MPa at the surface to about 2.2 MPa at 8 km, where it levels off and remains nearly constant in the midcrust down to about 20 km. However, the results at shallow depths could also be explained as reduced rupture velocities near the surface rather than a change in stress drop. Spatially coherent variations in median stress drop are observed, with generally low values for the Imperial Valley and Northridge aftershocks and higher values for the eastern Transverse ranges and the north end of the San Jacinto fault. We find no correlation between observed stress drop and distance from the San Andreas and other major faults. Significant along-strike variations in stress drop exist for aftershocks of the 1992 Landers earthquake, which may correlate with differences in main shock slip.

Chen, X, Shearer PM.  2011.  Comprehensive analysis of earthquake source spectra and swarms in the Salton Trough, California. Journal of Geophysical Research-Solid Earth. 116   10.1029/2011jb008263   AbstractWebsite

We study earthquakes within California's Salton Trough from 1981 to 2009 from a precisely relocated catalog. We process the seismic waveforms to isolate source spectra, station spectra and travel-time dependent spectra. The results suggest an average P wave Q of 340, agreeing with previous results indicating relatively high attenuation in the Salton Trough. Stress drops estimated from the source spectra using an empirical Green's function (EGF) method reveal large scatter among individual events but a low median stress drop of 0.56 MPa for the region. The distribution of stress drop after applying a spatial-median filter indicates lower stress drops near geothermal sites. We explore the relationships between seismicity, stress drops and geothermal injection activities. Seismicity within the Salton Trough shows strong spatial clustering, with 20 distinct earthquake swarms with at least 50 events. They can be separated into early-M(max) and late-M(max) groups based on the normalized occurrence time of their largest event. These swarms generally have a low skew value of moment release history, ranging from -9 to 3.0. The major temporal difference between the two groups is the excess of seismicity and an inverse power law increase of seismicity before the largest event for the late-Mmax group. All swarms exhibit spatial migration of seismicity at a statistical significance greater than 85%. A weighted L1-norm inversion of linear migration parameters yields migration velocities from 0.008 to 0.8 km/hour. To explore the influence of fluid injection in geothermal sites, we also model the migration behavior with the diffusion equation, and obtain a hydraulic diffusion coefficient of approximately 0.25 m(2)/s for the Salton Sea geothermal site, which is within the range of expected values for a typical geothermal reservoir. The swarms with migration velocities over 0.1 km/hour cannot be explained by the diffusion curve, rather, their velocity is consistent with the propagation velocity of creep and slow slip events. These variations in migration behavior allow us to distinguish among different driving processes.

Lin, GQ, Shearer P.  2006.  The COMPLOC earthquake location package. Seismological Research Letters. 77:440-444.   10.1785/gssrl.77.4.440   Website
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.

Shearer, PM, Abercrombie RE, Trugman DT, Wang W.  2019.  Comparing EGF methods for estimating corner frequency and stress drop from p wave spectra. Journal of Geophysical Research-Solid Earth. 124:3966-3986.   10.1029/2018jb016957   AbstractWebsite

Empirical Green's functions (EGFs) are widely applied to correct earthquake spectra for attenuation and other path effects in order to estimate corner frequencies and stress drops, but these source parameter estimates often exhibit poor agreement between different studies. We examine this issue by analyzing a compact cluster of over 3,000 aftershocks of the 1992 Landers earthquake. We apply and compare two different analysis and modeling methods: (1) the spectral decomposition and global EGF fitting approach and (2) a more traditional EGF method of modeling spectral ratios. We find that spectral decomposition yields event terms that are consistent with stacks of spectral ratios for individual events, but source parameter estimates nonetheless vary between the methods. The main source of differences comes from the modeling approach used to estimate the EGF. The global EGF-fitting approach suffers from parameter trade-offs among the absolute stress drop, the stress drop scaling with moment, and the high-frequency falloff rate but has the advantage that the relative spectral shapes and stress drops among the different events in the cluster are well resolved even if their absolute levels are not. The spectral ratio approach solves for a different EGF for each target event without imposing any constraint on the corner frequency, f(c), of the smaller events, and so can produce biased results for target event f(c). Placing constraints on the small-event f(c) improves the performance of the spectral ratio method and enables the two methods to yield very similar results.

Plesch, A, Shaw JH, Benson C, Bryant WA, Carena S, Cooke M, Dolan J, Fuis G, Gath E, Grant L, Hauksson E, Jordan T, Kamerling M, Legg M, Lindvall S, Magistrale H, Nicholson C, Niemi N, Oskin M, Perry S, Planansky G, Rockwell T, Shearer P, Sorlien C, Suss MP, Suppe J, Treiman J, Yeats R.  2007.  Community fault model (CFM) for southern California. Bulletin of the Seismological Society of America. 97:1793-1802.   10.1785/0120050211   AbstractWebsite

We present a new three-dimensional model of the major fault systems in southern California. The model describes the San Andreas fault and associated strike-slip fault systems in the eastern California shear zone and Peninsular Ranges, as well as active blind-thrust and reverse faults in the Los Angeles basin and Transverse Ranges. The model consists of triangulated surface representations (t-surfs) of more than 140 active faults that are defined based on surfaces traces, seismicity, seismic reflection profiles, wells, and geologic cross sections and models. The majority of earthquakes, and more than 95% of the regional seismic moment release, occur along faults represented in the model. This suggests that the model describes a comprehensive set of major earthquake sources in the region. The model serves the Southern California Earthquake Center (SCEC) as a unified resource for physics-based fault systems modeling, strong ground-motion prediction, and probabilistic seismic hazards assessment.

Fan, WY, Shearer PM.  2018.  Coherent Seismic Arrivals in the P Wave Coda of the 2012 M(w)7.2 Sumatra Earthquake: Water Reverberations or an Early Aftershock? Journal of Geophysical Research-Solid Earth. 123:3147-3159.   10.1002/2018jb015573   AbstractWebsite

Teleseismic records of the 2012M(w)7.2 Sumatra earthquake contain prominent phases in the P wave train, arriving about 50 to 100s after the direct P arrival. Azimuthal variations in these arrivals, together with back-projection analysis, led Fan and Shearer (, ) to conclude that they originated from early aftershock(s), located approximate to 150 km northeast of the mainshock and landward of the trench. However, recently, Yue et al. (, ) argued that the anomalous arrivals are more likely water reverberations from the mainshock, based mostly on empirical Green's function analysis of a M6 earthquake near the mainshock and a water phase synthetic test. Here we present detailed back-projection and waveform analyses of three M6 earthquakes within 100km of the M(w)7.2 earthquake, including the empirical Green's function event analyzed in Yue et al. (, ). In addition, we examine the waveforms of three M5.5 reverse-faulting earthquakes close to the inferred early aftershock location in Fan and Shearer (, ). These results suggest that the reverberatory character of the anomalous arrivals in the mainshock coda is consistent with water reverberations, but the origin of this energy is more likely an early aftershock rather than delayed and displaced water reverberations from the mainshock.

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.

Earle, PS, Shearer PM.  1994.  Characterization of Global Seismograms Using an Automatic-Picking Algorithm. Bulletin of the Seismological Society of America. 84:366-376. AbstractWebsite

An automatic phase picker is useful for quickly identifying and timing phase arrivals in large seismic data bases. We have developed an automatic phase picker that is sensitive to small changes in amplitude and applied it to over 7 yr of global data distributed by the National Earthquake Information Center (NEIC). Our phase-picking algorithm is based on a short-term-average to long-term-average ratio (STA/LTA) taken along an envelope function generated from the seismogram. The algorithm returns arrival times and corresponding pick qualities. The procedure requires few input parameters and is easily adapted to various types of data. We produce global travel-time plots from both high-frequency (20- or 40-Hz sample rate) and low-frequency (1-Hz sample rate) data. These plots clearly image the predominant high- and low-frequency phases in the NEIC data base. Picks made from the long-period seismograms are less precise, but they reveal far more phase arrivals than the short-period picks. A number of phases resulting from reflections and phase conversions at upper mantle discontinuities can be identified in the low-frequency picks; however, a search of the short-period picks for upper mantle discontinuity phases, between P and PP and prior to P'P', has so far been unsuccessful. In the long-period S and SS picks, we observe a discrepancy in SV and SH travel times, a possible result of upper mantle anisotropy. To check the accuracy and consistency of our algorithm, we present comparisons between hand-picked times and automatic-picked times for identical seismograms. Travel-time residuals from the short-period automatic picks and data reported to the International Seismological Centre (ISC) picks exhibit a comparable amount of scatter. Histograms of the ISC residuals and automatic-pick residuals are similar in shape and width for P and PcP. These observations suggest that human picking errors are not a major contributor to the scatter observed in ISC travel times, although direct comparisons between ISC reported picks and automatic picks on particular seismograms occasionally identify operator mispicks.

Wolfe, CJ, Okubo PG, Ekstrom G, Nettles M, Shearer PM.  2004.  Characteristics of deep (<= 13 km) Hawaiian earthquakes and Hawaiian earthquakes west of 155.55 degrees W. Geochemistry Geophysics Geosystems. 5   10.1029/2003gc000618   AbstractWebsite

[ 1] High precision relocation of earthquakes recorded by the Hawaiian Volcano Observatory (HVO) seismic network provides new information on the characteristics of seismic faulting at this oceanic hot spot. Using waveform cross correlation, we have measured correlation coefficients and travel time differences for a set of 14,605 deep (greater than or equal to 13 km) earthquakes recorded from 1988 to 1998. We find that about half of the analyzed earthquakes are in similar event clusters that delineate fault zones in the lower crust and upper mantle. We suggest that much of this deep seismicity reflects rupture in the brittle lithosphere away from the magma pathways, although at Kilauea the stresses from magma movement may additionally help trigger mantle earthquakes on preexisting faults in regions with high differential ambient stresses. Focal mechanisms of similar event clusters throughout Hawaii display characteristic patterns and appear consistent with the hypothesis that deep earthquakes on preexisting faults reflect the stresses due to volcano loading and flexure. We also present the results of applying cross correlation analyses and relocation to -7000 earthquakes at all depths located west of 155.55degreesW and recorded from 1988 to 1998. The pattern of relocated earthquakes at the Kealakekua fault zone is consistent with the presence of a lowangle detachment on the west flank of Mauna Loa.

Zhang, J, Gerstoft P, Shearer PM, Yao HJ, Vidale JE, Houston H, Ghosh A.  2011.  Cascadia tremor spectra: Low corner frequencies and earthquake-like high-frequency falloff. Geochemistry Geophysics Geosystems. 12   10.1029/2011gc003759   AbstractWebsite

The discovery of non-volcanic tremor (NVT) has opened a new window to observe major Earth plate boundaries. However, the spectral characteristics of NVT have not been well studied due to poor signal-to-noise ratio (SNR) on individual seismograms. We estimate the spectral content of Cascadia tremor between 2.5 and 20 Hz by suppressing noise using array analysis, and compute empirical path corrections using nearby small earthquakes. We demonstrate that the displacement spectra of the Cascadia tremor have corner frequencies around 3-8 Hz and fall off at f(-2) to f(-3) at higher frequencies. Our results have the following implications. (1) The high-frequency falloff of tremor agrees with the observations of regular earthquakes, suggesting that tremor can be analyzed using standard spectral models. Prior analyses that have shown a tremor spectral falloff proportional to f(-1) may reflect only the spectral behavior over a limited frequency band. (2) Tremor may be no different from a swarm of microearthquakes with abnormally small stress drops on the order of kPa, likely due to the presence of fluids. Alternatively the low corner frequencies of tremor may reflect abnormally slow ruptures. (3) Fitting a standard Brune (1970) spectral model implies a moment release rate of Cascadia tremor of 3.8 x 10(10) N.m/s assuming the tremor signals are P waves (or 1.4 x 10(10) N.m/s assuming S-waves). This implies that a typical 20-day long tremor episode releases moment equivalent to Mw 5.1 (P-wave) or Mw 4.9 (S-wave), although these may be underestimates if the spectra deviate substantially from the Brune model at very low frequencies.

Lin, GQ, Thurber CH, Zhang HJ, Hauksson E, Shearer PM, Waldhauser F, Brocher TM, Hardebeck J.  2010.  A California Statewide Three-Dimensional Seismic Velocity Model from Both Absolute and Differential Times. Bulletin of the Seismological Society of America. 100:225-240.   10.1785/0120090028   AbstractWebsite

We obtain a seismic velocity model of the California crust and uppermost mantle using a regional-scale double-difference tomography algorithm. We begin by using absolute arrival-time picks to solve for a coarse three-dimensional (3D) P velocity (V(P)) model with a uniform 30 km horizontal node spacing, which we then use as the starting model for a finer-scale inversion using double-difference tomography applied to absolute and differential pick times. For computational reasons, we split the state into 5 subregions with a grid spacing of 10 to 20 km and assemble our final statewide V(P) model by stitching together these local models. We also solve for a statewide S-wave model using S picks from both the Southern California Seismic Network and USArray, assuming a starting model based on the V(P) results and a V(P)/V(S) ratio of 1.732. Our new model has improved areal coverage compared with previous models, extending 570 km in the SW-NE direction and 1320 km in the NW-SE direction. It also extends to greater depth due to the inclusion of substantial data at large epicentral distances. Our V(P) model generally agrees with previous separate regional models for northern and southern California, but we also observe some new features, such as high-velocity anomalies at shallow depths in the Klamath Mountains and Mount Shasta area, somewhat slow velocities in the northern Coast Ranges, and slow anomalies beneath the Sierra Nevada at midcrustal and greater depths. This model can be applied to a variety of regional-scale studies in California, such as developing a unified statewide earthquake location catalog and performing regional waveform modeling.

Chen, XW, Shearer PM.  2013.  California foreshock sequences suggest aseismic triggering process. Geophysical Research Letters. 40:2602-2607.   10.1002/grl.50444   AbstractWebsite

Foreshocks are one of the few well-documented precursors to large earthquakes; therefore, understanding their nature is very important for earthquake prediction and hazard mitigation. However, the triggering role of foreshocks is not yet clear. It is possible that foreshocks are a self-triggering cascade of events that simply happen to trigger an unusually large aftershock; alternatively, foreshocks might originate from an external aseismic process that ultimately triggers the mainshock. In the former case, the foreshocks will have limited utility for forecasting. The latter case has been observed for several individual large earthquakes; however, it remains unclear how common it is and how to distinguish foreshock sequences from other seismicity clusters that do not lead to large earthquakes. Here we analyze foreshocks of three M>7 mainshocks in southern California. These foreshock sequences appear similar to earthquake swarms, in that they do not start with their largest events and they exhibit spatial migration of seismicity. Analysis of source spectra shows that all three foreshock sequences feature lower average stress drops and depletion of high-frequency energy compared with the aftershocks of their corresponding mainshocks. Using a longer-term stress-drop catalog, we find that the average stress drop of the Landers and Hector Mine foreshock sequences is comparable to nearby swarms. Our observations suggest that these foreshock sequences are manifestations of aseismic transients occurring close to the mainshock hypocenters, possibly related to localized fault zone complexity, which have promoted the occurrence of both the foreshocks and the eventual mainshock.