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Fan, WY, Shearer PM.  2017.  Investigation of Backprojection Uncertainties With M6 Earthquakes. Journal of Geophysical Research-Solid Earth. 122:7966-7986.   10.1002/2017jb014495   AbstractWebsite

We investigate possible biasing effects of inaccurate timing corrections on teleseismic P wave backprojection imaging of large earthquake ruptures. These errors occur because empirically estimated time shifts based on aligning P wave first arrivals are exact only at the hypocenter and provide approximate corrections for other parts of the rupture. Using the Japan subduction zone as a test region, we analyze 46 M6-M7 earthquakes over a 10year period, including many aftershocks of the 2011 M9 Tohoku earthquake, performing waveform cross correlation of their initial P wave arrivals to obtain hypocenter timing corrections to global seismic stations. We then compare backprojection images for each earthquake using its own timing corrections with those obtained using the time corrections from other earthquakes. This provides a measure of how well subevents can be resolved with backprojection of a large rupture as a function of distance from the hypocenter. Our results show that backprojection is generally very robust and that the median subevent location error is about 25km across the entire study region (approximate to 700km). The backprojection coherence loss and location errors do not noticeably converge to zero even when the event pairs are very close (<20km). This indicates that most of the timing differences are due to 3-D structure close to each of the hypocenter regions, which limits the effectiveness of attempts to refine backprojection images using aftershock calibration, at least in this region.

Warren, LM, Shearer PM.  2000.  Investigating the frequency dependence of mantle Q by stacking P and PP spectra. Journal of Geophysical Research-Solid Earth. 105:25391-25402.   10.1029/2000jb900283   AbstractWebsite

Using seismograms from globally distributed, shallow earthquakes between 1988 and 1998, we compute spectra for P arrivals from epicentral distances of 40 degrees to 80 degrees and PP arrivals from 80 degrees to 160 degrees. Selecting records with estimated signal-to-noise ratios greater than 2, we find 17,836 P and 14,721 PP spectra. We correct each spectrum for the known instrument response and for an omega (-2) source model that accounts for varying event sizes. Next, we stack the logarithms of the P and PP spectra in bins of similar source-receiver range. The stacked log spectra, denoted as log(D'(P)) and log(D'(PP)), appear stable between about 0.16 and 0.86 Hz, with noise and/or bias affecting the results at higher frequencies. Assuming that source spectral differences are randomly distributed, then for shallow events, when the PP range is twice the P range, the average residual source spectrum may be estimated as 2 log(D'(P))- log(D'(PP)), and the average P wave attenuation spectrum may be Estimated as log(D'(PP)) - log(D'(P)). The residual source spectral estimates exhibit a smooth additional falloff as omega (-0.15+/-0.05) between 0.16 and 0.86 Hz, indicating that omega (-2.15+/-0.05) is an appropriate average source model for shallow events. The attenuation spectra show little distance dependence over this band and have a P wave (t) over bar* value of similar to0.5 s. We use (t) over bar* measurements from individual P and PP spectra to invert for a frequency-independent Q model and find that the upper mantle is nearly 5 times as attenuating as the lower mantle. Frequency dependence in Q, is difficult to resolve directly in these data but, as previous researchers have noted, is required to reconcile these values with long-period Q estimates. Using Q model QL6 [Durek and Ekstrom, 1996] as a long-period constraint, we experiment with fitting our stacked log spectra with an absorption band model. We find that the upper corner frequency f(2) in the absorption band must be depth-dependent to account for the lack of a strong distance dependence in our observed (t) over bar* values. In particular, our results indicate that f(2) is higher in the top 220 km of the mantle than at greater depths; the lower layer is about twice as attenuating at 1 Ha than at 0.1 Hz, whereas the upper mantle attenuation is relatively constant across this band.

Shearer, PM.  2009.  Introduction to Seismology. :396., Cambridge: Cambridge University Press
Kiser, E, Ishii M, Langmuir CH, Shearer PM, Hirose H.  2011.  Insights into the mechanism of intermediate-depth earthquakes from source properties as imaged by back projection of multiple seismic phases. Journal of Geophysical Research-Solid Earth. 116   10.1029/2010jb007831   AbstractWebsite

This study investigates the spatial and temporal distribution of energy release of large, intermediate-depth earthquakes using a modified back projection technique first used to study the 2004 Sumatra-Andaman megathrust event. Multiple seismic phases are included in the back projection analysis, which provides the capability to determine the energy distribution with respect to depth and time. A total of 22 intermediate-depth earthquakes with moment magnitudes greater than or equal to 6.5 are investigated with hypocentral depths between 100 and 300 km. For most of these events, the vertical extent of energy release is either below the resolution of this study (<= 5 km) or slightly above (<= 15 km). This observation agrees with previous studies that find large, intermediate-depth earthquakes have subhorizontal rupture planes. The results also show a significant portion of the events have multiple rupture planes that are well separated in depth. The closeness in time of the ruptures on separate planes and the distance between the planes suggest dynamic triggering where the P waves from the first rupture initiate rupture on the second plane. We propose that a dehydration embrittlement mechanism combined with preferentially hydrated subhorizontal faults can explain the observations of dominant subhorizontal rupture planes and the frequent occurrence of rupture complexity involving multiple subevents.

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.

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.

Aster, RC, Shearer PM.  1992.  Initial Shear-Wave Particle Motions and Stress Constraints at the Anza Seismic Network. Geophysical Journal International. 108:740-748.   10.1111/j.1365-246X.1992.tb03465.x   AbstractWebsite

We use focal plane solutions to constrain principal stress directions in the vicinity of six Anza Network stations which show evidence for shallow shear wave anisotropy in the vicinity of the Anza seismic gap region of the San Jacinto fault. Faulting near all stations is consistent with approximately N-S maximum compressive stress. Five of these stations show nearly N-S initial particle motion alignment, consistent with the anisotropy-stress relationship expected for stress-aligned microcracks. However, one station (KNW) has a well-defined preferred initial shear wave polarization direction of N40-degrees-W. Although this polarization direction differs dramatically from the local maximum compressive stress direction, it is consistent with the anisotropy expected due to a local alignment of anisotropic bedrock minerals, particularly biotite. Thus, anisotropy observed at this station most likely reflects a fixed, palaeostrain alignment of anisotropic minerals and/or microcracks and does not require a dependence on the current stress field.

Cochran, ES, Shearer PM.  2006.  Infrasound events detected with the Southern California Seismic Network. Geophysical Research Letters. 33   10.1029/2006gl026951   AbstractWebsite

We examine continuous data from the Southern California Seismic Network from 2003 and identify infrasound acoustic waves from 76 previously undetected events. Using waveform cross-correlation of the signal envelope functions, we determine their relative arrival times and estimate source locations. The waves travel at acoustic speeds of 320 m/s and are observed in seismic records up to 450 km from their probable source locations off the west coast of southern California. The dominant daylight occurrence of the events points to a man-made source related to military activity. The events are mostly recorded in the winter and spring when atmospheric conditions trap acoustic energy near the Earth's surface and favor propagation to the west. These results suggest that data from regional and global seismic networks can supplement observations from infrasound arrays for Comprehensive Test Ban Treaty monitoring and geophysical applications such as volcano monitoring, bolide detection, atmospheric acoustic sources and atmospheric tomography.

Shearer, PM.  1997.  Improving local earthquake locations using the L1 norm and waveform cross correlation: Application to the Whittier Narrows, California, aftershock sequence. Journal of Geophysical Research-Solid Earth. 102:8269-8283.   10.1029/96jb03228   AbstractWebsite

Experiments with different earthquake location methods applied to aftershocks of the October 1, 1987, Whittler Narrows earthquake in California (M-L=5.9) suggest that local event locations can be greatly improved through the use of the L1 norm, station corrections and waveform cross correlation. The Whittler Narrows sequence is a compact cluster of over 500 events at 12 to 18 km depth located within the dense station coverage of the Southern California Seismic Network (SCSN), a telemetered network of several hundred short-period seismographs. SCSN travel time picks and waveforms obtained through the Southern California Earthquake Center are examined for 589 earthquakes between 1981 and 1994 in the vicinity of the mainshock. Using a smoothed version of the standard southern California velocity model and the existing travel time picks, improved location accuracy is obtained through use of the L1 norm rather than the conventional least squares (L2 norm) approach, presumably due to the more robust response of the former to outliers in the data. A large additional improvement results from the use of station terms to account for three-dimensional velocity structure outside of the event cluster. To achieve greater location accuracy, waveforms for these events are resampled and low-pass filtered, and the P and S wave cross-correlation functions are computed at each station for every event pair. For those events with similar waveforms, differential times may be obtained from the cross-correlation functions. These times are then combined with the travel time picks to invert for an adjusted set of picks that are more consistent than the original picks and include seismograms that were originally unpicked. Locations obtained from the adjusted picks show a further improvement in accuracy. Location uncertainties are estimated using a bootstrap technique in which events are relocated many times for sets of picks in which the travel time residuals at the best fitting location are used to randomly perturb each pick. Improvements in location accuracy are indicated by the reduced scatter in the residuals, smaller estimated location errors, and the increased tendency of the locations to cluster along well-defined fault planes. Median standard errors for the final inversion are 150 m in horizontal location and 230 m in vertical location, although the relative locations within localized clusters of similar events are better constrained. Seismicity cross sections resolve the shallow dipping fault plane associated with the mainshock and a steeply dipping fault plane associated with a M-L=5.3 aftershock. These fault planes appear to cross, and activity began on the secondary fault plane prior to the large aftershock.

Shearer, PM.  2001.  Improving global seismic event locations using source-receiver reciprocity. Bulletin of the Seismological Society of America. 91:594-603.   10.1785/0120000238   AbstractWebsite

The leading source of error in seismic event locations is travel-time perturbations caused by three-dimensional Earth structure. The reciprocity of travel times between sources and receivers provides a method for testing the effectiveness of empirical methods for improving event locations that rely on nearby calibration events of known location. We apply this approach to travel-time residuals obtained by Engdahl et al, (1998) for almost 100,000 teleseismic events. By analyzing the residual patterns at thousands of seismic stations of known location, we characterize the spatial coherence of station/event mislocation vectors. We find that, on average, calibration events are likely to improve locations only if they are located within 100-150 km of the target events. For 84 events of known location, we find that applying source-receiver reciprocity can significantly reduce location errors by correcting for the teleseismic residual pattern observed at stations close to the target events. These results have implications for efforts to improve event locations for nuclear explosion monitoring purposes.

Wang, W, Shearer PM.  2019.  An improved method to determine coda-Q, earthquake magnitude, and site amplification: Theory and application to Southern California. Journal of Geophysical Research-Solid Earth. 124:578-598.   10.1029/2018jb015961   AbstractWebsite

Seismic coda waves can be used to constrain attenuation, estimate earthquake magnitudes, and determine site amplification factors. We have developed a new multistation and multievent method to determine these three important seismic parameters simultaneously. We analyze 642 representative local (100 km) and shallow (20 km) earthquakes with catalog magnitudes between 1.8 and 5.4 in southern California at multiple frequency bands centered at 1.5, 3, 6, and 12 Hz. We find that the length of the moving average time window can affect the measurement of coda attenuation Q(C), but our tests indicate that the optimal window length is about 15 times the dominant data period. We use linear regression to fit each coda section and use only those portions that agree with the model decay rate with a correlation coefficient larger than 0.9. For a frequency-dependent coda-Q(C) model (Q(C) = Q(0)f(n)) at 1-Hz reference frequency, our results yield estimates for Q(0) and n of 107-288 and 0.42-1.14, respectively. Our coda magnitude estimates are linearly correlated with catalog magnitudes, and our observed lateral variations in coda-Q(C) and our site amplification factors are in general agreement with previous results, although there are notable differences at some locations. This approach provides a unified, accurate, and stable method to measure coda-Q(C), earthquake magnitude, and site amplification using coda waves of locally recorded earthquakes.

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

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

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

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

Shearer, PM.  1991.  Imaging Global Body Wave Phases by Stacking Long-Period Seismograms. Journal of Geophysical Research-Solid Earth. 96:20353-&.   10.1029/91jb00421   AbstractWebsite

Stacked record sections combining over 5 years of long-period Global Digital Seismograph Network (GDSN) seismograms from shallow events show dozens of seismic body wave phases. Separate images are obtained for the vertical, radial, and transverse components using an automatic gain control algorithm which normalizes the amplitude of each trace prior to stacking. These images provide an indication of the relative signal-to-noise characteristics of different parts of the GDSN data set. Core-mantle boundary diffracted phases are particularly prominent in these stacks, which show P(diff) to 150-degrees, PKP(diff) to 200-degrees, PP(diff) to 230-degrees, S(diff) to 160-degrees, and SS(diff) to 220-degrees. High-order S surface multiples can be seen extending through the surface wave train up to ranges of 540-degrees. Two examples of possible upper mantle discontinuity reflected phases are visible, one following PPP between about 95-degrees and 125-degrees, the other following ScS3 between about 210-degrees and 240-degrees. An apparent offset between SH and SV arrivals is seen which becomes larger for the higher-order S multiples (SSS, SSSS, etc.), and appears consistent with models containing transverse isotropy in the upper mantle.

Walker, KT, Shearer PM.  2009.  Illuminating the near-sonic rupture velocities of the intracontinental Kokoxili M-w 7.8 and Denali fault M-w 7.9 strike-slip earthquakes with global P wave back projection imaging. Journal of Geophysical Research-Solid Earth. 114   10.1029/2008jb005738   AbstractWebsite

The Denali and Kokoxili strike-slip earthquakes are two of the longest recent intracontinental ruptures. Previous studies report a range of rupture velocities. Here we image these earthquakes by reverse time migration of the intermediate-frequency P wave train recorded by global broadband seismometers. This technique permits a relatively direct measure of rupture velocity (speed and direction) as constrained by the radiated seismic energy, free from restrictive assumptions or rupture speed bounds placed on the solution. We compare our results with published seismic, GPS displacement, and surface slip inversion results. Both ruptures were initially subshear and transitioned over a distance no longer than 40 km to supershear speeds close to the P wave speed of similar to 5.6 km/s. We investigate the accuracy of our results with synthetic data and experiment with using different imaging parameters and seismic subnetworks. These tests allow us to rule out the possibility of subshear speeds along the supershear segments. Although we cannot exclude supershear speeds of 4.5-6.5 km/s, our most reliable rupture velocities of similar to 5.6 km/s are close to the local P wave speeds. We hypothesize that these intracontinental faults have weak shear strengths or high breakdown slips or crustal rigidities and experience at least moderate slip or slip rate weakening. Our observations and previous published results lead us to speculate that very long, surface-extending faults with general homogeneity in prestress and fault strength, together with smaller adjacent fault segments to provide triggering, may be necessary ingredients for the sub-Rayleigh to supershear rupture speed transition in strike-slip earthquakes.